Photostability of D--A nonlinear optical chromophores containing a benzothiazolium acceptor
Résumé
For the practical application of second-order NLO materials, not only a high molecular quadratic hyperpolarizability but also good thermal, chemical and photochemical stabilities are required. Most of the state-of-the-art chromophores with high NLO response cannot be put to use because they are photochemically highly unstable. Good thermal and photochemical stabilities with preserved high hyperpolarizabilities can be achieved by replacement of an aromatic ring with easily delocalizable heteroaromatics, e.g. with benzothiazole. Furthermore, desirable modifications of the benzothiazole fragment lead to improvement in values. Here we report results of a comprehensive investigation of the photochemical stability of seven D--A push-pull molecules based on a N-methylbenzothiazolium acceptor and a N,N-dimethylaminophenyl donor with a different length of conjugated bridge and different acceptor strength. The quantum yield () and the kinetic parameters of photoreactions were determined for existing photodegradation pathways on irradiation at 300-850 nm in MeOH. Trans¬-cis photoisomerization is proposed as a fast but inefficient photobleaching mechanism for these irradiation wavelengths. Self-sensitized photooxidation by 1O2 makes very slow parallel photodegradation pathway and, albeit to small value of Ф, plays a dominant role in the photodegradation of the compounds investigated. Both structural modifications (extension of conjugated bridge and an additional acceptor group bonded to heterocycle) resulting in an increase of NLO response led to a decrease in photostability due to the self-sensitized 1O2 photooxidative attack. Thus a compromise should be found between an increase in NLO response and a decrease in photostability to make a choice of studied compounds for practical applications.
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