Intersystem Crossing Rates in Photoexcited Rose Bengal: Solvation versus Isolation

We compare the intersystem crossing rate, k_ISC, 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.