Tetrastyrylarene derivatives: Comparison of one- and two-photon spectroscopic properties with distyrylarene analogues

The one-photon and two-photon absorption properties of cross-shaped chromophores consisting of four donor-substituted styryl branches linked to an aromatic core (benzene or pyrazine) have been investigated and compared with those of linear analogues with only two branches (donor-pi-donor distryrylarenes). The areas of the lowest energy two-photon absorption bands of the compounds with four branches were less than twice those of analogues with two branches. The spectral features observed in these chromophores suggest that electronic coupling between the branches is effective but does not lead to significant enhancement of the two-photon cross section when the branches extend in more than one dimension. In a chromophore with two donor-substituted and two acceptor-substituted branches the two-photon cross section is smaller than in the corresponding linear analogues. The main characteristics of both the one-photon and two-photon spectra of multibranched compounds of the type discussed here can be explained qualitatively within the molecular exciton description. In contrast to the case of one-photon absorptivities, the model shows that pure additivity of the two-photon absorption cross section should not be expected when two monomer units are coupled and that the cross section of the dimer depends on the relative orientation of the constituent units and on the strength and sign of the coupling interaction. In particular, the type of coupling effective in the four-branch chromophores presented here should result in a subadditivity of two-photon cross section of the monomers, in agreement with the experimental findings.