▎ 摘　　要

Using hybrid density functional theory methods, we investigate the second hyperpolarizabilities (gamma) of hexagonal shaped finite graphene fragments, which are referred to as hexagonal graphene nanoflakes (HGNFs), with two types of edge shapes: zigzag (Z) and armchair (A) edges. It is found that Z-HGNF, which gives intermediate diradical characters (y), exhibits about 3.3 times larger orthogonal components of gamma (gamma(xxxx) = gamma(yyyy) in this case) than A-HGNF, which gives zero y value (closed-shell system). The gamma density analysis reveals that this enhancement originates in the significant contribution of gamma densities on edge regions in Z-HGNF. These observations strongly indicate that Z-HGNF is a promising candidate of open-shell singlet NLO systems. (C) 2009 Elsevier B. V. All rights reserved.