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QM/MM studies on luminescence mechanism of dinuclear copper iodide complexes with thermally activated delayed fluorescence.

ABSTRACT: The QM/MM method is employed to investigate the photophysical mechanism of two dinuclear copper iodide complexes with thermally activated delayed fluorescence (TADF). The S₁-T₁ energy differences (ΔE _ST) in these two complexes are small enough so that repopulating the S₁ state from T₁ becomes energetically allowed. Both forward and reverse intersystem crossing (ISC and rISC) processes are much faster than the corresponding radiative fluorescence and phosphorescence processes [k _ISC (10⁸ s^-1) > k ^F _r (10⁶ s^-1), k _rISC (10⁵ s^-1) > k ^P _r (10³ s^-1)]. The faster rISC process than the phosphorescence emission enables TADF. Moreover, the diphosphine ligands are found to play an important role in regulating the electronic structures and thereto the radiative and nonradiative rate constants. The present work rationalizes experimental phenomena and helps understand the intrinsic luminescence properties. The obtained insights could be useful for tuning the luminescence performance of dicopper-based luminescence materials.

SUBMITTER: Wang Q

PROVIDER: S-EPMC9065726 | biostudies-literature | 2019 Jul

REPOSITORIES: biostudies-literature

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QM/MM studies on luminescence mechanism of dinuclear copper iodide complexes with thermally activated delayed fluorescence.

Wang Qian Q Gao Yuan-Jun YJ Zhang Ting-Ting TT Han Juan J Cui Ganglong G

RSC advances 20190703 36

The QM/MM method is employed to investigate the photophysical mechanism of two dinuclear copper iodide complexes with thermally activated delayed fluorescence (TADF). The S1-T1 energy differences (ΔE ST) in these two complexes are small enough so that repopulating the S1 state from T1 becomes energetically allowed. Both forward and reverse intersystem crossing (ISC and rISC) processes are much faster than the corresponding radiative f ...[more]

PMID: 35515523

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Project description:The high luminescence efficiency of cyclometallated iridium(iii) complexes, including those widely used in OLEDs, is typically attributed solely to the formally spin-forbidden phosphorescence process being facilitated by spin-orbit coupling with the Ir(iii) centre. In this work, we provide unequivocal evidence that an additional mechanism can also participate, namely a thermally activated delayed fluorescence (TADF) pathway. TADF is well-established in other materials, including in purely organic compounds, but has never been observed in iridium complexes. Our findings may transform the design of iridium(iii) complexes by including an additional, faster fluorescent radiative decay pathway. We discover it here in a new dinuclear complex, 1, of the form [Ir(N^C)2]2(μ-L), where N^C represents a conventional N^C-cyclometallating ligand, and L is a bis-N^O-chelating bridging ligand derived from 4,6-bis(2-hydroxyphenyl)-pyrimidine. Complex 1 forms selectively as the rac diastereoisomer upon reaction of [Ir(N^C)2(μ-Cl)]2 with H2L under mild conditions, with none of the alternative meso isomer being separated. Its structure is confirmed by X-ray diffraction. Complex 1 displays deep-red luminescence in solution or in polystyrene film at room temperature (λem = 643 nm). Variable-temperature emission spectroscopy uncovers the TADF pathway, involving the thermally activated re-population of S1 from T1. At room temperature, TADF reduces the photoluminescence lifetime in film by a factor of around 2, to 1 μs. The TADF pathway is associated with a small S1-T1 energy gap ΔEST of approximately 50 meV. Calculations that take into account the splitting of the T1 sublevels through spin-orbit coupling perfectly reproduce the experimentally observed temperature-dependence of the lifetime over the range 20-300K. A solution-processed OLED comprising 1 doped into the emitting layer at 5 wt% displays red electroluminescence, λEL = 625 nm, with an EQE of 5.5% and maximum luminance of 6300 cd m-2.

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QM/MM studies on luminescence mechanism of dinuclear copper iodide complexes with thermally activated delayed fluorescence.

Publications

QM/MM studies on luminescence mechanism of dinuclear copper iodide complexes with thermally activated delayed fluorescence.

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