title: Synthesis of two-photon absorbing unsymmetrical branched chromophores through direct tris(bromomethylation) of fluorene.
abstract: Branched fluorene-based chromophores bearing electron-donating diphenylamino or electron-withdrawing nitro groups were synthesized as well as their linear analogues. An efficient synthetic method was developed via a novel 2,4,7-tris(bromomethyl)-9,9-diethylfluorene intermediate. The bromomethyl groups in this key intermediate were converted to either phosphonate or carboxaldehyde moieties, facilitating preparation of a high functionality branched structure. It was found that the reactivity at position 4 is attenuated in the bromomethyl and phosphorylated derivates, facilitating the selective and systematic functionalization of the fluorenyl system. All compounds were stable up to ca. 350 degrees C, except for a sterically crowded branched derivative. The linear optical properties of the compounds were investigated by UV-visible, steady-state fluorescence, and excitation anisotropy spectroscopic measurements. Fluorescence quantum yields were greater than or equal to 0.84 for symmetric linear and unsymmetric branched derivatives. Very high two-photon absorption (2PA) cross-sections were achieved (5765 GM at 520 nm and 4194 GM at 570 nm), as determined with use of picosecond and femtosecond laser excitation sources, respectively.
authors.affiliation: Department of Chemistry and College of Optics and Photonics, CREOL & FPCE, University of Central Florida, P.O. Box 162366, Orlando, FL 32816-2366, USA.
authors.forename: Sheng
authors.initials: S
authors.lastname: Yao
authors.forename: Kevin D
authors.initials: KD
authors.lastname: Belfield