Hund’s multiplicity rule states that for a given electronic configuration, a higher spin state has a lower energy. Rephrasing this rule for molecular excited states predicts a positive energy gap between spin-singlet and spin-triplet excited states, which has been consistent with numerous experimental observations over almost a century. Here, we report a fluorescent molecule that disobeys Hund’s rule, possessing a negative singlet–triplet energy gap of –11 meV. The energy inversion of the singlet and triplet excited states results in delayed fluorescence with short time constants of 0.2 μs, which anomalously decrease with decreasing temperature due to the emissive singlet character of the lowest-energy excited state. Organic light-emitting diodes using this molecule exhibited a fast transient electroluminescence decay with a peak external quantum efficiency of 17%, demonstrating potential implications for optoelectronic devices, including displays, lighting, and lasers.
Physical Sciences - Article
Delayed Fluorescence from Inverted Singlet and Triplet Excited States for Efficient Organic Light-Emitting Diodes
https://doi.org/10.21203/rs.3.rs-478258/v1
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published 14 Sep, 2022
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light-emitting diodes
Hund’s multiplicity rule
optoelectronic devices
The spin multiplicity of molecular excited states plays a crucial role in organic optoelectronic devices. In the case of organic light-emitting diodes (OLEDs), the recombination of charge carriers leads to the formation of singlet and triplet excited states in a 1:3 ratio. This spin statistic limits the internal quantum efficiency of OLEDs and leads to energy loss due to the spin-forbidden nature of triplet excited states to emit photons. To overcome this issue, two strategies have been established for harvesting photons from the ‘dark’ triplet excited states. The first relies on organometallic complexes with transition metals, such as iridium and platinum, which induce a large spin–orbit coupling to allow triplet states to emit photons as phosphorescence2–4. The other utilizes organic molecules that exhibit thermally activated delayed fluorescence (TADF)5–7. This class of materials has energetically close singlet and triplet excited states, where ambient thermal energy upconverts the triplet states into singlet states through reverse intersystem crossing (RISC). Although the concept of TADF has the advantage of eliminating the need for transition metals, the resultant temporally delayed fluorescence typically has a time constant in the microsecond or even millisecond range, which is long enough for detrimental bimolecular annihilations, such as triplet–triplet annihilation and triplet–polaron annihilation, to compete with delayed fluorescence. These bimolecular annihilations lead to a decrease in device efficiency under high current densities, known as efficiency roll-off in OLEDs8.9, and generate high-energy excitons that are suspected to cause chemical degradation of materials, particularly in blue OLEDs10,11. The research community has thus focused on minimizing the singlet–triplet energy gap (DEST) to accelerate upconversion by thermal activation7. Alternatively, an ideal case would be thermodynamically favourable downconversion with negative DEST, which is not expected if Hund’s multiplicity rule applies to the lowest-energy excited state. Herein, we demonstrate experimental evidence of the existence of highly fluorescent organic molecules that disobey Hund’s rule and possess negative DEST for constructing efficient OLEDs.
Numerous observations of positive DEST in molecular excited states are generally understood in terms of the exchange interaction, the quantum-mechanical effect involving Pauli repulsion, which stabilises triplet states relative to singlet states12. DEST is simply equal to twice the positive exchange energy if the lowest-energy singlet and triplet excited states (S1 and T1) have the same single-excitation configuration13. Although there is general agreement that DEST must be positive, the potential for negative DEST in nitrogen-substituted phenalene analogues, such as cycl[3.3.3]azine and heptazine has been discussed during the last two decades14–20. Recent theoretical studies have also suggested the possibility of negative DEST in these molecules by accounting for double-excitation configurations where two electrons of occupied orbitals have been promoted out to virtual orbitals (Supplementary Fig. 1)16–20. Because the Pauli exclusion principle restricts the accessible double-excitation configurations in T1, an effective admixture of such configurations stabilises S1 relative to T1. If this stabilisation overcomes the exchange energy, DEST could have a negative value (Fig. 1a). However, to the best of our knowledge, none of the molecules have been experimentally identified with negative DEST and the resultant delayed fluorescence from inverted singlet and triplet excited states (DFIST).
