Electric-field-induced polarization current studies in guest–host polymers

We apply polarization current measurements to study dipole reorientation of nonlinear chromophores that are embedded in a polymer matrix. In particular, we apply a step voltage to a thin film of poly(methyl methacrylate) that is doped with a well-known dipolar chromophore, Disperse Red 1, or an apparently centrosymmetric squarylium chromophore, and measure the isothermal temporal behavior of the current at several temperatures. By correlating the current with electric-field-induced second-harmonic generation, we find that at very early times, polarization current dominates, while at later times, charge hopping mechanisms probably dominate. For the centrosymmetric squarylium chromophore dopant, we find an early-time current suggesting polarization decay with no corresponding second-harmonic signal. This suggests that the squarylium molecule has a dipole moment that is perpendicular to the dominant axis of the polarizability tensor and is consistent with the presence of the cis isomer. A time–temperature–voltage complimentarity relationship is also observed.