Using hybrid density functional theory methods, the static first (beta) and second (gamma) hyperpolarizabilities of a donor-acceptor disubstituted singlet diradical system built from phenalenyl radicals linked by an acetylene pi-conjugated bridge, DA-BPLE, is investigated and compared to an analogous closed-shell system, DA-BPRY, built from pyrene rings. DA-BPLE presents an intermediate diradical character and exhibits beta and gamma values about 14 and 7 times larger than those of DA-BPRY having a similar pi-conjugation length, respectively. These results suggest the applicability of the structure-property relation in gamma of symmetric diradical systems to beta and gamma of asymmetric diradical systems. (c) 2007 Elsevier B.V. All rights reserved.