Evaluating the Role of Chromophore Side Group Identity in Mediating Solution-Phase Rotational Motion

Abstract

We report on the rotational diffusion dynamics of the chromophore 7-nitrobenz-2-oxa-1,3-diazole (NBD) in a series of protic and polar aprotic solvents, as a function of the identity of the side group appended to the chromophore amine functionality. The central issue we address is whether or not the side groups play a role in mediating the anisotropic reorientation dynamics of the chromophore. To understand the motional properties of the chromophores in detail, we use both one-photon and two-photon excited fluorescence anisotropy decay measurements, and from these complementary excitation methods, we extract two of the Cartesian components of the rotational diffusion constant, D. The experimental data indicate that, regardless of the functionality of the pendant side group, the reorienting moieties exhibit ratios of Dz/Dx in the range 1.8-2.0. There is a small but discernible difference between the substituted chromophores. For all of the substituted NBD chromophores, dielectric friction plays a discernible role in determining their reorientation dynamics.