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 o f poly(p-phenylenevinylene) (PPV) nanocomposites dispersed with 5-nm-diam s ilica particles shows an apparent redistribution toward the nominal 0-0 mod e with increasing silica volume fraction. Franck-Condon analysis of this va riation, corrected for refractive index dispersion, reveals the presence of overlapping emission from two excited electronic states separated by 180 m eV. The principal emission arises from the molecular exciton while the lowe r-lying one is assigned to a dipole-dipole coupled two-chain aggregate exci ton. The quantum yield of the aggregate emission decreases monotonically wi th silica loading up to 50 vol %, whereas that of the molecular state exhib its a maximum at 15 vol %. When the samples are photoexcited below the pi-p i (*) localization edge, both of these emissions jointly redshift without a change in their relative intensities. When cooled below a transition tempe rature centered at 120 K, the yield of the aggregate exciton decreases shar ply 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 excitatio n and thus direct population of the aggregate domains. Using classical diel ectric medium theories to correct for the effects of refractive index, the radiative lifetime (tau (r)) of the molecular exciton in the various PPV co mpositions can be estimated. Together with the experimentally determined et a (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 % sil ica, 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.