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Optical Materials 35 (2013) 837–842

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Liquid crystalline chromophores for photonic band-edge laser devices Stephen M. Morris a,⇑, Malik M. Qasim a, Damian J. Gardiner a, Philip J.W. Hands a, Flynn Castles a, Gouli Tu b, Wilhelm T.S. Huck b, Richard H. Friend c, Harry J. Coles a a

Centre of Molecular Materials for Photonics and Electronics, Department of Engineering, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0FA, UK Melville Laboratory for Polymer Synthesis, Department of Chemistry, Lensfield Road, Cambridge CB2 1EW, UK c Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK b

a r t i c l e

i n f o

Article history: Received 8 June 2012 Received in revised form 28 September 2012 Accepted 21 October 2012 Available online 25 December 2012 Keywords: Liquid crystals Lasers Dyes

a b s t r a c t We present results on laser action from liquid crystal compounds whereby one sub-unit of the molecular structure consists of the cyano-substituted chromophore, {phenylene-bis (2-cyanopropene)}, similar to the basic unit of the semiconducting polymer structure poly(cyanoterephthalylidene). These compounds were found to exhibit nematic liquid crystal phases. In addition, by virtue of the liquid crystalline properties, the compounds were found to be highly miscible in wide temperature range commercial nematogen mixtures. When optically excited at k = 355 nm, laser emission was observed in the blue/green region of the visible spectrum (480–530 nm) and at larger concentrations by weight than is achievable using conventional laser dyes. Upon increasing the concentration of dye from 2 to 5 wt.% the threshold was found to increase from Eth = 0.42 ± 0.02 lJ/pulse (20 mJ/cm2) to Eth = 0.66 ± 0.03 lJ/pulse (34 mJ/ cm2). Laser emission was also observed at concentrations of 10 wt.% but was less stable than that observed for lower concentrations of the chromophore. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Research on band-edge laser devices based upon liquid crystals (LCs) has garnered interest in recent years due to the remarkable combination of characteristics that are achievable with this type of organic laser [1–3]. Studies carried out in the literature have shown that LC laser devices are characterized by features such as single-mode emission, wide-band wavelength tunability, and micron-sized resonator structures [1]. Consequently, these compact, wavelength tunable organic laser devices are of significant interest for a wide range of applications including large-area, compact display devices and medical diagnostic techniques. In general, these lasers typically consist of a chiral LC, which self-organizes to form a helical structure with periodic optical properties, and a fluorescent compound in the form of a laser dye that is dispersed into the host matrix. Previously, the materials that have typically been used as the chromophores in the LC lasers are the p-conjugated small molecules that were developed for conventional liquid ‘jet-stream’ dye lasers [4]. Typically, these dyes are push–pull chromophores consisting of electron-donating and electron-withdrawing functionalities which, when dispersed into solvents, exhibit high quantum yield and fluorescence lifetimes of the order of a few nanoseconds. Research on LC lasers has demonstrated that these high quantum efficiencies can be retained in ⇑ Corresponding author. E-mail address: [email protected] (S.M. Morris). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.10.046

chiral LC hosts for dyes such as pyrromethene 597 and, as a result, this can lead to relatively high slope efficiencies of approximately 30% for single-pass device architectures [5] and 60% for LC lasers on silicon backplanes [6]. However, one drawback with these dyes is their miscibility in liquid crystalline media, which is usually limited to about 3 wt.% in the LC host before aggregation of the dye begins to occur. Recent studies have been carried out on alternative chromophores in the form of pyrene dyes, oligothiophene analogues and oligofluorene structures that have been functionalized with pendant alkyl chains so that they are soluble in a LC host [7–12]. These chromophores typically possess an absorption band at long ultraviolet wavelengths (360–450 nm) with a peak emission wavelength at around 470–500 nm and, due to the preferential alignment with the director, have been shown to exhibit a high order parameter of the transition dipole moment (0.6–0.7) when dispersed into a nematic LC host. Consequently, the laser threshold is lowest at the long-wavelength photonic band-edge, which has been confirmed experimentally [7,8]. All these structures have high quantum yields and this is of importance for low-threshold laser emission. As an example, by comparing the pyrene laser dye with the DCM (4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostryl)-4H-pyran) laser dye, it was shown that the pyrene-based LC laser had a substantially lower threshold [12]. Additional benefits of the oligofluorene dyes have also been reported [8–11]. Specifically, a study has shown that an oligofluorene dye dispersed into a commercially available chiral nematogen

