Orientation and dynamics of chainlike dipole arrays:: Donor-acceptor-substituted oligophenylenevinylenes in a polymer matrix

We have investigated the orientational behavior of a series of novel oligophenylenevinylenes which carry a sequence of hyperpolarizable and dipolar donor-acceptor (DA) pairs, designed to allow for a free rotation of the DA orientation in a plane perpendicular to the long molecular axis. The studies include both the measurement of effective dipole moments, which are relevant for the degree of polar ordering required for second-harmonic generation, and the assessment of the time scales involved in the orientational motion of the effective dipole moment. The comparison between calculated dipole moments and those measured for different oligomers in toluene solution and in a polystyrene matrix confirm that a DA-pair separation of approximate to 10 Angstrom along the oligomer leads to a high chromophore concentration in the sample, yet without the unfavorable effect of aggregation or antiparallel alignment of neighboring dipole pairs. Both the bulkiness of the oligomer molecules and the electrostatic coupling of adjacent dipoles give rise to an improved stability of the field-induced chromophore alignment. In contrast to the monomeric stilbene-like model compounds, the time scale of oligomer reorientation parallels the structural relaxation time of the polymeric host material.