The nature of optical excitation and the degree of intramolecular charge transfer (ICT) as well as the dynamics of excited ICT states of two new tribranched donor-pi-acceptor molecules with acceptor-terminated (DA(3)) and acceptor-centered (AD(3)) geometries have been investigated by steady-state and femtosecond time-resolved stimulated emission fluorescence depletion (FS TR-SEP FD) measurements in different polar solvents. The interpretation of the experimental results is based on the comparative investigation of the two D-pi-A compounds with respect to the model monomer counterpart (DA). The larger solvatochromic effects and stronger solvent dependence of spectral properties of DA(3) than that of AD(3) indicate that the excited ICT state of DA(3) possesses higher polarity and larger dipole moments compared to those of AD(3). The similarity of absorption and strong solvent-dependent fluorescence spectra of DA(3) and DA reveals that the excited-state properties of DA(3) are identical to that of the model DA, which localized on one of the branches in DA(3). In contrast to DA(3), the large red shift in the absorption and the small Stokes shift of AD(3) suggest the formation of a delocalized ICT state to a certain extent in the excited state of AD(3). The dynamic behavior of excited ICT states for all three compounds are also investigated by femtosecond time-resolved stimulated emission depletion (FS TR-SEP FD) measurements, where the excited-state relaxations are highly dependent on both solvent polarity and the polar degree of the excited ICT states. Furthermore, the steady-state fluorescence excitation anisotropy shows that the intramolecular excitation transfer among the three disorder-induced localized ICT states with nondegenerate transition dipole moments is involved within DA(3). Compared to DA(3), a substantial red shift in the absorption of AD(3) results from the formation of a delocalized ICT state, where the specific excitation anisotropy spectrum shows that the excitation energy is mainly redistributed between the localized ICT state and the delocalized ICT state.

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.