Time-Resolved Electron Paramagnetic Resonance and Theoretical Investigations of Metal-Free Room-Temperature Triplet Emitters

Utilization of triplets is important for preparing organic light-emitting diodes with high efficiency. Very recently, both electrophosphorescence and electrofluorescence could be observed at room temperature for thienyl-substituted phenazines without any heavy metals (Ratzke et al. J. Phys. Chem. Lett., 2016, 7, 4802). It was found that the phosphorescence efficiency depends on the orientation of fused thiophenes. In this work, the thienyl-substituted phenazines are investigated in more detail by time-resolved electron paramagnetic resonance (EPR) and quantum chemical calculations. Spin dynamics, zero-field splitting constants, and electron-spin structures of the excited triplet states for the metal-free room-temperature triplet emitters are correlated with phosphorescence efficiency. Complete active space self-consistent field (CASSCF) calculations clearly show that the electron spin density distributions of the first excited triplet states are strongly affected by the molecular geometry. For the phosphorescent molecules, the electron spins are localized on the phenazine unit, in which the sulfur atom of the fused thiophene points upward. The electron spins are delocalized onto the thiophene unit just by changing the orientation of the fused thiophenes from upward to downward, resulting in the suppression of phosphorescence. Time-resolved EPR measurements and time-dependent density functional theory (TD-DFT) calculations demonstrate that the electron spins delocalized onto the thiophene unit lead to the acceleration of nonradiative decays, in conjunction with the narrowing of the singlet-triplet energy gap.