Photochemical transformation of dichloromethane to chloroform: insights into the mechanism of aza[5]helicene photoprotonation

Abstract

Aza[5]helicenes of the furoquinoline series have two important features: (a) possibility to modulate photophysical properties by protonation, and (b) they are good fluorophores that exhibit exceptional photostability in chlorine-free organic solvents. Light irradiation in chlorine-containing solvents was known to result in photoprotonation. In this work photophysics and photochemistry of a typical aza[5]helicene, namely 3-methoxy-6-(4-methoxyphenyl)naphtho[1′,2′:4,5]furo[2,3-c]quinoline (compound Hel) in dichloromethane (DCM) was studied using stationary photolysis with UV–Vis and NMR registration, time-resolved fluorescence and nanosecond laser flash photolysis. The free base aza[5]helicene was found for the first time to promote the transformation of dichloromethane to chloroform upon irradiation with UV light (308 nm). Studies of the quantitative characteristics of Hel and HelH⁺ (fluorescence quantum yields, spectra and kinetic properties of the triplet states –showed that photoprotonation occurs in two ways. The first pathway is started by an electron transfer from an excited Hel molecule to a solvent molecule, followed by an H atom transfer from a solvent molecule to the Hel⁺ radical cation. The second pathway of HelH⁺ formation is the reaction between Hel and HCl formed at the first stage. A quantitative mechanism of photoprotonation is proposed that explains all the observed experimental data.