Pioneering computational calculations16 inspired us to focus on heptazine as a potential class of molecules that exhibit DFIST. Correlated wave function theories suggested that S1 of heptazine lies 0.2–0.3 eV below T1, though S1 is a ‘dark’ state, meaning that the electronic transition to the ground state (S0) is dipole-forbidden and the oscillator strength (f) is zero in the D3h symmetry point group. Interestingly, the heptazine core is shared by several synthesised molecules that exhibit intense TADF with positive DEST21–24. We thus hypothesized that appropriate chemical substitutions would lead to heptazine analogues with negative DEST and sufficient f for intense fluorescence. As such, we introduced 186 different substituents to heptazine to generate 34,596 candidate molecules for computational screening. The structures of all substituents are available in Supplementary Fig. 2. To ensure synthetic feasibility, at most two distinct types of substituents were introduced to the heptazine core, R1 and R2 (Fig. 1b). We used standard linear-response time-dependent density functional theory (TDDFT) to calculate DEST and f, which are more affordable in computational cost than those calculated by correlated wave function theories. While the commonly used adiabatic approximation in TDDFT does not account for double-excitation character17,25, the properties calculated by TDDFT are still useful for pre-screening to narrow the list of candidate molecules prior to high-cost calculations and experimental evaluation, as both S1 and T1 of heptazine are almost entirely dominated by the single-excitation configuration between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)16.
Fig. 1c shows the statistics of the screened molecules as a function of DEST and f calculated by TDDFT. A well-known trade-off between small DEST and large f is evident from this particular data set of heptazine analogues. While balancing this trade-off is a key concern in recent synthetic efforts on TADF materials, Fig. 1c demonstrates the optimal combinations of DEST and f, for which one parameter cannot be further improved without sacrificing the other. Fig. 1d further visualizes the trade-off between DEST and f for each fluorescence colour. The screening data suggest 5,264 promising candidates to display fluorescence across the entire visible spectrum with DEST < 0.35 eV and f > 0.01. Setting the range of the S1–S0 energy gap to 2.70–2.85 eV for blue fluorescence further narrows down the candidates to 1,028 molecules, corresponding to 2.97% of all screened molecules. We then assessed their synthetic feasibility and selected two heptazine analogues, HzTFEX2 and HzPipX2 (Fig. 2a), for further evaluation. We note that the unsymmetrical substitutions in these molecules recover f while retaining small DEST (f = 0.010, 0.015 and DEST = 210, 334 meV for HzTFEX2 and HzPipX2, respectively).
To examine whether HzTFEX2 and HzPipX2 could have negative DEST, we computed their S1 and T1 by correlated wave function theories. Equation-of-motion coupled cluster with single and double excitation (EOM-CCSD)26 calculations predict that HzTFEX2 possesses a negative DEST of –12 meV, affirming its potential for exhibiting DFIST. In comparison, the value of DEST calculated for HzPipX2 remains at a positive value of 10 meV, which is comparable to those of the current state-of-the-art TADF materials27–38. Figs. 2b, c show the dominant pair of natural transition orbitals (NTOs)39 for S1 and T1. In both molecules, the hole orbitals are exclusively localized on the peripherical six nitrogen atoms of the heptazine core, whereas the electron orbitals are localized on the central nitrogen atom and the carbon atoms of the core, as well as on the substituents. The spatial separation of these orbitals indicates that the exchange interaction is weak, resulting in nearly degenerate S1 and T1 in the single-excitation scenario. In this situation, the stabilisation of S1 by including the double-excitation configurations becomes more dominant in determining the sign of DEST. Indeed, S1 of both molecules comprises double-excitation configurations with weights of approximately 1%, which are slightly higher than those of T1. Two other wave-function-based calculations using second-order algebraic diagrammatic construction (ADC(2))40 and complete active space with second-order perturbation theory (CASPT2)41 further validate the inversion of S1 and T1 in HzTFEX2 with calculated DEST values of –34 meV and –184 meV, respectively. However, for HzPipX2, the two methods also invert DEST (–12 meV with ADC(2) and –171 meV with CASPT2) compared to the positive value of 10 meV predicted with EOM-CCSD (Supplementary Table 1). This variation in the estimates of DEST highlights the current limitations of excited-state calculations and demands conclusive experimental evaluation.
