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 oligop henylenevinylenes which carry a sequence of hyperpolarizable and dipolar do nor-acceptor (DA) pairs, designed to allow for a free rotation of the DA or ientation in a plane perpendicular to the long molecular axis. The studies include both the measurement of effective dipole moments, which are relevan t 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 m oments and those measured for different oligomers in toluene solution and i n 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 antiparal lel alignment of neighboring dipole pairs. Both the bulkiness of the oligom er molecules and the electrostatic coupling of adjacent dipoles give rise t o an improved stability of the field-induced chromophore alignment. In cont rast to the monomeric stilbene-like model compounds, the time scale of olig omer reorientation parallels the structural relaxation time of the polymeri c host material.