Ultrafast twisting dynamics of photoexcited auramine in solution

Subpicosecond fluorescence up-conversion and transient absorption spectroscopy is applied to study the excited-state dynamics of auramine, a diphenylmethane dye, in liquid solutions. The fluorescence decays, on a time scale of a few picoseconds to a few tens of picoseconds, are found to be nonexponential and solvent viscosity dependent. They can be fitted as a sum of two exponentials in ethanol and three exponentials in decanol with a larger average Lifetime in the more viscous solvent. The decays exhibit wavelength-dependent time constants, whereas the fluorescence rise time is instrument limited (150 fs) at all wavelengths. The average decay time increases with the wavelength across the steady-state emission spectrum. The spectral reconstruction indicates a few hundred wavenumbers dynamic Stokes shift accompanied by a drop in the intensity in both solvents. From transient absorption experiments, the fluorescent state population is shown to decay to an intermediate dark state and then to the ground state, with a viscosity-dependent rate. A barrierless or quasi-barrierless photoreaction involving the rotational diffusion of the phenyl rings, with a change in the radiative transition rate along the reaction path, is proposed to explain the wavelength-dependent nonexponential fluorescence decays. Both fluorescence and transient absorption data are discussed in support of an adiabatic photoreaction involving internal twisting and charge shift.