Nature of the Lowest Singlet and Triplet Excited States of Organic

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The Nature of the Lowest Singlet and Triplet Excited States of Organic Thermally Activated Delayed Fluorescence Emitters: A Self-Consistent Quantum Mechanics/Embedded Charge Study Zeyi Tu, Guangchao Han, Taiping Hu, Ruihong Duan, and Yuanping Yi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00824 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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Chemistry of Materials

The Nature of the Lowest Singlet and Triplet Excited States of Organic Thermally Activated Delayed Fluorescence Emitters: A SelfConsistent Quantum Mechanics/Embedded Charge Study Zeyi Tu,†,‡ Guangchao Han,† Taiping Hu,†,‡ Ruihong Duan,†,‡ and Yuanping Yi*,†,‡ †

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of

Organic Solids, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡

*

University of Chinese Academy Sciences, Beijing, 100049, China

Corresponding author. E-mail: [email protected].

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ABSTRACT Thermally activated delayed fluorescence (TADF) is dictated by the properties of the lowest singlet (S1) and triplet (T1) excited states. Both small energy difference (ΔEST) and large spin-orbit coupling (SOC) between S1 and T1 are desired to increase the rate for reverse intersystem crossing (RISC). In this work, we have investigated the groundand excited-state electronic properties of three representative D-(π)-A type TADF molecules in solid phase by means of a self-consistent quantum-mechanics/embeddedcharge (QM/EC) approach, which consists of a series of iterative QM/EC single-point computations to account for the solid-state polarization effect. The results show that, unless the D and A units are perpendicular to each other, both the S1 and T1 states are characteristic of mixed charge transfer (CT) and local excitation (LE). Thereby, the ΔEST values are relatively large in gas phase. Importantly, the CT contribution is relatively larger in the S1 state than in the T1 state, thus the S1 energy is more stabilized by electronic polarization, leading to smaller ΔEST in solid phase. At the same time, the SOC can be considerable due to the difference in the nature of the S1 and T1 states. These results shed light on the origin of fast RISC in efficient organic TADF systems.

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INTRODUCTION Organic light-emitting diodes (OLEDs) have demonstrated practical applications in displays for smartphones and televisions due to highly efficient electroluminescence, mechanical flexibility, lightweight, as well as possibility of solution processing and low-cost manufacture.1-4 The third-generation OLEDs based on thermally activated delayed fluorescence (TADF) have attracted a great deal of attention owing to the capability to utilize both singlet and triplet excitons, which allows the charge-to-photon internal quantum efficiency (IQE) of the devices to reach nearly 100%.5-7 The key process for TADF is up conversion of the lowest triplet excited state (T1) to the lowest singlet excited state (S1) through reverse intersystem crossing (RISC). Therefore, the energy difference between the S1 and T1 states (ΔEST) needs to be minimized. To this end, organic TADF emitters are usually designed as electron donor-acceptor (D-A) structures that can separate the wavefunction distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) to reduce the exchange energy. This molecular design strategy relies on the assumption that both the S1 and T1 states are dominated by the HOMO→LUMO transition. However, if this is the fact, the S1 state will have a small oscillator strength and low fluorescence radiative rate, and in the meantime the spin-orbit coupling (SOC) between S1 and T1 will be very weak due to the similar transition characteristics,8-11 and the RISC rate should be also limited. On the other hand, some works have reported predominant LE characteristics in the T1 state.9,10,12 Thus, the microscopic mechanisms of efficient RISC are still controversial for organic TADF. 3



To deepen the understanding of the RISC process in organic TADF systems, it is highly desirable to obtain a reliable description of the excited-state electronic structures in solid state. It has been found by many experiments that the solid-state environment can strongly impact the photophysical properties of π-conjugated organic molecules. Rajamalli et al observed much higher photoluminescence quantum yields (PLQY) in thin films than in cyclohexane solutions for two benzoylpyridine-based TADF emitters bearing two carbazolyl or t-butyl carbazolyl groups as the electron-donating units.13 Santos et al and Xie et al pointed out that optimization of the emitter and host combination is important to minimize the RISC barrier and thus maximize TADF.14,15 Cotts et al reported that the lifetimes of both prompt and delayed fluorescence as well as the PLQY can be finely tuned by solid-state solvation.16,17 In contrast, the influence of solid-state environment is relatively less investigated in the theoretical studies of organic TADF, especially in a polarizable way. Since TADF molecules are usually featured with CT transitions, the molecular electric dipole moment of the S1 state is much larger than that in the ground state and thus the polarization effect becomes very significant and is strongly affected by the environment.18 Sun et al showed that the environmental polarization described by dielectric constant can make the S1 and T1 states dominated by charge-transfer excitations and result in a substantial reduction in ΔEST.19 Olivier et al found that both the S1 and T1 states have mixed CT/LE characters but the S1 state exhibits more dynamic and larger CT component, which is beneficial to increase the probability of small ΔEST.20 These calculations are very impressive, but the

