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Thermal equilibration between singlet and triplet excited states in organic fluorophore for submicrosecond delayed fluorescence

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Science Advances  12 Feb 2021:
Vol. 7, no. 7, eabe5769
DOI: 10.1126/sciadv.abe5769
  • Fig. 1 Fluorescence from equilibrated excited states.

    Simplified electron spin configurations representing fluorescence from thermally equilibrated S1 and T1 states. Unlike conventional TADF systems, this equilibrium model is valid only when both kISC and kRISC are significantly higher than kr.

  • Fig. 2 Molecular structures and photophysical properties.

    (A) Molecular structures of MCz-TXT and MCz-XT. (B) Steady-state absorption and photoluminescence (PL) spectra of MCz-TXT and MCz-XT in toluene (1.0 × 10−5 M). a.u., arbitrary units. (C) Steady-state PL spectra of their 10 wt % (weight %) doped films in 3,3′-bis(carbazol-9-yl)biphenyl (mCBP) host matrices. (D and E) Transient PL decay profiles of the doped films of MCz-TXT (D) and MCz-XT (E) measured at varying temperatures. (F) Temperature dependence of the delayed fluorescence rate constants (kDF) in the doped films. The solid lines in (F) represent theoretically predicted values using the equilibrium model described by Eq. 1, where the experimentally determined radiative decay rate constants (kr = 1.3 × 107 and 2.0 × 107 s−1) and energy gaps between the S1 and T1 states (ΔEST = 30 and 10 meV for MCz-TXT and MCz-XT, respectively) are adopted for simulations.

  • Fig. 3 Excited-state kinetics and electronic configurations.

    (A) Schematic potential energy surfaces of excited states depicting the nonadiabatic spin-flipping RISC and ISC. (B and C) Temperature dependence of the rate constants of RISC (B) and ISC (C) of MCz-TXT, MCz-XT, and 4CzIPN in 10 wt % emitter:mCBP doped films. The solid lines in (B) and (C) represent the fits to the nonadiabatic RISC and ISC models represented by Eqs. 2 and 3, respectively. (D and E) Natural transition orbitals for the excited states of MCz-TXT (D) and MCz-XT (E) at the lowest-energy crossing points between the potential energy surfaces of the singlet and triplet excited states.

  • Fig. 4 OLED characteristics.

    (A) Schematic device architecture and chemical structures of the materials used. (B) EL spectra measured at 10 mA cm−2. The inset of (B) displays a photo of sky-blue EL emission from the MCz-TXT–based device. Photo credit: Naoya Aizawa, Kyushu University. (C) Current density–voltage–luminance (J-V-L) characteristics and (D) external EL quantum efficiency (ηext)–L characteristics of the fabricated devices. The inset of (D) shows viewing angle dependence of the EL intensities exhibiting a Lambertian distribution.

  • Table 1 Parameters relevant to spin-flipping RISC and ISC.

    RISC and ISC rate constants (kRISC and kISC) at 300 K, reorganization energies (λRISC and λISC), activation energies (Ea,RISC and Ea,ISC), and SOC strengths (HSO) between S1 and T2 at the crossing point are listed for MCz-TXT, MCz-XT, and 4CzIPN.

    EmitterkRISC (s−1)kISC (s−1)λRISC (meV)λISC (meV)Ea,RISC (meV)Ea,ISC (meV)HSO (meV)
    MCz-TXTExp.1.1 × 1089.4 × 10859417573330.29
    Calc.3.4 × 1081.5 × 10950340120570.70
    MCz-XTExp.2.7 × 1063.4 × 1076401331150.020
    Calc.6.2 × 1062.8 × 1074834095300.057
    4CzIPNExp.1.9 × 1067.7 × 107891192250.035
    Calc.1.1 × 1062.6 × 1072635160800.074
  • Table 2 EL performance data.

    λEL, EL emission maximum at 10 mA cm−1; Von, turn-on voltage at a luminance of >1 cd m−2; Lmax, maximum luminance; ηext,max, maximum external EL quantum efficiency; ηext,100/1000/10000, external EL quantum efficiencies at luminance of 100, 1000, and 10,000 cd m−2; CE, maximum current efficiency; PE, maximum power efficiency; CIE, Commission Internationale de l’Éclairage color coordinates recorded at 10 mA cm−2.

    EmitterλEL (nm)Von (V)Lmax (cd m−2)ηext,max (%)ηext,100/1000/10000 (%)CE (cd A−1)PE (lm W−1)CIE (x, y)
    MCz-TXT4973.822,10025.825.8/24.6/21.667.252.0(0.21, 0.46)
    MCz-XT4893.817,70025.525.5/23.7/19.762.650.0(0.19, 0.42)
    4CzIPN5243.418,70023.123.1/22.1/16.777.664.9(0.31, 0.60)
    FIrpic4723.812,50025.525.5/23.2/17.552.238.4(0.15, 0.36)

Supplementary Materials

  • Supplementary Materials

    Thermal equilibration between singlet and triplet excited states in organic fluorophore for submicrosecond delayed fluorescence

    Naoya Aizawa, Akinobu Matsumoto, Takuma Yasuda

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