When Push Comes to Shove: Unravelling the Mechanism and Scope of Nonemissive meso-Unsaturated BODIPY Dyes

We report herein spectroscopy and computational results that illustrate an efficient intramolecular deactivation pathway for meso-unsaturated boron-dipyrromethene (BODIPY) dyes in their singlet excited state. Our results show that the mechanism hinges on the structural flexibility imparted by the boron atom and on the energetic stabilization conferred by extending the conjugation into the meso substituent, which is otherwise unconjugated in the ground state. Following photoexcitation, rotation along the dihedral angle of the meso-unsaturated group results in its conjugation at the expense of shifting one pyrrole moiety in dipyrrin out of the plane. Internal conversion to an energetically hot, ground-state species efficiently competes with emission. The mechanism applies to meso-vinyl, -formyl, and -iminyl moieties. The presence of methyl groups at positions C1 and C7 exacerbates the energetic penalty toward conjugation of the meso groups leading to a small energy gap between relaxed excited state and ground state and undetected emission quantum yields. Importantly, methyls at C1 and C7 prevent nonradiative deactivation in meso-aryl moieties, illustrating that when push comes to shove, the energetic (kinetic) barrier toward reaching conjugation is too large for aryl moieties but low enough for smaller groups to effectively compete with radiative transitions. Wisely chosen meso-unsaturated BODIPY dyes may serve as richly sensitive platforms for the preparation of novel fluorogenic substrates to monitor chemical reactions or to probe the rigidity of their surrounding environment.