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

Takanori Nawata,Takuya Hosokai,Hiroki Noda,Hajime Nakanotani,Yasuo Nakayama,Hiroyuki Matsuzaki,Chihaya Adachi

doi:10.1117/1.jpe.8.032102

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 ST(LE) . Relative to the ΔE ST(LE) values for the CzBN derivatives in the low polarity solvent toluene ( ϵ ∼ 2 ), a reduction of ΔE ST(LE) for the CzBN derivatives in the polar solvent acetonitrile ( ϵ ∼ 37 ) was confirmed while maintaining fairly constant Δ E 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 ST(LE) was limited, however, as an excessive decrease in ΔE ST(LE) facilitates the nonradiative deactivation of the triplet states, leading to a loss of the TADF efficiency. This paper shows that ΔE ST(LE) provides a measure of TADF activation and that appropriate regulation of ΔE ST(LE) is required to achieve high TADF efficiency.

Highlights

Activated delayed fluorescence (TADF) of organic molecules has attracted tremendous attention owing to its potential application to organic light-emitting diodes (OLEDs) and has renewed interest in photochemistry and photophysics.[1,2] Uoyama et al.[3] established a design concept for the synthesis of highly efficient thermally activated delayed fluorescence (TADF) molecules by connecting electrondonating and -accepting substituents with a large torsion angle
We investigated the solvent dependence of the photophysical properties of six types of carbazole benzonitrile (CzBN) derivatives
We previously investigated the photophysical properties of CzBN derivatives in toluene, whose dielectric constant (ε) is ∼2 at room temperature

Summary

Introduction

Activated delayed fluorescence (TADF) of organic molecules has attracted tremendous attention owing to its potential application to organic light-emitting diodes (OLEDs) and has renewed interest in photochemistry and photophysics.[1,2] Uoyama et al.[3] established a design concept for the synthesis of highly efficient TADF molecules by connecting electrondonating and -accepting substituents with a large torsion angle. S1 and T1 states is essential to achieve a high kRISC; known as El-Sayed’s rule, it has been shown that high kRISC values can be achieved by exploiting the singlet charge-transfer (1CT) and triplet locally excited (3LE) states.[18,19,21,25] In addition, some groups have proposed that second-order spin-orbit coupling enables utilization of higher order triplet states, which further facilitates RISC.[18,20,23] Conventional and time-dependent density functional theory calculations based on such orbital selection rules and/or second-order perturbation theory have succeeded in reproducing the experimentally determined kRISC values of some, but not all, of the organic molecules investigated.[21] a broad consensus of the TADF mechanism has yet to be reached Another aspect of the selection rules for RISC is our recent proposal based on experimental data that the energy gap between 1CT and 3LE, so-called ΔESTðLEÞ, is an important parameter to signify TADF [Fig. 1(a)].24. The large ε of acetonitrile is expected to stabilize the 1CT and 3CT states and, tune the energy levels, in particular ΔESTðLEÞ, of the CzBN derivatives

Experimental

Results and Discussion

Conclusion