A dipolar molecule and two series of fluorene derivatives have been theoretically studied with B3LYP/6-31G(d) associated with ZIN-DO-SOS method. The different peripheral groups and lengthening the bridge have effect on the one-photon absorption. The maximum two-photon absorption cross sections delta(max) for the fluorene derivative (1B) is more than three times as that of the dipolar counterpart (S1). The delta(max) are obviously increased as the conjugation length increases from the series 1 molecules to the series 2 counterparts. Meanwhile, the delta(max) are increased with the stronger electron-donating (accepting) capability. (C) 2008 Elsevier B.V. All rights reserved.