| Abstract
| - Unsymmetrical fluorenyl-based chromophores, flanked on either side by electron-acceptor (A) and/orelectron-donating (D) groups, separated by conjugated (π) moieties, were synthesized. Heck couplingand Wittig reactions were utilized to prepare target fluorene derivatives that possess very high solubilityin organic solvents, high thermal stability (up to 395 °C), and generally high fluorescence quantum yields(0.6−0.9). The systematic alteration of the structural design allowed for investigation of numerous factorsthat affect the optical nonlinearity, as well as probing effects of molecular symmetry, solvent polarity,strengths of electron donating and/or withdrawing end groups, and π-conjugation length. A detailedspectroscopic study of these molecules, including absorption, fluorescence emission, excitation, andexcitation anisotropy, was conducted. Two-photon absorption (2PA) cross sections (δ) were determinedby a nonlinear transmission method employing a femtosecond white-light continuum (WLC) pump−probe method, with all compounds exhibiting high two-photon absorption under femtosecond excitation.Compound 3 had a peak 2PA cross section, δ, of 1093 × 10-50 cm4 s photon-1 molecule-1 at 828 nm.In addition, excitation anisotropy studies permitted the determination of the spectral position of the S0 →S1 and S0 → S2 electronic transitions.
- Systematic photophysical studies were performed for a series of fluorene derivatives. Compound 3 had a peak 2PA fs cross section, δ, of 1093 × 10-50 cm4 s photon-1 molecule-1 at 828 nm, over sixfold higher than a lesser conjugated analog. In addition, excitation anisotropy studies permitted the determination of the spectral position of the S0 → S1 and S0 → S2 electronic transitions.
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