Nonplanar Ladder-Type Polycyclic Conjugated ... - ACS Publications

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Nonplanar Ladder-Type Polycyclic Conjugated Molecules: Structures and Solid-State Properties Kuojin Liu,†,‡ Feng Qiu,†,§,‡ Chongqing Yang,§ Ruizhi Tang,§ Yubin Fu,§ Sheng Han,*,† Xiaodong Zhuang,§ Yiyong Mai,§ Fan Zhang,*,§ and Xinliang Feng§,∥ †

School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Haiquan Road 100, 201418, Shanghai, P. R. China § School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Dongchuan Road 800, 200240, Shanghai, P. R. China ∥ Technische Universität Dresden, Helmholtzstraße 10, 01062, Dresden, Germany S Supporting Information *

ABSTRACT: Using an efficient intramolecular carbon− carbon cross-coupling reaction, a series of new ladder-type conjugated molecules have been prepared successfully in high yields. Such a pyran-fused polycylic structure possesses an extended π-conjugated backbone with flexible conformation, which gives these molecules interesting properties, including high solubility in common organic solvents, excellent thin filmforming abilities, blue fluorescent emission with good quantum yields, and aggregate formation in a binary solvent. The selfassembly behaviors of these molecules as well as various nanostructures can be finely tailored by varying the substituted group on the molecular periphery. The powder and singlecrystal X-ray diffraction analyses revealed that the synergetic effect of π−π stacking and van der Waals interactions play a key role in controlling the morphologies of these aggregates. More importantly, self-assembled molecules exhibit good fluorescent performance, due to their twist backbone conformation.

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In comparison with the aromatic π-conjugated molecules, πconjugated polycyclic aromatic carbonhydrates (PAHs) with saturated carbon atoms, featuring reduced aromaticity, has been still rarely documented. Incorporation of heteroatoms into the π-conjugated backbone of a molecule can not only tune the molecular energy levels but also provide noncovalent intermolecular interactions favorable for the formation of stable superstructures, compared with the full-carbon analogues.10−15 Very recently, pyran has emerged as a highly available moiety accessing to fused polycyclic π-conjugated molecules (e.g., compound (II))16 or D (donor)-A (acceptor) type conjugated macromolecules,17−19 for achieving high-performance electronic devices. For example, organic solar cell fabricated from cyclopenta[2,1-b;3,4-b′]dithiophene (III)-based copolymers exhibited a very high power conversion efficiency up to 10.6%.17 Obviously, the pyran moiety, featuring one small-radius oxygen atom and one saturated sp3-hybridized carbon atom, serves as the electrondonating role as well as reducing the rigidity of the skeleton in a conjugated system, with the consequence of the improvement

INTRODUCTION Heterocycle-fused π-conjugated molecules have drawn much attention as organic semiconductor materials for applications in next-generation flexible devices, such as organic field-effect transistors (OFETs), organic light emitting diodes (OLEDs), and organic solar cells (OSCs), due to their outstanding optoelectronic properties.1−5 Their rigid architectures typically endow them with extended π-conjugated systems, strong intermolecular interactions, and thus exhibiting versatile selfassembly behaviors and excellent charge-carrier mobility. However, such kinds of molecules are prone to form excimers or aggregates through π−π interactions, leading to fluorescence quenching or photoluminescence with additional undesired long wavelength emission.6 Rational complementary of the advantages in one molecule, demanding for a promising functionality, is still a big challenge for molecular design. Recently, supplanting of unsaturated sp2-hybridized carbon atoms with saturated sp3-hybridized carbon atoms in a molecular backbone is an amazing approach to narrow the optical band gap or weaken the aggregation effect through reducing the aromatic characters or the planar rigidity of a πconjugated system, respectively.7 For example, dimethylene C2bridged acene (Chart 1, I) containing nonplanar moieties exhibits high fluorescence performance in the aggregate state.8,9 © 2015 American Chemical Society

