Symmetric and asymmetric charge transfer process of two-photon absorbing chromophores: bis-donor substituted stilbenes, and substituted styrylquinolinium and styrylpyridinium derivatives

Two-photon absorption properties of a series of symmetrically substituted stilbenes and asymmetrically substituted stilbene-type derivatives with the same conjugated length have been investigated. The effective two-photon absorption cross sections, delta (TPA), as large as 62.0 x 10(-48) cm(4) s photon(-1) for D-pi -D molecules and 48.5 x 10(-48) cm(4) s photon(-1) for D-pi -A counterparts have been observed. The effect of these two types of chromophores on the peak position of the linear absorption, one-photon fluorescence as well as two-photon absorptivity is reported. Dipole moment change between the ground and the first excited states (Delta mu (ge)), and the transition dipole moment between the first and second excited states (M-ee') have also been calculated. It was found that the asymmetrically substituted derivatives possess relatively large Delta mu (ge), whereas the symmetrical counterparts show an increase in M-ee'. Although a large two-photon absorption resonance is due to the simultaneously high values of M-ee' and Delta mu (ge), correlated to intramolecular charge transfer, the former function is larger. These results obtained have demonstrated that the magnitude and the peak position of two-photon absorption depend not only on the amount but also on the direction of the intramolecular charge transfer.