We report a series of stiff dendrimers (referred to as T1, T2, T3, and T4) that have both gigantic two-photon absorption (TPA) cross sections up to 25,000 GM and strong two-photon excited fluorescence (TPEF) with fluorescence quantum yield of ∼0.5. The large TPA cross sections and high quantum yields of these dendrimers are directly related to their geometrical structures, where the polycyclic aromatic pyrene is chosen as the chromophoric core because of its planar and highly π-conjugated structure, fluorene moieties as dendrons extend the conjugation length through the planar structure, and carbazole moieties are modified at three-, six-, and nine-positions as electron donor. All of these groups are linked with acetylene linkage for effective π-electron delocalization, leading to large TPA cross section and high fluorescence quantum yield. The spectral properties of all dendrimers are investigated by one- and two-photon excitations. Furthermore, steady-state fluorescence excitation anisotropy and quantum chemical calculation are also employed to determine the structure-related mechanism of these dendrimers with gigantic TPA cross sections and high TPEF efficiency. We then show that the improvement of branched chains in the T-series dendrimers enhances the light-harvesting ability. The core emission spectra, fluorescence quantum yield, and fluorescence lifetime are almost invariable by directly exciting the dendrons. These results will provide a guideline for the design of useful two-photon materials with structural motifs that can enhance the TPA cross-section and fluorescence quantum yield of a molecule without causing a red shift of the one- and two-photon excitation wavelengths for specific applications.

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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 3 a, C-C=C-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≡C expansion results in an approximately twofold increase in the σ2PA value, from 336 to 656 GM (1 GM = 10(-50) cm(4) s molecule(-1) photon(-1)) at λ = 800 nm. The first excited states of Au and Ag symmetry accounting for 1PA and 2PA, respectively, were calculated at the TDDFT level of theory and used for sum-over-state estimations of σ2PA(λi), in which λi = 2 hc/Ei, h is Planck's constant, c is the speed of light, and Ei is the energy of the 2PA-allowed transition. The calculated 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.