Mj. Van Der Meer et al., Femtosecond fluorescence upconversion studies of barrierless bond twistingof auramine in solution, J CHEM PHYS, 112(6), 2000, pp. 2878-2887
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].