Rotational relaxation of linear pi-systems with flexible side chains in solution.

The classical hydrodynamic theory for Brownian rotational motion is applied to model compounds of conjugated polymers with alkoxy side chains of variable length. Theory predicts two rotational relaxation times for these types of molecules with the dipole transition moment parallel to the longest axis whereas experiments reveal only one. The rotational relaxation times and their relative amplitudes were calculated for a wide span of axial ratios of a general ellipsoid. In this way, the range in the axial ratios is obtained such that there is a chance to detect both rates experimentally. Rotational relaxation times of five particular molecules were measured in liquid n-butane. Theoretical calculations using ellipsoid parameters obtained from molecular dynamics calculations compare well with experimental results. Calculation of the rotational relaxation times from the autocorrelation function of the transition dipole moment vector requires significantly greater computational effort.