Article pubs.acs.org/JPCA

Mechanism of Intersystem Crossing of Thermally Activated Delayed Fluorescence Molecules Toshinari Ogiwara,†,‡ Yusuke Wakikawa,§ and Tadaaki Ikoma*,†,∥,⊥ †

Graduate School of Science and Technology, §Center for Fostering Innovative Leadership, and ⊥Center for Instrumental Analysis, Niigata University, 2-8050 Ikarashi, Nishi-ku, Niigata 950-2181, Japan ‡ The Electronic Materials Department, Idemitsu Kosan Co., Ltd., 1280 Kami-izumi, Sodegaura, Chiba 299-0293, Japan ∥ Core Research for Evolutionary Science and Technology, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi 332-0012, Japan S Supporting Information *

ABSTRACT: The spin sublevel dynamics of the excited triplet state in thermally activated delayed fluorescence (TADF) molecules have not been investigated for high-intensity organic light-emitting diode materials. Understanding the mechanism for intersystem crossing (ISC) is thus important for designing novel TADF materials. We report the first study on the ISC dynamics of the lowest excited triplet state from the lowest excited singlet state with chargetransfer (CT) character of TADF molecules with different external quantum efficiencies (EQEs) using time-resolved electron paramagnetic resonance methods. Analysis of the observed spin polarization indicates a strong correlation of the EQE with the population rate due to ISC induced by hyperfine coupling with the magnetic nuclei. It is concluded that molecules with high EQE have an extremely small energy gap between the 1CT and 3CT states, which allows an additional ISC channel due to the hyperfine interactions. (IQE) for fluorescent OLEDs is limited to 25%, the so-called singlet−triplet bottleneck. The out-coupling efficiency under a plane cavity configuration with a metal and a transparent electrode is approximately 20% if the emission pattern is assumed to be Lambertian.2 Consequently, the upper limit of the external quantum efficiency (EQE) for fluorescent OLEDs is only 5%. Harvesting the nonradiative triplet states (triplet harvest) generated at room temperature by carrier recombination is an important issue for the development of highefficiency fluorescent OLEDs. In recent years, thermally activated delayed fluorescence (TADF), which is caused by a reverse intersystem crossing (ISC), has attracted attention as a high-efficiency technology for OLEDs. The utilization of TADF from the lowest excited singlet (S1) state with strong chargetransfer (CT) character reported recently by Adachi et al. is a prospective way to achieve triplet harvest.3−8 Technology based on TADF may achieve a 100% IQE without the need for rare metals, in contrast to phosphorescence technology. However, to realize TADF technology, it is important to experimentally elucidate the kinetics and the electronic structures of the lowlying excited states. In particular, ISC between the singlet and triplet excited states, which is illustrated by the red arrow in Figure 1a, is a key dynamic of the TADF mechanism. Here, we clarify both the mechanism of ISC from the S1 state to the lowest excited triplet (T1) state at low temperature, which still prevails under room temperature, and the electronic structure

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rganic electroluminescence has been studied extensively with the goal of realizing thin, stable display devices with fast responses and wide viewing angles.1 As shown in Figure 1a,

Figure 1. (a) Elementary processes in OLED and (b) molecular structure and abbreviation of TADF molecules.

the population ratio between the singlet and triplet encounter pairs of electrons and holes (1,3e-h) is known to be 1:3 due to the spin statistics in organic light-emitting diodes (OLEDs). If the encounter pairs have an equal recombination yield to excitons, then the branching ratio of the singlet and triplet excitons is also 1:3. Therefore, the internal quantum efficiency © XXXX American Chemical Society

