Shining Light on the Solution- and Excited-State Dynamics of Chalcogenopyrylium Polymethine Dyes

Despite 40 years of interest in chalcogenopyrylium polymethine dyes, significant gaps remain in our understanding of their photophysical properties. These gaps hinder efforts to apply such dyes as photodynamic therapy and/or biomedical sensing agents where a complete understanding of their excited-state dynamics and chemistry are important. For example, despite previous reports that establish singlet oxygen yields as high as 12% for certain dyes, we observe no evidence for 1O2 from direct phosphorescence measurements. We now fill in many of these gaps through steady-state and pulsed-laser kinetic experiments on a family of 14 dyes, including six novel dyes, selected to vary physical and electronic structure. These structural changes encompass the 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 ps, suggesting rapid relaxation out of their excited states. Notably, 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. Our work establishes that even within the same class of compounds (e.g., pyrylium dyes), properties and reactivities may differ significantly yet the origins of these differences are not apparent from photophysical measurements alone. When combined with studies of structural dynamics, we have obtained a complete structure–function relationship that we can now apply to much broader classes of dyes and serves as a reliable foundation for developing the applications of such species.