Effect of tunable π bridge on two-photon absorption property and intramolecular charge transfer process of polycyclic aromatic hydrocarbons

The influences of the conjugation effect on the charge transfer and nonlinear optical (NLO) properties of polycyclic aromatic hydrocarbons (PAHs) are comprehensively investigated at the microscopic molecular level. We found that the conjugation effect of π bridge is negatively correlated with molecular planarity, excitation energy, two-photon absorption (TPA) cross-section, and the second hyperpolarizability. For the first time, the charge transfer matrix (CTM) is applied to the molecular two-photon transition process. Combining the charge difference density (CDD) diagram with CTM heat map to visually quantitative investigate the characteristics of excited states, the charge transfer path and transfer amount between atoms. During the two-photon transition of all molecules, the electronic excited state is locally excited. Compared with the first process, the range of intramolecular charge transfer in the second process of the two-photon transition is expanded. Comprehensive results prove that the π bridge with large conjugation effect distorts the molecular structure, which is not conducive to the intramolecular charge transfer. Therefore, the molecule DBP-1 with a carbon-carbon double bond as the π bridge has the largest transition dipole moments, TPA cross-section, and second static hyperpolarizability. Our research method can provide effective guidance for the design and optimization of nonlinear organic conjugated molecular materials.