TADF activation by solvent freezing: The role of nonradiative triplet decay and spin-orbit coupling in carbazole benzonitrile derivatives

• Published in: Synthetic metals Vol. 252; pp. 62 - 68
• Main Authors: Hosokai, T., Nakanotani, H., Santou, S., Noda, H., Nakayama, Y., Adachi, C.
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 01-06-2019; Elsevier BV
• Subjects: Activation; Atomic energy levels; Benzonitrile; Carbazole benzonitrile; Carbazoles; Coupling (molecular); Derivatives; Diodes; Emitters; Energy conservation; Fluorescence; Freezing; Liquid nitrogen; Orbit decay; Organic chemistry; Organic light emitting diodes; Phosphorescence; Photoluminescence; Solvents; Spin-orbit interactions; Temperature dependence; Thermally activated delayed fluorescence (TADF); Toluene; Temperature dependence; Phosphorescence; Thermally activated delayed fluorescence (TADF); Photoluminescence; Carbazole benzonitrile
• ISSN: 0379-6779; 1879-3290
• DOI: 10.1016/j.synthmet.2019.04.005

•Low temperature dependent photoluminescence properties of carbazole benzonitrile derivatives in toluene were investigated.•A new liquid nitrogen cryostat enabling the direct measurement of solution temperature was developed.•TADF was activated if the solution was frozen, irrespective to the TADF activity of the derivatives at room temperatures.•An elongation of the lifetime of excited triplet state was examined by transient photoluminescence measurements.•A spin-orbit coupling also affected to the TADF activity in low temperatures.
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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 (T1) and lowest excited singlet state (S1), so-called ΔEST, 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 T1 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 T1, probing the suppression of nonradiative decay path of T1. 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 T1 and an increase of spin-orbit coupling together with reducing ΔEST to achieve a high TADF emission efficiency.