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former does not provide an explicit atomistic description of the environment effect and the latter is lack of considering the impact of polarization effect on excitation characters. In this contribution, to gain insight into the influence of solid-state environment on the RISC process of organic TADF emitters, we have investigated the electronic structures and polarization energies of the lowest singlet and triplet excited states by a selfconsistent quantum-mechanics/embedded-charge (QM/EC) approach. For this approach, the atomic charges of the emitter molecule and the environmental molecules can be redistributed by a series of iterative QM/EC single-point computations and thus the electronic polarization effect is explicitly taken into account in a polarizable way. Recently, the QM/EC approach has been successfully applied to evaluate the chargetransport levels in both organic molecular crystals and amorphous materials.21-23 Here, this approach is further used to characterize the electronic excitation in a polarizable solid-state environment. We have calculated the ground-state and excited-state electronic-structure properties in both gas phase and crystal phase for three representative organic TADF emitters, i.e., TXO-TPA24, TPA-QCN25 and PXZ-TRZ26 (see Figure 1), which exhibit planar D-A, planar D-π-A, and orthogonal D-A structures, respectively. Although the TADF emitters usually have to be doped in host materials for the TADF OLED devices, the primary reason is to reduce the concentration of triplet excitons and thus to suppress triplet-triplet annihilation rather than to modulate the electronic-structure properties of the emitters. In addition, dipolar molecules are often used as ambipolar host materials,27,28 and TADF molecules themselves can act as host materials for phosphorescent OLEDs.29,30 Therefore, considering that our main goal 5



here is to reveal the different nature of the lowest singlet and triplet excitations in the TADF emitters and resultant changes in energy alignments from gas to solid phase, we have performed calculations on model systems extracted from the emitter crystal structures to reduce the complexities and computational costs caused by numerous molecular conformations and packing morphologies in doped films. Our calculations point to that both the S1 and T1 states are characteristic of hybrid charge-transfer and local excitation; thereby the ΔEST is relatively large in gas phase. However, because of the larger CT contribution, the S1 state exhibits stronger polarization effect with respect to the T1 state in the solid crystal phase, thus the ΔEST is substantially decreased.

Figure 1. Chemical structures of the studied TADF molecules exacted from the experimental crystals. The donor (electron-rich), acceptor (electron-deficient), and πbridge units are shown in blue, red, and black, respectively. The torsion angles between the donor and acceptor units or between the π-bridge and the donor [acceptor] unit are illustrated.

METHODOLOGY The ground and excited states are calculated by density functional theory (DFT) and time-dependent DFT (TDDFT) with the M06-2X functional31 and def2-SVP basis set32, 6


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respectively. For gas phase, the calculations are performed on a single molecule extracted from the experimental crystals.24-26 To account for the electronic polarization effect in solid phase, self-consistent QM/EC calculations of the ground and excited states are carried out on a cluster extracted from the experimental crystals. The selfconsistent procedure consists of iterative QM/EC single-point computations taking each molecule as the QM part in the presence of atomic point charges of the other molecules (see Figure S1). In the first QM/EC calculation, the central molecule (i.e. the emitter) is treated at the DFT (TDDFT) level for the ground (excited) state, in the presence of the atomic charges of the surrounding molecules that are initialized by DFT calculations on the isolated molecules. Subsequently, the atomic charges of the central molecule are fitted with the Merz-Singh-Kollman scheme to reproduce the QM/EC derived electrostatic potential.33 Then, the QM/EC calculation is performed consecutively and repeatedly for each molecule with the atomic charges of other molecules updated from the previous QM/EC runs, until the total energy change is smaller than 10-5 Hartree between two adjacent QC/EC calculations for the central molecule. Through this self-consistent QM/EC approach, the electronic states can be obtained in a “state-specific” way,34 which has been proved to be necessary, especially for the CT excitations.35,36 All these calculations are carried out by using the Gaussian 16 package.37 The cluster radius is set to be as large as 60 Å to account for the long-range Coulomb interaction, and there are total 1553, 1432, and 1500 molecules for TXO-TPA, TPAQCN, and PXZ-TRZ, respectively. To reduce the computational cost, only the 7