Received: March 8, 2015 Revised: March 14, 2015

A

DOI: 10.1021/acs.jpca.5b02253 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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those of 4Cz-IPN and PXZ-TRZ, and indicate an EEE/AAA phase. The extent and phase of the spectrum reflect the zerofield splitting and the spin polarization of the T1 state, respectively. As presented in Table 1 and Figure S3 (Supporting Information), the apparent decay times (Td) for 4Cz-IPN and PXZ-TRZ are shorter than those for Cz-T and PXZ-TRZ. The difference of Td suggests that the magnetic kinetics for TADF molecules with high EQE, which may be caused by fluctuation of molecular conformations, are relatively fast, although Td is governed by multiple processes such as the spin−lattice and spin−spin relaxations and the deactivation of the T1 state. The green curves in Figure 2 are the spectra calculated for the spin polarized T1 state through ISC induced by the spin− orbit coupling (SOC), as illustrated in Scheme 1a. The

of the T1 state of the TADF molecules shown in Figure 1b using time-resolved electron paramagnetic resonance (EPR) spectroscopy. Time-resolved EPR is useful for studies on the photoexcited short-lived triplet states and it enables detection of the spin polarization of the T1 state developed from ISC.9−12 Figure 2 shows transient EPR spectra of the T1 states for the TADF molecules, the absorption and emission spectra of which

Scheme 1. Spin Polarization Mechanisms for the T1 State Due to ISC from the S1 State under External Magnetic Field along the Z Axis of the Molecule

Figure 2. Transient EPR spectra for (a) 4Cz-IPN, (b) PXZ-TRZ, (c) Cz-T, and (d) PIC-TRZ in toluene detected at 550 ns after a laser flash at 77 K (red solid lines). The dotted lines are the simulation spectra: green, SOC-induced polarization; blue, HFC-induced polarization; black, SOC plus HFC-induced polarization.

populations of the spin sublevels TX, TY, and TZ in the T1 state are proportional to the direct integral of |⟨S|HSO|Ti⟩|2 (PSOC ). i Those populations are redistributed over the spin eigenstates under finite magnetic fields. The method of spectral simulation is described in Supporting Information. The EPR parameters used for the simulations, in which zero-field splitting parameters of positive D and negative E values are assumed on the basis of the ππ* electronic structure,13 are summarized in Table 1. The spectra for Cz-T and PIC-TRZ were reproduced well using positive large D values, and preferential population to the TX0 and TY0 sublevels denoted by pX0 and pY0. For 4Cz-IPN and PXZ-TRZ, the calculations with the zero-field splittings in Table 1 simulate the canonical fields of the

are shown in Figure S2 (Supporting Information), and the EQEs at room temperature4−7 are listed in Table 1. For 4CzIPN with the highest EQE among the four TADF molecules, the |ΔmS| = 1 transitions appear in a narrow field range from 280 to 370 mT and indicate a phase of AEE/AAE at the six canonical fields, where A and E denote the absorption and stimulated emission of microwaves. PXZ-TRZ also has a relatively narrow spectrum with the same phase of AEE/AAE. In contrast, the spectra of Cz-T and PIC-TRZ are broader than

Table 1. Maximum External Quantum Efficiency (EQE) for TADF Molecules and g-Value, Zero-Field Splittings (D and E), Populations of Spin Sublevels (pi and Pm), and Decay Time (Td) of the T1 States Obtained by Simulation of the Spin Polarized EPR Spectra Detected at 77 K molecule

EQEa (%)

g

D (cm−1)

E (cm−1)

B1/2b (mT)

pX0

pY0

pZ0

PmHFC

Tdc (μs)

4Cz-IPN PXZ-TRZ Cz-T PIC-TRZ

19.3 12.5 6.0 5.3

2.003 2.003 2.003 2.003

0.043 0.046 0.091 0.091

−0.012 −0.005 −0.008 −0.009

5 10 12 10

0.00 0.00 0.45 0.50

0.50 0.30 0.55 0.50

0.00 0.33 0.00 0.00

0.50 0.36 0.00 0.00

1.2 1.0 3.2 1.9

a

Taken from refs 4−7. bA half-width at half-height of Gaussian resonance line (eq S2, Supporting Information). cEstimated by fitting the time profiles of spin-polarized EPR signals using a convolution method (Figure S3, Supporting Information). B