Received: March 27, 2015 Revised: May 8, 2015 Published: May 19, 2015 3332

DOI: 10.1021/acs.cgd.5b00429 Cryst. Growth Des. 2015, 15, 3332−3338

Crystal Growth & Design

Article

Chart 1. Typical Examples of Nonplanar ansa-Bridged Acene Derivatives

different substituents was obtained by silica-gel column chromatography as a red liquid in yields of 19% and 23%, respectively. The target compounds (4a, 4b, and 4c) were prepared by intramolecular carbon−carbon coupling reactions of 3a, 3b, and 3c, respectively, using palladium diacetate as the catalyst in DMF at 130 °C for 48 h.22,23 The pure compounds were afforded as a white solid after purification by silica gel column chromatography in good yields (83%−90%). Detailed preparation information on intermediates and targeted molecules is presented in the Supporting Information. All targeted compounds were fully characterized by 1H and 13C NMR spectroscopy as well as high-resolution mass spectrometry. In the 1H NMR spectra, the proton signals of the aromatic skeletons of compounds 4a−4c are shifted to low field in comparison to those of compound 3a−3c, demonstrating that the conjugated lengths of the molecules increased after the formation of the pyran-fused polycyclic structures by cyclization reaction. In addition, the proton peak at 5.44 ppm is assigned to the methane group of pyran, indicating the nonaromatic structure of the pyran fragment. Thermal Behaviors. These molecules show good thermal stability with a weight loss of 5% over 290 °C (Figure S1, Supporting Information) as estimated by thermal gravimetry analysis (TGA) measurements. On the basis of differential scan calorimetry (DSC) analyses (Figure 1), all of conjugated molecules show two endothermic peaks on the heating curve and one exothermic peak on the cooling curve. The first small endothermic peak corresponds to the molecular motility in the solid state, indicating the flexible conjugated backbone of these pyran-fused compounds. The second endothermic peaks are attributed to their melting points. With the increase of the length of alkyloxy chains on the molecular periphery, the melting point of the molecule decreases, caused by the motility of alkyloxy chains. The unique phase transitions for such type of molecules seem to be attributable to their less-rigid backbones arising from the pyran moieties. Crystal Structure Analysis. The chemical structures of the as-prepared compounds were further confirmed by X-ray crystallographic analysis (Figure 2). Single crystals of 4a and 4c were obtained at ambient temperature by slowly diffusing hexane and methanol into their chloroform solutions, respectively. The central naphthyl ring of 4a or 4c shows slightly twisted deformation with an angle of around 13.4°. For compound 4a, the backbone exhibits large torsion angles of 30.3° and 34.2° between the central naphthyl ring and the outer benzene rings, respectively. In compound 4c, these torsion angles drop down to 25.8° and 27.7°, respectively. The oxygen atoms in the pyran units deviate from the mean planes of the naphthyl cores by about 24.1°, 25.0° for compound 4a and 21.1°, 19.8° for compound 4c, respectively. Such twisted backbone structures are attributed to the nonaromatic character

of the solubility and the tuning of the intermolecular interactions, etc. for the resulting materials. These intrinsic characteristics give the pyran moiety great promise for the construction of advanced functional materials. In the present work, we designed and synthesized a series of nonplanar ladder-type pyran-fused conjugated molecules (IV). The geometric and electronic structures of these molecules have been fully characterized by single crystal X-ray diffraction and optical spectroscopies. Their molecular packing in solid state is highly dependent on the side chains of the molecular periphery. In these molecules, the pyran-fused structures essentially generated the extended π-conjugated systems and rich optical properties. Meanwhile, self-organization of such types of molecules enables the formation of different stable superstructures, and their morphologies can be tailored through the introduction of alkyloxy chains on the molecular periphery.

