Photolysis of m-phenylene-bis(chlorodiazirine)+2-vinylpyridine: does an indolizine chromophore inhibit the photolysis of a diazirine?

The photolysis of m-phenylene-bis(chlorodiazirine) in the presence of 2-vinylpyridine (VP) yields the m-phenylene-bis(indolizine) by a mechanism involving two consecutive photoreactions. Photolysis of a first diazirine ring generates a carbene which reacts with 2-VP to give, via a sequence of fast thermal reactions, a first product including both the indolizine and the chlorodiazirine moieties. Although in the lowest excited singlet state of the primary product, the excitation is localized on the indolizine unit, the photolysis of this product induces the decomposition of the second diazirine ring to give a second carbene which yields the final product via the same sequence of reactions: formation of a 2-vinylpyridinium ylide, cyclization and elimination of HCl. Analysis of the absorption and fluorescence spectra indicates that an upper excited singlet state, with the excitation localized on the diazirine ring, is only a few kJ/mol above S1. It can therefore be populated by thermal activation of S1 so that there is, seemingly, an endothermic intramolecular energy transfer from the indolizine moiety to the diazirine ring.