Two-photon absorption and fluorescence in nanoscale multipolar chromophores:: effect of dimensionality and charge-symmetry

A series of structurally related chromophores of different symmetry (quadrupolar, C-2nu, octupolar....) and shape (rod-like, propeller-shaped, Y-shaped, dendritic....) were investigated and compared for optimization of molecular two-photon absorption (TPA). Their design is based on the functionalization of linear or branched conjugated backbones with electron-releasing and/or electron-withdrawing peripheral groups. Their TPA spectra were determined by investigating their two-photon-excited fluorescence properties in the NIR region using pulsed excitation in the femtosecond regime. These studies provide evidence that the charge symmetry plays an important role in determining the TPA magnitude, the quadrupolar chromophore leading to the highest TPA cross-section. However, higher-order charge symmetries and branched structures provide an interesting route for improvement of the non-linear absorptivity/transparency range trade-off as well as for TPA spectral broadening in the NIR region. (C) 2004 Elsevier B.V. All rights reserved.