Molecular engineering of nanoscale quadrupolar chromophores for two-photon absorption

Our aim has been the design of optimized NLO-phores with very high two-photon absorption (TPA) cross-sections (sigma(2)) in the red-NIR region, while maintaining high linear transparency and high fluorescence quantum yield. Our molecular engineering strategy is based on the push-push or pull-pull functionalization of semi-rigid nanoscale conjugated systems. The central building blocks were selected as rigid units that may assist quadrupolar intramolecular charge transfer by acting either as a (weak) donor or acceptor core. Quadrupolar molecules derived either from a phenyl unit, a rigidified biphenyl moiety or a fused bithiophene unit have been considered. Conjugated oligomers made of phenylene-vinylene and/or phenylene-ethynylene units were selected as connecting spacers between the core and the electroactive end groups to ensure effective electronic conjugation while maintaining suitable transparency/fluorescence. The TPA cross-sections were determined by investigating the two-photon-excited fluorescence properties using a Ti:sapphire laser delivering fs pulses. Both the nature of the end groups and of the core moiety play an important role in determining the TPA spectra. In addition, by adjusting the length and nature of the conjugated extensor, both amplification and spectral tuning of TPA cross-sections can be achieved. As a result, push-push fluorophores which demonstrate giant TPA cross-sections (up to 3000 GM) in the visible red, high fluorescence quantum yields and good transparency in the visible range have been obtained.