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RESULTS AND DISCUSSION Synthesis of Ladder-type PAHs. The synthetic route of nonplanar ladder-type pyran-fused conjugated molecules is described in Scheme 1. Starting from 2,6-dimethylnaphthalene Scheme 1. Synthetic Route to the Nonplanar Ladder-Type Polycyclic Conjugated Molecules

by using a two-step sequence of bromination with bromine and N-bromobutanimide, respectively, the key intermediate 1,5dibromo-2,6-bis(bromomethyl)naphthalene (2) was prepared in a good yield (74%).20 Afterward, the intermediate ether compounds (3a−3c) were prepared by condensation coupling of 2 with various phenolic compounds in dimethylformamide (DMF) at 80 °C by using an improved synthetic protocol.21 Here, except for phenol, 4-hexyloxy phenol and 4-dodecyloxy phenol (5a−5b) were synthesized by condensation reaction of hydroquinone with 1-bromohexane and 1-bromododecane, respectively, under the catalysis of potassium hydroxide (Scheme S1, Supporting Information). The pure 5a−5b with 3333



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Figure 1. DSC curves of compound 4a (a), 4b (b), and 4c (c).

parallel columns, the shortest distance between the neighboring molecules is 3.49 Å between the α-terminal carbon atoms of the benzene moieties from the neighboring molecules. No apparent π−π stacking intermolecular interactions of the central naphthyl rings were observed, due to the twist backbone of the pyran-fused conjugated molecule. The solid-state packing of compound 4c is significantly different from that of compound 4a. The molecules adopt staggered packing motif, in which each molecule is partly overlapped with two neighboring molecules in a face-to-face fashion with the shortest distance of 3.60 Å, which suggests the presence of π−π stacking interactions. The two adjacent molecules are facial isomers and in mirror symmetry, relative to the mean plane of the backbone. In addition, the presence of C−H···π interactions from dodecyloxy groups to the central naphthyl rings of the neighboring molecules also stabilize the close-packed structures in solid state. The parallel stacking columns are interdigitated with each other through the alky side chains, manifesting the distinct effect of the substituted groups on the solid state of such kinds of molecules. Optical Properties. The optical properties of compound 4a−4c in chloroform were investigated. In UV−vis spectra of compound 4a, a set of absorption bands appears at around 340,

Figure 2. Crystal structures of compound 4a (a, b) and 4c (c, d) from front- and side-view representations, respectively.

of the fused pyran units. The lengths of C−C (1.50/1.49 Å) and C−O bonds (1.45/1.43 Å and 1.39/1.38 Å) in pyran moieties are longer than those of C−C bond lengths in the naphthyl and benzene rings, further manifesting the less aromatic character of the pyran parts. Moreover, two terminal dodecyloxy groups in compound 4c are oriented away from the attached benzene rings with a dihedral angle of around 62.5°. In Figure 3, the packing diagram of compound 4a reveals a slipped stacking in an edge-to-edge fashion. In each of the

Figure 3. Crystal packing of compound 4a (a, b) and 4c (c, d) from front- and side-view representations, respectively. 3334



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Figure 4. Normalized absorption spectra (a−c) and PL spectra (d−f) of 4a−4c in CHCl3 solution and thin films, respectively.

Table 1. Photophysical Data of Pyran-Fused Conjugated Molecules 4a−4c in CHCl3 in thin film

in CHCl3 solution 4a 4b 4c

λabsa [nm]

εmax [m−1 cm−1]

λemb [nm]

SSc [nm]

Φfd %

λabsa [nm]

λemb [nm]

SSc [nm]

Φfd %

357 369 369

16730 21770 20200

418 453 453

61 84 84

22.8 15.8 15.4

362 372 372

426 438 436

64 66 64

9.4 17.1 15.2

a

The longest absorption maxima. bEmission maxima upon excitation at the longest absorption maxima. cStokes shift, evaluated from the absorption maxima and emission spectra maxima. dquantum yields determined by using an integrating sphere.

