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...

Photo‐induced geometric isomerisation of azobenzene based chromophores can be utilised in a variety of molecular control procedures including photo‐induced poling. In such processes additional mobility is imparted to chromophores trapped in a glassy matrix through the photo‐induced trans‐cis‐trans cycle. We show that by using selected narrow wavelength bands of light to induce the isomerisation cycle during photo‐induced poling, enhanced levels of polar order can be obtained in comparison to those obtained with monochromatic light. We attribute this to an increase in the fraction of chromophores which undergo isomerisation via an inversion mechanism. As a result, a higher proportion of the chromophores are able to participate in the photo‐induced poling process since the free volume for isomerisation is reduced from that required for the rotation mechanism.

Photorefractive polymers which incorporate azo-dyes as the non-linear chromophore element, can be used not only for generating gratings by the photorefractive effect, but also by photoisomerization of the azo-dye. In the latter mechanism, repeated trans-cis isomerization causes the chromophore molecules to become aligned at right angles to the laser polarization direction, thereby making the material birefringent.These two phenomena are to a large degree independent, and can be studied separately, by appropriate choice of polarization direction of the interacting beams. Furthermore, the diffraction efficiency of the photorefractive gratings is a very sensitive function of the poling field strength, while that of the photoisomerization gratings is less so.In this work, we investigate the components diffracted from each of these gratings formed in a hybrid photorefractive polymer material PVK:TNF:DEACST:disperse red 1. We then explore the possibility of performing some simple optical processing applications, exploiting the flexibility provided by this multiple grating process. A scheme for producing a novelty filter, which displays only the moving parts of a scene is considered.The limitations of these films for such processing applications are discussed.

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