Molecular Structures and Second-Order Nonlinear Optical ... - MDPI

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Molecular Structures and Second-Order Nonlinear Optical Properties of Ionic Organic Crystal Materials Xiu Liu, Zhou Yang *, Dong Wang and Hui Cao Beijing Key Laboratory of Function Materials for Molecule & Structure Construction, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China; [email protected] (X.L.); [email protected] (D.W.); [email protected] (H.C.) * Correspondence: [email protected]; Tel.: +86-10-62333759 Academic Editor: Helmut Cölfen Received: 1 September 2016; Accepted: 25 November 2016; Published: 14 December 2016

Abstract: In recent years, there has been extensive research and continuous development on second-order nonlinear optical (NLO) crystal materials due to their potential applications in telecommunications, THz imaging and spectroscopy, optical information processing, and optical data storage. Recent progress in second-order NLO ionic organic crystal materials is reviewed in this article. Research has shown that the second-order nonlinear optical properties of organic crystal materials are closely related to their molecular structures. The basic structures of ionic organic conjugated molecules with excellent nonlinear optical properties are summarized. The effects of molecular structure, for example, conjugated π electron systems, electronic properties of donor-acceptor groups, and different counter-anion effects on second order NLO properties and crystal packing are studied. Keywords: second-order nonlinear optical material; ionic organic crystal; D-π-A conjugated molecule

1. Introduction There has been considerable interest in organic nonlinear optical (NLO) materials with large second-order optical nonlinearities due to their attractive potential applications in optical frequency conversion, integrated photonics, high-speed information processing, and THz wave generation and detection [1–3]. These materials have low dispersion of their dielectric constants (refractive index) and nonlinear optical susceptibility from direct current (low frequency) to the optical frequency range, because of their dominant electronic contribution to linear and nonlinear optical material polarizability [4–7]. This permits an ultra-fast polarizability response time compared to inorganic materials, whose dominant response usually comes from intrinsically slower acoustic and optical lattice vibrations. Additionally, the nonlinear optical figures of merit of organic materials may be several orders of magnitude higher than those of their inorganic counterparts and they have almost unlimited design possibilities [8,9]. All these advantages make organic NLO materials extremely attractive for the above mentioned applications. It has become an important research focus to design and synthesize new organic NLO materials with excellent performance. Among them, single crystals are one of the most attractive materials owing to their typically large macroscopic nonlinearities, high packing densities, and excellent long-term orientational and photochemical stabilities, as well as their superior optical quality. Second harmonic generation (SHG) is a nonlinear optical process, in which photons with the same frequency interacting with a nonlinear material are effectively combined to generate new photons with twice the energy, and therefore twice the frequency and half the wavelength of the initial photons [10]. To obtain large second-order NLO effects, it is necessary to investigate both microscopic and macroscopic NLO properties of organic materials. On the microscopic level, molecular nonlinearity is determined by the molecular structures and their electronic properties. In general, Crystals 2016, 6, 158; doi:10.3390/cryst6120158

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dipolar molecule units containing highly delocalized conjugated systems and strong electron acceptors (–NO2 , –COOH, –SO3 H, –CN, etc.) and electron donors (–NR2 , –OR, etc.) lead to large molecular first hyperpolarizabilities (β). In addition to a large first-order hyperpolarizability of the molecules, the macroscopic second order susceptibilities (χ(2) ) are strongly dependent on the relative alignment of the π-conjugated chromophores [11]. However, about 75% of the nonchiral organic chromophores crystallize centrosymmetrically and thereby exhibit no macroscopic second-order optical nonlinearity. The non-centrosymmetric arrangement of the chromophores in the crystalline lattice is one of the most crucial issues in developing ideal and highly successful second-order NLO crystalline materials [12]. There have been several approaches used to factitiously achieve non-centrosymmetry. The utilization of strong coulomb interactions, for example, the crystallization of ionic salts [13] to overcome the symmetry problem has been proved to be a simple and efficient strategy to obtain materials with large χ(2) . Inside D-π-A conjugated organic ionic species, the cation is the main source of nonlinear properties, whereas the counter-anion is used to adjust the crystal packing through coulombic interactions [14–17]. The suitable combination of cation and counter-anion would lead to high second-order optical nonlinearity. Stilbazolium organic salt occupy an important position in the field of nonlinear optics owing to their larger nonlinear optical effect. Especially, 4-N,N-dimethylamino-4,-N,-methyl-stilbazolium tosylate (DAST); this is one of the best NLO crystals with large second-order nonlinear optical susceptibilities χ111 (2) = 2020 ± 200 pm/V at 1.3 µm and χ111 (2) = 420 ± 110 pm/V at 1.9 µm due to the appropriate orientation of the chromophores in the crystal [18–20]. DAST consists of a positively charged nonlinear optical chromophore stilbazolium and a negatively charged tosylate anion. However, the upper limits of theoretical calculated macroscopic susceptibilities for DAST are by far not yet achieved [21]. Besides, the growth of highly nonlinear optical quality bulk or thin films of DAST single crystals still remains an urgent challenge. It usually takes one or two months to grow high-quality and large-size DAST crystals, and the growth process requests high precision in temperature fluctuations. Therefore it is desirable to develop new organic crystals with larger NLO properties and better crystal growth abilities compared with those of DAST [22]. 2. Physical Explanations and NLO Measurement Method 2.1. Physical Explanations A laser is a source of light and the light emitted by a laser has a very high degree of collimation and coherence. The light of a laser beam is highly monochromatic and can be focused to a minimum spot size on a material. Therefore, a focused laser beam can provide a high power per unit area. The laser light can stimulate atomic or molecular systems to produce transition and release the stored energy also as light. Therefore what we have is a process of controlling the light by light in the atomic or molecular system. When the electromagnetic field of a laser beam is illuminated on an atom or a molecule, it induces electrical polarization that gives rise to many of the unusual and interesting properties that are optically nonlinear [23]. A relationship between the polarization P induced in a molecule and the applied electric field E can be written by P = ε0 αE + ε0 βE2 + ε0 γE3 + . . .

(1)

where ε0 is the vacuum dielectric constant, α is the linear polarization, β and γ are the first and second hyperpolarizability, respectively. Furthermore, the even order tensor β, responsible for second-order NLO effects (for example SHG), requests non-centrosymmetric molecular structure and crystal packing. It is also well established that the macroscopic polarization of a bulk material under an applied strong electric field can be regarded as an averaged sum of the individual molecular polarizations [24]: (1)

(2)

(3)

P = χIJK E + χIJK E2 + χIJK E3 + . . .

(2)

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(2)

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(3)

where χIJK and χIJK are the second and third order susceptibility tensors, respectively. The expected powder measurement efficiency of a novel crystal could be estimated in some ways through a mathematical model. Firstly, some reasonable assumptions were made for calculation. Any influence of the intermolecular interactions and any contribution of the counter-anion to the nonlinearity is not considered. To estimate suppose that the stilbazolium chromophore is essentially unidimensional, with only one leading component β of the first-hyperpolarizability tensor along (2)

the orientation of the long axis in chromophore. The susceptibility tensor χIJK can be related to the microscopic first hyperpolarizabilities βijk using the oriented-gas model [25] as: (2)

χIJK = NFIJK βeff IJK

(3)

where N is the number density of the chromophores, FIJK is the correction factors due to intermolecular interactions, and βeff IJK is the effective hyperpolarizability of the crystalline system, which can be calculated by the following formula: βeff IJK = Σcosθ(I, i)·cosθ(J, j)·cosθ(K, k)βIJK

(4)

Take DAST for example, the four chromophores in the unit cell are aligned along the 3a + b and along the 3a − b crystallographic vectors, and therefore make an angle of about θ = 20◦ with respect to the polar axis a. The effective hyperpolarizability of the unit cell in the crystal of DAST only depends on the following components: 3 eff eff eff 2 βeff 111 = βcos θ = 0.83β and β221 = β212 = β122 = βcosθsin θ = 0.11β.