4a−4c were observed in the thin films compared with those in solution (Figure 4a−c), indicating the strong intermolecular interactions and the tendency to aggregate in solid state for such kinds of molecules.24 The molecules 4a−4c show strong blue fluorescence emissions in CHCl3 (Figure 4d−f). The emission maxima of compound 4a locate at 418 nm, while the emission bands of 4b and 4c exhibit a significant bathochromic shift of 35 nm, with respect to that of 4a. The large Stokes shifts (SS) with 61−84 nm for these molecules manifest their remarkable structural deformations between the ground and excited states of the molecules, likely due to the reduced rigidity of the pyran-fused backbones.25 The moderate fluorescence quantum yields of compounds 4a−4c (from 0.23 to 0.15) are comparable to those of pyran-fused PAHs.16 Interestingly, compounds 4a−4c also exhibited strong blue emission in the thin films formed through spin-coating on the quartz plate (Figure 4d−f). The fluorescence quantum yields of compounds 4a−4c in solid state were also determined by using an integrating sphere (Table 1). Compounds 4a−4c in the solid state still maintained certain quantum yields, likely attributed to the twisted backbones associated with the pyran moieties as shown in their single crystal structures. Compared with the value of ∼23% in solution, compound 4a shows a much lower quantum yield (∼9%) in the solid state, reasonably arising from the stronger intermolecular interactions. The quantum yields of 17.1% for 4b and 15.2% for 4c in solid state are quite similar to those of 15.8% and 15.4% for 4b and 4c in solution, respectively, indicating that the alkyloxy chains on the molecular peripheries indeed enable effectively suppressing

357, and 372 nm, and a small shoulder peak at around 321 nm is observed (Figure 4a−c), which is typically attributed to the π−π* transitions of the aromatic backbones in a conjugated system. Compared with that of compound 3a prior to cyclization, the main absorption bands of 4a are significantly red-shifted (Figure S2), manifesting the presence of an extended π-conjugated system after the formation of the pyran-fused ring. Such vibronically split features of these absorption bands for 4a arise from the rigid molecular structure, in which the intramolecular rotation of phenyl group is restricted by the methyleneoxy moiety. In comparison to that of compound 4a at 357 nm, the maximum absorption of 4b is red-shifted by 12 nm, due to the electron-donating effect of the attached hexyloxy groups. As expected, UV−vis absorption profile of 4c is similar to that of compound 4b. Notably, the absorption spectra of all compounds exhibit an almost solvent-independent effect, revealing their nonpolar characters in the ground state (Figures S3−S5). The UV−vis absorption spectra of pyran-fused PAHs were further calculated at the time-dependent density functional theory (TD-DFT) level (Figures S6−S8). The profiles of UV−vis spectra are in good agreement with their experimental results, in which the main absorption bands at the low-energy window are dominated by electronic transitions between HOMO and LUMO orbitals. Regarding to their excellent solubility in some common organic solvents, compounds 4a−4c are facile to form thin films under spin-coating treatment. And, attaching long chains onto the peripheries of the molecules enable essentially improving their film-forming abilities. Remarkable red shifts and peak broadening of the absorption spectra for compounds 3335