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S.M. Morris et al. / Optical Materials 35 (2013) 837–842

mixture can lead to improved temporal and spatial stability of the LC laser output when compared with the laser dye DCM, although the excitation threshold and slope efficiency were found to be the same for both dyes for the so-called single transverse mode [8]. A further study demonstrated that, using oligofluorene structures, it is possible to combine fluorescence with an intrinsic chirality which, depending upon the handedness, can assist in forming the helical structure. Many of the oligofluorene compounds that were studied did not exhibit any liquid crystalline behaviour and were limited to low concentrations. There were found to be some exceptions namely that the polyfluorene polymer and the oligofluorene compounds containing chiral centres did show liquid crystalline behaviour at high temperatures (T > 150 °C) [10,11]. Oligofluorene structures are also particularly effective for use in LC lasers based upon cholesteric glasses where it was demonstrated that the laser output of the glass film was stable with time at high input powers unlike a conventional fluid LC laser consisting of the same dye and concentration thereof [9]. A poly-phenylenevinylene derivative with triptycene groups has also been considered for LC lasers [18,19]. In this case, the addition of the triptycene groups prevented strong interpolymer interactions, which generally limits the solubility and the efficiency. The results showed that low thresholds were observed as a result of a high order parameter of the transition dipole moment in the liquid crystalline host. The polyfluorenes and the polyphenylenevinylene have also received considerable attention from the point of view of semiconducting polymer devices, for which they were first developed [13–17]. As a result, it is well known that such structures exhibit high gain and laser action either in blends or when confined to high Q-factor micro-cavities. Nevertheless, despite their high quantum yields, both the oligofluorene compounds and the poly-phenylenevinylene derivatives are not readily miscible with low-molar mass liquid crystals up to large concentrations. Therefore, the motivation for this study was to develop new compounds that combine liquid crystalline properties with the photoluminescence of the chromophores from the semiconducting polymers so as to ensure a greater compatibility with low molar mass materials. Towards this end, we have synthesized liquid crystalline nematogens that consist of the phenylene-bis (2-cyanopropene) chromophore, which is the repeating unit of the cyano-substituted poly(p-phenylenevinylene) polymer. These compounds are found to exhibit enantiotropic nematic LC phases as well as broadband photoluminescence when optically excited at wavelengths between 355 nm and 450 nm. Moreover, when combined with a wide temperature range chiral nematic host these compounds exhibit laser emission up to large concentrations by weight (10 wt.%). Because of the inherent liquid crystallinity of the chromophore, this approach enables a higher concentration of the dye to be dispersed into the LC host than is achievable with the small conjugated organic molecules that are used as laser dyes. Herein, the photoluminescence properties of the structures are discussed and laser emission is observed under optical excitation.

2. Synthesis and characterization Three different compounds based upon the phenylene-bis (2cyanopropene) chromophore were synthesised each one exhibiting a nematic liquid crystalline phase. The synthetic route was as follows: 2,40 -Difluorobiphenyl-4-ol (1 eq.) was condensed with 1,9dibromononane (1 eq.) in the presence of acetone and potassium carbonate. The reaction mixture was refluxed for 3 days and was monitored by thin layer chromatography. After the completion of the reaction, the mixture was extracted with dichloromethane (DCM) three times. The organic layers were separated and evapo-

