Unraveling the Emission Mechanism of Radical-Based Organic Light

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Spectroscopy and Photochemistry; General Theory

Unraveling the emission mechanism of radical based organic light-emitting diodes Chuhuan He, Zhendong Li, Yibo Lei, Wenli Zou, and Bingbing Suo J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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The Journal of Physical Chemistry Letters

Unraveling the emission mechanism of radical based organic light-emitting diodes Chuhuan He,† Zhendong Li,∗,‡ Yibo Lei,¶ Wenli Zou,† and Bingbing Suo∗,† †Shaanxi Key Laboratory for Theoretical Physics Frontiers, Institute of Modern Physics, Northwest University, Xi’an, Shaanxi 710027, China ‡Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China ¶Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, Northwest University, Xi’an, Shaanxi, 710027, China E-mail: [email protected]; [email protected]

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Abstract Stable doublet radical molecules have recently emerged as a promising new type of emitters in organic light-emitting diodes (OLEDs) to approach 100% internal quantum efficiency. However, the detailed emission mechanism of these open-shell emitters remains elusive. Through theoretical model analysis and first-principle calculations, we unraveled the emission mechanism of a typical emitter, (4-N-carbazolyl-2,6dichlorophenyl)bis(2,4,6-trichlorophenyl)methyl (TTM-1Cz). Our study showed that the electroluminescence arises from the first doublet excited state generated by injecting one electron into the singly occupied molecule orbital (SOMO) and one hole into the highest doubly occupied molecule orbital (HDMO). Due to the distinct charge transfer rates in charge-injection processes, the puzzle of 100% formation ratio of the emissive doublet exciton in experiments is revealed. Based on these understandings, we proposed simple molecular designs via substitutions that can tune the HDMO-SOMO gap and hence shift the emission wavelength to the region of yellow and green light.

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The development of Organic Light-Emitting Diodes (OLEDs) has attracted wide attentions due to their advantages such as low-energy cost, lightweight, high-quality color, wide viewing angle and flexibility. 1–10 In conventional fluorescent OLEDs, however, most of the energies are wasted due to the ratio of light emissive singlet and non-emissive triplet excitons in electrical excitation being 1:3, which limits the internal quantum efficiency (IQE) to 25%. In order to enhance the electroluminescent efficiency, plenty of studies endeavored to harvest the triplet exciton. 3,9–11 Contrary to the efforts that focused exclusively on closed-shell molecules, Peng et. al. 12 recently proposed a new type of fluorescent material based on neutral radicals, where a chemical stable radical (4-N-carbazolyl-2,6-dichlorophenyl)bis(2,4,6trichlorophenyl)methyl (TTM-1Cz) was successfully used as the emitter in OLED. This work was followed up by the successful proposal of a series of new luminescent radicals as emitters. 13–18 In particular, Ai et al. 16 have achieved the maximum external quantum efficiency (EQE) of as large as 27% in a new radical-based OLED, indicating that the 100% quantum yield of the emissive exciton has been obtained. It is believed that the spontaneous radiation in radical arises from the spin-allowed excited doublet states and thus could circumvent the triplet harvesting problem in the conventional materials. However, to the best of our knowledge, there is no elaborate theoretical and computational work supporting this assumption and revealing the detailed luminescent mechanism. The major goal of this work is to unravel the emission mechanism of TTM-1Cz through both theoretical model analysis and first-principle calculations. Theoretical model analysis. The ratio of the singlet and triplet excitons in a closed-shell molecule has been quite well understood, but the formation of excitons and their ratios in radical molecules are still unsettled. 12,13,19 In their first experimental work on the radical emitter, Peng et al. 12 assumed that the open-shell orbital splits into a singly occupied molecular orbital (SOMO) and a singly unoccupied molecular orbital (SUMO), and the charge injection removes an electron from SOMO and attaches an electron into SUMO, see Scheme A of Figure 1, which gives rise to a SUMO→SOMO transition in emission. Moreover, the 3



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orbital energies for SOMO and SUMO from an unrestricted Kohn-Sham (UKS) calculation were used to support their assumption. 12 However, we should point out that although the energy gap between SOMO and SUMO with a particular functional agrees well with the energy of the first absorption peak of TTM-1Cz, this UKS-based picture is theoretically incorrect, because the energy splitting of the open-shell orbital is artificial due to the broken of spin symmetry.

