Effects of hydrodynamic coupling on electro-optical transients.

The effects resulting from coupling between translational and rotational diffusion on electro-optical transients were analyzed by Brownian dynamics simulations. Diffusion tensors, including translational-rotational coupling tensors, were derived from bead model simulations. Optical and electrical parameters were assigned according to models and experimental data. Hydrodynamic coupling has a strong impact on the shape of electro-optical transients for objects with a nonsymmetric structure. As expected, hydrodynamic coupling in general does not affect the time constants derived from decay curves. However, special attention is required at singular points, where amplitudes are inverted. Under these conditions, transients are observed with artificially reduced time constants, suggesting compact structures. False conclusions may be avoided, when experimental data are analyzed over a sufficiently wide range of conditions. Because transients induced upon reversal of the field vector are strongly affected by hydrodynamic coupling, dipole types cannot be assigned from these transients simply according to standard rules, when the shape of the objects is nonsymmetric. The time constants obtained from exponential fitting of rise-curves show unexpected dependencies on the field strength, which are mainly due to superposition of individual components with amplitudes of opposite sign that are not resolved upon fitting. Dipole parameters obtained from stationary degrees of orientation via orientation functions may also be strongly affected by hydrodynamic coupling. The high sensitivity of electro-optical transients on details of molecular shape and optical and electrical parameters provides a powerful approach to molecular analysis, but quantitative assignments require special care. When symmetry is lost, which must be expected because of bending in many cases, hydrodynamic coupling effects cannot be neglected.