rated to dryness under reduced pressure. The resultant product was chromatographed (silica gel, DCM:hexane, 1:1) and the desired product [1-bromo-9-(20 ,4-difuorobiphenyl-40 -yloxy)nonane] was obtained as a major fraction in good yield (63%). This was followed by a reaction with an equimolar amount of 4-hydroxybezaldehayde in the presence of acetone and potassium carbonate, refluxed for 2 days. At the completion of the reaction, the mixture was extracted with DCM (3 100 mL) and organic layers were separated and evaporated to dryness. The resultant product was isolated through column chromatography (silica gel, DCM:hexane, 2:1). The major fraction was collected as a desired product p-[1-(20 ,4-difuorobiphenyl-40 -yloxy)-9-nonyloxy]bezaldehayde (a), (81%). This compound was further condensed with 1,4-phenylenediacetonitrile and 4hexyloxybenzaldehyde in equimolar quantities in the presence of potassium tert.butoxide (0.1 eq) and ethanol. The resultant mixture was refluxed under nitrogen atmosphere for 10 h. At the completion of the reaction, the solvent was removed under reduced pressure and chromatographed (silica gel, DCM:hexane, 1:1) to obtain desired products (b,c, and d) as three separate fractions. 2.1. Compound b Yield (18%) 1H NMR (500 MHz, CDCl3) d 7.97–7.83 (m, 4H, ArH), 7.69 (s, 4H, ArH), 7.49 (s, 2H, CH), 7.00–6.93 (m, 4H, ArH), 1.84– 1.76 (m, 4H, OCH2), 1.51–1.44 (m, 4H, CH2), 1.39–1.32 (m, 8H, CH2), and 0.96–0.89 (m, 6H, CH3). 13C NMR (126 MHz, CDCl3) d 161.43 142.31(CH), 135.18, 131.52, 126.29, 126.17, 118.45 (CN), 115.05 (ArC), 107.42 (C), 68.40, 31.69, 29.23, 25.80, 22.73 (CH2), and 14.17 (CH3). 2.2. Compound c Yield (19%), 1H NMR (500 MHz, CDCl3) d 7.90 (dd, J = 8.9, 1.9 Hz, 4H, ArH), 7.71 (s, 4H, ArH), 7.51 (s, 2H, CH), 7.46 (ddd, J = 8.9, 5.4, 1.9 Hz, 2H, ArH), 7.29 (t, J = 8.9 Hz, 1H, ArH), 7.10 (t, J = 8.7 Hz, 2H, ArH), 6.98 (dd, J = 8.9, 1.9 Hz, 4H, ArH), 6.75 (dd, J = 8.9, 1.9 Hz, 1H, ArH), 6.70 (dd, J = 8.9, 1.9 Hz, 1H, ArH), 4.05–4.02 (m, 4H, OCH2), 3.98 (t, J = 6.5 Hz, 2H, OCH2), 1.86–1.76 (m, 6H, CH2), 1.53–1.45 (m, 6H, CH2), 1.43–1.33 (m, 10H, CH2), and 0.92 (t, J = 6.5, 3H, CH3). 13C NMR (126 MHz, CDCl3) d 162.21 (d, J = 246.9 Hz, F–C), 161.47, 161.43, 160.28 (d, J = 246.9 Hz, F–C), 159.99, 159.91, 159.30, 142.39 (CH) 142.36 (CH), 135.27, 135.24, 131.92, 131.54, 130.96, 130.92, 130.53, 130.51, 130.47, 130.44, 126.34, 126.24, 126.20, 120.31 (d, J = 15.6 Hz, ArC), 118.47 (CN), 115.45 (d, J = 15.6 Hz, ArC), 115.09, 111.02, 111.00, 107.55 (C), 107.50, 102.76, 102.55 (ArC), 68.56, 68.43, 68.36 (OCH2), 31.71, 29.59, 29.41, 29.33, 29.25, 26.13, 25.83, 22.75 (CH2), and 14.19 (CH3). 2.3. Compound d Yield (25%), 1H NMR (500 MHz, CDCl3) d 7.90 (dd, J = 8.7, 2.0 Hz, 4H, ArH), 7.71 (s, 4H, ArH), 7.51 (s, 2H, CH), 7.49–7.43 (m, 4H, ArH), 7.29 (t, J = 8.7 Hz, 2H, ArH), 7.13–7.08 (m, 4H, ArH), 6.98 (dd, J = 8.7, 2.0 Hz, 4H, ArH), 6.80–6.73 (m, 2H, ArH), 6.70 (dd, J = 8.7, 2.0 Hz, 2H, ArH), 4.04 (t, J = 6.5 Hz, 4H, OCH2), 3.98 (t, J = 6.5 Hz, 4H, OCH2), 1.86–1.77 (m, 8H, CH2), 1.54–1.44 (m, 8H, CH2), and 1.39 (m, 12H, CH2). 13C NMR (126 MHz, CDCl3) d 162.44 (d, J = 251.2 Hz, F–C), 161.49, 161.44, 160.28 (d, J = 251.2 Hz, F–C), 160.00, 159.92, 159.57, 159.31, 154.88, 154.79, 142.38 (CH), 135.26, 133.64, 131.93, 131.82, 131.55, 131.02, 130.97, 130.93, 130.74, 130.53, 130.48, 130.45, 129.72, 126.44, 126.36, 126.25, 120.71, 120.32(d, J = 14.9 Hz), 118.48 (CN), 115.74, 115.56 (d, J = 14.9 Hz), 115.37, 115.10, 114.74, 111.03, 111.01, 110.62, 107.57 (C), 102.76, 102.55, 102.09, 101.86, (ArC), 69.70, 68.56, 68.38, 68.28, 29.86, 29.59, 29.56, 29.52, 29.41, 29.39, 29.34, 29.26, 29.09, 29.03, 26.13, and 26.04 (CH2).