molecules within a cutoff distance around the emitter are allowed to be polarized, namely, to be involved in the self-consistent procedure, considering the fact that the dipole moment induced polarization diminishes rapidly with distance38 and that we are primarily interested in the feedback effect of the induced polarization on the emitter.39 After careful tests, the excitation energies are converged at 40 Å for the polarizable radius (see Figure S2), with 464, 432 and 439 molecules included for TXO-TPA, TPAQCN, and PXZ-TRZ, respectively. To elucidate the excited-state transition characters, the hole and electron densities, the overlap (𝑂ℎ,𝑒 ) between hole and electron wave functions, and the distance (∆𝑟) between the centroids of hole and electron are obtained by the Multiwfn program.40 The SOC matrix elements between singlet and triplet states are calculated in the framework of the single-particle Breit-Pauli (BP) operator with an effective charge approximation,41 as implemented in the PySOC program.42,43 To further verify the calculated excitedstate properties, TDDFT calculations at the Tamm-Dancoff approximation (TDA) 44 are also performed for the molecules in gas phase with the ωB97X functional45 and def2SVP basis set; in these calculations, the range-separation parameter ω is optimally tuned by minimizing the value of 𝐽2 ,46,47 1 2

𝐽 = ∑[𝜀H (𝑁 + 𝑖) + IP(𝑁 + 𝑖)]2 𝑖=0

where IP and 𝜀H denote the ionization potential and the HOMO energy of a given molecule, respectively; N is the number of electrons in the neutral molecule. Hereafter,

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the tuned range-separated functional is labeled as ωB97X*. This method has been widely used to calculate the excited-state properties of organic TADF molecules.9,46

RESULTS AND DISCUSSION Nature of the S1 and T1 States. Figure 2a displays the TDDFT-M06-2X/def2-SVP calculated hole and electron densities for the lowest singlet and triplet excited states in gas phase, which agree very well with those calculated at the TDA-ωB97X*/def2-SVP level (see Figure S3). We note that the energy differences between the S1 and S2 states in TXO-TPA are very small (ca. 0.02 eV). Also, similar energies are found for the T1 and T2 states in PXZ-TRZ, which is consistent with the results in toluene calculated by Samanta et al at the TDA-LC-ωPBE*/6-31+G(d) level combined with the polarizable continuum model (PCM).9 Like most of TADF molecules, the S1 state shows remarkable electron transfer from the donor unit to the acceptor unit and non-negligible locally-excited component. The overlap between hole and electron wave functions for the S1 state is 0.30, 0.29 and 0.05 in TXO-TPA, TPA-QCN and PXZ-TRZ, respectively. The hybrid CT/LE character can enhance the oscillator strengths and fluorescence rates, especially in TXO-TPA and TPA-QCN (see Table 1). In contrast, although T1 is also a CT/LE hybridized state, the LE component is obviously larger with the overlap between hole and electron wave functions reaching 0.51, 0.52, and 0.16 in TXO-TPA, TPA-QCN, and PXZ-TRZ, respectively.

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Figure 2. Hole (in yellow) and electron (in purple) density distribution for the lowest singlet and triplet excited states calculated in gas phase (a) and solid state (b). Excitation energies and overlap between hole and electron wave functions are also indicated.

Table 1. Oscillator Strengths of the S1 State (f), Energy Differences (ΔEST) and SpinOrbit Couplings (SOC) between the S1 and T1 States Calculated in Gas Phase and Solid State. Gas Solid -1 f f ΔEST (eV) SOC (cm ) ΔEST (eV) 0.093 0.57 2.89 0.082 -0.05 TXO-TPA a a a 0.154 0.59 2.81 1.049 0.59 0.22 0.540 0.12 TPA-QCN 0.005 0.02 0.57 0.005 0.01 PXZ-TRZ b b -0.02 1.12 a b Results related to the S2 state; Results related to the T2 state.

