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Absence of delayed fluorescence and triplet–triplet annihilation in organic light emitting diodes with spatially orthogonal bianthracenes

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Abstract

Two compounds, 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN), which has a single anthracene unit, and 10,10′-diphenyl-9,9'-bianthracene (PPBA), which has two spatially orthogonal anthracene units, were compared and investigated in terms of photoelectric characteristics and the reverse intersystem crossing (RISC) process in organic light emitting diodes (OLEDs). Transient electroluminescence (EL) measurements indicated large contributions of triplet–triplet annihilation (TTA) for MADN but almost no contribution of TTA for PPBA. The magnetic field dependence of EL for the two anthracene compounds was also different. The EL of MADN was sensitive to the magnetic field at high current density, but PPBA showed less dependence, which indicated the absence of the TTA process for PPBA. TD-DFT calculations revealed that PPBA has doubly degenerate lowest triplet states (T1 and T2) with a much lower energy than S1, which is unfavorable for thermally activated delayed fluorescence (TADF). The near-zero ΔEST between highly excited states Sm≥1 and Tn>2 is favorable to RISC at a highly excited state. Oxygen quenching of photoluminescence only for PPBA and decreasing EL intensity with decreasing temperature only for PPBA support the existence of the RISC path from Tn>2 to Sm≥1. A high external quantum efficiency of 11% in blue OLEDs with PPBA was obtained, indicating that this orthogonal anthracene type of molecular design for RISC at a highly excited state would expand the material development of compounds emitting blue fluorescence for OLEDs.

Graphical abstract: Absence of delayed fluorescence and triplet–triplet annihilation in organic light emitting diodes with spatially orthogonal bianthracenes

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Publication details

The article was received on 20 Nov 2018, accepted on 03 Jan 2019 and first published on 03 Jan 2019


Article type: Paper
DOI: 10.1039/C8TC05817B
Citation: J. Mater. Chem. C, 2019, Advance Article
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    Absence of delayed fluorescence and triplet–triplet annihilation in organic light emitting diodes with spatially orthogonal bianthracenes

    Y. Pu, R. Satake, Y. Koyama, T. Otomo, R. Hayashi, N. Haruta, H. Katagiri, D. Otsuki, D. Kim and T. Sato, J. Mater. Chem. C, 2019, Advance Article , DOI: 10.1039/C8TC05817B

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