In this letter we report the three-photon absorption-induced upconversion fluorescence emission and the three-photon absorption cross-section of two fluorene derivatives with D-pi-D (9,9-didecyl-2,7-bis-(N,N-diphenylamino)fluorene) and D-pi-A ((7-benzothiazol-2-yl-9,9-didecylfluoren-2-yl)diphenylamine) structural motifs. The three-photon absorption cross-section of the D-pi-D analog (sigma(3)(') = 82 x 10(-78) cm(6) s(2) photon(-2)) is 2.2 times greater than that of its D-pi-A counterpart (sigma(3)' = 37 x 10(-78) cm(6) s(2) photon(2)), showing that symmetric charge transfer enhances 3PA. The three-photon-excitation of these two compounds in hexane solution (9.8 x 10(-3) M) was accomplished with a tunable OPG pumped by picosecond laser pulses. The three-photon absorption coefficients were measured using an open aperture Z-scan technique. (C) 2004 Elsevier B.V. All rights reserved.