For TXO-TPA and TPA-QCN, the molecular twist between the donor and acceptor units is relatively small, the π-conjugation is quite extended, especially in TPA-QCN that has a phenyl bridge (see Figure S4). This is beneficial to enhance the overlap between hole 10


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and electron and the LE component in the S1 and T1 states. Therefore, large ΔEST values of 0.57 and 0.59 eV are found for TXO-TPA and TPA-QCN, respectively. On the contrary, the perpendicular structure of PXZ-TRZ inhibits the conjugation between the donor and acceptor moieties, making the S1 and T1 states dominated by charge-transfer transition; thus the ΔEST of PXZ-TRZ gets very small (0.02 eV), but the oscillator strength of the S1 state nearly vanishes.48 Consistent with the results reported by Samanta et al,9 the calculated spin-orbit coupling between S1 and T1 is considerable for the three compounds. This can be attributed to the essential difference in the CT component between S1 and T1. Figure 2b shows the hole and electron densities as well as the excitation energies of the lowest singlet and triplet excited states in the solid crystals. The calculated excitation energies compare well with the available experimental values measured from the first absorption peak of the thin films (2.76 and 2.48 eV for TXO-TPA and TPA-QCN, respectively24,25). Compared with those in gas phase, the overlaps between hole and electron wave functions for both S1 and T1 get smaller in solid state, and the oscillator strengths of the S1 state are decreased. This is in agreement with the results calculated by Sun et al at the TDA PCM-tuned-ωB97X*/cc-pVDZ level.19 Nonetheless, the S1 and T1 states still have an important LE component and the LE contribution is larger in the T1 state relative to the S1 state for TXO-TPA and TPA-QCN in solid state. Despite of the difference in the transition nature of the S1 and T1 states, the ΔEST is substantially decreased to -0.05 and 0.12 eV in solid state from 0.57 and 0.59 eV in gas phase for TXO-TPA and TPA-QCN, respectively. At the same time, the SOC between S1 and T1 11



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should be considerable due to their different transition characters according to the ElSayed’s rule.8,11 This rationalizes the fast RISC for efficient TADF in TXO-TPA and TPA-QCN. For PXZ-TRZ in solid state, complete charge transfer is found for both the S1 and T1 states and thereby the SOC can be negligible. However, the T2 state may contribute to the RISC process due to the large SOC with the S1 state.9 Besides, it is noteworthy that both the S1 and S2 states for TXO-TPA have significant nπ* transitions in gas phase. However, in solid phase, the S1 and S2 states mainly correspond to ππ* and nπ* transitions, respectively, and their energy gap is increased to 0.97 eV from 0.02 eV in gas phase. In the case of PXZ-TRZ, different from the charge-transfer nature of T1, T2 is dominated by local excitation on the PXZ donor unit, and the energy gap between them is increased to 0.29 eV in solid phase from 0.04 eV in gas phase. These results indicate that the solid-state polarization effects on the excited states are strongly dependent on the transition nature and the transition nature can be also changed by polarization effects.

Table 2. Polarization Energies (𝑃 ) and the Electrostatic and Induction Components (𝑃𝑒𝑙𝑒𝑐 and 𝑃𝑖𝑛𝑑 ) for the S1 and T1 Excitation (in Unit of eV).