HzTFEX2 and HzPipX2 were synthesised by nucleophilic aromatic substitution of 2,5,8-trichloroheptazine with the corresponding alcohol or amine, followed by Friedel–Crafts reactions with m-xylene. The details of the synthesis and characterisation are given in the Supplementary Information. The photophysical properties of the two molecules were evaluated in deaerated toluene solutions (Fig. 3a, Extended Data Table 1). The steady-state absorption spectra of HzTFEX2 and HzPipX2 comprise the lowest-energy absorption bands centred at 441 nm and 429 nm, respectively, with small molar absorption coefficients on the order of 103 M–1 cm–1, reflecting the spatial separation between the hole and electron NTOs computed for S1 of each molecule. Upon photoexcitation, HzTFEX2 exhibits blue emission with a peak wavelength (lPL) of 449 nm and a PL quantum yield (FPL) of 74%, while a slightly blueshifted lPL of 442 nm and a similar FPL of 67% are observed for HzPipX2. These energy differences in absorption and emission are also predicted by TDDFT calculations and are attributed to the piperidyl group in HzPipX2 having a stronger electron-donating effect than the 2,2,2-trifluoroethoxy group in HzTFEX2. In aerated toluene solutions, the FPL values of HzTFEX2 and HzPipX2 decreased to 54% and 37%, respectively. Since atmospheric O2 can quench molecular triplet excited states, we ascribe the blue emissions of the two molecules at least partially to delayed fluorescence through forward intersystem crossing (ISC) and RISC between S1 and T1. This assumption is supported by transient absorption decay measurements on HzTFEX2, which probed ISC from S1 to T1 as the signal decay of S1 at 700 nm and the signal growth of T1 at 1,600 nm, followed by the persistent signal decays of both S1 and T1 (Extended Data Fig. 1). We also note that both decays have similar time constants (223 ns for S1 and 210 ns for T1), indicating steady-state conditions with the constant population ratio maintained by ISC and RISC.
To reveal the excited-state kinetics of the two molecules in detail, we performed transient PL decay measurements at varying temperatures (Figs. 3b, c). Both molecules exhibited biexponential transient PL decays, which comprise nanosecond-order prompt fluorescence followed by sub-microsecond delayed fluorescence with temperature-dependent time constants. Remarkably, the time constant of delayed fluorescence (tDF) of HzTFEX2 gradually decreased from 217 ns to 195 ns with decreasing temperature from 300 K to 200 K (Fig. 3d). This anomalous temperature dependence of tDF indicates that S1 lies energetically below T1, where lowering the temperature shifts the steady-state population towards emissive S1 relative to dark T1 and thus accelerates the delayed fluorescence (i.e., decreases tDF). In comparison, the tDF of HzPipX2 increased from 565 ns to 1,372 ns during the same temperature lowering, as has been similarly observed in conventional TADF materials5–7. It is worth noting that the tDF of HzTFEX2 is much shorter than the emission time constants ever reported for TADF materials27–38 and phosphorescent materials2–4 used for efficient OLEDs, which are typically in the microsecond range.
We further analysed the temperature-dependent PL decay kinetics with the underlying rate equation. In the absence of phosphorescence and non-radiative decay of T1 to S0, the rate equation for the populations of S1 and T1 is given by
where kr, knr, kISC, and kRISC are the rate constants of radiative decay of S1 to S0, non-radiative decay of S1 to S0, ISC of S1 to T1, and RISC of T1 to S1, respectively. By numerically fitting Eq. (1) to the PL decay data at 300 K, we found that RISC was faster than ISC in HzTFEX2 (kRISC = 4.2 ´ 107 s–1 vs kISC = 2.3 ´ 107 s–1) but slower than ISC in HzPipX2 (kRISC = 2.2 ´ 107 s–1 vs kISC = 8.9 ´ 107 s–1) (Figs. e, f). Furthermore, the temperature dependence of kISC and kRISC follows the Arrhenius equation, k = A exp(–Ea/kBT), where k is the rate constant, A is the pre-exponential factor, Ea is the activation energy, kB is the Boltzmann constant, and T is the absolute temperature (Extended Data Fig. 2). The best-fit parameters of the Arrhenius equation yield the activation energies of ISC and RISC (Ea,ISC and Ea,RISC) (Extended Data Table 1). Subtracting Ea,ISC from Ea,RISC, we determined the DEST of HzTFEX2 to be –11 meV, which is in marked contrast to the positive DEST values previously observed in numerous molecules including HzPipX2 (DEST = 52 meV).
