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Evidence and mechanism of efficient thermally activated delayed fluorescence promoted by delocalized excited states

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Science Advances  10 May 2017:
Vol. 3, no. 5, e1603282
DOI: 10.1126/sciadv.1603282
  • Fig. 1 EL mechanism.

    (A) Schematic of electrical exciton generation and EL mechanism in TADF-OLEDs. (B) Relationship between experimentally determined kRISC and ΔEST in previous works (5, 1319) and this work. The kRISC values of (4) to (6), treating to ~0 s−1, are below the limit of the estimation because of nearly no or undetectably weak DF intensity (Fig. 2A, inset). (C) Molecular structures of CzPN and CzBN derivatives, highlighting linearly positioned Cz moieties (blue). The numbering of the substituent positions of the BN core is depicted for 2CzBN. (D) D and A units of the CzPN and CzBN derivatives and D-A-D structure constructed with a linearly positioned Cz pair. R indicates substituents with electron-accepting properties, such as cyano groups.

  • Fig. 2 Photophysical characteristics.

    (A) PL decay curves of 2CzBN, o-3CzBN, m-3CzBN, p-3CzBN, 4CzBN, and 5CzBN in toluene at 295 K. 2CzBN, o-3CzBN, and m-3CzBN showed only prompt fluorescence, whereas p-3CzBN, 4CzBN, and 5CzBN exhibited prompt and delayed fluorescence (TADF). Right: Disappearance of TADF due to deactivation of T states caused by O2. (B) ΔEST with an error of ±0.03 eV of CzBN derivatives, estimated from the threshold energy difference between the fluorescence and phosphorescence spectra (77 K) for each molecule (right). Note that only the energy of the phosphorescence spectrum of 2CzBN was chosen from a first peak top (0-0 peak). For 2CzBN, the phosphorescence spectrum of TCzB (black) is also shown to display the spectral similarities with 2CzBN. Inset: Chemical structure of TCzB. (C) ηEQE as a function of current density for OLEDs [ITO/TAPC (35 nm)/mCP (10 nm)/dopant (15 wt %):PPT (30 nm)/PPT (40 nm)/LiF (0.8 nm)/Al (100 nm)] containing o-3CzBN (green), m-3CzBN (light blue), p-3CzBN (light green), 4CzBN (light brown), and 5CzBN (red) as the emitter dopant.

  • Fig. 3 Excited-state dynamics of CzPN derivatives.

    (A) Selected TAS spectra of 4CzIPN (Δt = 3 ps and 4.6 μs) and 2CzPN (Δt = 3 ps and 30 μs). div, division. (B) Contour maps of TAS results of 4CzIPN and 2CzPN obtained by different TAS techniques: microsecond-TAS (top) and nanosecond-TAS (bottom) (29, 30). For (B), ΔOD (color intensity) in each figure is normalized arbitrarily for better visualization. (C) Time profile of TR-PL and ΔOD in TAS results at 860 nm (1Cz+) and 1070 nm (3Cz+) of 2CzPN. The TR-PL shown to overlap with the profile of T feature (3Cz+) illustrates the coincidence of their τ. (D) Schematic explanation of the CR band formed by Cz2+ in terms of energy-level diagram.

  • Fig. 4 RISC mechanism of TADF-active molecules.

    (A) Contour maps of nanosecond-TAS results of 2CzBN, o-3CzBN, m-3CzBN, p-3CzBN, 4CzBN, and 5CzBN in toluene. (B) TAS spectra of the S (Δt = 0 to 1 ns) and T (Δt = 50 to 100 ns) states of 4CzBN. ΔOD is averaged one in each time range. (C) Ground-state absorption spectra of the CzBN derivatives in toluene. (D) Relation between kRISC and ΔEST(LE) of TADF-inactive (left) and TADF-active (right) molecules in energy-level diagram, respectively. Flu., fluorescence; Phos., phosphorescence.

  • Table 1 PL characteristics and rate constants of CzBN and CzPN derivatives in solution and doped film.
    MaterialΦPL Air*
    (%)
    ΦPL Degassed*
    (%)
    ΦPL Solid film
    (%)
    τprompt
    (ns)
    τDF
    (μs)
    Embedded Image‡§
    (×107 s−1)
    kRISC‡§
    (×105 s−1)
    kISC‡§
    (×108 s−1)
    Embedded Image‡§
    (×104 s−1)
    ΔEST‡║
    (eV)
    2CzBN152310.91.31.400.878.30.21
    o-3CzBN213126/3318.5151.100.46.50.21
    m-3CzBN151733/263.6394.202.42.60.24
    p-3CzBN101435/311.2358.30.127.52.70.22
    4CzBN96294/761.6365.61.85.71.70.22
    5CzBN98589/783.8392.42.42.40.420.17
    2CzPN42.346.589**27281.60.060.213.60.21
    4CzIPN109482**164.60.6320.30.561.50.04

    *PLQY at a concentration of 10−4 M in toluene.

    †PLQY for PPT host matrix doped with 3 wt % (left) or 15 wt % (right) of the emitter.

    ‡Solution samples.

    §Rate constant of radiative decay of singlets Embedded Image, Embedded Image, kRISC, and kISC of the solution samples determined by the method described by Masui et al. (18).

    ║Energy gap calculated from S1 and T1 energy levels estimated from the threshold of fluorescence spectra and peak (2CzBN) or threshold (others) of phosphorescence spectra (Fig. 2B), respectively. The error is ±0.03 eV. The data of fluorescence and phosphorescence spectra of 2CzPN and 4CzIPN are depicted in fig. S7.

    ¶Virtual values as determined by measuring a time profile of decay curve of triplet state absorption bands in microsecond-TAS (see figs. S1 and S8).

    **Value of 2CzPN-mCP [1,3-bis(N-carbazolyl)benzene] (6 wt %) by Masui et al. (18) and of 4CzIPN-CBP (4,4-N,N′-dicarbazole-biphenyl) (6 wt %) by Uoyama et al. (5).

    Supplementary Materials

    • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/5/e1603282/DC1

      Synthesis and characterization of 2CzBN, o-3CzBN, and p-3CzBN

      fig. S1. Time profiles of TAS of various triplet states of 4CzIPN and 2CzPN in toluene.

      fig. S2. Steady-state absorption spectra.

      fig. S3. Laser power dependence in TAS spectra.

      fig. S4. TAS spectra of the CzBN derivatives.

      fig. S5. TAS results of 3, 4, 6-p-3CzBN.

      fig. S6. Energy position of CR band.

      fig. S7. Emission spectra of CzPN derivatives.

      fig. S8. Time profiles of TAS of triplet states of CzBN derivatives.

    • Supplementary Materials

      This PDF file includes:

      • Synthesis and characterization of 2CzBN, o-3CzBN, and p-3CzBN
      • fig. S1. Time profiles of TAS of various triplet states of 4CzIPN and 2CzPN in toluene.
      • fig. S2. Steady-state absorption spectra.
      • fig. S3. Laser power dependence in TAS spectra.
      • fig. S4. TAS spectra of the CzBN derivatives.
      • fig. S5. TAS results of 3, 4, 6-p-3CzBN.
      • fig. S6. Energy position of CR band.
      • fig. S7. Emission spectra of CzPN derivatives.
      • fig. S8. Time profiles of TAS of triplet states of CzBN derivatives.

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