Conformationally tuned large two-photon absorption cross sections in simple molecular chromophores

We investigate here the relationship between molecular architecture and two -photon absorption (TPA) processes in a class of alkyl-substituted 4-quinop yran chromophores. We find that TPA cross sections diverge as the one-photo n gap energy nears one-half of the two-photon gap. The molecular strategy p roposed here to tune these two-excitation gaps for maximizing TPA cross sec tions is to twist the molecule about the bond connecting the chromophore do nor and acceptor phenylene fragments. Extremely large TPA cross sections, d etermined by the absorption bandwidth, can then be realized (imaginary part of the third-order polarizability similar to 2.6 x 10(5) x 10(-36) esu) fo r fundamental photon energies near 1.0 eV, when the torsional angle approac hes 104 degrees. The required torsional angle is achieved by introduction o f sterically encumbered 2,2',2",2'" tertiary alkyl substituents.