One- and two-photon stimulated emission depletion of a sulfonyl-containing fluorene derivative.

One- and two-photon stimulated emission transitions were investigated by the fluorescence quenching of a sulfonyl-containing fluorene compound, 2,7-bis(4-(phenylsulfonyl)styryl)-9,9-didecyl-9H-fluorene (1), in solution at room temperature using a picosecond pump-probe technique. The nature of stimulated transitions under various fluorescence excitation and quenching conditions was analyzed theoretically, and good agreement with experimental data was demonstrated. Two-photon stimulated transitions S1-->S0 were shown for 1 at lambdaq=1064 nm, representing the first report of two-photon stimulated emission depletion (STED) in a molecular system. The two-photon stimulated emission cross section, delta2PE(lambdaq), of fluorene 1 was estimated to be approximately 240-280 GM, suggesting that this compound may be a good candidate for use in two-photon STED microscopy.