Mechanochromic luminescence quenching: force-enhanced singlet-to-triplet intersystem crossing for iodide-substituted difluoroboron-dibenzoylmethane-dodecane in the solid state.

A lipid derivative of difluoroboron-iododibenzoylmethane (BF(2)dbm(I)OC(12)H(25)) was synthesized via Claisen condensation and boronation. Green photoluminescence is observed for the complex in the solid state. Unlike the previously reported difluoroboron-avobenzone (BF(2)AVB) complex, which exhibited significantly red-shifted fluorescence upon mechanical perturbation, the emission of a BF(2)dbm(I)OC(12)H(25) solid film is quenched when the sample is smeared under air but becomes orange under nitrogen. Spectroscopic and lifetime studies suggest that smearing brings the singlet excited state closer to the triplet state, thus increasing the coupling between the two states. As a result, intersystem crossing from the singlet to the triplet excited state is facilitated, and the total luminescence intensity is quenched at room temperature.