Various copolymers of MA-TIN 1, 2-[2-hydroxy-3-tert-butyl-5-(O-[2-hydroxy-3-(2-methylpropenoyloxy)propyl]-2-carbonyloxyethyl)phenyl]benzotriazole, and MA-TZ 1, 2,4-bis(2,4-dimethylphenyl)-6-[2-hydroxy-4-(2-hydroxy-3-[2-methylpropenoyloxy])propoxyphenyl]-1,3,5-triazine, with styrene, methyl methacrylate, and methacrylic acid have been synthesized by radical polymerization. Their absorption spectra in the long-wavelength UV region appear unchanged compared to those of the monomeric UV absorbers, indicating the stabilizer chromophore to remain unimpaired in the course of the polymerization. Both the monomeric and the polymeric stabilizers exhibit a strongly Stokes-shifted, temperature-dependent, low-quantum-yield fluorescence which arises from an intermediate species formed by intramolecular proton transfer. The intramolecular hydrogen bond which is essential for the photostability of this type of UV absorbers thus is still intact in the copolymers. Activation energies for the radiationless deactivation process can be evaluated from the temperature dependence of the proton-transferred fluorescence. These energies lie between 4 and 5 kJ/mol for most of the benzotriazole and triazine stabilizers investigated and show hardly any matrix dependence. Fluorescence-decay measurements with crystalline MA-TIN 1 at different temperatures reveal a close correspondence of the temperature dependence between decay times and relative quantum yields. The radiationless process thence is concluded to originate from the proton-transferred level S1‘. The decay time at room temperature is estimated at 70 ps, close to the value for crystalline TIN P. The proton-transferred fluorescence of MA-TIN 1, in contrast, exhibits a biexponential decay profile.

Photochemical and thermal cis/trans isomerization is reported for a stilbazolium betaine (M). Obtained from synthesis in the trans configuration, M cannot be isomerized directly but only via the 0-protonated form MH&ns. Photoisomerization of MH;,,, yields the cis isomer MH& (structure established by IH NMR) which can be deprotonated to the cis betaine. M,i, represents the first example of a cis isomer in this class of compounds. Though fairly stable in aqueous solution, M,i, can be fully reverted to the trans isomer either thermally or photochemically. The sequence M,,,,, s MH:,,, F! MH;, ~i M,i, -M,,,,, constitutes a complete molecular reaction cycle which may serve as a chemical model for the storage of information and subsequent regeneration of the information carrier, for instance, in biological systems.

No abstract

No abstract

No abstract