A novel strain-induced transition of the director orientation in a monodomain cross-linked side-chain liquid-crystal polymer system is reported which is analogous to the Fréedericksz transition in simple nematics. This new type of transition has a characteristic threshold strain which arises from an internal elastic barrier to rotation and is related to the level of anisotropy of the polymer chain trajectory.

The influence of substituents and media polarity on the photoinducedE→Z geometrical isomerisation of the stilbene, azobenzene and N-benzylideneaniline chromophores has been compared and assessed. The efficiency of the process in all three systems is markedly dependent on the presence and characteristics of electron-donor and electron-acceptor substituents at the 4- and 4′-positions. The results are discussed in terms of relaxation of the E-excited singlet state. In the absence of a nitro substituent, relaxation to the S ₁ orthogonal state competes effectively with non-productive intramolecular electron transfer; in the presence of a nitro substituent, the T ₁ orthogonal state is formed from inter-system crossing. For systems with a 4-nitro and a 4′-electron-donor substituent, access to the triplet state is inhibited by polar solvents promoting formation of the inactive charge-transfer state from the S ₁ state, and no isomerisation is observed. Similar effects are observed in both solution and polymer films. Such variations in behaviour have important implications for the utilisation of the chromophores in nonlinear optical phenomena including photorefractivity.