Locked alkenes with a short triplet state lifetime

Substituent effects on radiationless decay rates in four-membered ring alkenes have been studied by the spin-restricted ensemble-referenced Kohn–Sham method. The substituents at the C1 and C2 double-bond positions vary from strong π-acceptors, like BH2, to strong π-donors, like NH2. Unlike π-acceptors, π-donors cause stronger pyramidalization of the C1 and C2 positions in the triplet state, and the resulting vertical de-excitation energies between T1 and S0 states are considerably smaller in the π-donor-substituted cyclobutenes. The crossing between T1 and S0 potential energy surfaces in these compounds occurs near the T1 minima, resulting in high probability of radiationless transition from the T1 to S0 state.