(5)

The powder measurement efficiency depends on N and the squared effective β components, averaged over all possible orientations . In case that only two effective components βeff 111 and eff eff eff )2 > can be expressed as βeff = β = β are non-zero, =

1 2 eff 2 eff eff (19(βeff 111 ) + 13β111 β221 + 44(β221 ) ), 105

(6)

which gives DAST = 0.14β2 for DAST [26]. 2.2. NLO Measurement Method 2.2.1. Kurtz and Perry Powder Technique for SHG Efficiency Kurtz and Perry powder technique [27] is a simple and quick experimental method which only requires the material in powder form for evaluating the SHG efficiency of nonlinear optical materials. Powders were made from single crystals using a Spex vibrating ball mill, and then graded using standard sieves. A laser beam of fundamental wave-length 1064 nm from a Q-switched Nd:YAG laser, 8 ns pulse width, with 10 Hz pulse rate was made to fall normally on the pre-packed microcapillary tube [28]. Second harmonic radiation generated by the randomly oriented microcrystals were focused by a lens and detected by the photomultiplier tube. The efficient responses for SHG mainly depend on the particle size, field-gain coefficient, the power of the fundamental beam, and the minimum beam waist [29]. 2.2.2. Maker-Fringe Technique for Optical Tensor Element The nonlinear optical tensor elements were tested by the standard Maker-fringe technique generalized for anisotropic absorbing materials. In order to avoid absorption of the generated second-harmonic wave, the experiment must be performed with a fundamental wavelength longer than twice the absorption edge of the sample. For example the measurement of the 4-N,N-dimethylamino-40 -N 0 -methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS) [30], fundamental wavelength of 1907 nm generated by stimulated Raman scattering in pressurized

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hydrogen gas was used, pumped by a Q-switched Nd:YAG laser operating at 1064 nm. A typical Maker-fringe element of of DSTMS DSTMS is is demonstrated demonstrated in in Figure Figure 1. 1. Maker-fringe data data obtained obtained by by measuring measuring the the d d111 111 element With With cc plates plates of ofDSTMS DSTMSthe thenonlinear nonlinearoptical opticalsusceptibility susceptibilityelements elementsd111 d111, , dd212 212,, and and dd122 122 could could be be measured by selecting the appropriate polarization of the fundamental and of the second harmonic measured by selecting the appropriate polarization of the fundamental and of the second harmonic waves. Quartz with with dd111 0.277pm/V pm/V µm was chosen reference substance compared 111==0.277 waves. Quartz atat 1.91.9 μm was chosen forfor thethe reference substance compared to to measurement. The nonlinear opticaltensor tensorelement elementd111 d111isisthe thelargest largestdue due to to the the preferential thethe measurement. The nonlinear optical preferential alignment alignment of of the the chromophores chromophores along along the the xx11 axis axis of of the the DSTMS. DSTMS.

Figure 1. (Color online) Maker-fringe curve obtained by rotating a c plate 4-N,N-dimethylamino-4′Figure 1. (Color online) Maker-fringe curve obtained by rotating a c plate 4-N,N-dimethylamino-40 N′-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate(DSTMS) (DSTMS) crystal around the dielectric x1 N 0 -methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate crystal around the dielectric x1 axis. axis. The impinged fundamental beam at 1.9 μm as well as the generated second-harmonic light at The impinged fundamental beam at 1.9 µm as well as the generated second-harmonic light at 0.95 µm 0.95 μm were s polarized [27]. were s polarized [27].

3. Design Design of of Second-Order Second-Order NLO NLO Organic Organic Crystal Crystal Materials Materials 3. 3.1. Design Design of of Anionic Anionic Change Change 3.1. There has has been been intensive intensive research research and and continuous continuous development development in in second-order second-order NLO NLO organic organic There crystal materials, aimed at obtaining new organic materials with large nonlinear, excellent ability of crystal materials, aimed at obtaining new organic materials with large nonlinear, excellent ability of crystal growth, growth,and andhigh hightemperature temperatureand andlight light stability. Variation of counter-ions in organic crystal stability. Variation of counter-ions in organic saltssalts has has been confirmed to be an easy and high-efficiency approach for creating crystals with large been confirmed to be an easy and high-efficiency approach for creating crystals with large second-order second-order NLO activity. Studythat haseven indicated that even veryinminor changesof inthe thecounter-anion structures of NLO activity. Study has indicated very minor changes the structures the counter-anion play important role in stacking macroscopical lattice stacking Research on using play an important role in an macroscopical lattice [31]. Research on using [31]. different counter-anions different counter-anions to induce molecule arranging with the same cation of DAST has made to induce molecule arranging with the same cation of DAST has made some achievements [32–37]. some achievements [32–37]. Well-designed counter-anions are more likely to form Well-designed counter-anions are more likely to form non-centrosymmetric structures having even non-centrosymmetric having even higher NLO properties than DAST. DASTcations derivatives higher NLO propertiesstructures than DAST. DAST derivatives (Figure 2) with ethyl-substituted were (Figure 2) with ethyl-substituted cations were designed and synthesized by Okada et al. in the early designed and synthesized by Okada et al. in the early years [38]. Counter anion exchange afforded years [38]. anion exchange afforded several SHG active crystals even ethyl-substituted several SHGCounter active crystals even in ethyl-substituted derivatives. Among them, theincrystal structure of derivatives. Among them, the crystal structure of (2d) belongs to the monoclinic space group P21 (2d) belongs to the monoclinic space group P21 with a polar b-axis. Crystal 1c gives an isomorphous with a polar b-axis. Crystal 1c gives an isomorphous crystal structure with DAST, it has excellent crystal structure with DAST, it has excellent second-order nonlinear optical properties similar to that second-order nonlinear optical properties similar to that of DAST. Moreover, it can also be applied of DAST. Moreover, it can also be applied for terahertz-wave generation. for terahertz-wave generation. In the early years, study on the appeal of acid anion replacement of DAST tended to be more In the early years, study onnew the appeal of acidsalts anion replacement of DAST to be more casual. Yang et al. investigated stilbazolium which were grouped intotended three series with casual. Yang investigated which were grouped into three 3) series with various kindsetofal. counter anionsnew [39].stilbazolium Comparisonsalts of the experimental results (Figure revealed various kinds of counter anions [39]. Comparison of the experimental results (Figure 3) revealed that introducing especially long counter anions into stilbazolium derivatives does not only restrain thatSHG introducing long counter stilbazolium derivatives does not only restrain the activityespecially but also influences theanions growthinto of single crystals. On the other hand, it is more the SHG activity but also influences the growth of single crystals. On the other hand, it is more probable to induce a non-centrosymmetic structure with higher SHG activity than that of DAST, probable to induce a non-centrosymmetic structure with higher SHG activity than that of DAST, while using a bulky counter anion compared to the tosylate of DAST, even a relatively bulk while using a may bulky compared to theoftosylate of DAST,For even a relatively counter-anion stillcounter lead to anion difficulty of nucleation the compounds. example, DSNSbulk [34] 0 0 counter-anion may still lead to difficulty of nucleation of the compounds. For example, DSNS (4-N,N-dimethylamino-4 -N -methyl-stilbazolium 2-naphthalenesulfonate) was highlighted for[34] its (4-N,N-dimethylamino-4′-N′-methyl-stilbazolium was nm, highlighted its second-order nonlinearity, which is 50% higher 2-naphthalenesulfonate) than that of DAST at 1907 but it isfor quite 0 -N 0 -methyl-stilbazolium second-order 50% higher than that of DAST at 1907 nm, but it is quite difficult to get nonlinearity, its bulk singlewhich crystal.isDSTMS [40] (4-N,N-dimethylamino-4 difficult to get its bulk single crystal. DSTMS [40] (4-N,N-dimethylamino-4′-N′-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate) with similar second-order susceptibilities with DAST possesses has 2,4,6-trimethylbenzenesulfonate) with similar second-order susceptibilities with DAST possesses has better growth characteristics. A size of 33 × 33 × 2 mm3 of a DSTMS bulk single crystal can be obtained by a solution growth method without using a seed crystal. The development of DSTMS has

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better growth characteristics. A size of 33 × 33 × 2 mm3 of a DSTMS bulk single crystal can be Crystals 6, 158 5 of 19 has obtained by2016, a solution growth method without using a seed crystal. The development of DSTMS Crystals 2016, 6, 158 5 of 19 demonstrated that the long-term crystal growth problem hindering development of NLO materials demonstrated that the long-term crystal growth problem hindering development of NLO materials can in principle be solved. A strong effect of intermolecular interactions suchofasNLO hydrogen bonds demonstrated long-term crystal problem hindering development materials can in principlethat be the solved. A strong effectgrowth of intermolecular interactions such as hydrogen bonds on on macroscopic non-linerarity of stilbazolium salts has been observed. Both the results of powder can in principle be solved. Aofstrong effect ofsalts intermolecular interactions as hydrogen bonds on macroscopic non-linerarity stilbazolium has been observed. Bothsuch the results of powder SHG SHG macroscopic measurement and crystal growth showed that minor modification of tosylate can induce non-linerarity of stilbazolium salts has been observed. Both the results of powder SHG measurement and crystal growth showed that minor modification of tosylate can induce new new measurement and crystal growth showed that minor modification of tosylate can induce new stilbazolium derivatives with high SHG efficiencies and improved crystal-growing characteristics. stilbazolium derivatives with high SHG efficiencies and improved crystal-growing characteristics. stilbazolium derivatives with high SHG efficiencies and improved crystal-growing characteristics.

Figure 2. Molecule structures of ionic salts synthesized. Figure 2. 2. Molecule of ionic ionicsalts saltssynthesized. synthesized. Figure Moleculestructures structures of

Figure 3. Molecule structures of the stilbazolium derivatives (a) and single crystals of 4-N,NFigure 3. Molecule structures of the stilbazolium derivatives (a) and single crystals of 4-N,Ndimethylamino-4′-N′-methyl-stilbazolium 2-naphthalenesulfonate (DSNS) (b) (a) and 4-N,N-dimethylaminoFigure 3. Molecule structures of the stilbazolium derivatives and single crystals of dimethylamino-4′-N′-methyl-stilbazolium 2-naphthalenesulfonate (DSNS) (b) and 4-N,N-dimethylamino4′-N′-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS) (c). 0 -N 0 -methyl-stilbazolium 4-N,N-dimethylamino-4 2-naphthalenesulfonate (DSNS) (b) and 4-N,N4′-N′-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS) (c).

dimethylamino-40 -N 0 -methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS) (c).

