Effect of terminal substituents on intramolecular charge transfer in one- and two-photon absorption of zinc phthalocyanine derivatives

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Zinc phthalocyanine derivatives have unique structure and photophysical properties, which make them a remarkable potential in the fields of optoelectronics as well as biomedicine. In order to reveal the effect of the terminal substituents on the electron transition mechanisms, one-photon absorption (OPA) and two-photon absorption (TPA) processes of three zinc phthalocyanines modified with different terminal substituents (PcZn-0, PcZn-1 and PcZn-2) have been analyzed by using density-functional theory (DFT) in combination with the sum-over-states (SOS) model, the visualization of the transition density matrix (TDM) as well as charge density difference (CDD) calculations. Red-shifted OPA absorption peaks with enhanced intensities, have been observed in molecules PcZn-1, PcZn-2 with long terminal substituents. However, for TPA process, a more effective charge transfer over almost the entire molecule has been observed in PcZn-1 molecule with 4-methoxybenonic acid as terminal substituents. When H-atoms in the hydroxyl groups of four terminal 4-Methoxybenzoic acid groups were substituted by dispersed red-one chains, a limited range of intramolecular charge transfer and localized excitation inside the dispersed red-one chain were observed, leading to a localized excitation absorption peak and expanding the wavelength region of TPA response. Furthermore, terminal substituents with conjugated branches exhibit a stronger cooperative effect between the terminal chains and shows larger TPA response. On the contrary, excessive dihedral angles between different groups can weaken the conjugation degree of molecules and affect intramolecular charge transfer as well as TPA response.