Photoisomerization of 1-phenyl-2-(pyridin-2-yl)indole BMes₂: The dark isomer

A new member of photochromic N,C-chelate organoboron compounds, BMes2(Py-N-Ph-In) (1), where Mes = mesityl, Py-N-Ph-In = 1-Ph-2-(2-Py)-indolyl, has been synthesized, and its photo-thermal isomerization properties have been investigated. Upon irradiation by UV light (365 nm), compound 1 isomerizes to compound 1a, changing color from light yellow to dark turquoise-green with the disappearance of the green fluorescence of 1. This process involves the breaking/formation of a B-C bond and a C-C bond and is thermally reversible. Compound 1a finally allowed us to obtain suitable crystals. The crystals of the dark isomer 1a were isolated, and its structure was determined by a single-crystal X-ray diffraction analysis, which confirms the formation of a C-C bond between the indolyl ring and a mesityl ring, the dearomatization of the mesityl ring, and the presence of a BC2 triangle in 1a. NMR and DFT computational data further support that 1a is an analogue of the dark isomers generated from previously reported N,C-chelate BMes2 compounds.