The influence of substituents and media polarity on the photoinduced E 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.There is currently considerable interest in initiating and controlling changes in polymer properties by the use of photoinduced molecular rearrangements of chromophores which are incorporated either as tethered side chains or in a guest-host system.1,2 The E-Z photointerconversions of aryl-substituted 2p systems has attracted particular attention for this application, and the volume of literature describing the use of azobenzene-based dyes and of arylpropenoates for this purpose is considerable.2,3,4 The photoinduced geometrical changes of enhancing the relaxation of the excited singlet state to a low-N-benzylideneanilines and stilbenes also have potential in this lying intramolecular charge-transfer state and/or promoting a area. Azobenzene and stilbene and their derivatives have been higher degree of electron transfer in this state (as shown in extensively studied in the area of nonlinear optical properties, Scheme 1). The resulting dipolar species 2 has low energy and including second-order phenomena such as photoinduced undergoes neither intersystem crossing to the triplet state nor poling5 and frequency doubling6 and in third-order phenomena conversion to the orthogonal excited state from which either such as degenerate four-wave mixing7 and photorefractivity.8 geometrical isomer may be formed. Thus no, or very inefficient, The trends found in these investigations may have important E-Z photoconversion is observed. implications in the exploitation of stilbenes and azobenzenesHere we evaluate the way in which substituents can influence in these emerging technologies.the solvent polarity-sensitive photoisomerisation of stilbenes, While photoinduced geometrical isomerisations can be useful and consider whether such sensitivity to the polarity of the to tailor polymer properties for specified purposes,4,5 they can media is relevant to the azobenzenes and t...

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