Spectroscopy and rotational dynamics of oxazine 725 in alcohols: A test of dielectric friction theories

The spectroscopy and rotational dynamics of oxazine 725 in alcohols are analyzed in terms of dielectric friction theories. From the static spectroscopy, the difference between the ground and electronically excited state dipole moment is found to be 4.5 D. MNDO calculations reveal a ground state moment of 1.7 D. Using dielectric friction theory, the dipole moment difference can also be evaluated from the difference between the ground and electronically excited state rotational times. A commonly used continuum theory expression for evaluating the dielectric friction constant is found to be inconsistent with the static spectroscopic data. Calculation of the dielectric friction constant using the diffusive model developed by Hubbard and Wolynes gives better agreement with the experimental data. The model developed by van der Zwan and Hynes — which connects dielectric friction to time dependent solvation dynamics — results in quantitative agreement with the data obtained from static spectroscopy. Using this model, an accurate measure of the contribution of dielectric friction to the rotational dynamics can be calculated.