Stepwise two-photon absorption processes utilizing photochromic reactions

Stepwise two-photon absorption (2PA) is one of the nonlinear photoresponses where two photons are sequentially absorbed to proceed a photophysical or photochemical reaction. The stepwise 2PA can selectively produce a higher excited state with relatively high spatial selectivity, and therefore, it can be an efficient tool to explore and realize nonlinear photoresponsive materials beyond Kasha's rule. Moreover, if a photogenerated chemical species is used as an intermediate state of the stepwise 2PA, the power threshold to promote the stepwise 2PA can be greatly reduced compared with that of a simultaneous 2PA, and conventional LEDs and even sunlight could be used to induce the stepwise 2PA. Because of these advantages, the stepwise 2PA has been widely studied more than a half century ago not only for fundamental physics and chemistry but also for applications to optical memory, holography, and artificial photosynthesis. In this review, we overview various types of stepwise 2PA processes from the viewpoints of history, fundamentals, and applications. Especially we focus on the stepwise 2PA processes related to photochromic reactions because the combination of photochromic reactions and stepwise 2PA processes has realized various potentially-applicable nonlinear photoresponsive materials and found several anomalous phenomena beyond the one-photon photophysical and photochemical reactions. The stepwise 2PA processes are mainly divided into two categories involving an electronic excited state or a photogenerated transient species as the intermediates states of the stepwise 2PA. These two types of the stepwise 2PAs are explained respectively, and finally, the summary and outlook are presented. (C) 2017 Elsevier B.V. All rights reserved.