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3.2. Design and Change of the Stilbazolium Cation 3.2. Design and Change of the Stilbazolium Cation The benzene of the electron donor and the pyridine of the electron acceptor are ideal The benzene benzene of the electron donor and the pyridine of theacceptor are of ideal the electron donor and pyridine ofthe thebeginning electron areacceptor ideal combinations combinations for of one other in the cation ofthe DAST. So at ofelectron grouping of changes the combinations other cation of at DAST. So at part the beginning grouping of changes of the for onecation, other for in one the cationinofthe DAST. So beginning grouping of changes of the DAST DAST most researches were inclined tothe replace Aofand E asofmentioned below, while (2) values DAST cation, most researches were inclined to changed. replace A significantly and mentioned below, while cation, most researches were inclined to not replace partpart A and EE asasmentioned while main channel of electron transport was The large χbelow, of the (2) (2) main electron transport was not changed. significantly large cations of the channel of electron transport was changed. The significantly χ values values above-mentioned ionic organic crystals arenot due to high βThe values of stilbazolium and due to above-mentioned ionic due stilbazolium and due to above-mentioned ionic organic organiccrystals crystals are duetotohigh high βvalues values of stilbazolium cations and due preferred non-centrosymmetric packing.are Generally, theβlarge β of values of ionic cations chromophores are preferred non-centrosymmetric packing. Generally, the ββvalues chromophores are to preferred non-centrosymmetric packing. Generally, thelarge large valuesof ofionic ionicof chromophores mainly provided by strong electron acceptor and electron donor characteristics the cation parts. mainly provided by strongspecies electroncan acceptor and donor characteristics of cation provided acceptor and electron electron donor characteristics of the the cation parts. parts. Thus ionic π-conjugated be highly polarized when an appropriate substituent is ionic π-conjugated species can be highly polarized when an appropriate substituent is Thus ionic π-conjugated species can be highly polarized when an appropriate substituent is introduced introduced into the cation parts. Therefore, numerous studies on modifying the cation of introduced into theTherefore, cation parts. on of modifying thecompounds cation of into the cation parts. numerous studies onand modifying the cation stilbazolium stilbazolium compounds have been Therefore, carried outnumerous somestudies interesting achievements have been stilbazolium compounds been carried out and some interesting achievements have been attained [37–39]. As shown in Figure 4, the cation of DAST could be attained divided into five As parts (A–E). have been carried out andhave some interesting achievements have been [37–39]. shown in attained [37–39]. As shown in Figure 4, the cation of DAST could be divided into five parts (A–E). Figure the cation of DAST could be divided into five partson (A–E). Many research have Many 4, research investigations have been concentrated changing some of investigations the five parts to Many research investigations haveproperty concentrated on changing some of nonlinear the five property parts to been concentrated onorder changing some ofbeen the five to improve the second order improve the second nonlinear ofparts the cation molecule. improve the second order nonlinear property of the cation molecule. of the cation molecule.

Figure 4. Molecular structure of the modification of DAST cation. Figure 4. Molecular Molecular structure of the modification of DAST cation.

Part A Part A This will be more conductive for the formation of charge transfer system resonances and the This willof be more conductive the formation system resonances and the broadening the scope for for π-electric chargeof charge flow, transfer while the electron-withdrawing/ broadening scope for π-electric charge flow, while electron-withdrawing/ ofof the the scope forofπ-electric charge flow, whilestronger. the electron-withdrawing/electron-repulsing electron-repulsing ability donor/acceptor becomes Large βthe values can be obtained by electron-repulsing ability of donor/acceptor becomes stronger. Large β values can be obtained by ability of donor/acceptor becomes stronger. Large groups β valuesinto cantheir be obtained by structure. introducing stronger introducing stronger electron donor and acceptor molecular Because of introducing stronger electron donor into and their acceptor groups intocharge theirBecause molecular Because of electron donor and acceptor groups molecular structure. astructure. stronger push–pull a stronger push–pull NLO chromophores enable an efficient transferoffrom the donor to the aNLO stronger push–pull NLO an chromophores enable an efficient from the donor to the chromophores enable efficient charge transfer from thecharge donor transfer to the acceptor upon excitation. acceptor upon excitation. acceptor upon excitation. Okada et synthesized ion-complex crystal composed of protonated merocyanine and etal. al. synthesizedan anorganic organic ion-complex crystal composed of protonated merocyanine Okada et al. synthesized an organic ion-complex crystal composed of protonated merocyanine p-toluenesulfonate anion, i.e.,i.e., 1-methyl-4(2-(4-hydroxyphenyl)vinyl) pyridinium 4-toluenesufonate and p-toluenesulfonate anion, 1-methyl-4(2-(4-hydroxyphenyl)vinyl) pyridinium 4-toluenesufonate and p-toluenesulfonate anion, i.e., 1-methyl-4(2-(4-hydroxyphenyl)vinyl) pyridinium 4-toluenesufonate (MC-PTS shown in in Figure Figure 5). MC-PTS MC-PTS showed showed aa SHG SHG intensity intensity of of 0.14 0.14 times times as as as shown 5). large as that of (MC-PTS as shownby inthe Figure 5). MC-PTS showed a SHG intensity of as large as that of DAST determined method at aatpumping wavelength of 0.14 1064 nm. Single crystal X-ray determined by thepowder powder method a pumping wavelength of times 1064 nm. Single crystal DAST determined the MC-PTS powder method at in aspace pumping wavelength of which 1064 nm. Single crystal analysis showsshows thatby MC-PTS crystallizes in the group of P1,ofin which molecular dipoles are X-ray analysis that crystallizes the space group P1, in molecular dipoles X-ray analysis shows MC-PTS crystallizes inindicated the space group P1, insulfonate which molecular dipoles ideally aligned in one direction. It was alsoalso indicated thatthat the tetrahedral anion plays the are ideally aligned in that one direction. It was the of tetrahedral sulfonate anion plays are ideally in one direction. It was also indicated that the tetrahedral part of a of chiral handle to cause non-centrosymmetric space groups [40].[40]. sulfonate anion plays the part aaligned chiral handle to cause non-centrosymmetric space groups the part of a chiral handle to cause non-centrosymmetric space groups [40]. HO HO SO3SO3-

N+ N+

MC-PTS MC-PTS

Figure 5. 5. Molecular Molecular structure structureofof1-methyl-4(2-(4-hydroxyphenyl)vinyl) 1-methyl-4(2-(4-hydroxyphenyl)vinyl) pyridinium pyridinium 4-toluenesufonate 4-toluenesufonate Figure Figure 5. Molecular structure of 1-methyl-4(2-(4-hydroxyphenyl)vinyl) pyridinium 4-toluenesufonate (MC-PTS). (MC-PTS). (MC-PTS).

The compound 3′,4-dihydroxy-4-N-methylstilbazolium tosylate (Figure 6) was prepared by The compound 30 ,4-dihydroxy-4-N-methylstilbazolium tosylate (Figure 6) was prepared by The wasofprepared by Marder etcompound al. in the 3′,4-dihydroxy-4-N-methylstilbazolium early days [41]. A large powder SHG tosylate efficiency(Figure of 0.1 6) times DAST was Marder et al. in the early days [41]. A large powder SHG efficiency of 0.1 times of DAST was Marder et al. in the early days [41]. A large powder SHG efficiency of 0.1 times of DAST was

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observed for this thiscompound. compound.X-ray X-ray studies exhibited crystallizes the triclinic space observed for studies exhibited thatthat this this crystallizes in theintriclinic space group group with one cation and one in anion in the unit cell. P1 withP1one cation and one anion the unit cell.

Figure Figure 6. 6. Molecular Molecular structure structure of of 3′, 30 , 4-dihydroxy-4-N-methylstilbazolium 4-dihydroxy-4-N-methylstilbazolium tosylate. tosylate.

