Picosecond and Steady-State Emission of [Ru(phen)2dppz]2+ in Glycerol:  Anomalous Temperature Dependence

The excited-state deactivation of the “light-switch” compound [Ru(phen)2dppz]2+, where phen = 1,10-phenanthroline and dppz = dipyrido[3,2-a:2‘,3‘-c]phenazine, has been investigated in glycerol using single-photon counting at picosecond time resolution. Relaxation back to the ground state occurs in about 8 ns at 20 °C, which is much faster than previously reported in monohydric alcohols, though still slow compared to that in water. Multivariate kinetic analysis reveals three distinct excited species involved in the relaxation process in glycerol. Using a matrix exponential approach for the kinetic data analysis, including global fitting of the relaxation data collected at many wavelengths, individual emission spectra for all three excited species could be resolved. The resolved emission profile for the most short-lived species was found to resemble the steady-state emission spectrum of [Ru(phen)3]2+ in glycerol whereas the emission profile of the intermediate species resembled that of [Ru(phen)2dppz]2+ in ethanol. The spectrum of the third species is considerably red-shifted compared to those of the other two. The longest lifetime as well as the emission quantum yield show pronounced nonmonotonic variations with temperature in apparent conflict with the Arrhenius equation. This anomalous temperature dependence can be accounted for by a model based on the equilibrium between two excited species, corresponding to the two resolved emission spectra retrieved at 20 °C. Thermodynamic data indicates that transfer to the fast-relaxing, red-shifted species is accompanied by a substantial lowering in enthalpy. The thermodynamic data, as well as an abnormally high preexponential factor for the back reaction from the third to the second excited species, could be explained in terms of the formation of two hydrogen bonds, one to each of the aza nitrogens of the dppz moiety.