Intra- and Intermolecular Charge Transfer in a Novel Dimer: Cooperatively Enhancing Second-Order Optical Nonlinearity

Based on s-indaceno­[1,2,3-cd;5,6,7-c′d′]­diphenalene (1) consisting of two phenalenyl moieties, the monomer 2 and its dimer 2₂ are designed by boron and nitrogen atoms substituting the central carbon atoms of phenalenyl moieties. Calculated energy decompose analysis (EDA) shows that the orbital interaction for 2₂ possesses a large attractive contribution of −18.31 kcal mol^–1, which is dominated by the π–π stacking interaction between the upper and the lower π-conjugated units. Interestingly, the natural population analysis (NPA) charge and the transition density matrix (TDM) show that both intramolecular charge transfer and intermolecular charge transfer (CT) exist in 2₂. Further, the first hyperpolarizability (β_tot = 4.56 × 10⁴ au) of 2 with intramolecular CT is greatly larger than that of reported molecule 3 (5.45 × 10³ au) with intermolecular CT. Significantly, 2₂ exhibits the largest β_tot value to be 1.42 × 10⁵ au, which is caused by combining the intra- and intermolecular CT transitions (β_x = 1.40 × 10⁵ au and β_z = 2.27 × 10⁴ au). Correspondingly, highest occupied molecular orbital (HOMO) → lowest unoccupied molecular orbital (LUMO) (intramolecular CT) in the low-energy electronic transition of 2₂ is 68%, while HOMO → LUMO + 1 (intermolecular CT) is 18%, which demonstrates that the intramolecular CT effect on the β_tot value is stronger than the case of the intermolecular CT effect. The present work might provide rich insight into designing and developing potential second-order optical nonlinearity materials with inter- and intramolecular CT characters.