Charge transfer and first hyperpolarizability: cage-like radicals C59X and lithium encapsulated Li@ C59X (X= B, N)

摘要

Very recently, two new cage-like radicals (C59B and C59N) formed by a boron or nitrogen atom substituting one carbon atom of C-60 were synthesized and characterized. In order to explore the structure-property relationships of combination the cage-like radical and alkali metal, the endohedral Li@ C59B and Li@ C59N are designed by lithium (Li) atom encapsulated into the cage-like radicals C59B and C59N. Further, the structures, natural bond orbital (NBO) charges, and nonlinear optical (NLO) responses of C59B, C59N, Li@ C59B, and Li@ C59N were investigated by quantum chemical method. Three density functional methods (BHandHLYP, CAMB3LYP, and M05-2X) were employed to estimate their first hyperpolarizabilities (beta(tot)) and obtained the same trend in the beta(tot) value. The beta(tot) values by BHandHLYP functional of the pure cage-like radicals C59B (1.30x103 au) and C59N (1.70x 103 au) are close to each other. Interestingly, when one Li atom encapsulated into the electron-rich radical C59N, the beta(tot) value of the Li@ C59N increases to 2.46x103 au. However, when one Li atom encapsulated into the electron-deficient radical C59B, the beta(tot) value of the Li@ C59B sharply decreases to 1.54x102 au. The natural bond orbital analysis indicates that the encapsulated Li atom leads to an obvious charge transfer and valence electrons distribution plays a significant role in the beta(tot) value. Further, frontier molecular orbital explains that the interesting charge transfer between the encapsulated Li atom and cage-like radicals (C59B and C59N) leads to differences in the beta(tot) value. It is our expectation that this work will provide useful information for the design of highperformance NLO materials.