Tunable third-order NLO properties of acene derivatives with molecular structural modification

The substitution of heteroatoms and the expansion of π-conjugated units have significant effects on the photoelectric properties of polycyclic aromatic hydrocarbons (PAHs). In this study, based on the experimental molecule PBC, 10 acene derivatives containing the pyrrole group were designed by three strategies: (1) changing the connection position of the pyrrole group, (2) exchanging the position of the N atom, and (3) increasing the length of the π-linker. Density functional theory (DFT) was used to optimize the molecular geometric structure. Time-dependent density functional theory (TD-DFT) was used to calculate the relevant parameters of the excited states. The results show that both of the pyrrole group and the N atom in the para-position, and the addition of the π-linker can reduce the energy gap, cause redshift of the linear absorption peak, and increase the two photon absorption (TPA) cross-section. The analyses of the charge density difference (CDD) and the charge-transfer matrix (CTM) proved that the electron transfer is mainly concentrated in the π-linker. Moreover, the participation of the multi-ring skeleton on both sides decreases gradually with the increase of the length of the π-linker. The length of the π-linker changes the dominant transition channel and intramolecular charge-transfer (ICT) characteristics of TPA, thus affecting the transition dipole moments and nonlinear absorption properties. The second hyperpolarizability of the designed molecule PBI5-p5 is also significantly superior to that of similar materials reported. It is expected that the above molecular design strategies and comprehensive analysis of nonlinear optical (NLO) properties could provide theoretical support for the research on acene derivatives.