Compound b consists of the chromophore at the centre of the molecule with terminal alkyl chains: the conjugation extends from oxygen to oxygen within the chromophore. In this case the cyano units act as the electron withdrawing groups and the oxygen atoms in the ether linkage is the electron donor. For compound c the chromophore sits between the terminal alkoxy chain and the bridging alkoxy chain that links to the fluorinated biphenyl unit. Finally, compound d contains the chromophore at the centre of the molecule, with two terminal difluorobiphenyl mesogenic units. It should be noted that compounds c and d are a similar generic structure to the bimesogenic compounds that have been developed for the flexoelectro-optic effect [20]. It can be seen that this chromophore in each case is similar to the repeating unit of the semiconducting/ light-emitting polymer structure poly(cyanoterephthalylidene), typically abbreviated as CN-PPV. The CN-PPV derivatives were chosen for two reasons: (1) they are known to exhibit high quantum yields as semi-conducting polymers and (2) it was considered that the relatively elongated shape of the chromophore would be compatible with calamitic (rod-shaped) liquid crystals enabling a greater dispersity in the host. The three compounds presented in Fig. 1 exhibited enantiotropic nematic liquid crystalline phases: compound c also exhibited an underlying smectic phases. For compound b, the onset of a nematic LC phase is observed upon heating from the crystalline solid phase at 146 °C and is found to extend to 197 °C at which point the sample clears to the isotropic phase. On the other hand, compound d is found to be at the lower temperature of 187 °C. By moving the chromophore to the centre of the molecule, d the nematic phase is found to appear at a lower temperature: on heating, the crystalline to nematic transition occurs at 120 °C, which then clears to the isotropic phase at 151 °C. The lower transition temperature is a result of the fact that compound d is dominated by bent conformers in the nematic phase, which leads to a reduction in the entropy change at the nematic to isotropic transition. This is similar to the observations of odd-spaced bimesogenic compounds discussed elsewhere [21]. The nematic phase of compound d was found to be rather unstable and so further measurements were carried out only on compounds b and c.

