Heavy atom effects on synthetic pyranoanthocyanin analogues

Pyranoflavylium cations are synthetic analogues of pyranoanthocyanins, the much more color-stable compounds that are formed spontaneously from grape anthocyanins during the maturation of red wines. In the present work, our studies of the photophysical properties of pyranoanthocyanin analogues are extended to include nine pyranoflavylium cations substituted with one or two bromo and/or iodo heavy atoms. The room temperature fluorescence, 77 K fluorescence and phosphorescence, triplet formation in solution, and sensitized singlet oxygen formation, with excited state acidity suppressed by the addition of trifluoroacetic acid, are compared to those of similar pyranoflavylium cations that do not contain a heavy atom. Heavy atom effects on the photophysics of the S₁ state of pyranoflavylium cations are found to be relatively small, which is attributed to the nodal properties of the orbitals involved, which prevent effective mixing of the spin-orbit coupling on the heavy atoms into the excited singlet state, S₁, of the pyranoflavylium chromophore. Heavy atom effects on the phosphorescence of these heavy atom-substituted pyranoflavylium cations at 77 K are somewhat larger, consistent with a spin-orbit coupling-induced increase in the radiative rate constant for phosphorescence, as are the triplet-sensitized singlet oxygen formation quantum yields in fluid solution.