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.
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