As electron donor donor of of the the stilbazolium stilbazolium cation, cation, the the dimethylamino dimethylamino group group [-N(CH [-N(CH3))2]] is much As electron 3 2 is much stronger dihydroxyl groups, of the the reported reported stilbazolium stilbazolium cations cations stronger than than the the hydroxyl hydroxyl and and dihydroxyl groups, so so most most of possess a dimethylamino group at the position of Part A, for example, DAST, DSNS and DSTMS. possess a dimethylamino group at the position of Part A, for example, DAST, DSNS and DSTMS. Part Part B B In studies found thethe replacement of part A and E did In recent recent years, years,ititisisprobable probablethat thatasasmany many studies found replacement of part A and E not promising results, following investigations turnedturned to parttoB part and D which a critical did produce not produce promising results, following investigations B and D play which play a role in role nonlinear effects.effects. Research showsshows that that a hetero atom cancanpromote critical in nonlinear Research a hetero atom promoteoror prevent prevent the the pushing/withdrawing pushing/withdrawing effect effect of of aa donor/acceptor donor/acceptor[42]. [42].Thereby Therebythe theintroduction introduction of of heterocyclic heterocyclic rings rings can improvesecond-order second-ordernonlinearity. nonlinearity. It effective is effective to introduce an aromatic heterocyclic ringa can improve It is to introduce an aromatic heterocyclic ring with with a smaller resonance energy instead of a benzene ring into chromophores, because the higher smaller resonance energy instead of a benzene ring into chromophores, because the higher resonance resonance energy of the ring maynonlinear easily cause nonlinear to reduce or energy of the benzene ringbenzene may easily cause susceptibility to susceptibility reduce or saturate, whereas saturate, whereas the different electron density of the aromatic heterocyclic ring can adjust the the different electron density of the aromatic heterocyclic ring can adjust the second-order nonlinear second-order nonlinear optical susceptibility as an auxiliary to the acceptor [43]. optical susceptibility as an auxiliary to the acceptor [43]. Quantum Quantum mechanical mechanical studies studies have have suggested suggested that that thiophene thiophene and and pyrrole pyrrole are are more more electron-rich electron-rich heteroaromatic rings than benzene ring [15] series of second-order NLO chromophores with heteroaromatic rings than benzene ring [15] series of second-order NLO chromophores with pyrrole pyrrole and auxiliary electron donors was was synthesized synthesized by Ma et et al. al. Hyper-Rayleigh Hyper-Rayleigh and thiophene thiophene as as (Figure (Figure 7) 7) auxiliary electron donors by Ma scattering scattering measurements measurements at at an an incident incident wavelength wavelength of of 780 780 nm nm revealed revealed moderate moderate to to very very large large −30−esu −30 esu) 30 − 30 molecular hypolarizabilities (151 × 10 and 155 × 10 for chromophores of PPTs TPTs, molecular hypolarizabilities (151 × 10 esu and 155 × 10 esu) for chromophores ofand PPTs and respectively. Their Their magnitude of second-order optical nonlinearities is is indicated TPTs, respectively. magnitude of second-order optical nonlinearities indicatedbybyaa first first (2)) at the hyperpolarizability and aa second-order second-order NLO NLO susceptibility susceptibility (χ (χ(2) hyperpolarizability ββ at at the the molecular molecular level level and ) at the (2), the optimal combination of both the molecular obtain materials materials with with large large χχ(2) molecular integral integral level level [44]. [44]. To To obtain , the optimal combination of both the molecular molecular second-order second-order hyperpolarizability hyperpolarizability ββ and and the the orientation orientation in in the the bulk bulk are are required. required. In In other other words, crystals of organic chromophores with large β belonging to non-centrosymmetric space words, crystals of organic chromophores with large β belonging to non-centrosymmetric space groups groups are of theforms expected forms for bulk materials of largeonly χ(2)a. few However, only a with few are one of theone expected for bulk materials of large χ(2) . However, chromophores chromophores with large β have been developed into beneficial bulk crystalline materials for large β have been developed into beneficial bulk crystalline materials for practical NLO applications. practical NLO applications. This is because most of the chromophores tend to pack in crystals with This is because most of the chromophores tend to pack in crystals with centrosymmetric space groups centrosymmetric groupsofwhich components of χ(2), or althoughstructures, they achieve which remove all space components χ(2) , orremove althoughallthey achieve non-centrosymmetric the non-centrosymmetric structures, the β is also direction-sensitive, and the orientation of their β is also direction-sensitive, and the orientation of their molecular dipole moments in the crystalline molecular in the crystalline with respect to the polar acrystal axes areNLO not solid with dipole respectmoments to the polar crystal axes aresolid not optimum for maximizing second-order optimum for maximizing a second-order NLO response [45]. response [45]. Li et Li et al. al. synthesized synthesized two two series series of of thienyl-substituted thienyl-substituted pyridinium pyridinium salts salts (Figure (Figure 8) 8) with with different different counter-anions of the the hydrous/anhydrous hydrous/anhydrous phase counter-anions to to investigate investigate the the effect effect of phase on on crystal crystal packing packing [31]. [31]. Single crystals of two kinds of salts (Figure 9) were successfully obtained from methanol by a slow Single crystals of two kinds of salts (Figure 9) were successfully obtained from methanol by a slow evaporation method.Crystal Crystalstructure structure analysis revealed crystallized the monoclinic evaporation method. analysis revealed thatthat theythey crystallized in theinmonoclinic space space and P⎯1, respectively. TPNSa forms a hydrated non-centrosymmetric phase with group group P1 andP1 P 1, respectively. TPNS forms hydrated non-centrosymmetric phase with powder powder SHG efficiency 0.40 times of DAST at a wavelength of λ = 2109 nm. TPDMS gives an SHG efficiency 0.40 times of DAST at a wavelength of λ = 2109 nm. TPDMS gives an anhydrous anhydrous phase and crystallizes in centrosymmetric structures without a SHG signal. Their study phase and crystallizes in centrosymmetric structures without a SHG signal. Their study indicated indicated that thienyl-substituted pyridinium salts tend to crystallize in non-centrosymmetric structure with a hydrous phase [46].

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that thienyl-substituted pyridinium salts tend to crystallize in non-centrosymmetric structure with a hydrous phase [46]. Crystals 2016, 6, 158 8 of 19 Crystals 2016, 6, 158 Crystals 2016, 6, 158

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Figure7.7.Molecular Molecular structures of of TPTSS and Figure andPPT PPTSSS.. . Figure 7. Molecular structures structures of TPT TPTS and PPT Figure 7. Molecular structures of TPTS and PPTS.

Figure 8. Molecular structures of TPNS and TPTMS. Figure 8. Molecular Molecular structures of TPNS and TPTMS. Figure of TPNS TPNS and and TPTMS. TPTMS. Figure8. 8. Molecular structures structures of

Figure 9. Single crystals of TPNS (a) and TPTMS (b). Figure 9. Single crystals of TPNS (a) and TPTMS (b). Figure 9. Single crystals of TPNS (a) and TPTMS (b). Figure 9. Single crystals of TPNS (a) and TPTMS (b).

Tsuji et al. replaced dimethylamino by methoxyl as electron donor and used a naphthalene Tsuji et al. replaced dimethylamino by methoxyl as electron donor and used a naphthalene Tsuji et al. replaced dimethylamino by methoxyl as sulfonic electron acid donor used a naphthalene ringTsuji to increase the conjugated system by (Figure 10). After ionand replacement, the novel et al. replaced dimethylamino methoxyl as electron donor a naphthalene ring ring to increase the conjugated system (Figure 10). After sulfonic acid and ion used replacement, the novel −30 ring to increase the conjugated system (Figure 10). After sulfonic acid ion replacement, the novel crystal with a large hyperpolarizability β0 of 870 × 10 −30 esu was found to give second-order NLO tocrystal increase theaconjugated system (Figureβ010). After sulfonic acid ion replacement, the novel crystal with large hyperpolarizability of 870 × 10 esu was found to give second-order NLO crystalcrystals with a [47]. large hyperpolarizability β0 of 870 × 10−30 esu was found to give second-order NLO active active crystals [47]. with a large hyperpolarizability β0 of 870 × 10−30 esu was found to give second-order NLO active active crystals [47]. crystals [47].

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Figure 10. Molecular structure of ionic salts with naphthalene ring. Figure 10. Molecular structure of ionic salts with naphthalene ring.

Part C Part C

Figure 10. Molecular structure of ionic salts with naphthalene ring.

Many research studies have been carried out on introducing different electron delocalization

Part C research studies have been carried out on introducing different electron delocalization Many channels into D-π-A structures. There are several widely used electron delocalization channels, channels into D-π-A There are several widely used electron delocalization channels, Many research studies been carried out on introducing electron delocalization Figure 10.have Molecular of ionic salts with naphthalene ring. among them, C=C, structures. C=N, C≡C, and N=Nstructure (Figure 11) which have beendifferent fully studied [34,45]. Different channels into D-π-A structures. There are several widely used electron delocalization channels, among them, C=C, C=N, C ≡ C, and N=N (Figure 11) which have been fully studied [34,45]. electron delocalization channels have different effects on the nonlinear susceptibility of the among them, C=C, C=N, C≡C, and N=N (Figure 11) which have been fully studied [34,45]. Different Part C Different electron channels have effects onofthe nonlinear susceptibility of molecule. Theredelocalization are two main reasons which leaddifferent to the phenomenon second order polarizability electron delocalization channels have different effects on the nonlinear susceptibility of the increasing rapidly with growing conjugated chain length: on one hand, the second order the molecule. There are two main reasons which lead to the phenomenon of second order polarizability Many research studies have been carried out on introducing different electron delocalization molecule. There are two main reasons which lead to the phenomenon of second order polarizability susceptibility the molecules is directly proportional to on theused square of the length of order the conjugated increasing rapidly growing conjugated chain length: one hand, thedelocalization second susceptibility channels intoofwith D-π-A structures. There are several widely electron channels, increasing rapidly with growing conjugated chaindonor length: on one groups hand, the second order system. On the other hand, steric hindrance between and accepter will reduce as the among them, C=C, C=N, C≡C, and N=N (Figure 11) which have been fully studied [34,45]. Different of the molecules is directly proportional to the square of the length of the conjugated system. On the susceptibility of the molecules is directly proportional to the square of the length of the conjugated conjugated system elongates. electron delocalization channels have different effects on the nonlinear susceptibility of the othersystem. hand, On steric hindrance between donor and accepter groups will reduce as the conjugated the other hand, steric hindrance between donor and accepter groups will reduce as the molecule. There are two main reasons which lead to the phenomenon of second order polarizability system elongates. conjugated system elongates. increasing rapidly with growing conjugated chain length: on one hand, the second order susceptibility of the molecules is directly proportional to the square of the length of the conjugated system. On the other hand, steric hindrance between donor and accepter groups will reduce as the Styryl-pyridinium Azobenzene Schiff base Tolan conjugated system elongates. Figure 11. Molecular structures of electron delocalization channels. Styryl-pyridinium Azobenzene Schiff base

