TY - JOUR
T1 - TADF activation by solvent freezing
T2 - The role of nonradiative triplet decay and spin-orbit coupling in carbazole benzonitrile derivatives
AU - Hosokai, T.
AU - Nakanotani, H.
AU - Santou, S.
AU - Noda, H.
AU - Nakayama, Y.
AU - Adachi, C.
N1 - Funding Information:
This work was supported in part by the JST-ERATO , Adachi Molecular Exciton Engineering Project , the International Institute for Carbon Neutral Energy Research (WPI-I2CNER) sponsored by Ministry of Education, Culture, Sports, Science and Technology (MEXT) , and Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number JP16K14102 , 17H03137 , 18K03902 and 18H05267 .
Publisher Copyright:
© 2019 Elsevier B.V.
PY - 2019/6
Y1 - 2019/6
N2 - Thermally activated delayed fluorescence (TADF) materials have attracted considerable attentions as a new kind of emitters in organic light-emitting diodes. While it is requisite to minimize an energy difference between the lowest excited triplet state (T 1 ) and lowest excited singlet state (S 1 ), so-called ΔE ST , a deeper understanding of the emission mechanism is desirable to clarify the comprehensive molecular design. In this paper, we present that the TADF ability and efficiency of (rare-)metal free organic molecules are surely influenced by both the nonradiative decay of T 1 and spin-orbit coupling. By investigating a temperature dependent photoluminescence of carbazole benzonitrile derivatives in toluene solutions using a newly developed liquid nitrogen cryostat, we demonstrate the activation of TADF by solvent freezing for room temperature-TADF inactive molecules. Transient photoluminescence measurements of the frozen samples show a significant increase of a lifetime of T 1 , probing the suppression of nonradiative decay path of T 1 . A magnitude of the TADF activation by the solvent freezing is closely related to the degree of spin-orbit coupling of the molecules. The present results emphasize the importance of suppression of nonradiative decay of T 1 and an increase of spin-orbit coupling together with reducing ΔE ST to achieve a high TADF emission efficiency.
AB - Thermally activated delayed fluorescence (TADF) materials have attracted considerable attentions as a new kind of emitters in organic light-emitting diodes. While it is requisite to minimize an energy difference between the lowest excited triplet state (T 1 ) and lowest excited singlet state (S 1 ), so-called ΔE ST , a deeper understanding of the emission mechanism is desirable to clarify the comprehensive molecular design. In this paper, we present that the TADF ability and efficiency of (rare-)metal free organic molecules are surely influenced by both the nonradiative decay of T 1 and spin-orbit coupling. By investigating a temperature dependent photoluminescence of carbazole benzonitrile derivatives in toluene solutions using a newly developed liquid nitrogen cryostat, we demonstrate the activation of TADF by solvent freezing for room temperature-TADF inactive molecules. Transient photoluminescence measurements of the frozen samples show a significant increase of a lifetime of T 1 , probing the suppression of nonradiative decay path of T 1 . A magnitude of the TADF activation by the solvent freezing is closely related to the degree of spin-orbit coupling of the molecules. The present results emphasize the importance of suppression of nonradiative decay of T 1 and an increase of spin-orbit coupling together with reducing ΔE ST to achieve a high TADF emission efficiency.
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U2 - 10.1016/j.synthmet.2019.04.005
DO - 10.1016/j.synthmet.2019.04.005
M3 - Article
AN - SCOPUS:85064219389
SN - 0379-6779
VL - 252
SP - 62
EP - 68
JO - Synthetic Metals
JF - Synthetic Metals
ER -