Comparative study of diradical characters and third‐order nonlinear optical properties of linear/cyclic acenes versus phenylenes

A long‐range corrected spin‐unrestricted density functional theory (LC‐UDFT) approach has been used to investigate the diradical character ( y i ) and third‐order nonlinear optical properties of linear/cyclic [ N ]acenes and [ N ]phenylenes in their ground states, where N is the number of benzene rings in each linear and cyclic configuration. It has been found that linear and cyclic acenes with N = 5–10 show singlet diradical characters, while linear and cyclic phenylenes with equivalent number of benzene rings have closed‐shell configurations. The amplitudes of third‐order nonlinear optical polarizability (γ zzzz ) for open‐shell linear/cyclic acenes are larger especially in intermediate range of diradical character than those of closed‐shell phenylene counterparts. For example, the γ zzzz values of [5]cyclic acene ( y 0 = y 1 = 0.320) is 4.89 × 10 3 a.u. which is about 6 times larger than those of 0.80 × 10 3 a.u. for closed‐shell [5]cyclophenylene. Similarly, γ zzzz value of [5]linear acene is also about 9 times larger than its closed‐shell [5]linear phenylene counterpart. Our results show interesting insights into the relationship among the architectures, diradical characters and γ zzzz values of different acenes and phenylenes. © 2012 Wiley Periodicals, Inc.