On the Determination of Transition-Moment Directions from Absorption Anisotropy Measurements

A formula is derived for the absorption anisotropy K = (A − A⊥)/(A + 2A⊥) (where A and A⊥ are the absorbances parallel and perpendicular to the stretching direction of the polymer film, respectively) as a function of the stretch ratio, Rs, of the film and the angle φ between the absorption transition moment direction and the long axis of a prolate molecule. Employing this relation, absolute transition moment directions (the angles φ) were determined experimentally for the following compounds: 1,8-diphenyloctatetraene (DPO), 1,6-diphenylhexatriene (DPH), 1,4-diphenylbutadiene (DPB), 4-dimethylamino-4′-nitrostilbene (DNS), 4-dimethylamino-4′-chlorostilbene (DCIS) and p-terphenyl (TP). The directions were found to be along the long molecular axis in the long-wave absorption band. Small deviations of the angles obtained from φ = 0°, which were of the order of several degrees, are due to the incomplete linearity of the molecules under investigation.