Three-photon absorption enhancement in a symmetrical charge transfer fluorene derivative

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–π–D (9,9-didecyl-2,7-bis-(N,N-diphenylamino)fluorene) and D–π–A ((7-benzothiazol-2-yl-9,9-didecylfluoren-2-yl)diphenylamine) structural motifs. The three-photon absorption cross-section of the D–π–D analog (σ3′=82×10−78 cm6 s2 photon−2) is 2.2 times greater than that of its D–π–A counterpart (σ3′=37×10−78 cm6 s2 photon−2), showing that symmetric charge transfer enhances 3PA. The three-photon-excitation of these two compounds in hexane solution (9.8×10−3 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.