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Thermally Activated Delayed Fluorescence in a Y₃ N@C₈₀ Endohedral Fullerene: Time-Resolved Luminescence and EPR Studies.

ABSTRACT: The endohedral fullerene Y₃ N@C₈₀ exhibits luminescence with reasonable quantum yield and extraordinary long lifetime. By variable-temperature steady-state and time-resolved luminescence spectroscopy, it is demonstrated that above 60 K the Y₃ N@C₈₀ exhibits thermally activated delayed fluorescence with maximum emission at 120 K and a negligible prompt fluorescence. Below 60 K, a phosphorescence with a lifetime of 192±1 ms is observed. Spin distribution and dynamics in the triplet excited state is investigated with X- and W-band EPR and ENDOR spectroscopies and DFT computations. Finally, electroluminescence of the Y₃ N@C₈₀ /PFO film is demonstrated opening the possibility for red-emitting fullerene-based organic light-emitting diodes (OLEDs).

SUBMITTER: Zalibera M

PROVIDER: S-EPMC5751888 | biostudies-literature |

REPOSITORIES: biostudies-literature

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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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Thermally Activated Delayed Fluorescence in a Y₃ N@C₈₀ Endohedral Fullerene: Time-Resolved Luminescence and EPR Studies.

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Thermally Activated Delayed Fluorescence in a Y3 N@C80 Endohedral Fullerene: Time-Resolved Luminescence and EPR Studies.

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Thermally Activated Delayed Fluorescence in a Y₃ N@C₈₀ Endohedral Fullerene: Time-Resolved Luminescence and EPR Studies.