Synthesis, characterization, and optical properties of new two-photon-absorbing fluorene derivatives

The synthesis of a series of four new compounds containing fluorenyl chromophores is presented, along with the results of spectroscopic and photochemical studies aimed at understanding the two-photon absorption properties and energetics of their electronically excited states. The molecular structures of the compounds were systematically varied to allow comparison of molecules possessing high and low molecular symmetry, short and long alkyl chains, and a fluorenyl conjugated pi-system. Solvent-dependent absorption and emission were investigated along with pi-conjugation length. Preliminary measurements of two-photon absorption (2PA) using a two-photon fluorescence method indicate that these chromophores exhibit high two-photon absorptivity. A symmetrical molecule (3), possessing a relatively large pi-conjugated system, flanked on either side by electron-withdrawing groups (benzothiazole), exhibited a peak 2PA cross section (delta) of 6000 x 10(-50) cm(4) s photon(-1) molecule(-1) at 600 nm. Excitation anisotropy studies revealed the position of the S-0 --> S-1 and S-0 --> S-2 electronic transitions. Consistent with quantum mechanical selection rules, the two-photon allowed transition (S-0 --> S-2) was dominant.