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

The absorption and fluorescence of substituted distyrylbenzene (DSB) derivatives and segmented poly(phenylene vinylene) (PPV) derivatives are characterized by long-wavelength absorption maxima and absorption coefficients of lambda(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 central phenylene ring of DSB groups increase the bathochromic shift in the spectra in comparison to DSB, without a significant decrease in the high DSB fluorescence quantum yield. Both phenyl and cyano substitutions at the vinylene 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 related segmented PPV derivatives show nearly the same absorption spectra, fluorescence spectra, and radiative rate constants k(f) = Phi(f)/tau, indicating 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/Berg formula are in reasonable agreement. This supports the possibility of interpreting the properties of the polymers in terms of their DSB units. The decrease in the emission anisotropy can be ascribed to multistep energy transfer processes between different oriented segments.