Femtosecond fluorescence upconversion studies of barrierless bond twisting of auramine in solution

Femtosecond fluorescence upconversion studies have been performed for auramine (a diphenylmethane dye), dissolved in ethanol, as a function of temperature. It is found that the (sub)picosecond decay components in the fluorescence slow down as the temperature is lowered from 293 K to 173 K. From the observation of a residual fluorescence, with a viscosity-dependent lifetime of about 30 ps (or longer at higher viscosity), and transient absorption results it is concluded that the two-state sink function model [B. Bagchi, G. R. Fleming, and D. W. Oxtoby, J. Chem. Phys. 78, 7375 (1983)] does not apply in the case of auramine. Comparison of the auramine fluorescence kinetics in ethanol and decanol shows that diffusional twisting and not solvation is the main cause for the (sub)picosecond excited state relaxation. To explain the experimental results, adiabatic coupling between a locally excited emissive state (F) and a nonemissive excited state (D) is considered. Torsional diffusion motions of the phenyl groups in the auramine molecule are held responsible for the population relaxation along the adiabatic potential of the mixed state, S-1 (comprised of the F and D states). Simulation of the excited state dynamics is feasible assuming a barrierless-shaped potential energy for S-1 and applying the Smoluchowski diffusion equation. The temporal behavior of the auramine band emission was simulated for the temperature range 293 K > T > 173 K, with the temperature, T, and the viscosity coefficient, eta, being the only variable parameters. The simulated temporal behavior of the emission in the investigated temperature range is compatible with that obtained experimentally. The rotational diffusion coefficient for the auramine phenyl groups as extracted from the simulations is found to follow the Einstein-Stokes relation. From the numerical calculations the effective radius of the twisting phenyl groups is determined as 1.0 Angstrom which compares well with the actual value of 1.2 Angstrom. (C) 2000 American Institute of Physics. [S0021-9606(00)51706-1].