Theoretical Insight into the Effect of Phosphorus Oxygenation on Nonradiative Decays: Comparative Analysis of P-Bridged Stilbene Analogs

Incorporation of phosphorus element into a 𝜋-conjugated skeleton offers valuable prospects for adjusting the electronic structure of the resulting functional 𝜋-electron systems. The trivalent phosphorus has the potential to decrease the LUMO level through the 𝜎∗-𝜋∗ interaction, which is further enhanced by its oxygenation to the pentavalent P center. This study shows that utilizing our computational analysis to examine excited-state dynamics based on radiative/nonradiative rate constants and fluorescence quantum yield (ΦF) is effective for analyzing the photophysical properties of P-containing organic dyes. We theoretically investigate how the trivalent phosphanyl group and pentavalent phosphine oxide moiety affect radiative and nonradiative decay processes. We evaluate four variations of P-bridged stilbene analogs. Our analysis reveals that the primary decay pathway for photo-excited bis-phosphanyl-bridged stil- bene is the intersystem crossing (ISC) to the triplet state and nonradiative. The oxidation of the phosphine moiety, however, suppresses the ISC due to the relative destabilization of the triplet states. The calculated rate constants match an increase in experimental ΦF from 0.07 to 0.98, as simulated from 0.23 to 0.94. The reduced HOMO–LUMO gap supports a red shift in the fluorescence spectra relative to the phosphine analog. The thiophene-fused variant with the non-oxidized trivalent P cen- ter exhibits intense emission with a high ΦF, 0.95. Our prediction indicates that the ISC transfer is obstructed owing to relatively destabilized triplet state induced by the thiophene substitution. Conversely, the thiophene-fused analog with the phosphine ox- ide moieties triggers a high-rate internal conversion mediated by conical intersection, leading to a decreased ΦF.