Intersystem Crossing Rates in Photoexcited Rose Bengal: Solvation versus Isolation.

Abstract

We compare the intersystem crossing rate, kISC, of Rose Bengal (RB) in an aqueous pH 12 solution with the corresponding relaxation rates of four different RB-derived anion and dianion species isolated in the gas phase: the doubly deprotonated dianion ([RB-2H]2-), the singly deprotonated monoanion ([RB-H]-), and the corresponding singly negatively charged sodium and cesium adducts ([RB-2H + Na]- and [RB-2H + Cs]-, respectively). Each of them was probed following photoexcitation of their first singlet excited states (S1) at or near room temperature. The solution was studied by transient absorption spectroscopy, whereas the mass-selected anions were characterized by time-resolved photoelectron spectroscopy─all with ca. 50 femtosecond temporal resolution. [RB-H]- shows an S1 lifetime of ca. 80 ps; the solution ensemble, thought to consist primarily of solvated dianion chromophores, shows a similar lifetime of ca. 70 ps. By contrast, the isolated dianion, [RB-2H]2-, has a much longer lifetime. Superimposed on S1 decay attributable mainly to intersystem crossing, all four isolated anions also show some rapid oscillatory features of the transient photoelectron signal on a 4-5 ps timescale after excitation. Interestingly, an analogous phenomenon is also seen in the transient absorption measurements. We attribute it to a librational oscillation as the S1 state, initially populated in the S0 geometry, relaxes into its excited state equilibrium structure. Some implications of these observations for RB photophysics and interpretation of solution measurements are discussed─also in terms of density functional theory and time-dependent density functional theory calculations of ground and excited states.