One- and Two-Photon Photophysical Properties, Ultrafast Dynamics, and DFT Study of Three Modified Zinc...

As two-photon absorption (TPA) materials, phthalocyanine molecules have promising application prospects due to their large TPA absorption cross-section, high third-order nonlinear optical susceptibility, and ultrafast response characteristics. In this work, optical properties and the ultrafast response of three modified zinc phthalocyanine molecules (P-HPcZn, Pc-P-Pc, and (DR1)₄PcZn) were analyzed. No obvious side-shoulder absorption peaks in the Q-band can be observed from the steady-state absorption spectra of the three molecules, confirming the lack of aggregation products in the solutions of our measurement. Open-aperture Z-scan results show relatively large TPA cross-section values of 136.4 and 55.3 GM for Pc-P-Pc and (DR1)₄PcZn, respectively. The nonlinear optical results show that the absorption process observed under the excitation of femtosecond pulses is a reverse saturable absorption (RSA) mechanism. Up-conversion fluorescence spectra of (DR1)₄PcZn in THF solution indicate that the fluorescence emission mechanism is TPA. In the study of ultrafast dynamics, the transient absorption spectra were investigated and the decay lifetime of the dynamic traces corresponding to some representative probe wavelengths was obtained through data fitting with a multi-exponential function. Finally, the charge transfer and excited state properties of the modified zinc phthalocyanine molecules were discussed in depth by the DFT method. The energy gaps of P-HPcZn, Pc-P-Pc, and (DR1)₄PcZn are 2.16, 1.39, and 2.13 eV, respectively. The results indicate that the Pc-P-Pc of donor-acceptor-donor (D-A-D) structure has the smallest energy gap as well as the best charge transfer properties.