Tolan

Figure 11. Molecular structures of electron delocalization channels. (1) Styryl-Pyridinium FigureDerivatives 11. Molecular structures of electron delocalization channels.

Styryl-pyridinium one of the most efficient electron delocalized conjugated Tolan skeletons. So far Azobenzene Schiff base (1) Styryl-pyridinium Styryl-Pyridinium is Derivatives most of the reported NLO molecules include this fragment. The carbon and nitrogen atoms on the (1) Styryl-Pyridinium Derivatives Figure 11. Molecular structures of electron delocalization channels. Styryl-pyridinium is one of the most electron delocalized conjugated skeletons. So far pyridine ring of styryl-pyridinium are inefficient sp2 hybrid orbitals, each atom on the ring forms a most of the system reportedwith molecules include this afragment. The and nitrogen atoms on theSo far Styryl-pyridinium isNLO one of π theorbital most efficient electron delocalized conjugated skeletons. conjugated one without lone pair of carbon electrons on the nitrogen atom. (1) Styryl-Pyridinium Derivatives are in sp2 hybrid orbitals, each atom on the ring forms a pyridine ring of styryl-pyridinium most of the NLO moleculesdoes include this fragment. The carbon and Marder nitrogen atoms Therefore, thereported formation of stilbazolium not destroy the cyclic conjugated system. [48] conjugated system with one π orbital without a lone pair of electrons on the nitrogen atom. Styryl-pyridinium is one of the most efficient electron delocalized conjugated skeletons. So far pointed out that the NLO charge transfer was orbitals, considered to beatom achieved by ring on the pyridine ring of molecule styryl-pyridinium are mechanism in sp2 hybrid each on the Therefore, the formation of molecules stilbazolium does not destroy cyclic conjugated system. Marder [48] most of the NLO include this fragment. The carbon nitrogen atoms on the resonance ofreported two limiting forms, ground state and the excited state. Second-order forms a conjugated system with one π orbital without a lone pairand of electrons onmolecular the nitrogen pointed that molecule charge mechanism was considered to be by pyridineout ring ofthe styryl-pyridinium are in transfer sp2 of hybrid orbitals, eachforms atomare onclose the ring forms a polarizabilities tend toNLO be larger as of the energies these two limiting toachieved each other; atom. Therefore, the formation stilbazolium does not destroy the cyclic conjugated system. resonance of two limiting forms, ground state and excited state. Second-order molecular conjugated system with one π orbital without a lone pair of electrons on the nitrogen atom. such as [48] dimethylamino-styrylpyridinium (Figure 12). Thetransfer molecules lost aromaticities from Marder pointed out that the NLO molecule charge mechanism considered to Therefore, form the formation stilbazolium does notofdestroy the cyclic conjugated [48] polarizabilities tend beof larger as the energies these two limiting forms aresystem. closewas toMarder each other; conjugated (i) totoconjugate form (ii). However, as first approximation (ignoring the interaction be achieved by resonance of two limiting forms, ground state and excited state. Second-order pointed out that the NLO molecule charge transfer mechanism was considered to be or achieved by such dimethylamino-styrylpyridinium (Figure 12). The ismolecules aromaticities from of the as corresponding anion), intramolecular charge-transfer not just aslost isolated as neutral molecular polarizabilities tend to be larger as the energies of these two limiting forms are close to resonance of two limiting forms, ground state and excited state. Second-order molecular conjugated form (i) to conjugate form (ii). However, as first approximation (ignoring the interaction molecules. The energy of the two kinds of styryl-pyridinium conjugate forms is close compared polarizabilities tend to be larger as the energies of these two limiting forms are close to each other; each other; such as dimethylamino-styrylpyridinium (Figure 12). The molecules lost aromaticities of the corresponding anion), intramolecular charge-transfer is not just as isolated or as neutral with the neutral molecule, which could improve hyperpolarizability β values. suchconjugated as dimethylamino-styrylpyridinium (Figure 12). The molecules lost aromaticities from the molecules. The energy two kinds of styryl-pyridinium conjugate forms is close compared from formof (i)the to conjugate form (ii). However, as first approximation (ignoring conjugated form (i) to conjugate form (ii). However, as first approximation (ignoring the interaction with the neutral molecule, which could improve hyperpolarizability β values. interaction of the corresponding anion), intramolecular charge-transfer is not just as isolated or of the corresponding anion), intramolecular charge-transfer is not just as isolated or as neutral as neutral molecules. The energy of the two kinds of styryl-pyridinium conjugate forms is close molecules. The energy of the two kinds of styryl-pyridinium conjugate forms is close compared compared with the neutral molecule, which could improve hyperpolarizability β values. with the neutral molecule, which could improve hyperpolarizability β values.

Figure 12. Two kinds of dimethylamino-styryl-pyridinium conjugate forms. 12. Two kinds of dimethylamino-styryl-pyridinium conjugate forms. (2) AzobenzeneFigure Derivatives

and its derivatives are characterized by reversible transformations [49] from the (2) Azobenzene Azobenzene Derivatives generally more stable form to less stable cis form upon irradiation with UV or visible light Figure 12. Two kinds of dimethylamino-styryl-pyridinium conjugate forms. Figure 12.trans Two kinds ofthe dimethylamino-styryl-pyridinium conjugate forms. Azobenzene its derivatives are characterized by reversible transformations [49] from[50]. the to yield a photo and stationary composition that is wavelength and temperature dependent generally more stable trans form to the less stable cis form upon irradiation with UV or visible light (2) Azobenzene Derivatives (2) Azobenzene Derivatives to yield a photo stationary composition that is wavelength and temperature dependent [50]. Azobenzene and its derivatives are characterized by reversible transformations [49] from the Azobenzene and its derivatives characterized byupon reversible transformations [49]light from the generally more stable trans form to are the less stable cis form irradiation with UV or visible generally more stable trans form to the less stable cis form upon irradiation with UV or to yield a photo stationary composition that is wavelength and temperature dependent [50].visible

light to yield a photo stationary composition that is wavelength and temperature dependent [50].

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Therefore, azobenzene becomes a typical case which demonstrates the property of the N=N double bond. The trans-cis photo isomerization of azobenzene is the basis of photo-responsive properties of 6,many Crystals 2016, 158 azo-functional materials [51], which produce photochromic [52], photo 10 of 19induced anisotropy, and photo-induced phase transition effects [53]. The isomerization mechanism Therefore, azobenzene becomes a typical case which demonstrates the property of the N=N double hasCrystals drawn attention recently. Many azobenzene derivatives have been10developed, 2016,extensive 6, 158 of 19 bond. The trans-cis photo isomerization of azobenzene is the basis of photo-responsive properties of including the D-π-A neutral molecule [54] and the azobenzene polymer [55] but only rare many azo-functional materials [51], which produce photochromic [52], photo induced anisotropy, Therefore, azobenzene becomes typical case whichionic demonstrates the property of the N=N double investigations have focused on athe azobenzene species derivatives. and photo-induced phase transition effects [53]. The isomerization mechanism has drawn extensive