TXO-TPA TPA-QCN PXZ-TRZ

𝑷 0.95 1.02 0.28

S1 𝑷𝒆𝒍𝒔𝒕 0.04 0.13 -0.17

𝑷𝒊𝒏𝒅 0.91 0.89 0.45

𝑷 0.31 0.55 0.26

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T1 𝑷𝒆𝒍𝒔𝒕 0.03 0.06 -0.15

𝑷𝒊𝒏𝒅 0.29 0.49 0.41

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Polarization Energies of the S1 and T1 States. As seen in Figure 2, the solid-state polarization induced decrease of ΔEST comes from the larger excitation energy reduction of the S1 state with respect to the T1 state. Similar to the definition of the polarization energy of charge carrier,49 the difference between the excitation energies in gas phase and solid phase is defined as the polarization energy of an exciton (𝑃 = 𝐸𝑔𝑎𝑠 − 𝐸𝑠𝑜𝑙𝑖𝑑 ). The calculated polarization energies of the S1 and T1 excitons are listed in Table 1. We note that the polarization energy of the S1 exciton in TXO-TPA is calculated from the excitation energies of the S2 state in gas phase and the S1 state in solid phase, since the latter is originated from the former. For the S1 excitation, the polarization energies are positive for all the compounds, indicating smaller excitation energies in solid phase than in gas phase. For TXO-TPA and TPA-QCN, the polarization energies are as large as 0.95 and 1.02 eV, respectively; these values are comparable to the polarization energies of charge carriers in organic molecular materials. This can be attributed to the long distance between the hole and electron centroids (7.54 Å for TXO-TPA and 9.77 Å for TPA-QCN). Compared with TXO-TPA and TPA-QCN, PXZ-TRZ exhibits much smaller polarization energy for the S1 exciton (ca. 0.28 eV). For all the compounds, the polarization energy of the T1 exciton is smaller than that of the S1 exciton; especially for TXO-TPA and TPA-QCN, the polarization energy of the T1 exciton is only about one-third or one half of that of the S1 exciton. This agrees with the finding by Olivier et al that the S1 state exhibits broader energy distribution compared to the T1 state.20 In contrast, the polarization energy of the T1 exciton is just slightly smaller than that of the S1 exciton for PXZ-TRZ, 13



due to the similar transition characters of the S1 and T1 states. To summarize, the calculated polarization energies show that the excitation energies of both the S1 and T1 states are decreased from gas phase to solid phase. Moreover, the decrease of the S1 excitation energy is more than that of the T1 excitation energy, especially for planar molecules, leading to smaller ΔEST in solid phase for all the TADF emitters. Thus the TADF efficiency could be strongly affected by the solid-state environment, and the emission color, which is closely related to the S1 excitation energy, can be also dramatically tuned.50

Figure 3. Dipole moments in gas phase and solid phase and induced dipole moments in solid phase for the S1 (a) and T1 (b) states.

Electrostatic and Induction Effects. To elucidate the sources of polarization energy, we partition it into two terms: 𝑃 = 𝑃𝑒𝑙𝑠𝑡 + 𝑃𝑖𝑛𝑑 ,49,51,52 where 𝑃𝑒𝑙𝑠𝑡 represents the electrostatic term (also known as static term) that is calculated from the Coulombic interaction of the so-called permanent charges derived from gas-phase electron density, 14


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and 𝑃𝑖𝑛𝑑 is the induction term (also known as dynamic term) that arises from the induced charge redistribution of both the emitter and the surrounding molecules. The calculated results are listed in Table 2. In general, the 𝑃𝑒𝑙𝑠𝑡 depends not only on the molecular electric multipoles (mainly dipoles for the molecules in this study) but also on molecular packing modes, as de Silva et al pointed out that the diverse packing modes can result in broadened distribution of the density of states in amorphous films due to electrostatic effect.53 For the studied crystals here, TXO-TPA belongs to the 𝑃1̅ space group, and TPA-QCN and PXZ-TRZ belong to the 𝑃21 /𝑐 space group (see Figure S5). All the three crystals have only one inequivalent molecule per unit cell. TXO-TPA exhibits slipped parallel π–π stacking motifs, and the π stacks are arranged in an anti-parallel mode. For TPAQCN, weak anti-parallel π–π stacking is also present and alternate edge to edge contacts are formed due to multipole hydrogen bonding interactions. PXZ-TRZ displays local anti-parallel donor-donor or acceptor-acceptor π-π stacking. For the S1 exciton, the 𝑃𝑒𝑙𝑠𝑡 displays a positive value of 0.04 eV in TXO-TPA and 0.13 eV in TPA-QCN, but a negative value of -0.17 eV in PXZ-TRZ. The opposite 𝑃𝑒𝑙𝑠𝑡 can be attributed to the different directions of the excited-state dipole moments among the emitters. For TXOTPA and TPA-QCN, the excited-state dipole moments are roughly in the same directions as the ground state, the corresponding excitons are likely to be stabilized by the dipole moments of the surrounding anti-parallel molecules as mentioned above, leading to a positive 𝑃𝑒𝑙𝑠𝑡 . On the contrary, the directions of the excited-state and ground-state dipole moments appear to be opposite to each other for PXZ-TRZ, thus 15