3. Experimental To confirm the existence of the nematic phase, example Schlieren textures that were observed between the crossed polarizers of

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an Olympus BH2 microscope, are shown in Fig. 2. The nematic phase is evident from the two and four-point brushes present in the Schlieren texture. Optical textures are shown for compounds b and c are shown in Fig. 2a and b, respectively. When capillary filled into a glass cell with unidirectionally rubbed polyimide alignment layers, these compounds were found to exhibit a uniform alignment of the director and optic axis in the plane of the cell when in the nematic phase. As an example, the steady-state absorbance spectrum of compound b when in the nematic phase at a temperature of 150 °C is shown in Fig. 3a. The sample was injected, through capillary action, into 5 lm cells consisting of anti-parallel rubbed polyimide alignment layers so as to promote a uniform alignment of the director along the rubbing direction. The absorbance spectrum was found to extend from 380 nm to 450 nm with a peak value at kP = 418 nm corresponding to the phenylene-bis (2-cyanopropene) chromophore. A similar result is also observed for compound c: this is to be expected as the chromophore remains the same and the only change is the length of the flexible spacer chain. The absorption of these compounds in the neat LC phase exists at longer wavelengths than that recorded for a concentration of 1 105 mol/L in ethanol (shown in Fig. 3b for compound c), whereby the peak absorption was found to occur at the shorter wavelength of kP = 376 nm. This solvatochromic shift is the result of the larger polarity of the LC host compared with ethanol. Also shown in Fig. 3a is the fluorescence spectrum which, overall, is rather featureless with a peak wavelength at 524 nm: this is a shorter wavelength than that recorded for the light-emitting polymer, poly(cyanoterephthalylidene) whereby the entire polymer backbone is conjugated (kP = 710 nm) [14]. The resulting Stokes shift of the chromophore is large (Dk = 104 nm). In contrast to the absorption spectrum, the fluorescence curve is very broad and is found to extend from 450 nm to 650 nm. There is some overlap between the long-wavelength region of the absorption spectrum and the short-wavelength side of the fluorescence and thus wavelengths below 450 nm would not be desirable for LC lasing experiments due to re-absorption of the emission. Because of the high temperature of the nematic phase of the compounds, samples were prepared where the LC chromophore was dispersed into wide temperature range nematogen mixtures. This allowed for both laser emission to be achieved at room temperature and for their characteristics as a laser active material to be studied. Mixtures were prepared by adding the compounds at different concentrations by weight to the commercially available

Fig. 1. The synthetic routes for the three dye compounds (b,c, and d) that consists of the phenylene-bis (2-cyanopropene) chromophore. All three compounds exhibit enantiotropic nematic phases.

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Fig. 2. Optical textures of the nematic phase of the compounds b recorded at T = 150 °C on an optical polarizing microscope. (a) The Schlieren texture of the nematic phase indicated by the two- and four-point brushes for compound b. (b) The Schlieren texture of compound c.

Fig. 4 demonstrates the steady-state absorbance and fluorescence spectrum of compound c in the ZLI-1840 host. The absorbance spectrum was found to be at shorter wavelengths to that of the neat compound (c.f. Fig. 3) in the nematic phase with a peak absorbance at kP = 400 nm. Two absorbance spectra are presented in Fig. 4a corresponding to the absorbance parallel (AII) and perpendicular (A\) to the nematic director, for the sample consisting of 2 wt.% of compound c in ZLI-1840. These results show that the transition dipole moment of the chromophore aligns preferentially with the director and the corresponding order parameter at 25 °C was found to be ST = 0.3 ± 0.05, using the relation

ST ¼

Fig. 3. (a) The steady-state absorbance (black line) and fluorescence (grey line) of compound b at a temperature of 150 °C in the nematic phase. (b) Steady-state absorbance of compound c in ethanol at a concentration of 1 105 mol/L.