bond. The trans-cis photo isomerization of azobenzene is the basis of photo-responsive properties of attention recently. Many azobenzene derivatives have been developed, including the D-π-A neutral molecule [54] and the azobenzene polymer [55] but only rare investigations have focused on the and photo-induced phase transition effects [53]. The isomerization mechanism has drawn extensive azobenzene ionic species derivatives. Schiff basesrecently. have the of intramolecular transferincluding through entire molecule. attention Manyability azobenzene derivatives have charge been developed, thethe D-π-A neutral In other words, electric charge can be transferred from part D to part A inside D–CH=N–A or (3) Schiff[54] Bases molecule and the azobenzene polymer [55] but only rare investigations have focused on the azobenzene(D-Donor, ionic species derivatives. molecule. From another aspect, styryl-pyridinium derivatives A–CH=N–D A-Acceptor) Schiff bases have the ability of intramolecular charge transfer through the entire molecule. In and(3) azobenzene derivatives to transferred form planar structures the solid state, while Schiff otherSchiff words, electric charge tend can be from part D toinpart A inside D–CH=N–A or bases Bases tend to be non-planar. Two benzene rings twist at a certain angle in the opposite direction along the A–CH=N–D (D-Donor, A-Acceptor) molecule. From another aspect, styryl-pyridinium derivatives Schiff bases have the ability of intramolecular charge transfer through the entire molecule. In 0 0 and azobenzene derivatives tend to form planar structures in the solid state, while Schiff bases tend C1-Cα C1 -Cα bond. That to say, the conjugation of the SchiffD–CH=N–A base molecule otherand words, electric charge canis be transferred from part degree D to part A inside or is not to be non-planar. Two benzene rings twist at a certain angle in value the opposite direction correspondingly. along the equal to other types of molecules, thus the contribution of β will diminish A–CH=N–D (D-Donor, A-Acceptor) molecule. From another aspect, styryl-pyridinium derivatives C1-Cα and C1′-Cα′ bond. That is to say, the conjugation degree of the Schiff base molecule is not and azobenzene derivatives to form planar structureswhich in the solid while bases tend of the However, the two benzenetend rings are not coplanar, maystate, make theSchiff birefringence equal to other types of molecules, thus the contribution of β value will diminish correspondingly. to be non-planar. Two benzene rings twist at a certain angle in the opposite direction the compound too large. more importantly, it isbirefringence becausealong the However, not the to twobebenzene rings Additionally, are not coplanar, which may make the of molecular the C1-Cα and C1′-Cα′ bond. That is to say, the conjugation degree of the Schiff base molecule is not structure of this oflarge. plane, symmetry of importantly, molecules is which maystructure give a higher compound not tokind be too Additionally, more it isreduced, because the molecular equal to other types of molecules, thus the contribution of β value will diminish correspondingly. of this of plane, symmetry of molecules is reduced, which may give a higher chance of chance of kind forming a non-centrosymmetric crystal. Meanwhile Schiff bases and their derivatives However, the two benzene rings are not coplanar, which may make the birefringence of the forming a non-centrosymmetric crystal. Meanwhile Schiff bases and theirabsorption derivatives have a strong have a strong frequency multiplication effect and a shorter optical cutoff wavelength, compound not to be too large. Additionally, more importantly, it is because the molecular structure frequency multiplication effect and a shorter optical absorption cutoff wavelength, which can which cankind improve their transparency [56]. Ais series of SHG were designed and of this of plane, symmetry of molecules reduced, whichchromophores may give a higher chance of improve their transparency [56]. A series of SHG chromophores were designed and synthesized by forming a non-centrosymmetric bases13, and their derivatives have SHG a strong synthesized by Coe et al. One ofcrystal. them,Meanwhile as shown Schiff in Figure was found to exhibit efficiency Coe et al. One of them, as shown in Figure 13, was found to exhibit SHG efficiency 0.43 times of multiplication and a shorter optical absorption cutoff wavelength, which can 0.43frequency timesatof DAST at 1907effect nm [57]. DAST 1907 nm [57]. improve their transparency [56]. A series of SHG chromophores were designed and synthesized by Coe et al. One of them, as shown in Figure 13, was found to exhibit SHG efficiency 0.43 times of DAST at 1907 nm [57].

(3) Schiff Bases many azo-functional materials [51], which produce photochromic [52], photo induced anisotropy,

Figure 13. Molecular structure of ionic salt with Schiff base structure.

Figure 13. Molecular structure of ionic salt with Schiff base structure. (4) Derivatives Tolan Derivatives (4) Tolan Figure 13. Molecular structure of ionic salt with Schiff base structure. In the case of common organic chromophores, the tolan derivatives were found to have larger

In the case of common organic chromophores, the tolan derivatives were found to have larger NLOTolan properties than a series of stilbene derivatives when compared at the same excitation energy (4) Derivatives NLO than a series of stilbene derivatives compared atofthe same [58].properties Since a similar effect was expected for DAST analogues,when the ethynyl analogue DAST andexcitation its In the case of common organic chromophores, the tolan derivatives were found to have larger energy [58]. Since a similar effect was expected for DAST analogues, the ethynyl analogue of related compounds with cation 1 (Figure 14) were prepared by Okada et al. and their properties NLO properties than a series of stilbene derivatives when compared at the same excitation energy wereand investigated. SHG active with salts based cation 1,14) including iodide andby4-substituted DAST its relatedSixcompounds cationon1 (Figure were prepared Okada et al. and [58]. Since a similar effect was expected for DAST analogues, the ethynyl analogue of DAST and its benzenesulfonates, were found to have non-centrosymmetric structures. The crystal structures of their properties were investigated. Six SHG active salts based on cation 1, including iodide and related compounds with cation 1 (Figure 14) were prepared by Okada et al. and their properties the p-chlorobenzenesulfonate salt and p-toluenesulfonate salt were analyzed as being isomorphous 4-substituted benzenesulfonates, were non-centrosymmetric structures. The crystal were investigated. Six SHG active saltsfound basedtoonhave cation 1, including iodide and 4-substituted to one another. As a result, the off-diagonal second-order nonlinear optical coefficient (d) values of benzenesulfonates, were found to have non-centrosymmetric structures. The crystal of structures of the p-chlorobenzenesulfonate salt and p-toluenesulfonate salt structures were analyzed as these two were estimated to be about twice as large as those of DAST [26]. However, tolan the p-chlorobenzenesulfonate salt and p-toluenesulfonate salt were analyzed as being isomorphous being isomorphous to one another. As a result, the off-diagonal second-order nonlinear optical derivatives have not been further investigated, perhaps because of the poor thermal and to one another. As a result, the off-diagonal second-order nonlinear optical coefficient (d) values of coefficient (d) values of these were estimated to be about twice as large as those of DAST [26]. photochemical stability of the two ethynyl compared to their ethenyl analogues. these two were estimated to be about twice as large as those of DAST [26]. However, tolan However, tolan derivatives have not been further investigated, the poor derivatives have not been further investigated, perhaps because ofperhaps the poorbecause thermal of and thermal and photochemical stability of the ethynyl compared to their ethenyl analogues. photochemical stability of the ethynyl compared to their ethenyl analogues. Figure 14. Molecular structure of ionic salt with tolan structure.

(5) Extension of the Conjugate Chain Length

Figure 14. Molecular structure of ionic salt with tolan structure.

14. Molecular structure of stilbazolium ionic salt with tolan structure. Tsuji et al. Figure synthesized a series of methoxy analogues (abbreviated as MOSA [1–3]) with π-conjugation extended byLength attaching a fused aromatic ring, i.e., 6-styrylisoquinolinium (5) Extension of the Conjugate Chain

(5) Extension of the Conjugate Chain Length

Tsuji et al. synthesized a series of methoxy stilbazolium analogues (abbreviated as MOSA

[1–3]) with π-conjugation extended by attaching a fused aromatic ring, i.e., 6-styrylisoquinolinium Tsuji et al. synthesized a series of methoxy stilbazolium analogues (abbreviated as MOSA [1–3]) with π-conjugation extended by attaching a fused aromatic ring, i.e., 6-styrylisoquinolinium

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and 4-[2-(2-naphthyl)ethenyl] pyridinium derivatives with methoxy substituents (Figure 15). Crystals 2016, 6, 158 11 of 19 and 4-[2-(2-naphthyl)ethenyl] derivatives 15). The The experimental β valuespyridinium of MOSA [1,3] at with 1064methoxy nm in substituents methanol (Figure were evaluated as −30 esu and − 30 − 30 experimental β values of MOSA [1,3] at 1064 nm in methanol were evaluated as 1090 × 10 1090 × 10 esu and 900 × 10 esu, respectively. The π-conjugation elongation using fused and pyridinium derivatives withusing methoxy (Figure 15). The 900 ×4-[2-(2-naphthyl)ethenyl] 10−30 esu, respectively. The π-conjugation elongation fusedsubstituents rings was confirmed to shift rings was confirmed to shift the absorption maximum wavelength to shorter wavelengths than in −30 esu and experimental β values of MOSA [1,3] at 1064 nm in methanol were evaluated as 1090 × 10 the absorption maximum wavelength to shorter wavelengths than in the case of π-conjugation the case of π-conjugation elongation by increasing the double-bond number. This study clarified 900 × 10−30 esu, respectively. π-conjugation elongation usingclarified fused rings wasfused-ring confirmedsystems to shift elongation by increasing theThe double-bond number. This study that the that the fused-ring systems possess large β, irrespective of their relatively short the absorption maximum wavelength to shorter wavelengths than in the case of π-conjugation possess large β, irrespective of their relatively short absorption wavelengths, compared absorption to the wavelengths, comparedsystem to the double-bond elongation system [33]. elongation byelongation increasing the double-bond number. This study clarified that the fused-ring systems double-bond [33]. possess large β, irrespective of their relatively short absorption wavelengths, compared to the double-bond elongation system [33].

