Fluorescence spectroscopy of potential electroluminescent materials: Substituent effects on DSB and segmented PPV derivatives

The absorption and fluorescence of substituted distyrylbenzene (DSB) deriva tives and segmented poly(phenylene vinylene) (PPV) derivatives are characte rized by long-wavelength absorption maxima and absorption coefficients of l ambda(a) = 380-450 nm, epsilon = 20,000-60,000 M-1 cm(-1) and fluorescence maxima, quantum yields, and decay times of lambda(f) = 440-530 nm, Phi(f) = 0.2-0.9, and tau = 0.8-2.5 ns, respectively. Alkoxy substituents at the ce ntral phenylene ring of DSB groups increase the bathochromic shift in the s pectra in comparison to DSB, without a significant decrease in the high DSB fluorescence quantum yield. Both phenyl and cyano substitutions at the vin ylene bridge lead to a further bathochromic shift of the fluorescence and a decrease in the quantum yield to ca. 0.4. The DSB derivatives and the rela ted segmented PPV derivatives show nearly the same absorption spectra, fluo rescence spectra, and radiative rate constants k(f) = Phi(f)/tau, indicatin g the efficacy of the segmentation of the polymer chain. The radiative rate constants determined by the Phi(f) and tau values and by the Strickler/Ber g formula are in reasonable agreement. This supports the possibility of int erpreting the properties of the polymers in terms of their DSB units. The d ecrease in the emission anisotropy can be ascribed to multistep energy tran sfer processes between different oriented segments.