Low-power, non-coherent light-triggered two-photon absorption via extending the lifetime of the transition state

Materials with multi-photon absorption (MPA) feature, are highly desirable for applications such as deep-seated tumor treatment, high spatiotemporal resolution bioimaging, sophisticated micro-nano fabrication, optical data storage, frequency-upconverting laser, and optical limiting. The classical two-photon absorption (TPA) process relies on an extremely short-lived virtue state, leading to the requirement of an ultrahigh power density of femtosecond pulsed laser. To break this application barrier, the key solution is to extending the lifetime of the transition state in TPA. Recently, the operation of TPA with low-power non-coherent excitation (LPNC-TPA) was achieved by leveraging the mono-reduced species (such as radical anion) and the triplet excited state of dye molecules as the transition state with a relatively long lifetime. In this review, the mechanism of these LPNC-TPA processes will first be introduced, followed by the approaches to extend the lifetime of the mono-reduced species and the triplet state. Then, considering its ability to tune the aggregation mode of dye molecules, the metal–organic framework (MOF) will be emphasized as an efficient tool to operate efficient LPNC-TPA in the solid state. The merits and features of LPNC-TPA materials will be revealed through their emerging applications in photoredox catalysis, photopolymerization, 3D printing, in vivo cancer treatment, bioimaging, and biosensing. Finally, the future directions and challenges of LPNC-TPA are proposed, along with the possible solutions.