Recent Advances in Organic Donor-Acceptor Two-Photon Absorption Materials: Mechanisms, Molecular Design, and Applications.

Organic materials have been widely recognized as highly promising candidates for two-photon absorption (TPA) applications owing to their strong optical absorption, readily tunable molecular structures, and excellent processability. By introducing electron donors (D) and acceptors (A) either within molecules or through intermolecular interactions, D-A-type organic TPA materials have been successfully developed, leading to significant advances in the understanding and enhancement of TPA performance. This review provides a comprehensive overview of several representative mechanisms employed to investigate TPA processes, including the damped response theory, few-photon irradiation TPA, low-power incoherent two-photon absorption, and charge-transfer mechanisms. Subsequently, various classes of organic D-A materials exhibiting TPA properties are discussed, encompassing organic small molecules, organic frameworks, and cocrystals. In addition, diverse applications of TPA are summarized, such as two-photon imaging and photodynamic therapy. Finally, the challenges and opportunities facing future TPA research are outlined, with particular emphasis on emerging strategies assisted by artificial intelligence.