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solvent mixtures were investigated. Upon addition of methanol (70% volume fractions) as a consequence of the formation of aggregates.26,27 Importantly, the hypochromatic shift of the absorption peak demonstrates that compound 4c likely undergoes Haggregation.28 In the fluorescent spectra of compound 4c, the emission maxima at ∼453 nm slightly red-shift, with a continual decrease of the intensity under the addition of methanol into its chloroform solution (Figure 6b), presumably due to the formation of aggregates in the mixed solvents. Similarly, the fluorescent intensities of compound 4a and 4b also decrease with the addition of methanol (Figure S10, Supporting Information). However, the fluorescence profiles of all compounds seem not to be obviously changed as the formation of the aggregates, probably attributable to the weak π−π stacking interaction in the aggregate state for these molecules, completely different from those for the typical PAHs.29,30 After the drop-casting chloroform/methanol solutions of compound 4a−4c on silicon wafers and the removal of the solvent by evaporation under ambient temperature,31 the morphologies of the resulting aggregates were investigated by scanning electron microscopy (SEM). Compound 4c could self-assemble into ribbon-like aggregates with a uniform width of approximately 300 nm and lengths ranging from 0.8 to 5 μm (Figure 7a). Transmission electron microscopy (TEM) image of the sample also confirms the formation of the ribbon-like aggregates (Figure 7b). Furthermore, atomic force microscopy (AFM) image reveals that the ribbons show a smooth surface with an average height of ∼78 nm (Figure 7c−d). Interestingly, with the decrease of the length of the side chains from dodecyl group to hexyl group, compound 4b self-organizes into helical fibers with the lengths of 10−20 μm and the diameters of 0.5− 5 μm. Such twisted aggregates were probably induced by the intrinsic twisting of the molecular backbone, due to the flexible character of the pyran rings. Notably, both left-hand and righthand helical superstructures coexist in the aggregates, suggesting that the aggregates are racemism, while in the case of compound 4a without side chain substituents, 2D nanosheets are formed using a similar self-assembly method. However, it has been difficult to control the formation of the superstructures with distinct edges and uniform sizes so far. To explore the influence of the molecular structure on the supramolecular arrangement and morphology, powder X-ray diffraction (XRD) was employed to analyze the aggregation behavior of compound 4c. As shown in Figure 8a, the intense and sharp peaks of the diffraction pattern indicated the high crystallinity of the assembled nanoribbons. Simulated by Materials Studio program version 6.1, both crystal cells of the aggregates and single crystal were monoclinic crystals, and the XRD pattern of the aggregates is also similar to that of single crystal (see Figure S11). Accordingly, the peaks at 2θ = 3.4° and 6.9° can be indexed as (100) and (200) reflections, respectively, and 2θ = 2.3°, 5.3°, 8.2° and 10.9° can be indexed as (001), (002), (003), (004) reflections, respectively, which are associated with a lamellar packing.27,32,33 In addition, the reflection peak at 2θ = 20.6° is typically originated from the ordered packing of alkyl chains, and the reflection peak at 2θ = 23.3° can be attributed to the π−π stacking of aromatic

the chromosphere interactions of compound 4b or compound 4c in solid state. These results are in agreement with their crystal structure analysis discussed above. The photophysical properties of 4a−4c were summarized in Table 1. Electrochemical Properties. The electrochemical behaviors of pyran-fused PAHs 4a−4c were subsequently investigated by cyclic voltammetry (CV) in CH3CN (Figure S9). In the CV profiles of 4a−4c, one irreversible oxidation processes are observed. The HOMO energy level of 4a is −5.63 eV, deriving from the oxidation potential (assuming Fc/Fc+ at 4.8 eV), while the LUMO energy level is −2.48 eV, which is calculated based on the absorption edges. Owing to the existence of alkyloxy chains with electron-donating ability, HOMO energy levels of 4b and 4c decrease to −5.50 eV and −5.48 eV, respectively. The LUMO and HOMO values of pyran-fused PAHs were summarized in Table 2. Table 2. Electrochemical Properties of Compounds 4a−4c

4a 4b 4c

HOMOa (eV)

LUMOb (eV)

Egoptb (eV)

HOMOc (eV)

LUMOc (eV)

Egoptc (eV)

−5.63 −5.52 −5.48

−2.48 −2.50 −2.46

3.15 3.02 3.02

−5.39 −5.25 −5.25

−1.66 −1.67 −1.66

3.73 3.58 3.59

For all of the molecules, HOMO = −[Eonset+4.8] eV, Eox(ferrocene) = 0.46 eV vs Ag/AgCl, Eonset is the onset values of oxidation potentials. b LUMO = HOMO − Egopt, Egopt: optical band gap, estimated from UV−vis absorption edge. cObtained by DFT calculations. a

To further insight into the geometric and electronic structures of these compounds, the theoretical calculations of pyran-fused PAHs have been performed based on TD-DFT (RB3LYP/6-31G (d) level). As shown in Figure 5, the LUMOs

Figure 5. Calculated molecular orbitals of compounds 4a−4c.

