[23]
[24]
[25] [26]
discussed for anti-BCH have also been performed for the syn-conformer. For both conformers similar results were obtained (see also: R. W. A. Havenith, L. W. Jenneskens, J. H. van Lenthe, Chem. Phys. Lett. 1998, 282, 39 ± 48). Gaussian 98 (Revision A.7), M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres, M. Head-Gordon, E. S. Replogle, J. A. Pople, Gaussian, Inc., Pittsburgh, PA, 1998.. GAMESS-UK, a package of ab initio programs, M. F. Guest, J. H. van Lenthe, J. Kendrick, K. Schˆffel, P. Sherwood, R. J. Harrison, 2002, with contributions from R. D. Amos, R. J. Buenker, H. J. J. van Dam, M. Dupuis, N. C. Handy, I. H. Hillier, P. J. Knowles, V. Bonacic-Koutecky, W. von Niessen, R. J. Harrison, A. P. Rendell, V. R. Saunders, A. J. Stone, D. J. Tozer, A. H. de Vries. It is derived from the original GAMESS code due to M. Dupuis, D. Spangler, J. Wendolowski, NRCC Software Catalog, Vol. 1, Program No. QG01 (GAMESS) 1980. For the syn-conformer of BCH, two excited 1B2 states are found at 6.70 and 7.50 eV with an oscillator strength ratio of 0.60. M. B. Robin, Higher Excited States of Polyatomic Molecules, Vol. 1, Academic Press, New York, NY, 1974; J. M. Zwier, A. M. Brouwer, W. J. Buma, A. Troisi, F. Zerbetto, J. Am. Chem. Soc. 2002, 124, 149 ± 158.
Received: October 24, 2002 [Z 556]
A Photochromic Liquid Crystal System Michel Frigoli and Georg H. Mehl*[a] KEYWORDS: liquid crystals ¥ molecular switches ¥ photochromism ¥ selfassembly
Understanding the folding behaviour of molecules or supramolecular entities of either synthetic or biological origin is of fundamental importance in a number of disciplines.[1] In this context, the light-induced folding and unfolding of molecules due to the reversible formation and cleavage of covalent bonds is a particularly attractive area of research, as not only is the spatial structure of the molecular entities changed, but other physical properties are modulated as well. These range from electromagnetic absorption behaviour to self-assembly proper[a] Dr. G. H. Mehl, Dr. M. Frigoli Department of Chemistry University of Hull Hull HU6 7RX (UK) Fax: ( 44) 1482 ± 466 ± 411 E-mail:
[email protected]
ties, thus making these materials not only highly functional but also potentially very useful as molecular switches. Photochromic organic compounds are very promising systems for achieving these important scientific goals. The great potential of liquid crystalline systems to be used as optical switches has been recognised: Research has concentrated predominantly on azo groups as the photoactive groups. However, there are reports of other photochromic groups being used, particularly in macromolecular systems,[2, 3] but, due to questions associated with the stability of such systems, they have not yet been included in optical switching devices. Hence, finding a reliable class of systems that have liquid crystalline phase behaviour close to ambient temperature, and which are structurally compatible with current liquid crystal systems and technology, is a very promising and necessary area of research. The use of photochromic groups of modified diarylethene structures, where the photochromism is due to a reversible electrocyclisation reaction, has recently been advanced.[4, 5] The thermal stability and versatility of such systems, particularly of heteroaryl systems, has been demonstrated for a large number of systems.[6] Efforts to synthesise systems which are miscible with liquid crystals have been made by number of researchers.[7] Our approach is modular, and makes use of a photochromic core, a 1,2-bis(2-methylbenzo[b]thiophen-3-yl)hexafluorocyclopentene system connected (using ether linkages, which are known to be beneficial for forming mesomorphic behaviour) to two cyanobiphenyl groups (the mesogens), via spacers of ten methylene groups. The system is shown in Scheme 1. The versatility of this new concept is apparent, and should lead to the systematic investigation of the influence of the photochromic group, the spacer lengths and the mesogens on the properties of such systems.[8] The synthetic path to the photochromic liquid crystal 1, which can switch to 1 a in an electrocyclisation reaction, is shown in Scheme 2.[6] The synthesis is convergent in the sense that the mesogenic groups are attached to the phenol groups of the photochromic core 6 in an etherification reaction in the final reaction step. The core, whose synthesis and structure has not yet been reported, was obtained in four reaction steps, starting from 6-methoxybenzothiophene, 2, where the phenol function is protected as a methoxy group. Methylation at the 2-position of the aromatic ring system, using n-butyllithium at 50 to 408C (see Scheme 2), followed by the addition of methyliodide at 78 8C resulted in 3. Bromination at the 3-position using bromine in chloroform resulted in compound 4, and introduced the functionality required for the assembly of the photochromic skeleton. Treating 4 in THF with n-butyllithium at 78 8C, followed by the addition of a semiequivalent amount of octafluorocyclopentene yielded 5. The subsequent reaction of the methoxy groups with boron tribromide resulted in the new functional photochrome 6 (1,2-bis(6-hydroxy-2-methylbenzo[b]thiophen-3-yl)hexafluorocyclopentene). Following this scheme, compound 1 was synthesised on a gram scale, in an overall yield of 34 % based on the starting material 2. The photochromic properties of compounds 5, 6 and the liquid crystal 1 were investigated in a manner described elsewhere, and the results are listed in Table 1.[9]
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Scheme 1. Photochromic liquid crystal system.
