Photophysics, photochemistry and thermal stability of diarylethene-containing benzothiazolium species

• Ring-size and solvent effects on photophysics, photo- and thermal-isomerisation of diaylethene-containing benzothiazolium moieties.
• Effect of N-methylation on photo-properties of neutral dyes.
• Investigation of mechanism of cis–trans thermal isomerisation reaction.

The photophysics, photochemistry and thermal stability of four 1-aryl-2-(N-methyl-2-benzothiazolium) ethene iodides (aryl: phenyl, 1-naphthyl, 9-phenanthryl and 9-anthryl) were studied. Although the absorption spectra are found to be hypsochromic-shifted, fluorescence spectra are bathochromic-shifted. The dipole moment in the relaxed excited state was found to be larger than that in the ground state. To investigate effects of N-methylation and the aryl-ring size, a detailed comparison was made between those in the present work (of charged compounds) and previous studies of their neutral analogues, with computed electron affinity and ionisation potentials serving to rationalise the experimentally observed bathochromic shifts in absorption and emission spectra. The kinetics of thermal isomerisation depend strongly on the nature of the aryl moiety and solvent; the larger the aryl ring, the slower the rate of isomerisation. The fastest isomerisation process was found to take place in MeOH. The anthryl derivative did not isomerize either by light- or heat-exposure, due to high energy barriers of rotation around the ethenic bond. Based on the significant blue-shift of the Z-isomer absorption maximum relative to that of the E-isomer, and the high percentage of Z-isomers in the photostationary state, these compounds may serve as potential promising candidates for optical data-storage applications.