A detailed study on the photophysical properties of a series of alkoxy substituted diphenylbutadienes in solution and in the solid state providing a molecular level understanding of the factors controlling their solid-state luminescence behavior is reported. Our studies provide clear evidence for exciton splitting in the solid state resulting in red-shifted emission for this class of materials. The role of the number of alkoxy substituents and the alkyl chain length in controlling the nature of the molecular packing and consequently their fluorescence properties has been elucidated. Whereas in the di-and tri-alkoxy substituted derivatives, the solid-state fluorescence was independent of the length of the alkyl chains, in the monoalkoxy substituted derivatives, increasing the length of the alkyl chain resulted in a visual change in fluorescence from green to blue. On the basis of the analysis of the molecular packing in the single crystals, this difference could be attributed to fluorescence arising from aggregates with an edge-to-face alignment in the molecules possessing short alkyl chains (methyl and butyl) to monomer fluorescence in the long alkyl chain containing derivatives.

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Reversible photochemical phase transition behavior of some alkoxy-cyano-substituted diphenylbutadiene liquid crystals has been studied. The EE isomers of these derivatives were found to undergo photoisomerization to yield thermally stable EZ and ZE isomers, which could be transformed back to the EE isomer using light of a different wavelength. On photolysis, the liquid-crystalline EE isomers of these derivatives exhibited isothermal phase transition to the isotropic phase due to the formation of the non-liquid-crystalline EZ and ZE isomers. The thermal stability of the EZ and ZE isomers ensures that reverse switching of these materials to the liquid-crystalline phase remains purely a photon-controlled process.

This paper studies the potential of 3 newly synthesized low molecular weight gelling agents (2 of them with azo groups) with the semicarbazide group as a H-bonding motif to gel the smectic phase of a commercial chiral smectic C * material. To detect gelation two methods were used: the stabilization of the director pattern, which measures the interaction of the physical gel network with the LC-director, and the suppression of the onset of electrohydrodynamic instabilities, which measures the increase of viscosity. Special emphasis was put on the influence of the photochemical trans-cis isomerization of the gelling agents on gelation.From the stabilization of the director pattern an elastic component can be seen by the memory effect (two different switching times). Photochemical trans-cis isomerization destroys this memory effect. The increase of the threshold field for the onset of electrohydrodynamic instabilities in free-standing films measures the increase of viscosity and the supression of flow. Both methods detect the same melting temperature of the gel and the same reduction of the gelling ability upon trans-cis isomerization. This proves that the gelling abilities of the new azo-containing gelling agents can be reversibly switched on and off by photoisomerization.