Sensitization of singlet oxygen by pyrylcarbocyanine dyes in the plasmonic field of metal nanoparticles.

Luminescence of pyrylcarbocyanine dyes with various chalcogen heteroatoms O, S, and Se series is studied in solutions and polymer films, including additional perturbations by the silver plasmonic nanoparticles. The main goal of the study is sensitization effect of the dyes on singlet oxygen (O2(Δ1g)) generation and its phosphorescence. Substitution of heteroatom (O → S → Se) leads to a sequential bathochromic shift of the dyes` absorption and fluorescence spectra accompanied by a decrease of quantum yield and lifetime of their fluorescence. The time-dependent DFT calculations with spin-orbit coupling (SOC) account explain this behavior by increase of the intersystem crossing (ISC) S1 → T1 rate due to the SOC enhancement. This internal heavy atom effect is analyzed in terms of the 4pz orbital inclination at the Se atom and some σ-π mixing in the almost planar chromophore moiety structure. The delayed fluorescence (DF) is observed in polymer films, because of reversed ISC from the T1 to S1 state. The DF mechanism is supported by direct phosphorescence observation of the Se-containing dye in the near-IR region. Photoexcitation of dye solutions in acetonitrile at atmospheric pressure leads to the singlet oxygen emission at 1270 nm with a lifetime typical for this solvent. In spite of low quantum yield of O2(Δ1g) generation (ΦΔ ≈ 0.03) the studied dyes could be useful sensitizers of singlet oxygen because of their NIR operation region, low toxicity and plasmon nanoparticles enhancement effects.