Shining Light on the Photophysics of Polymethine Dyes with Chalcogenopyrylium Termini

Previous reports of chalcogenopyrylium polymethine dyes establish singlet oxygen yields as high as 12%. Our studies of these systems suggested that the current understanding of the excited state dynamics is incomplete. In fact, we observe no evidence for singlet oxygen generation across a range of experiments. We have carried out steady-state and pulsed-laser kinetic experiments on a family of fourteen dyes, including six novel dyes, selected to vary physical and electronic structure. We have changed the identity of the chalcogen between selenium and tellurium heteroatoms, phenyl, thiophene, tert-butyl substituents, and methine linker length. Excited-state lifetimes were obtained by femtosecond transient absorption spectroscopy. Lifetimes were all sub-300 picoseconds, suggesting rapid relaxation. Furthermore, we observed no evidence of any triplet transient processes; phosphorescence was only observed in samples at 77 K. Variable-temperature NMR experiments implicate rotation of the pyran ring about the methine backbone as a critical determinant of the dynamics of these dyes that distinguishes their photophysics from more rigid analogues.