Solvent Effect on Thermally Activated Delayed Fluorescence by 1,2,3,5-Tetrakis(carbazol-9-yl)-4,6-dicyanobenzene

Abstract

Thermally activated delayed fluorescence (TADF) is fluorescence arising from a reverse intersystem crossing (RISC) from the lowest triplet (T1) to the singlet excited state (S1), where these states are separated by a small energy gap (ΔEst), followed by a radiative transition to the ground state (S0). Rate constants relating TADF processes in 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) were determined at four different solvent polarities (toluene, dichloromethane, ethanol, and acetonitrile). We revealed that the rate constant of RISC, kRISC, which is the most important factor for TADF, was significantly enhanced by a reduced ΔEst in more polar solvents. The smaller ΔEst was mainly attributable to a stabilization of the S1 state. This stabilization also induced a Stokes shift in fluorescence through a relatively large change of the dipole moment between S1 and S0 states (17 D). Despite of this factor, we observed a negative correlation between ΔEst and efficiency of the delayed fluorescence (φd). This was ascribed to a lower intersystem crossing rate, kISC, and increased nonradiative decay from S1, k(s)nrs, in polar solvents.

Solvent-dependent investigation of carbazole benzonitrile derivatives: does the LE3−CT1 energy gap facilitate thermally activated delayed fluorescence?

“…The change in the rate constants in acetonitrile is qualitatively similar to 4CzIPN, 34 implying that solvent effects are a general phenomenon for TADF molecules. The only difference is that the TADF efficiency in toluene, the most nonpolar solvent used by Ishimatsu et al, 34 demonstrates the best value, suggesting that the absolute value of ΔE STðLEÞ is a minimum in toluene for 4CzIPN, as for 5CzBN.…”

“…However, a strongly polar solvent is not always beneficial for TADF efficiency, as 5CzBN (and also 4CzIPN in Ref. 34) showed a larger decrease in TADF efficiency due to the consequent increase in k nr;T . As a result, the polar host environment has an advantage for k RISC , but the appropriate polarity is required for respective molecules to maximize the effect in TADF efficiency.…”

“…The only difference is that the TADF efficiency in toluene, the most nonpolar solvent used by Ishimatsu et al, 34 demonstrates the best value, suggesting that the absolute value of ΔE STðLEÞ is a minimum in toluene for 4CzIPN, as for 5CzBN. In fact, the 3 LE state of 4CzIPN is already above the 1 CT and 3 CT states (Fig.…”

Highly efficient hybrid warm white organic light-emitting diodes using a blue thermally activated delayed fluorescence emitter: exploiting the external heavy-atom effect

“…S4. The spectra of these fluorophors are identical as the concentration of the EHA perturbers increases, indicating that the effect of the polarity changing can be ruled out, 18 and therefore, the change in the PL characteristics of these films is mainly due to the EHA effect.…”

“…[17][18][19] Therefore, it would be important to understand how a TADF fluorophor would perform in the presence of a phosphorescent EHA perturber.…”