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-π-Acceptor) in hexane solution, pumping at wavelengths from 300 to 1200 nm, and its three-photon absorption cross-section at 1095 nm (σ3′=355×10-78cm6s2photon-2 for the transition S0 → S1). 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 σ3′. An increase of approximately four and nine fold, with respect to its Donor-π-Donor and Acceptor-π-Donor counterparts, respectively, was observed.