Degenerate nonlinear absorption and optical power limiting properties of asymmetrically substituted stilbenoid chromophores

Two-photon absorption (2PA) spectra (650-1000 nm) of a series of model chromophores were measured via a newly developed nonlinear absorption spectral technique based on a single and powerful femtosecond white-light continuum beam. The experimental results suggested that when either an electron-donor or an electron-acceptor was attached to a trans-stilbene at a para-position, an enhancement in molecular two-photon absorptivity was observed in both cases, particularly in the 650-800 nm region. However, the push-pull chromophores with both the donor and acceptor groups showed larger overall two-photon absorption cross-sections within the studied spectral region as compared to their mono-substituted analogues. The combined results of the solvent effect and the ¹H-NMR studies indicated that stronger acceptors produce a more efficient intramolecular charge transfer character upon excitation, leading to increased molecular two-photon responses in this model-compound set. A fairly good 2PA based optical power limiting behavior from one of the model chromophores is also demonstrated.