Mono- and biphotonic photochemistry in glass matrices

Abstract

Photochemistry in hard glassy solvent matrices gives different results than in gas matrices. It is performed at 83, 77, and ≥10 K by continuous irradiation and by pulsed multi MW cm −2 peak intensity excitation for those systems that do not react monophotonically. The highly structured matrix spectra should be taken as a basis for the interpretation of transient spectra to avoid ambiguities. Numerous [2.2]paracyclophanes are photolyzed. Most of them give stable diradical and quinodimethane spectra in addition to fluorescence and phosphorescence. Some benzylic diradicals undergo chemiluminescence after their photochemical generation. Matrix isolation spectroscopy is at variance with common interpretations in the lepidopterene case. A [2+4]-photocycloreversion of a substituted cyclohexene at 83 K leads to diene stereoisomers/rotamers that isomerize upon further irradiation. E/Z-photoequilibria are obtained in MTHF matrix from both sides with ω-nitrostyrene and α-benzylidene-γ-butyrolactone at 83 K, the latter stereoisomerization was also successfully studied at 10 K. Pulsed irradiation of technical photostabilizers at 10 K leads to stable zwitterion formation by proton migration that cannot be seen by continuous excitation. Inter- and intramolecular donor acceptor systems provide stable charge separation at 15 or 77 K upon pulsed laser irradiation and radicalanion spectra are recorded. Biphotonic photochemistry at ≥10 K allows for the formation of new ring systems such as dioxathiirane ( cyclo-SO 2), several aryldioxaziridines, and an electron rich triaziridine, compounds that revert upon thawing and could not be obtained by continuous irradiation, except cyclo-SO 2 that can also be formed after absorption of the long lived SO 2 triplet by another two-photon process.