Figure 15. Molecular structures with prolonged conjugate chains. Figure 15. Molecular structures with prolonged conjugate chains. Figure 15. Molecular structures with prolonged conjugate chains.

Part D

Part D

When introducing other electron acceptors in place of the pyridine ring, it is advantageous to Part D

When introducing electron acceptors in place of the is advantageous introduce stronger other electron withdrawing groups than the pyridine pyridine ring, ring. itThe volume of to When introducing other electron in place ofpyridine the pyridine ring, isof advantageous to introduce stronger withdrawing groups than the ring. Theitvolume substitutions substitutions in electron the molecule is anotheracceptors main factor that may improve asymmetry the of compound, introduce stronger electron withdrawing groups than the pyridine ring. The volume of in is enhancing possibility of chromophore packing into non-centrosymmetric in theresulting molecule another the main factor that may improve asymmetry of the compound, crystals resulting in substitutions in the molecule is another main factor that may improve asymmetry of the compound, [59–61]. enhancing the possibility of chromophore packing into non-centrosymmetric crystals [59–61]. resulting inetenhancing the possibility of to chromophore packing into non-centrosymmetric crystals al. introduced benzindole substitute the pyridinium as electron ChenChen et al. introduced benzindole to substitute the generally generallyused used pyridinium as electron [59–61]. acceptor in the conjugated molecule. Bulk single crystals of P-BI were obtained with sizes of up to acceptor Chen in theetconjugated molecule. Bulk single crystals of P-BI were obtained with sizes of up introduced to substitute used pyridinium as electron 3 using thebenzindole 17.0 × 6.0 × 2.0al.mm slow evaporation methodthe as generally shown in Figure 16. Kurtz powder tests 3 to 17.0 × 6.0 × 2.0 mm using the slow evaporation method shown in Figure with 16. Kurtz powder tests acceptor thethe conjugated Bulk efficiency single crystals ofas P-BI were obtained of up revealed in that maximummolecule. powder SHG of P-BI with the monoclinic spacesizes group P21 to is 3 revealed that the maximum powder SHG efficiency of P-BI with the monoclinic space group P2 17.0 6.0 × 2.0 using theDAST. slow evaporation method as shown in Figure 16. Kurtz powder 1.14 ×times the mm benchmark Besides, they changed the electron conjugated channel tests into 1 is 1.14 times the that benchmark Besides, they changed the electron into thiophene. revealed the maximum SHG efficiency P-BI with theconjugated monoclinic space group 1 is thiophene. S-BI-1 and DAST. S-BI-2powder (methanol and ethanolofsolvent, respectively) werechannel obtained byP2 slow 1.14 the benchmark DAST. Besides, theySHG changed the electron conjugated channel into S-BI-1 andtimes S-BI-2 (methanol ethanol respectively) were obtained evaporation crystal growthand method. Thesolvent, powder efficiency was found to be by 0.23slow timesevaporation that of thiophene. S-BI-1 and (methanol and ethanol solvent, respectively) were that obtained by crystal growth method. The powder SHG efficiency was found to bethe 0.23 times of DAST. There is DAST. There is only aS-BI-2 small difference in π-conjunction between molecular structure of slow P-BI growth method. Thebetween powder SHG efficiency was found beP-BI 0.23and times that ofa it is and S-BI difference butcrystal it is accompanied by a large change of molecular ordering in thetoof crystalline state and only evaporation a small in π-conjunction the molecular structure S-BI but DAST. There is only amacroscopic small difference in π-conjunction between thedemonstrate molecular structure of P-BI substantial change of physical properties. These results that the crystal accompanied by a large change of molecular ordering in the crystalline state and a substantial change and S-BI of but it is accompanied by asensitive large change of molecular instructure the crystalline and a packing this type ofproperties. salt is very to thedemonstrate nature of theordering molecular [62]. state of macroscopic physical These results that the crystal packing of this type of substantial change of macroscopic physical properties. These results demonstrate that the crystal salt ispacking very sensitive to the nature ofsensitive the molecular structure [62]. of this type of salt is very to the nature of the molecular structure [62]. (a)

(a) N

S

N

S

N+

I-

+

N

I-

S-BI S-BI

N

N+

I-

N

N+

I-

P-BI

Figure 16. Molecular structures of S-BI and P-BI (a) and single crystals of P-BI (b). P-BI Figure 16. Molecular structures of S-BI and P-BI (a) and single crystals of P-BI (b).

Figure 16. Molecular structures of S-BI and P-BI (a) and single crystals of P-BI (b).

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A series of π-conjugated push-pull chromophores was designed and synthesized by Jeong et al., A series of π-conjugated push-pull chromophores wasacceptor designedgroups and synthesized by Jeong which used various quinolinium derivatives as electron (Figure 17). The et same al., which used various quinolinium derivatives as electron acceptor groups (Figure 17). The same Crystals 2016, 6, 6,a 158 12 of of 19 high group reported new organic quinolinium crystal HMQ-TMS which possesses potential for Crystals 2016, 158 12 19 group reported a new organic quinolinium crystal HMQ-TMS which possesses potential for high photoelectricity efficiency and gap-free broadband THz wave generation. It is suitable for the HMQ photoelectricity and gap-free THz wave generation. is suitable for the HMQ A series ofefficiency π-conjugated push-pullbroadband chromophores was designed andItsynthesized Jeong et part to form acentric crystal structures based on Coulomb interactions, therefore by a large optical part to form acentric crystal structures based onasCoulomb interactions, therefore large al., which used various quinolinium derivatives electron acceptor groups (Figure a17). Theoptical same transparency range can bebe obtained. At usedpump pumpwavelength wavelength 800 HMQ-TMS transparency range can obtained. At the the widely widely of of 800 nm,nm, HMQ-TMS group reported a new organic quinolinium crystal used HMQ-TMS which possesses potential for high crystals exhibit excellent crystal characteristics, THz generation properties, and offered broader crystals exhibit excellent crystal characteristics, THzTHz generation properties,Itand offeredfor broader THzTHz photoelectricity efficiency and gap-free broadband wave generation. is suitable the HMQ spectral [63]. spectral [63]. crystal structures based on Coulomb interactions, therefore a large optical partcoverage to coverage form acentric transparency range can be obtained. At the widely used pump wavelength of 800 nm, HMQ-TMS crystals exhibit excellent crystal characteristics, THz generation properties, and offered broader THz spectral coverage [63].

Figure 17. Molecular structure of HMQ-TMS (a) and single crystal of HMQ-TMS (b).

Figure 17. Molecular structure of HMQ-TMS (a) and single crystal of HMQ-TMS (b).

A series of quinolinium compounds DA-DMQ1,4-T, DA-DMQ1,2-T and DA-DMQ2,3-T (Figure 18) Figure 17. Molecular structure of HMQ-TMS and single ofand HMQ-TMS (b). A series of quinolinium compounds DA-DMQ1,4-T, DA-DMQ1,2-T DA-DMQ2,3-T (Figure reveal large macroscopic nonlinear optical response(a)with high crystal hyperpolarizability β0 of 256, 233, 18) −30 reveal nonlinearThe optical response with high hyperpolarizability andlarge 122 ×macroscopic 10 esu , respectively. electron-withdrawing strength of the derivatives β follows the 233, 0 of 256, A quinolinium compounds DA-DMQ1,4-T, (Figure 18) −30ofesu ofseries DMQ1,2 > ,DMQ1,4 > DMQ2,3 19). TheDA-DMQ1,2-T increase of β0and for DA-DMQ2,3-T DA-DMQ1,2-T compared andorder 122 × 10 respectively. The(Figure electron-withdrawing strength of the derivatives follows reveal large macroscopic nonlinear optical response high hyperpolarizability β00 ofred-shifted 256, 233, DA-DMQ2,3-T having the same> conjugation lengthwith can explained by the the to order of DMQ1,2 > DMQ1,4 DMQ2,3 (Figure 19).beThe increase of strongly β0 for DA-DMQ1,2-T −30 −30 and 122 × 10 esu , respectively. The electron-withdrawing strength of the derivatives follows theof wavelength of maximumhaving absorption of the former, caused by the stronger by acceptor compared toDMQ1,2 DA-DMQ2,3-T the (Figure same conjugation length can be explained the strongly order of > DMQ1,4 > DMQ2,3 19). The increase of β 00 for DA-DMQ1,2-T compared 1,2-dimethylquinolinium. The further increase inofβthe 0 for DA-DMQ1, 4-T over DA-DMQ1, 2-T is red-shifted wavelength of maximum absorption former, causedbyby stronger acceptor of to DA-DMQ2,3-T same conjugation length can be explained thethe strongly red-shifted significant (~10%), having but lessthe than might be expected considering the remarkable increase in N-N* 1,2-dimethylquinolinium. Theabsorption further increase β0 for caused DA-DMQ1, 4-Tstronger over DA-DMQ1, wavelength of maximum of the informer, by the acceptor of2-T is distance alone, which can be attributed to the larger acceptor strength of DMQ1,2 [64]. significant (~10%), but less than might increase be expected the4-T remarkable increase 1,2-dimethylquinolinium. The further in β00 considering for DA-DMQ1, over DA-DMQ1, 2-TinisN-N* significant but be lessattributed than might considering the remarkable increase distance alone, (~10%), which can to be theexpected larger acceptor strength of DMQ1,2 [64]. in N-N* distance alone, which can be attributed to the larger acceptor strength of DMQ1,2 [64].

