Impact of terminal group on temperature-dependent excited state relaxation in cationic dyes

Cationic organic dyes carry a positive charge distributed along the molecule, and the localization of this charge significantly affects their symmetry and optical properties. Depending on the different factors (topology of the terminal groups, the polarity of the solvent, and the temperature) the polyene, polymethine, or donor-acceptor structure form in such dyes, and excited state relaxation for such systems is not fully explored, particularly at low temperatures. At room temperature, the studied cationic dyes, regardless of symmetry in the ground state, are mostly symmetrical in the excited state. At low temperatures, charge localization effects become evident, leading to symmetry breaking in both ground and excited states. In this paper, we distinguish how terminal groups at the end of the cationic dyes impact the relaxation of excited states by analyzing experimental low-temperature time-resolved spectra combined with quantum-chemical calculations. Distinctive emission (690 nm) in the anti-Stokes range of polymethine band (700–730 nm) features polyene structures forming depending on the temperature, solvent polarity, and charge-donating properties of the dye's terminal groups. Furthermore, in low-temperature time-resolved photoluminescence, a 760 nm band is distinguished and associated with intramolecular charge transfer. Our calculations revealed unequal distribution of total positive charge in different molecular fragments (polymethine chain and terminal groups) and formation of negative charge on polymethine chain. We propose a model of excited state relaxation transitions for linear cationic molecular systems that enable donor-acceptor features. This model offers valuable insights for designing new functional materials with tunable properties or efficient energy transfer systems for artificial photosynthesis.