In this Letter, we report the characterization of the pure three-photon absorption and the measurement of the three-photon absorption cross-section (sigma(3)') of a new series of halogenated fluorene derivatives. The three-photon absorption of these compounds corresponds to the S-0 -> S-1 electronic transition. All four compounds presented here have potential application as photosensitizers in deep photodynamic therapy induced via high order nonlinear absorption. The measurements were done using a tunable OPG pumped by a 25 ps Nd-YAG laser, and performing transmittance vs. input intensity as well as open aperture Z-scan at different wavelengths to determine the highest value of sigma(3)'. (c) 2006 Elsevier B.V. All rights reserved.