the 𝑃𝑒𝑙𝑠𝑡 is negative. This result indicates that the electrostatic term, which is neglected in implicit polarizable models (e.g. PCM), can contribute to the shift and broadening of absorption or emission spectra. The values of 𝑃𝑒𝑙𝑠𝑡 for the T1 exciton are comparable to those for the S1 exciton, suggesting that the different polarization energies for the S1 and T1 excitons are mainly from the induction term. The 𝑃𝑖𝑛𝑑 values are all positive and larger than the absolute values of 𝑃𝑒𝑙𝑠𝑡 . For the S1 exciton, the 𝑃𝑖𝑛𝑑 is as large as 0.9 eV in TXO-TPA and TPA-QCN and 0.45 eV in PXZ-TRZ. The significant stabilization of the S1 exciton could be related to the large molecular dipole moment in the excited states. As depicted in Figure 3, the dipole moments of the S1 state in gas phase are 17.6, 36.3, and 19.0 Debye for TXO-TPA (the S2 state), TPA-QCN and PXZ-TRZ, respectively, much larger than the ground-state dipole moments of 3.5, 10.2, and 3.0 Debye. In the solid crystals, the dipole moments of the S1 state for the three compounds are even increased to 35.2, 51.3 and 21.0 Debye due to enhanced CT characters. Compared with the S1 state, less CT contribution leads to smaller dipole moments in the T1 state, especially for TXO-TPA and TPA-QCN. Thus the 𝑃𝑖𝑛𝑑 for the T1 exciton is substantially decreased to 0.29 for TXO-TPA and 0.49 eV for TPA-QCN while is slightly decreased to 0.41 eV for PXZ-TRZ. By and large, larger excited-state dipole moments in solid state are beneficial to enhance 𝑃𝑖𝑛𝑑 to stabilize the excitons. However, it should be noted that the 𝑃𝑖𝑛𝑑 values can be also related to the dielectric properties and molecular packing structures.54,55

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CONCLUSIONS We have studied the excited-state properties of three representative D-(π)-A structured TADF molecules (TXO-TPA, TPA-QCN and PXZ-TRZ) in both gas phase and solid phase from first-principles calculations. To consider the polarization effect, both the ground and excited states in solid phase are calculated by a self-consistent QM/EC approach. This approach can explicitly take account of the environmental molecules and molecular packing in a polarizable way. The calculated results underline that (i) no matter in gas phase or solid phase, both S1 and T1 are a hybrid CT/LE state unless the donor and acceptor moieties are perpendicular to each other, and the difference in the CT degrees can result in considerable SOC between the S1 and T1 states; (ii) the solidstate polarization energy is larger for the S1 exciton with respect to the T1 exciton due to the more significant CT contribution, which brings about smaller ΔEST in solid phase than in gas phase. Thus, complete charge transfer in both the S1 and T1 states is not a necessary condition to obtain small ΔEST in solid phase. More importantly, large SOC and small ΔEST can be achieved simultaneously by the different nature and polarization effects of the S1 and T1 states. This work would be helpful for in-depth understanding of the RISC process in efficient organic TADF materials.

ASSOCIATED CONTENT Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.xxxxxxx. Schematic description of the self-consistent QM/EC approach; dependence of excitation energies on the radius of polarizable region; hole and electron density distribution for the lowest singlet and triplet excited states calculated at the TDA ωB97X*/def2-SVP level in gas phase; the frontier occupied and unoccupied orbitals and energy levels in gas phase; experimental crystal structures of the emitters; molecular dipole moments of the ground and excited states.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.Y.)

ORCID

Yuanping Yi: 0000-0002-0052-9364

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work has been supported by the National Natural Science Foundation of China (Grant No 91833305), Ministry of Science and Technology of China (Grant No 18


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2017YFA0204502), and Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No XDB12020200).

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Table of contents S1 Solid

S1

T1 QM/EC Gas

T1

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Nature of the Lowest Singlet and Triplet Excited States of Organic

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