Figure 18. Molecular structures of DA-DMQ1,4-T, DA-DMQ1,2-T, and DA-DMQ2,3-T. Figure Molecular structuresof ofDA-DMQ1,4-T, DA-DMQ1,4-T, DA-DMQ1,2-T, and DA-DMQ2,3-T. Figure 18. 18. Molecular structures DA-DMQ1,2-T, and DA-DMQ2,3-T.

Figure 19. Molecular structures of DMQ1,2, DMQ1,4, and DMQ2,3.

Figure 19.19. Molecular DMQ1,2,DMQ1,4, DMQ1,4, and DMQ2,3. Figure Molecularstructures structures of of DMQ1,2, and DMQ2,3.

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Shortly afterwards, new efficient nonlinear optical quinolinium-based single crystals OHQ-TMS developed by the same group. Remarkably high effective Shortlywere afterwards, new efficient nonlinear optical quinolinium-based singlehyperpolarizability crystals OHQ-TMS eff eff −30 tensor values β122 of the 27 ×same 10 group. esu were obtained for these crystals. Plate-shaped bulktensor quinolinium 122by were developed Remarkably high effective hyperpolarizability values 2 were successfully grown by the solution growth method as eff − 30 crystals with an area of up to 56 mm β122 of 27 × 10 esu were obtained for these crystals. Plate-shaped bulk quinolinium crystals 2 were shown Figure crystal structure of monoclinic space groupmethod P21 symmetry was with anin area of up20. to The 56 mm successfully grown by acentric the solution growth as shown in obtained measurement. As-grown acentric OHQ-TMS crystals a largewas transparency Figure 20.by TheXRD crystal structure of monoclinic space group exhibit P21 symmetry obtained byrange XRD from 550 nm toAs-grown 1600 nm OHQ-TMS in the optical region and aa relatively low absorption coefficient measurement. crystals exhibit large transparency range from 550 nmin to the 1600THz nm region. Thus OHQ-TMS crystals are prospective materials for use in second-order NLO in the optical region and a relatively low absorption coefficient in the THz region. Thus OHQ-TMS applications [65]. crystals are prospective materials for use in second-order NLO applications [65].

Figure20. Figure 20.Molecular Molecularstructure structureof ofOHQ-TMS OHQ-TMS(a) (a)and and single single crystals crystals of of OHQ-TMS OHQ-TMS (b). (b).

Part E For DAST DASTderivatives, derivatives,it it is common to use methyl groups Fewhave works have been is common to use methyl groups in PartinE.Part FewE. works been reported reported on substitutions Part E. substituted Coe et al. substituted by agroups. few other groups. They on substitutions of Part E. of Coe et al. the methyl the by amethyl few other They synthesized synthesized a total of sixteen saltsfour comprising separate of which includes a total of sixteen dipolar salts dipolar comprising separatefour series, each ofseries, whicheach includes four different four different N-substituted pyridinium electron acceptor structures (Figure Almost of the N-substituted pyridinium electron acceptor structures (Figure 21). Almost all21). of the salts all designed salts designed crystallize centrosymmetrically, leading tosecond-order crystals without NLO crystallize centrosymmetrically, leading to crystals without NLO second-order effects. Fortunately, effects. Fortunately, molecules (6)polar and (10) adopt theCpolar space group C C and show SHG activity molecules (6) and (10) adopt the space group and show SHG activity of 0.8 and 1.0 times C of 0.8ofand 1.0 [66,67]. times that of DAST [66,67]. Second-orderofNLO susceptibility of crystals made of that DAST Second-order NLO susceptibility crystals made of chromophore (6) with chromophore (6) with PF6-counter higher than that of DAST have been reported [68]. PF6 -counter anion higher than thatanion of DAST have been reported [68].

Figure 21. Molecular structures with modified Part E. Figure 21. Molecular structures with modified Part E.

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4. Stilbazolium Dyes in Other Matrices Although there are a large variety of NLO dyes with high second-order hyperpolarizability β constants have been developed. The principal requirement for SHG is a non-centrosymmetric structure, most of the chromophores have a strong tendency to crystallize centrosymmetrically that leads to a poor SHG effect [69]. Furthermore, novel materials are preferred not only to provide large SHG efficiencies, but also to attribute practical performance such as chemical stability and easy processability for making technological applications [70]. Therefore, researches utilize NLO dyes in other directions such as in self-assembled or multilayers and in polymer NLO dye composite materials. Along this direction, zeolites films and related polymer materials have been proved to be the hosts for aligned inclusion of organic NLO dyes to study novel organic-inorganic composite SHG materials [71]. 4.1. Zeolite Films Zeolites are microporous crystalline aluminosilicates whose solid structure includes channels and cavities at the molecular level. The porosity of the zeolite is open to the exterior and thus it is easiy to attract guests inside the lacunae of the zeolite particles. The ability of zeolites to incorporate organic NLO dyes has modified the dyes molecular properties and controlled the interaction with zeolites [72]. Stucky and co-workers gave the first report of the use of inorganic hosts and organic guests to form nonlinear optical materials. According to their study, para-nitroaniline (PNA), 2-methyl-4-nitroaniline (MNA), 2-amino-4-nitropyridine, and analogous compounds are readily introduced into the channels in one direction of AlPO4 -5, and the dye-incorporating AlPO4 -5 powders generate a second harmonic with the intensity far exceeding that of quartz powders [73]. 4.2. Polymers Polymers with NLO chromophores are required to be polarized to orient the chromophore in the field direction so that they can attain a macroscopic NLO effect. There are two main forms in which the NLO chromophores and poled polymer could combine together: host-guest polymer system and side-chain polarized polymers system [74]. The host-guest polymer system describes a kind of chromophore-doped polymeric in which the NLO chromophore and micromolecule (guest) can be dissolved in the polymer (host). Though it is quite simple to synthesize this polymeric, the density of the NLO chromophore is limited by the compatibility of host and guest such that the macroscopical NLO coefficient may be more difficult than expected [75]. A side-chain polarized polymers system is a method to make a chromophore bonded to the polymer backbone through a chemical reaction, which can greatly increase the chromophore content that significantly improves the stability of orientation. Besides, some interesting investigations have attempted to use acentric NLO active nanocrystals as building blocks of composite polymers. It has been proven that acentric NLO active nanocrystals can be oriented by a strong directional electric field when dispersed into a liquid host. The orientation process is accompanied by a significant increase of the second harmonic generation which is, however, totally transient due to the high mobility of the dipolar NLO active nanocrystals in the dispersion. Roberto Macchi and coworkers [76] reported that PMMA films containing 4 wt % of DAST obtained by spincoating show in situ growth of homogeneously dispersed oriented nanocrystals with sizes of less than 100 nm. The final film shows a significant and enduring SHG at room temperature. 5. Conclusions Progress in second-order NLO ionic organic crystal materials has been reviewed. They consist of a positively charged nonlinear optical cation (e.g., stilbazolium) molecule and a negatively charged anion molecule. The cation is the main source of nonlinear optical properties, whereas the counter-anion is used to adjust the crystal packing through coulombic interactions. Variation of counter-ions in organic salts has been confirmed to be an easy and high-efficiency approach for creating crystals

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with large second-order NLO activity. It is also effective to improve nonlinear susceptibility of cation molecules by increasing the strength of the donor/acceptor and extending the length of the π-electron conjugated bridge [77,78]. In this paper, the possibilities of modifying the cation chromophores have been discussed and recent examples reviewed. Many investigations have been conducted to synthesize new organic molecules based on DAST. In the last few years, most of them have tended to change the parts B and D to improve second-order optical nonlinearity of new organic materials and achieve promising goals. It has been found successfull to introduce heterocyclic groups such as thiophene and pyrrole into part B. The analogical aromatic furan, which has a similar structure to thiophene is expected to become the focus of future research. In order to develop new organic materials, another polycyclic aromatic hydrocarbon naphthalene with a planar structure just like benzene and larger aromaticity than thiophene is expected to be combined with different part D groups. Moreover, bulk size groups like benzoindoles have been previously considered as not being easy to form non-centrosymmetric crystal packing and are thus inappropriate for introduction into part D. The success of P-BI with 1.14 times SHG efficiency of DAST may draw attention to the modifications in part D with bulk groups. Acknowledgments: This work was supported by the National Natural Science Foundation of China (Grant No. 61370048, 51333001, 51373024, 51473020), the National Key Basic Research Program of China (2014CB931804), the Fundamental Research Funds for the Central Universities (Grant No. FRF-TP-15-003A3). Author Contributions: Xiu Liu collected data and wrote the manuscript. Zhou Yang, Dong Wang and Hui Cao gave suggestion and revised the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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