One- and Two-Photon Laser Photochemistry in Organic Solids

The criteria for site-selective low-temperature laser photochemistry in organic solids are discussed within a general framework that includes both one- and two-photon photochemical reaction schemes. We identify requirements for the use of this photochemistry in photochemical hole burning applications such as high-resolution spectroscopy and the study of low-temperature reaction mechanisms. The one- and two-photon reaction schemes are quite different from one another with respect to excited state geometries and short-lived photoreactive intermediates. The reversible tautomerism of quinizarin in hydrogen-bonded host materials is studied as a one-photon example. The photochemistry involves both intra- and intermolecular hydrogen bonds. The photodissociations of s-tetrazine (ST) and dimethyl-s-tetrazine (DMST) are studied as examples of the two-photon reaction and a kinetic scheme is proposed for the tetrazine case. The intensity dependence of the photochemistry in solids is modeled mathematically.