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To study the effect of terminal groups substituent on the two-photon absorption (TPA) properties of D - π - A - π - D structural dyes with diketo-pyrrolo-pyrrole (DPP) as central electron acceptor group, linear/transient absorption spectroscopy, one/two-photon fluorescence spectra, open-aperture Z -scan, and excited states quantum chemical calculations have been performed to systematically study the intermediate product electron acceptor J0 as well as three D - π - A - π - D structural dyes using benzene, triphenylamine and 2,3-methoxy-triphenylamine as terminal electron donors, named as J1, J2, and J3, respectively. The intermediate product electron acceptor itself exhibits a relatively large TPA response, which indicates the DPP group has a large conjugated system. J2, with strong electron-donating ability groups triphenylamine as terminal groups, shows a large TPA cross-section of 534 GM, which is approximately 10 times larger than that of J1 with weak electron pushing/pulling abilities groups benzene as terminal groups (51 GM). The result indicates that the electron-donating abilities of terminal groups do play a key role in TPA response. Furthermore, methoxy substituent on the terminal triphenylamine groups can further effectively increase the TPA response, J3 exhibits a 15-fold enhancement compared with that of J1. Transient absorption spectra and quantum chemical calculation results are well accorded with the experimental results. Terminal substituents with strong electron donor groups are beneficial for the formation of intramolecular charge transfer (ICT) process. Besides, methoxy substituent in the terminal of triphenylamine groups can further enhance the degree of ICT and induce AIE characteristics in the molecule. [ABSTRACT FROM AUTHOR]

[Display omitted] • A theoretical study on one-photon and two-photon absorption spectra of three zinc phthalocyanine derivatives. • Density-functional theory (DFT) and sum-over-states (SOS) model were applied to calculate OPA and TPA processes for several derivatives. • Transition density matrix (TDM) and charge density difference (CDD) maps visualized intramolecular charge transfer during OPA and TPA processes. 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. [ABSTRACT FROM AUTHOR]