Diindolocarbazole - achieving multiresonant thermally activated delayed fluorescence without the need for acceptor units.

David Hall,Stuart Warriner,Kleitos Stavrou,Yoann Olivier,Eimantas Duda,Andrew Danos,Sergey Bagnich,Eli Zysman-Colman,Andrew Monkman,Alexandra M Z Slawin,Anna Köhler,David Beljonne

doi:10.1039/d1mh01383a

David Hall, Stuart Warriner + Show 10 more

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In this work we present a new multi-resonance thermally activated delayed fluorescence (MR-TADF) emitter paradigm, demonstrating that the structure need not require the presence of acceptor atoms. Based on an in silico design, the compound DiICzMes4 possesses a red-shifted emission, enhanced photoluminescence quantum yield, and smaller singlet-triplet energy gap, ΔEST, than the parent indolocarbazole that induces MR-TADF properties. Coupled cluster calculations accurately predict the magnitude of the ΔEST when the optimized singlet and triplet geometries are used. Slow yet optically detectable reverse intersystem crossing contributes to low efficiency in organic light-emitting diodes using DiICzMes4 as the emitter. However, when used as a terminal emitter in combination with a TADF assistant dopant within a hyperfluorescence device architecture, maximum external quantum efficiencies of up to 16.5% were achieved at CIE (0.15, 0.11). This represents one of the bluest hyperfluorescent devices reported to date. Simultaneously, recognising that MR-TADF emitters do not require acceptor atoms reveals an unexplored frontier in materials design, where yet greater performance may yet be discovered.

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Communication applications in several other optoelectronic contexts.[5,6,7,8] reverse intersystem crossing (RISC) is achieved in D–A Thermally activated delayed fluorescence (TADF) materials through the reduction of the exchange integral by electronically decoupling the donor and acceptor moieties as a result of a highly twisted conformation,[2,4] with the HOMO situated on the donor and LUMO on the acceptor, combined with vibronic coupling between local and charge transfer triplet states to facilitate spin orbit coupling.[9]
RISC is achieved in D–A TADF materials through the reduction of the exchange integral by electronically decoupling the donor and acceptor moieties as a result of a highly twisted conformation,[2,4] with the HOMO situated on the donor and LUMO on the acceptor, combined with vibronic coupling between local and charge transfer triplet states to facilitate spin orbit coupling.[9]
We showed that decoration of this core with mesityl groups, Mes3DiKTa, can mitigate aggregation induced quenching (AIQ),[20] which is a common problem with these planar molecules

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Communication applications in several other optoelectronic contexts.[5,6,7,8] RISC is achieved in D–A TADF materials through the reduction of the exchange integral by electronically decoupling the donor and acceptor moieties as a result of a highly twisted conformation,[2,4] with the HOMO situated on the donor and LUMO on the acceptor, combined with vibronic coupling between local and charge transfer triplet states to facilitate spin orbit coupling.[9]. We optimized both the S1 and T1 states within the TDA using PBE0 functional and 6-31G(d,p) basis set, and computed the T1 and S1 excited state energies at the SCSCC2/cc-PVDZ level of theory for ICz, ICzMes[3] and DiICzMes[4] as well as for three literature MR-TADF compounds, DABNA-1, BCzBN and DiKTa. Quantitative agreement with the experiments is reached with DEST increasing for ICz, ICzMes[3] and DiICzMes[4], to 0.59 eV, 0.45 eV and 0.29 eV (Fig. 3), respectively, caused by a larger relaxation energy of the T1 state in line with a greater LE character for this state (Tables S9–S11, ESI‡).

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