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

Abstract

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 1 O 2 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.

Mechanism landscape in pyrylium induced organic afterglow systems

“…In the case of P1-PVK-TMTPB materials, the transient absorption spectra (excited by a 390 nm pump laser) collected at delay times from 200 fs to 500 ps show a broad band from 500 nm to 750 nm (Fig. 4I and S24 †), which can be mainly attributed to P1's excitons [59][60][61][62][63] and possibly the minor contribution of P1's radicals. [62][63][64] At such an early stage, the contribution of P1's radicals for the transient absorption spectra should be small, because the extent of intermolecular charge transfer from PVK to P1 should be small.…”

Exploring the Te(II)/Te(IV) Redox Couple of a Tellurorosamine Chromophore: Photophysical, Photochemical, and Electrochemical Studies

“…Such two-electron oxidations of Te(II) to Te(IV) are seen in other Te heterocycles as well. 47 The event at 0.92 V was assigned to further oxidation chemistry of the Te(IV) core. The events at −1.0 and −1.8 V were attributed to additional ring-centered reduction chemistry, in analogy to what has been observed for previous Te systems, as well as simple O-containing rhodamines.…”

Shining light on the photophysics of polymethine dyes with chalcogenopyrylium termini

“…[19][20][21] These dyes have been described as singlet oxygen sensitizers in air-saturated solutions, [7,22,23] but these prior studies do not rule out the generation of other reactive oxygen species. Herein and elsewhere, [24] we describe the electronic structure and excited-state dynamics of fourteen dyes, including six novel dyes. We have studied the effects of selenium versus tellurium; the pendant substituents; and the length of the polymethine backbone.…”