Photochemistry of ketocyanin dye — polyenic bis-ω,ω′-amino ketones substituted in the polyene chain

Abstract

The photochemistry of ketocyanin dyes, 15 polyenic bis-ω,ω′-amino ketones (PBAK), was studied by the methods of pulsed and laser photolysis. During the photoexcitation of the PBAK solutions, the formation of their cis-photoisomers is observed, whose relaxation kinetics into the initial trans-isomers is dependent on the intensity of the probing light. The energy of activation of the cis → trans-isomerization process in the dark increases with decrease in the polarity of the solvent. During the photoexcitation of fluorine-substituted PBAK with a six-membered central ring, their cyclization takes place with the formation of pyran forms (PF) absorbing in the UV region; the PF then convert into the initial dye. The formation of PF partially proceeds via the intermediate formation of a cis-isomer of the dye. During the photoexcitation of deoxygenated solutions of PBAK, the formation of their triplet states is also observed; the decay kinetics of the triplet states becomes accelerated with decrease in the solvating power of the solvent. This is explained by the decrease in the T-S₀ splitting due to increase in the energy barrier of the cis-trans isomerization process. A scheme has been proposed for the potential surfaces of PBAK to explain this phenomenon. For some PBAK, the formation is observed of cis-photoisomers not only from the S₁ but also from the T-state.