Abstract
Design principle to enhance two-photon absorption (TPA) of organic molecules and some leading strategies are reviewed. Relation between one- and two-photon absorption spectra is discussed in view of molecular symmetry with examples of centro- and noncentrosymmetric molecules. The lowest energy TPA band of noncentrosymmetric molecules can be described by the two-state model, resulting in the correspondence to the one-photon absorption peak, while the higher TPA bands and the TPA bands of centrosymmetric molecules originate from more complicated processes. Transition dipole moment is focused as a controlling factor to enhance TPA cross section by electron donor/acceptor substitution. Extension of π-conjugation system across the core of even-numbered branched systems and molecules having the partial open-shell electronic structures are also discussed as strategies to enhance TPA cross section.
