Photoluminescence of poly(p-phenylenevinylene)-silica nanocomposites:: Evidence for dual emission by Franck-Condon analysis

The vibronic mode intensity pattern of the photoluminescence (PL) spectra of poly(p-phenylenevinylene) (PPV) nanocomposites dispersed with 5-nm-diam silica particles shows an apparent redistribution toward the nominal 0-0 mode with increasing silica volume fraction. Franck-Condon analysis of this variation, corrected for refractive index dispersion, reveals the presence of overlapping emission from two excited electronic states separated by 180 meV. The principal emission arises from the molecular exciton while the lower-lying one is assigned to a dipole-dipole coupled two-chain aggregate exciton. The quantum yield of the aggregate emission decreases monotonically with silica loading up to 50 vol %, whereas that of the molecular state exhibits a maximum at 15 vol %. When the samples are photoexcited below the pi-pi (*) localization edge, both of these emissions jointly redshift without a change in their relative intensities. When cooled below a transition temperature centered at 120 K, the yield of the aggregate exciton decreases sharply relative to the molecular exciton and the overall PL quantum yield (eta (pl)) rises. The aggregate exciton therefore appears to be formed from the molecular exciton through a phonon-assisted mechanism. At room temperature, this directly competes with de-excitation of the molecular exciton. This behavior differs from the dialkoxy-PPVs which show site-selective excitation and thus direct population of the aggregate domains. Using classical dielectric medium theories to correct for the effects of refractive index, the radiative lifetime (tau (r)) of the molecular exciton in the various PPV compositions can be estimated. Together with the experimentally determined eta (pl), this gives the eta (pl)tau (r) product of the molecular exciton as a function of composition. This function exhibits a maximum at 15 vol % silica, indicative of a crossover behavior that shows the competing influence of morphological disorder on the population and radiative de-excitation of this state. (C) 2001 American Institute of Physics.