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

Abstract

, "Solvent-dependent investigation of carbazole benzonitrile derivatives: does the 3 LE − 1 CT energy gap facilitate thermally activated delayed fluorescence?," J. Photon. Energy 8(3), 032102 (2018), 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 STðLEÞ . Relative to the ΔE STðLEÞ 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.

Excited state engineering for efficient reverse intersystem crossing

“…While Hosokai et al . ( 24 ) already reported that the T 1 state of 5CzBN has CT character, we also confirmed the solvatochromism of the phosphorescence spectra of 5CzBN (fig. S2), indicating that the T 1 state of 5CzBN has more CT character than LE character despite the vibronic phosphorescence spectra.…”

Sensitizing TADF Absorption Using Variable Length Oligo(phenylene ethynylene) Antennae

“…Secondly, we point out that our measurements are made using chloroform as a solvent rather than toluene (commonly used for 4CzIPN) because the TADF was more intense for our molecules in chloroform. This result is consistent with literature findings for 4CzIPN and various carbazole benzonitrile derivatives where the reverse intersystem crossing rate is increased in solvents more polar than toluene (Ishimatsu et al, 2013;Nobuyasu et al, 2016;Hosokai et al, 2018). The decrease in the TADF emission for 4CzIPN in more polar solvents than toluene is attributed to increased nonradiative rates (Ishimatsu et al, 2013).…”

Extracting Design Principles for Efficient Thermally Activated Delayed Fluorescence (TADF) from a Simple Four-State Model

“…High-throughput screening approaches have been proposed [29][30][31] to find new emitters with an effective tradeoff between small Δ and fast radiative decay. Further experimental 15,[32][33][34] studies have shown that for many TADF molecules the lowest triplet state is in fact a local excitation (LE), which indicated that the understanding of the TADF mechanism is incomplete and known design strategies are not optimal. Since then, a number of theoretical works have analyzed the mixed CT/LE nature of the excited states [35][36][37][38][39][40] , importance of vibronic effects [41][42][43][44][45] , and influence of the interactions with the environment [46][47][48][49] , including the importance of static and dynamic disorder 50,51 .…”