nematogen mixture ZLI-1840, which was obtained from Merck KGaA and used without further purification. This host was chosen as it exhibits a wide temperature mesophase and because it does not absorb at the pump wavelength of k = 355 nm. The concentrations of the compound ranged from 2 to 10 wt.% and were dispersed into ZLI-1840 by placing the samples in an oven at a temperature close to the clearing point (100 °C) and left for a period of 24 h to ensure even mixing of the components. Afterwards, samples were capillary filled into the same 5 lm cells described previously. When dispersed into the low-molecular mass nematogen mixture, ZLI-1840, a high-quality, monodomain alignment was observed for all mixtures and there was no evidence of aggregation of the dye in the sample.

AII A? : AII þ 2A?

ð1Þ

The order parameter, which is plotted as a function of temperature in the inset of Fig. 4a, was found to decrease slightly as the concentration was increased. In addition, an example of a polar plot of the absorbance for 2 wt.% of compound c in ZLI-1840 is shown in Fig. 4b. The profile of the absorbance spectrum was not found to differ significantly as the concentration of the compound was increased from 2 wt.% to 10 wt.%. Upon increasing the concentration of either compound b or c in the liquid crystalline host, the absorption was found to increase in accordance with the Beer–Lambert relation and no aggregation of the dye was observed for the concentration range described herein. An example of the increase in absorption is shown in Fig. 4c for compound c. A similar result was found for compound b. Fig. 4d shows that the fluorescence spectrum is shifted to shorter wavelengths compared to that observed for the neat compound (c.f. Fig. 3a). In the nematic host, ZLI-1840, the peak fluorescence is shifted by 36 nm from kP = 524 nm to kP = 490 nm. Consequently, the emission is mostly in the blue region of the visible spectrum and this can be seen from the inset, which shows a photograph of the blue emission from the sample when optically excited with ultraviolet light at k = 365 nm. Upon increasing the concentration from 2 wt.% to 10 wt.% the fluorescence peak is found to shift from 470 nm to 490 nm, indicating that the chromophore is forming excimers through molecular p-stacking. Samples were prepared for laser emission by dispersing approximately 4 wt.% of the high twisting power chiral dopant (BDH1281, Merck) into the dye-doped mixtures so as to form a chiral nematic phase. The concentration was chosen so that the long-wavelength band-edge of the photonic band gap overlapped the fluorescence maximum of the chromophore between 470 nm and 490 nm so as to maximize the efficiency of the laser. In this case, the mechanism responsible for laser generation in these micron-sized films is


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Fig. 4. The absorbance anisotropy of compound c when dispersed into a wide temperature range nematogen mixture (ZLI-1840). (a) The absorbance parallel (AII) and perpendicular (A\) to the director for a concentration of 2 wt.% in the LC host (inset shows the temperature dependence of the order parameter of the transition dipole moment), (b) a polar plot of the absorbance at T = 25 °C. (c) Absorbance perpendicular to the director for the three different concentrations of compound c in the ZLI-1840 nematic host. (d) Fluorescence of the 10 wt.% compound c/ZLI-1840 mixture. The inset of the figure is a photograph of the cell revealing the blue emission of the chromophore when excited by an ultra-violet light source. Measurements were carried out at 25 °C.

due to the large gain at the edges of a photonic band gap, i.e., photonic band-edge lasing. [2,3] The dye-doped chiral nematic mixtures were capillary-filled into 10 lm reflective cells with silicon backplanes so as to improve thermal conductivity and stability of the laser when optically excited at 355 nm. The configuration of the cells is reported elsewhere [6]. These cells had rubbed polyimide alignment layers on the inner surfaces of both substrates in order to promote a Grandjean texture (the helix axis is aligned to the normal of the substrates). The cells were optically excited using an excitation wavelength of 355 nm (third harmonic of an Nd:YAG laser (CryLas)). The pulses were