Having experimentally determined negative DEST, we conclude that HzTFEX2 exhibits ‘delayed fluorescence from inverted singlet and triplet excited states (DFIST)’. Further synthetic efforts replacing the xylyl groups in HzTFEX2 with either phenyl or tolyl groups led to HzTFEP2 and HzTFET2, which similarly display DFIST with measured DEST values of –14 meV and –13 meV, respectively (see Extended Data Table 1 and Supplementary Fig. 3 for detail), indicating the potential of heptazines for further developing efficient DFIST materials. Thus, we propose to refer to the present type of emissions as ‘H (heptazine)-type delayed fluorescence’ by analogy with ‘E (eosin)-type delayed fluorescence’ referred to as TADF42 and ‘P (pyren)-type delayed fluorescence’ involving triplet–triplet annihilation43.
Finally, we evaluated the electroluminescence (EL) properties of HzTFEX2 in OLEDs fabricated by thermal evaporation. The details of the fabrication procedures and the device structures are given in the Supplementary Information. Figs. 4a and b show the EL spectra, current density–voltage–luminance characteristics, and external quantum efficiency–luminance characteristics of the OLED. Intense blue EL originating from HzTFEX2 was observed with spectral peak wavelengths (lPL) at 450 nm and 479 nm and Commission Internationale de l¢éclairage (CIE) coordinates of (0.17, 0.24). The maximum external quantum efficiency reached 17%, corresponding to an internal quantum efficiency of 80% for a bottom-emission OLED with a typical light-outcoupling efficiency of 20%44. We note that the viewing-angle dependence of the luminance followed the Lambertian distribution (Fig. 4b inset), ensuring accurate estimation of the external quantum efficiency from the forward emission. Remarkably, HzTFEX2 exhibited fast transient EL decay, reflecting the sub-microsecond H-type delayed fluorescence (Fig. 4c). In comparison, much slower transient EL decays were observed for the E-type delayed fluorescence of 2,4,5,6-tetra(carbazol-9-yl)isophthalonitrile (4CzIPN)6 and P-type delayed fluorescence of 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN)45, though the EL of MADN initially decayed faster by the prompt fluorescence solely from S146. It is thus evident that the fast triplet harvesting of HzTFEX2 with negative DEST can be retained even in actual OLEDs. Although the efficiency roll-off in this preliminary device concerning the large hole-injection barrier caused by the high ionization potential of HzTFEX2 (6.3 eV) is still significant, we anticipate that further optimization of molecular design will address this issue and allow a conclusive exploration of the effects of negative DEST on efficiency roll-off and device stability.
In conclusion, we have demonstrated fluorescent heptazine molecules that possess negative DEST. We observed their blue delayed fluorescence in both PL and EL with anomalous features: (I) very short decay time constants (tDF ~ 0.2 ms), (II) a decreasing trend in tDF with decreasing temperature, and (III) the rate inversion of RISC and ISC (kRISC > kISC). These features indeed arose from negative DEST and led to the terminology ‘delayed fluorescence from inverted singlet and triplet excited states (DFIST)’ or ‘H (heptazine)-type delayed fluorescence’. We envisage that further development of DFIST materials will offer stable and efficient OLEDs based on fast triplet-to-singlet downconversion with great implications for displays, lighting, and lasers.
Quantum chemical calculations For the 34,596 heptazine molecules, the T1 geometries were optimised using spin-unrestricted DFT with the LC-BLYP functional and the 6-31G basis set. The vertical excitation energies of S1 and T1 were calculated using linear-response TDDFT with the LC-BLYP functional and the 6-31G(d) basis set. The range-separated parameter for the LC-BLYP functional was set to 0.18 bour–1 to incorporate a reasonable amount of exact exchange. The T1 geometries of HzTFEX2 and HzPipX2 were also optimised using spin-unrestricted second-order Møller–Plesset perturbation theory (MP2) with the correlation consistent cc-pVDZ basis set. The vertical excitation energies of S1 and T1 at the MP2 geometries of HzTFEX2 and HzPipX2 were calculated using EOM-CCSD26, ADC(2)40, and CASPT241 with the cc-pVDZ basis set. The CASPT2 calculations were performed with the fully internally contracted scheme over the state-averaged complete active space self-consistent field (CASSCF) wave functions with an active space of 12 electrons and 12 orbitals. The DFT, TDDFT, MP2, and EOM-CCSD calculations were performed using Gaussian 16 RevC.01. The ADC(2) calculations were performed using Q-Chem 5.3.0. The CASSCF and CASPT2 calculations were performed using Orca 4.2.1.
