Difluorenyl carbo-benzenes: Synthesis, electronic structure, and two-photon absorption properties of hydrocarbon quadrupolar chromophores
Résumé
The synthesis, crystal and electronic structures, and one- and two-photon absorption properties of two quadrupolar fluorenyl-substituted tetra-Ph carbo-benzenes are described. These all-hydrocarbon chromophores, differing in the nature of the linkers between the fluorenyl substituents and the carbo-benzene core (C[n.63743]C bonds for 3 a, C[n.63743]C[n.63741]C[n.63743]C expanders for 3 b), exhibit quasi-superimposable one-photon absorption (1PA) spectra but different two-photon absorption (2PA) cross-sections σ2PA. Z-scan measurements (under NIR femtosecond excitation) indeed showed that the C[n.63741]C expansion results in an approx. twofold increase in the σ2PA value, from 336 to 656 GM (1 GM=10-50 cm4 s mol.-1 photon-1) at λ=800 nm. The first excited states of Au and Ag symmetry accounting for 1PA and 2PA, resp., were calcd. at the TDDFT level of theory and used for sum-over-state estns. of σ2PA(λi), in which λi=2 hc/Ei, h is Planck's const., c is the speed of light, and Ei is the energy of the 2PA-allowed transition. The calcd. σ2PA values of 227 GM at 687 nm for 3 a and 349 GM at 708 nm for 3 b are in agreement with the Z-scan results.