A long-range corrected spin-unrestricted density functional theory (LC-UDFT) approach has been used to investigate the diradical character (yi) 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 (γzzzz) 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 γzzzz values of cyclic acene (y0 = y1 = 0.320) is 4.89 × 103 a.u. which is about 6 times larger than those of 0.80 × 103 a.u. for closed-shell cyclophenylene. Similarly, γzzzz value of linear acene is also about 9 times larger than its closed-shell linear phenylene counterpart. Our results show interesting insights into the relationship among the architectures, diradical characters and γzzzz values of different acenes and phenylenes. © 2012 Wiley Periodicals, Inc.