Materials and synthesis Commercially available reagents and solvents were used without further purification unless otherwise noted. 4CzIPN and MADN were purchased from Luminescence Technology Corporation and e-Ray Optoelectronics Technology, respectively. The synthetic procedures and characterization data of the heptazine molecules are detailed in the Supplementary Information.
Photophysical measurements Steady-state UV-vis absorption spectra were recorded on a SHIMADZU UV-3600i Plus spectrophotometer. Steady-state PL spectra were acquired on a HORIBA FL3 spectrofluorometer with 370 nm photoexcitation from a Xe arc lamp. The absolute PL quantum yields were determined using a Hamamatsu Photonics C9920 integrated sphere system with 370 nm excitation from a Xe arc lamp. Transient absorption decay measurements were performed by a randomly interleaved plus train method47 on a UNISOKU picoTAS system with a 355 nm Q-switched laser pump source (pulse width < 350 ps) and a supercontinuum white probe source (pulse width < 100 ps). Transient PL decay measurements were performed by time-correlated single photon counting on a HORIBA FL3 spectrofluorometer with a 370 nm LED pump source (pulse width < 1.2 ns) and a UNISOKU CoolSpek cryostat using liquid nitrogen as the coolant. Ionization potentials were determined using a RIKEN KEIKI AC-3 ultraviolet photoelectron yield spectrometer.
Analysis of transient PL decay kinetics The time constants of prompt and delayed fluorescence (tPF and tDF) were determined by biexponential decay fitting and deconvolution with the instrument response function. It is common when determining the rate constants of the transitions involved in TADF to assume kISC >> kRISC such that the contribution of RISC to the prompt fluorescence is negligible48. However, this assumption does not hold true for DFIST materials with kRISC > kISC. Thus, kr + knr, kISC, and kRISC were determined without assuming kISC >> kRISC by fitting Eq. (1) to the transient PL decay data using the scipy.integrate.odeint and scipy.optimize.curve_fit functions in Python 3.749. The values of kr and knr were determined from FPL = kr/(kr + knr), assuming negligible non-radiative decay of T1 to S0. The activation energies of ISC and RISC (Ea,ISC and Ea,RISC) were determined by fitting the Arrhenius equation to the temperature dependence of kISC and kRISC, respectively. DEST was determined by subtracting Ea,ISC from Ea,RISC.
OLED fabrication and evaluation The fabrication procedures of OLEDs are detailed in the Supplementary Information. EL spectra were recorded using a Hamamatsu Photonics PMA-12 photonic multichannel analyser. Current density–voltage–luminance characteristics were measured using a Konica Minolta CS-200 luminancemeter and a Keithley 2400 source meter. The viewing-angle dependence of luminance was measured using a home-built spectrogoniometer with a Konica Minolta CS-2000 spectroradiometer. Transient EL decay measurements were performed using a home-built setup with a Hamamatsu Photonics H7826 silicon photomultiplier tube (time response =1.5 ns) and an Agilent 33220A function generator for pulse OLED operation (square-wave voltages = 8, –4 V and frequency = 2 kHz)50.
Acknowledgements This work was supported in part by an Industrial Technology Research Grant for Young Researchers from NEDO (Grant No. 09151455 for Y.-J.P.), JST PRESTO (Grant No. 13417316 for Y.-J.P. and JPMJPR17N1 for N.A.), and JSPS KAKENHI (Grant No. 24685029, 17H03103, 20H02554 for Y.-J.P. and 20K15252 for N.A.). The authors thank Dr. Minjun Kim at RIKEN CEMS, Prof. Takayuki Chiba, and Prof. Masakatsu Hirasawa at Yamagata University for their support in OLED fabrication and evaluation. The computations were partially performed using the HOKUSAI Big Waterfall system at RIKEN.
Author contribution N.A. and D.M. conceived the project. N. A. and Y.-J.P. performed the quantum chemical calculations. N. A., H.I., and Y.-J.P. performed the photophysical measurements. N.A., Y.K., and Y.-J.P. fabricated and evaluated the OLEDs. B.D., R.I., H.I., and A.N. performed synthetic experiments and assisted with the characterisation of the synthesised compounds and the analysis of data. N.A., Y.-J.P., F.A. and D.M. designed the experiments, and analysed the data. N.A., Y.-J.P., and D.M. wrote the manuscript.
Competing interests The authors declare no competing interests.
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There is NO Competing Interest.
- AizawaNatureESI.pdf
Supplementary information
- ExtendedData.pdf