Spin dynamics of thiophene-fused π-systems with delayed emission

Abstract

Triplet excited states in organic molecules are generally optically inactive due to spin-forbidden transitions to the singlet ground state. Recent studies have extensively investigated triplet states to circumvent limitations imposed by spin statistics in organic light-emitting diodes. Room-temperature electrophosphorescence and electrofluorescence have previously been observed in thienyl-substituted phenazines without heavy metals; however, phosphorescence was absent when spin density was localized on the thiophene units, despite the expected heavy-atom effect from the sulfur atom. In this study, we investigate the spin dynamics of thiophene-fused π-systems lacking phenazine units to enable the effective utilization of triplet states in thiophene-containing π-systems. This investigation reveals delayed emission in the excited dimers of fused thiophenes, likely associated with triplet–triplet annihilation. Optical and EPR measurements, together with quantum chemical calculations, indicate that the fused thiophenes form head-to-tail dimers in both the ground and excited states.