Femtosecond fluorescence upconversion studies of barrierless bond twistingof auramine in solution

Femtosecond fluorescence upconversion studies have been performed for auram ine (a diphenylmethane dye), dissolved in ethanol, as a function of tempera ture. It is found that the (sub)picosecond decay components in the fluoresc ence 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 r esults 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 ap ply in the case of auramine. Comparison of the auramine fluorescence kineti cs in ethanol and decanol shows that diffusional twisting and not solvation is the main cause for the (sub)picosecond excited state relaxation. To exp lain the experimental results, adiabatic coupling between a locally excited emissive state (F) and a nonemissive excited state (D) is considered. Tors ional diffusion motions of the phenyl groups in the auramine molecule are h eld responsible for the population relaxation along the adiabatic potential of the mixed state, S-1 (comprised of the F and D states). Simulation of t he excited state dynamics is feasible assuming a barrierless-shaped potenti al energy for S-1 and applying the Smoluchowski diffusion equation. The tem poral behavior of the auramine band emission was simulated for the temperat ure range 293 K > T > 173 K, with the temperature, T, and the viscosity coe fficient, eta, being the only variable parameters. The simulated temporal b ehavior of the emission in the investigated temperature range is compatible with that obtained experimentally. The rotational diffusion coefficient fo r the auramine phenyl groups as extracted from the simulations is found to follow the Einstein-Stokes relation. From the numerical calculations the ef fective 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 America n Institute of Physics. [S0021-9606(00)51706-1].