Excited State Relaxation in Cationic Pentamethine Cyanines Studied by Time-Resolved Spectroscopy

In this work, we examined the nature and pathways of electronic excitation relaxation at low temperature in cationic pentamethine pyrylocyanine dye, which in the ground state is characterized by symmetric and nonsymmetric localization of cationic charge on the carbon chain between the terminal groups. The carbon chain in such symmetric and non-symmetric charge localizations is close to polymethine and polyene conformations, respectively. The effect of symmetry breaking during the two-way transition polymethine - polyene in the optically excited state on the electronic properties of dyes is considered. Combined quantum-chemical calculations and time-resolved spectral measurements allowed us to characterize the symmetric and non-symmetric structures in the ground and excited states with specific peaks of transitions and lifetimes. The features of the polymethine structure of the dye are attributed to large-radius polaron formation, whereas for the polyene structures the model of excitons formed on the double bonds of the carbon chain can be used to analyze the relaxation pathways. The energy diagram model for relaxation pathways of optically excited electronic states has been proposed. The proposed model can be characteristic for pentamethine cyanine dyes with the symmetric and non-symmetric distribution of cationic charge at the carbon chain between the terminal groups in the ground and excited states.