Difluorenyl carbo‐Benzenes: Synthesis, Electronic Structure, and Two‐Photon Absorption Properties of Hydrocarbon Quadrupolar Chromophores

The synthesis, crystal and electronic structures, and one-and two-photon absorption properties of two quadrupolar fluorenyl-substituted tetraphenyl 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-C bonds for 3a, C-C C-C expanders for 3b), exhibit quasi-superimposable one-photon absorption (1PA) spectra but different two-photon absorption (2PA) cross-sections alpha(2PA). Z-scan measurements (under NIR femtosecond excitation) indeed showed that the C C expansion results in an approximately twofold increase in the alpha(2PA) value, from 336 to 656 GM (1 GM=10(-50) cm(4)s molecule(-1) photon(-1)) at lambda=800 nm. The first excited states of A(u) and A(g) symmetry accounting for 1PA and 2PA, respectively, were calculated at the TDDFT level of theory and used for sum-over-state estimations of sigma(2PA)(lambda(i)), in which lambda(i)=2hc/E-i, h is Planck's constant, c is the speed of light, and E-i is the energy of the 2PA-allowed transition. The calculated sigma(2PA) values of 227 GM at 687 nm for 3a and 349 GM at 708 nm for 3b are in agreement with the Z-scan results.