for all compounds primarily reside on the core of 1,5-diphenyl naphthalene, indicative of a similar LUMO level. The HOMOs for all compounds are distributed over the 1,5-diphenyl naphthalene with partly contributed by pyran, demonstrating that pyran is not an effective π-electrons delocalized structure on the molecular skeleton. Compared with that of compound 4a, the oxygen atom in alkyloxy chains contributes to the HOMO level in compounds 4b and 4c, revealing its electrondonating effect. Therefore, the HOMO levels of 4b and 4c are lower than that of 4a. The calculated HOMO energy levels of pyran-fused PAHs are in good agreement with the values estimated from CVs. Self-Assembly Behaviors. To elucidate the influence of the solvent polarity on the aggregation behaviors of 4a−4c, the optical properties of these molecules in a series of binary 3336



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Figure 6. UV−vis absorption (a) and fluorescence emission (b) spectra of compound 4c (5 × 10−5 M) with varied intensities in chloroform/ methanol mixtures with different methanol fractions.

be composed of 20 layers of the repeat units. The proposed molecular arrangement for compound 4c was evaluated on the basis of the single crystal analysis, as depicted in Figure 8b, clearly disclosing the nanostructure formation mechanism.

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CONCLUSIONS

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ASSOCIATED CONTENT

A new family of nonplanar ladder-type conjugated molecules has been prepared successfully in good yields by using Pdcatalyzed intramolecular carbon−carbon cross-coupling reaction. The geometric and electronic structures of these molecules have been fully characterized by single crystal Xray analysis and optical spectroscopies. The pyran-fused polycyclic structures give such kinds of molecules unique backbones featuring extended π-conjugated systems and flexible conformation, and thus leading to interesting physical properties including large Stokes shifts, blue fluorescent emission with good quantum yields, and a strong tendency to aggregate in binary solvents. Their excellent solubility in common organic solvents endows them with good film-forming properties, and these molecules exhibit strong intermolecular interaction in the thin films. The self-assembly behaviors of these molecules with various superstructures can be finely tailored by varying the substituted group in the molecular periphery. The excellent properties of these pyran-fused conjugated molecules in aggregate state may allow them to be used as promising active materials in electronic device fabrications, such as OLEDs with blue emitting, p-type building blocks for OSCs devices, etc. Meanwhile, the construction of the pyran-fused polycyclic conjugated system through intramolecular cyclization might pave a new avenue to form organic functional molecules with much complex structures.

Figure 7. SEM (a), TEM (b), and AFM images (c) and height profile (d) of ribbon-like aggregates formed by compound 4c; SEM images of nanostructure of compound 4b (e) and 4a (f).

S Supporting Information *

Experimental information, NMR spectra, HRMS-ESI spectra, UV−vis spectra, fluorescence spectra, TGA data, DSC data, and X-ray diffraction patterns. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00429. X-ray crystallographic data for compound 4a (CCDC 1048145) and 4c (CCDC 1048146), can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/ datarequest/cif.

Figure 8. (a) X-ray diffraction pattern of nanostructure from compound 4c plotted against the angle 2θ; (b) illustration of the ribbon-like nanostructures on the basis of the molecular packing pattern in the single crystals of compound 4c.

segments.34 Given that the d spacing of the (100) reflection value is 3.9 nm calculated from the XRD pattern, the assembled ribbons of compound 4c with the thickness of ∼78 nm should 3337



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AUTHOR INFORMATION

Corresponding Authors

*(F.Z.) E-mail: [email protected]. *(S.H.) E-mail: [email protected]. Author Contributions ‡

K.L. and F.Q. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (973 Program: 2013CBA01602, 2012CB933404), the Natural Science Foundation of China (21174083 and 21102091), the Shanghai Committee of Science and Technology (11JC1405400), Shanghai Jiao Tong University (211 Project), China Postdoctoral Science Foundation (2013M540356), and Postdoctoral Research Foundation of Shanghai Jiao Tong University (AE606203).

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