DOI: 10.1021/acs.jpca.5b02253 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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for the lowest 3ππ* states for the unit molecules in the TADF compounds, such as carbazole (|D| = 0.1022 cm−1),16 11,12diphenyl-11H,12H-indolo[2,3-a]carbazole (|D| = 0.088 cm−1, see Figure S4, Supporting Information), triphenyl-s-triazine (|D| = 0.124 cm−1),17 phenoxazine (|D| = 0.1247 cm−1),18 and cyanobenzene (|D| = 0.1371 cm−1),19,20 whereas Cz-T and PICTRZ, respectively, have |D| values close to that of carbazole and indolocarbazole rather than s-triazine. Comparison of the zero field splitting parameter indicates that the T1 states of Cz-T and PIC-TRZ have an electronic structure of 3ππ* localized in the carbazole part (loT1). The |D| value for 4Cz-IPN and PXZ is too large compared with the typical values for radical pairs having well-separated spin centers. The size of |D| and the nonvanishing |E| value for 4Cz-IPN and PXZ-TRZ indicates the delocalization of excitation over the entire molecule (deT1) rather than CT character. ) It is noteworthy that the population ratio of PHFC (=ΣPHFC i to PSOC (=Σp0i ) increases with the EQE. The spin polarization of the T1 state populated by the anisotropic ISC from the S1 state with a CT character (CTS1) can be understood by the two mechanisms shown in Scheme 1. The TADF molecules have efficient SOC-induced ISC from the CTS1 state to the T1 state. It is probable that the nitrogen nuclei of triazines in PXZ-TRZ, Cz-T, and PIC-TRZ, and the nitrogen and/or carbon atoms in cyano groups may give rise to SOC due to their effective onecentered integrals. In addition, the high efficiency 4Cz-IPN and PXZ-TRZ TADF molecules have HFC-induced ISC through a higher triplet (Tn) state that should be located closer in energy to the CTS1 state, which is followed by a spin conservative internal conversion (IC) to the T1 state. The small energy gap between the CTS1 and Tn states, which is less than the Zeeman energy range of 0.21−0.40 cm−1 for the present EPR experiments, is necessary for HFC-induced ISC from the CT S1 state. This energy requisite suggests that the Tn state has the same CT character as the CTS1 state (CTTn) because the CT state can have a small gap between the singlet and triplet states, which is denoted by 2J, where J is the coupling constant for the exchange interaction. Such an extremely small |J| < 0.1 cm−1, even in the intramolecular CT state with a charge separation of less than 1 nm, supports a twist conformation between the donor and acceptor chromophores. The twist conformation causes the overlap between the singly occupied orbitals to be small. The strong correlation between the EQE and the PmHFC/ PSOC ratio shown in Figure 3 confirms that HFC-induced ISC is an important pathway for TADF. Therefore, we can conclude that the HFC-induced ISC enhances the delayed fluorescence from the triplet manifolds using thermal energy at room temperature. The involvement of magnetic nuclei in the molecules, such as nitrogens in carbazole and triazine, as well as many protons bonded on aromatic rings, are another essential factor for enhancing EQE based on the TADF technology.

observed spectra, but none of the calculations reproduces all of the AEE/AAE phases observed in the spectra. The disagreement of the spectral phase indicates that there is another pathway to produce the populations in the T1 state for 4CzIPN and PXZ-TRZ with relatively large EQE. To clarify the origin of the AEE/AAE phase, hyperfine coupling (HFC)-induced ISC illustrated in Scheme 1b, which is effective in the case of molecules that possess almost degenerate singlet and triplet states such as radical pairs,14,15 was taken into account together with SOC-induced ISC. Pi(B0 ) = PiSOC(B0 ) + PiHFC(B0 )

(1)

