Multimode-photochromism based on strongly coupled dihydroazulene and diarylethene

Synthesis and photophysical/photochemical investigations of 1,8a-dihydro-2, 3-bis(2,5-dimethy-3-thienyl)-azulene-1,1-dicarbonitrile (1A) and 1,8a-dihyd ro-2,3-diphenylazulene-1,1-dicarbonitrile (2A) are reported. The photoproce sses and thermal reactions of systems 1 and 2 were studied by time-resolved and steady-state techniques under various conditions. The dihydroazulene ( DHA)-dithienylethene (DTE) conjugate 1A is photochemically converted into t he dihydrothienobenzothiophene (DHB) isomer 1C and the vinyl-heptafulvene ( VHF) isomer 1B. System 2 exhibits exclusively DHA/VHF photochromism. For bo th systems the VHF form thermally reverts back into the DHA form. Their rat e constant (k(B -->A)) increases with the solvent polarity and the relaxati on kinetics proceed by means of an activation barrier of 65-80 kJ mol(-1); k(B -->A) and the activation parameters of the isomerisation reactions are rather similar. The photostationary state of the 1A --> 1B and 1A --> 1C ph otoisomerisation is sensitive to the irradiation wavelength. The concept of cycloswitching is discussed.