Synthesis, electronic structure, linear and nonlinear photophysical properties of novel asymmetric branched compounds

A series of novel asymmetric branched compounds that utilize a 1,3,5-triazine core and feature D-π-A-(π-D′-π-A′)0-2 configurations (D = donor, A = acceptor, π = conjugated bridge) were designed, successfully synthesized, and fully characterized by 1H NMR, 13C NMR, FT-IR, and HRMS. Their photophysical properties including linear absorption, one-photon excited fluorescence, two-photon absorption, and frequency up-converted fluorescence, were systematically investigated in different solvents. With a rise in the polarity of solvents, the peak positions of the one-photon excited fluorescence are red-shifted and the Stokes shifts increase, while the linear absorption wavelengths change slightly. In addition, the target compounds except CZ show the positive solvatokinetic effect. With a rise in the number of branches, the red shifts of the absorption and emission maxima, the hyperchromicity of the molar absorption coefficients, and the decrease of the Stokes shifts are observed. The peripheral electron donors (carbazole, phenothiazine) and acceptors (pyridine, benzimidazole) also exert an important influence on the photophysical properties. Under excitation of 690–930 nm fs laser pulses, all the target compounds emit frequency up-converted fluorescence with the maximal peaks at 471–575 nm, and the two-photon absorption cross-sections in THF are 132 (PTZ), 182 (CZ), 453 (CZ-Py1), 844 (CZ-Py2), 1244 (CZ-BI1), and 2072 (CZ-BI2) GM, respectively. Their two-photon response is found to be nearly additive with respect to the number of branches. The time-dependent density functional theory calculations were conducted to gain an insight into their electronic structures and to better understand the structure-photophysical property relationships. The results clearly indicate the importance of appropriate structural units on the enhancement of two-photon absorption properties.