Hot Vibrational States in a High-Performance Multiple Resonance Emitter and the Effect of Excimer Quenching on Organic Light-Emitting Diodes

The photophysics of multiple resonance thermally activated delayed fluorescence molecule nu-DABNA is described. We show coupling of a 285 cm(-1) stretching/scissoring vibrational mode of peripheral phenyl rings to the S-1 state, which dictates the ultimate emission full-width at half maximum. However, a separate high amplitude mode, 945 cm(-1) of the N-biphenyl units, mediates the reverse intersystem crossing (rISC) mechanism. Concentration-dependent studies in solution and solid state reveal a second emission band that increases nonlinearly with concentration, independent of the environment assigned to excimer emission. Even at concentrations well below those used in devices, the excimer contribution affects performance. Using different solvents and solid hosts, rISC rates between 3-6 x 10(5) s(-1) are calculated, which show negligible dependence on environmental polarity or host packing. At 20 K over the first 10 ns, we observe a broad Gaussian excimer emission band with energy on-set above the S-1 exciton band. An optical singlet-triplet gap (Delta E-ST) of 70 meV is measured, agreeing with previous thermal estimates; however, the triplet energy is also found to be temperature-dependent. A monotonic increase of the exciton emission band full-width at half maximum with temperature indicates the role of hot transitions in forming vibrational excited states at room temperature (RT), and combined with an observed temperature dependency of Delta E-ST, we deduce that the rISC mechanism is that of thermally activated reverse internal conversion of T-1 to T-N (n >= 2) followed by rapid rISC of T-N to S-1. Organic light-emitting diodes with nu-DABNA as a hyperfluorescent emitter (0.5 wt % and 1 wt %) exhibit an increase of maximum external quantum efficiency, reaching 27.5% for the lower nu-DABNA concentration. On the contrary, a Forster radius analysis indicated that the energy transfer ratio is smaller because of higher donor-acceptor separation (>2.4 nm) with weak sensitizer emission observed in the electroluminescence. This indicates excimer quenching in 1 wt % devices.

Automated summary

The photophysics of the nu-DABNA molecule involves coupling between vibrational modes and electronic states, with concentration-dependent emission behavior. The reverse intersystem crossing mechanism is mediated by specific vibrational modes. Temperature plays a role in exciton and triplet energy states, affecting the efficiency of organic light-emitting diodes.