Ab initio and first principles theoretical investigations of triplet–triplet fluorescence in trimethylenemethane biradicals

Abstract

Theoretical studies on triplet–triplet (T₁ → T₀) fluorescence of the arylated trimethylenemethane (TMM) biradicals, ³2˙˙, were carried out using post-Hartree–Fock ab initio and various first principles density functional theory methods. Analysis of optimized geometries including bond alternations and spin distributions indicates that the triplet ground (³2˙˙) and excited (³2˙˙*) states of these biradicals have aromatic and quinoidal characteristics, respectively. Inspection of their calculated electronic structures shows that, in comparison to ³2˙˙, one of the spins in ³2˙˙* is more delocalized onto arene-rings linked to the TMM framework.