Three-photon absorption enhancement in symmetrical charge transfer pull-pull fluorene derivatives

We report the multiphoton-induced upconversion fluorescence of 2,7-bisbenzothiazole-9,9-didecylfluorene (Acceptor-pi-Acceptor) in hexane solution, pumping at wavelengths from 300 to 1200 nm, and its three-photon absorption cross-section at 1095 nm (sigma(3)' = 355 x 10(-78) cm(6) s(2) photon(-2) for the transition S-0 -> S-1). The multiphoton-induced fluorescence emission and the absorption measurements of 2,7-bisbenzothiazole-9,9-didecylfluorene in hexane were performed with a tunable OPG pumped by a Nd-YAG picosecond laser. It is demonstrated that the strong symmetrical electron-transfer from the electron rich fluorene core to the two electron-withdrawing benzothiazole end groups enhances sigma(3)'. An increase of approximately four and nine fold, with respect to its Donor-pi-Donor and Acceptor-pi-Donor counterparts, respectively, was observed. (c) 2005 Elsevier B.V. All rights reserved.