PHFC i

Here stands for the populations of the Ti sublevel induced by the HFC. HFC-induced ISC is dependent on the external magnetic field, differently from SOC-induced ISC. The energy shift of the Th,l sublevels of the triplet state due to the Zeeman interaction significantly reduces the ISC rate from the singlet state S to the Th,l sublevels of the higher triplet (Tn) state. The resonance fields for the X-band EPR experiments are much larger than the HFC for organic molecules (a few mT); therefore, HFC-induced ISC occurs between the singlet state S, and the middle spin sublevel Tm, which can be approximated to T0 and maintain almost degeneracy with the S state, even under a magnetic field. According to the conservation rule of spin angular momentum in the transition between the Tn and T1 states, the T1 state may have an overpopulation on the Tm state (PHFC m ), due to the HFC-induced ISC mechanism: PmHFC ≠ 0

and

PhHFC = PlHFC = 0

(2)

The spectra calculated using eqs 1 and 2 reproduce the observed spectra very well, as shown by the black dotted curves in Figure 2a,b. The spectra calculated with only PHFC in positive i D value show the AEE/AAE pattern, as illustrated by the blue dotted curves in Figure 2, whereas the spectral pattern changes to EAA/EEA in negative D value. The calculated spectral pattern indicates that the assumption of positive D value for the T1 state is reasonable. However, the simulation only using PHFC i in positive D does not fit the observed total spectral shape. Thus, the combination of PSOC and PHFC was necessary for the i i spectral simulation. The population ratios optimized by the simulations are summarized in Table 1. Figure 3 shows the decrease in the zero-field splitting |D|, with increasing EQE for the TADF molecules. The |D| values for 4Cz-IPN and PXZ-TRZ are significantly smaller than those

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, absorption and emission spectra, simulation for EPR spectrum of polarized triplet state, time development of polarized EPR signals, and time-resolved EPR spectrum of indolocarbazole. This material is available free of charge via the Internet at http://pubs.acs.org

Figure 3. Correlation between the magnetic properties of the T1 state (HFC-induced ISC (red circle), zero-field splitting parameter |D| (blue square), and decay time (green triangle)) and the external quantum efficiency. C

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Acceptor Molecule for Artificial Photosynthesis. J. Am. Chem. Soc. 2011, 133, 1240−1243. (16) Siegel, S.; Judeikis, H. S. Triplet State Zero-Field Splittings of Some Structurally Related Aromatic Hydrocarbon and Heterocyclic Molecules. J. Phys. Chem. 1966, 70, 2201−2204. (17) Brinen, J. S.; Koren, J. G.; Hodgson, W. G. ESR and Phosphorescence Spectra of the Triplet States of Phenyl s-Triazines and Phenyl Benzenes. J. Chem. Phys. 1966, 44, 3095−3099. (18) Lhoste, J. M.; Haug, A.; Ptak, M. Electron Paramagnetic Resonance Studies of Photoselected Triplet Molecules. I. Phenoxazine. J. Chem. Phys. 1966, 44, 648−654. (19) Hirota, N.; Wong, T. C.; Harrigan, E. T.; Nishimoto, K. Studies of Hyperfine Splittings and Spin Distributions of the Lowest Excited Triplet States of Substituted Benzenes and Pyridines. Mol. Phys. 1975, 29, 903−919. (20) Niizuma, S.; Kwan, L.; Hirota, N. E.P.R. and Zero-field ODMR Studies of the T1 States of Halogen Containing Benzonitriles, Anisoles and Anilines. Mol. Phys. 1978, 35, 1029−1046.

AUTHOR INFORMATION

Corresponding Author

*T. Ikoma. Graduate School of Science and Technology. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Katsumi Tokumaru for fruitful discussions and comments. This work was supported by a grant from Idemitsu Kosan Co., Ltd., a CREST grant from JST, a Grant-in-Aid for Scientific Research (No. 2610088) from MEXT, and a grant from the Network Joint Research Center for Materials and Devices.

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DOI: 10.1021/acs.jpca.5b02253 J. Phys. Chem. A XXXX, XXX, XXX−XXX