Photochemical protonation of an azahelicene in 1,2-dichloroethane

Heterocyclic analogues of phenanthridine are an important class of nitrogen-containing compounds with promising optical properties including high quantum yield of luminescence, high molar absorption coefficients and sensitivity to solvent medium. The luminescent properties of phenanthridine and its derivatives can be changed by introducing appropriate substituents into various positions of the phenanthridine framework. Recently developed luminescent azahelicenes of furoquinoline series demonstrate strong acidochromic effect both on absorption and emission spectra. One of the features of these compounds is the photochemical formation of a protonated form in chlorine-containing solvents under UV irradiation. In this work we perform a mechanistic study of 3-methoxy-6-(4-methoxyphenyl)naphtho[1′,2′:4,5]furo[2,3-c]quinoline (compound 1) in 1,2-dichloroethane (1,2-DCE) by means of stationary and laser flash photolysis. In addition to the triplet state formation observed in typical organic solvents photoexcitation in 1,2-DCE was found to result in an electron transfer from the excited compound 1 to a solvent molecule. Reactions of resulting intermediates lead to the formation of the protonated azahelicene. The quantitative mechanism of photoprotonation is put forward.