Rotational dynamics of excited probes: Analysis of rate constants based on the predictions of hydrodynamic theory

摘要

The rotational diffusion of benzo(rst)pentaphene (RST) in n-hexadecane solvent has been investigated using time-resolved linear dichroism spectroscopy. Theoretical models of rotational diffusion predict biexponential decays of the observed dichroism for this planar polycyclic aromatic probe in solution. Consistent with theory, two time constants of tau(1) = 12 +/- 5 and tau(2) = 180 +/- 10 ps are observed in the loss of dichroism for RST. The ratio of these two time constants is tau(2)/tau(1) approximate to 15 and they occur with approximately equal weights within the decay. Hydrodynamic theory, using either slip or stick boundary conditions, would suggest a ratio of only ca. 2-4. Detailed analysis of the preexponential factors for the decay components according to current hydrodynamic theory reveals that there are no physically allowed conditions that would result in a ratio of time constants that exceed ca. 4 while simultaneously allowing the two exponentials to be observed with nearly equal weights within the decay. Based on this finding, it is unlikely that both observed decay components can be due to random diffusional motion.