Solvent-dependent investigation of carbazole benzonitrile derivatives: does the 3LE−1CT energy gap facilitate thermally activated delayed fluorescence?

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

doi:10.1117/1.JPE.8.032102

2 February 2018 Solvent-dependent investigation of carbazole benzonitrile derivatives: does the ³LE−¹CT energy gap facilitate thermally activated delayed fluorescence?

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

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Journal of Photonics for Energy, Vol. 8, Issue 3, 032102 (February 2018). https://doi.org/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, ΔEST(LE). Relative to the ΔEST(LE) values for the CzBN derivatives in the low polarity solvent toluene (ϵ∼2), a reduction of ΔEST(LE) for the CzBN derivatives in the polar solvent acetonitrile (ϵ∼37) was confirmed while maintaining fairly constant ΔEST 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 ΔEST(LE) was limited, however, as an excessive decrease in ΔEST(LE) facilitates the nonradiative deactivation of the triplet states, leading to a loss of the TADF efficiency. This paper shows that ΔEST(LE) provides a measure of TADF activation and that appropriate regulation of ΔEST(LE) is required to achieve high TADF efficiency.

CC BY: © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

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Takuya Hosokai, Hiroki Noda, Hajime Nakanotani, Takanori Nawata, Yasuo Nakayama, Hiroyuki Matsuzaki, and Chihaya Adachi "Solvent-dependent investigation of carbazole benzonitrile derivatives: does the ³LE−¹CT energy gap facilitate thermally activated delayed fluorescence?," Journal of Photonics for Energy 8(3), 032102 (2 February 2018). https://doi.org/10.1117/1.JPE.8.032102

Received: 16 November 2017; Accepted: 10 January 2018; Published: 2 February 2018

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