Solvent-dependent investigation of carbazole benzonitrile derivatives: does the LE3−CT1 energy gap facilitate thermally activated delayed fluorescence?
Journal of Photonics for Energy
Solvent-dependent investigation of carbazole benzonitrile derivatives: does the 3 LE − 1 CT energy gap facilitate thermally activated delayed fluorescence?," Abstract. The photophysical properties of six types of carbazole benzonitrile (CzBN) derivatives are investigated in different solvents to examine the thermally activated delayed fluorescence (TADF) activation via reducing the energy gap between the singlet charge-transfer and triplet locally excited states, ΔE STðLEÞ . Relative to the ΔE
... TðLEÞ values for the CzBN derivatives in the low polarity solvent toluene (ε ∼ 2), a reduction of ΔE STðLEÞ for the CzBN derivatives in the polar solvent acetonitrile (ε ∼ 37) was confirmed while maintaining fairly constant ΔE ST values. Notably, TADF activation was observed in acetonitrile for some CzBN derivatives that are TADF inactive in toluene. A numerical analysis of various rate constants revealed the cause of TADF activation as an increase in the reverse intersystem crossing rate and a suppression of the non-radiative decay rate of the triplet states. The positive effect of ΔE STðLEÞ was limited, however, as an excessive decrease in ΔE STðLEÞ facilitates the nonradiative deactivation of the triplet states, leading to a loss of the TADF efficiency. This paper shows that ΔE STðLEÞ provides a measure of TADF activation and that appropriate regulation of ΔE STðLEÞ is required to achieve high TADF efficiency.