Scheme 2. Scheme showing the synthesis of the photochromic compounds 5 and 6 and the liquid crystalline material 1.
Absorption maxima of 267 nm for 5, 269 nm for 6 and 280 nm for 1 (the open forms of this series) are close to the results reported for the unsubstituted system, for which a value of 258 nm has been reported.[10] The very large extinction coefficient of 1 (e 62 000 mol 1 L cm 1), compared to the values for the other compounds (5, e 23 800; 6, e 22 000 mol 1 L cm 1), can be attributed to the presence of the cyanobiphenyl groups in the molecule; this is supported by the observation that the absorption spectrum of 1 corresponds to the sum of the absorption spectra of the photochromic core and the cyanobiphenyl groups. The absorption maxima of the closed forms (namely 1 a, 5 a and 6 a) were found to be in the range 519 ± 520 nm for all the compounds in this series, and are shown in Table 1. Figure 1 depicts the absorption spectra of 1 and 1/1 a in the photostationary state.
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Absorption maxima values in the photostationary state (APS) under irradiation with light at 313 nm are interesting and an indication of the conversion between the two states: A higher APS indicates a preferred switching to the closed form. The APS values are 0.18 for 5 a and 0.24 for 6 a, in line with results calculated from reported data.[10] The result for 1 a, 0.39, is surprising and indicates that the degree of conversion, from 1 to 1 a, in the photostationary state is higher for 1 than for the other members in the series. In a control experiment, where an alkyloxycyanobiphenyl was added to a solution of 5, no enhancement of the photoconversion took place. This suggests that the process responsible for enhancing the conversion of 1 to 1 a requires activity in the nanometer range: the distance on a molecular level between the photochromic core and the mesogens in 1 and 1 a.[11] The results of the investigation of the mesomorphic properties of 1 and 1/1 a under photostationary conditions using differential scanning calorimetry (DSC) spectroscopy are summarised in Table 2. The solid state properties of 1 are characterised by the presence of a highly ordered liquid crystal or crystal phase below 33.3 8C. In the temperature interval up to 74.5 8C, optical polarising microscopy revealed broken focal conical defects in conjunction with schlieren tex-
Table 1. Absorption behaviours. Compound
lmax, open form [nm] (e [mol 1 L cm 1])
lmax, closed form [nm]
APS[a] [lmax, closed form]
1[b] 5[b]
280 267 305 269 307 ± ± ±
± ±
± ±
±
±
520 520 519
0.39 0.18 0.24
6[c] 1 a[d] 5 a[d] 6 a[e]
(62 000) (23 800) (2900) (22 000) (3200)
[a] Absorption maximum obtained at the photostationary state under irradiation with 313 nm light. [b] Absorption maxima and coefficients of the open-ring isomers in cyclohexane. [c] Absorption maxima and coefficients of the open-ring isomers in THF. [d] Absorption maximum of the closed-ring isomer in cyclohexane. [e] Absorption maximum of the closed-ring isomer in THF.
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spacers, indeed the transition temperatures are very close to that of a comparable alkoxy-cyanobiphenyl compound.[12, 13] Further, repeated heating and cooling cycles of the samples did not have any influence on the transition temperatures and enthalpies, verifying the thermal stability of the system. These results indicate that the concept of separate individual functionalities of a photochromic core and mesogens, linked by flexible spacers in a modular approach, allows the design of materials where the mesomorphic phase structure and range, and, additionally, the absorption characteristics, can be modulated in a controlled manner by irradiation. We acknowledge the EPSRC for funding and Professors R. Guglielmetti and A. Samat (Marseille, France) for the very generous access to their photochromic laboratory.
Figure 1. Photoirradiation induced absorption spectral changes of 1 (3.33 10 5 mol L 1) in cyclohexane: Open-ring isomer 1 (- - - -);The photostationary state 1/1 a under irradiation with 313 nm light (––).
Table 2. Transition temperatures as determined by DSC (2nd heating run) Compound
Thermal transitions [ 8C] (Transition enthalpies [J g 1])
1 1/1 a[a]
Cr 33.3 (0.6) SmC 74.5 (1.1) N 78.5 (1.0) Iso Tg 26.5 SmX 54.6 (0.2) N 75.9 (2.6) Iso
N nematic, SmC smectic C, SmX smectic X, Cr crystalline, Tg glass transition, Iso isotropic. [a] Measured under photostationary conditions.
tures, indicating the formation of a smectic C phase (the molecules are arranged in layers and are tilted on average to the layer normal). In the temperature interval from 74.5 to 78.5 8C, the temperature where the material melts into an isotropic state, the presence of a schlieren texture, with two and four brush defects (strengths s of 1³2 and 1), shows the formation of a nematic phase (orientational ordering of the molecules, but no positional ordering) as the highest stable liquid crystal phase. The value of the transition enthalpies, 1.0 and 1.1 J g 1, are indicative of first-order transitions and are in line with those observed for other mesomorphic systems. The sample 1/1 a, a deep purple colour, obtained under photostationary conditions by removing the solvent, is characterised by a different phase behaviour. A glass transition at 26.5 8C was followed on heating by a highly ordered liquid crystalline phase which melted into a nematic phase at 54.6 8C, characterised by the typical schlieren texture. The transition to the isotropic state at 75.9 8C is associated with a melting enthalpy of 2.6 J g 1, however, annealing sample 1/1 a for several days led to the slow crystallisation of the system. The isotropic temperature of 75.9 8C is close to that obtained for 1, and this similarity is attributed to the decoupling effect of the flexible
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