Figure 1: Two schemes of excitons in a doublet state. In fact, the excitons in radical systems should be more appropriately described by scheme B in Figure 1. In this spin-restricted picture, which correctly handles the spin symmetry in a doublet reference, the injection of electron-hole pairs can yield three different holeelectron distributions depending on the involved orbital pairs. Both the excitons (I) and (II) lead to doublet excited states, which are spin-allowed to transit back to the double ground state. The last type of exciton (III) containing three singly occupied orbitals can yield three states, viz., two different doublet states (denoted by Da and Db ) as well as one quartet state Q. 20,21 For instance, the MS = 1/2 configurations can be written as Da =

√1 (Ψa i 2

+ Ψ¯ai¯ ), Db =

√1 (−Ψa i 6

+ Ψ¯ai¯ + 2Ψ¯tit¯a ), and Q =

√1 (Ψa i 3

− Ψ¯ai¯ + Ψ¯tit¯a ), where

D0 = Ψ = |i¯iti represents the ground state, i, t, and a label HDMO (highest doubly occupied molecular orbital), SOMO, and LUMO (lowest unoccupied molecule orbital), respectively, 4


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and the label without (with) bar i (¯i) represents α (β) spin orbital. It can be seen that Da √ is a bright state if hD0 |~r|Da i = 2hi|~r|ai 6= 0, whereas Db always has a vanishing transition dipole hD0 |~r|Db i = 0, similar to the quartet state Q with hD0 |~r|Qi = 0, since it arises from the so-called triplet-coupled single excitation. 20,21 Only the doublet arising from the singlet-coupled single excitation may contribute to luminescence, being similar to the singletsinglet emission in the closed-shell case. Therefore, assuming that only one type of exciton in Figure 1 may take place, the IQE can reach up to 100% for the types (I) and (II) but merely 25%(=

2 ) 2+2+4

for the type (III). Note that Ref. 19 neglected the fact that Db is a dark state

and predicted the IQE to be 50%(=

2+2 ) 2+2+4

for the type (III). In practice, whether mixed

types of excitons can be generated simultaneously depends on the energy levels of specific doublet molecules as well as the charge transfer rates, and hence the theoretical IQE may range from 25% to 100% depending on the mixture. The two new experiments on the radicalbased OLEDs have clearly demonstrated that the IQE of the doublet exciton may exceed 50%,, 13,16 which inspires us to investigate the emission mechanism of TTM-1Cz in details to understand why the quantum yield of the emissive doublet could be so high. In this work, we investigate the electronic structure of TTM-1Cz by density functional theory (DFT) and its excited states by time-dependent density functional theory (TDDFT) to examine the energy scales of different types of excitons. Apart from the unclear emission mechanism of TTM-1Cz, the emission wavelengths of the present radical-based materials lie in the deep-red region, 12–16,22 which hinders its applications in a wider range. Therefore, it is highly desirable to look for new radicals with broader emission wavelengths while maintaining the chemical stability. Based on the knowledge obtained from theoretical calculations, we will propose a molecular design strategy to sieve the derivatives of TTM-1Cz. Four TTM-1Cz’s derivatives with strong emissions in the region between orange and green are proposed for future study. Absorption and emission spectra. Since the unrestricted TDDFT (U-TDDFT) for openshell molecules may suffer from the spin-contamination problem in excited states, the explic5



itly spin-adapted TDDFT (X-TDDFT) 20,21,23–26 is performed first and compared with the U-TDDFT results to check whether the spin-contamination problem is relevant for TTM1Cz. Several exchange-correlation (XC) functionals have been tested, and finally PBE0 27 is adopted since it can well reproduce the experimental spectra (see Sec. 1 in the supporting information (SI)). All these calculations were carried out using the Beijing Density Functional (BDF) package. 28–30 In Figure 2, we present the absorption spectra of TTM-1Cz in chloroform solution by X-TDDFT and U-TDDFT with the 6-31G* basis set, 31,32 and the natural transition orbitals (NTO) of the first excited state D1 are also shown. Table S4 in SI collects the X-TDDFT results for doublet excited states with oscillator strength (f ) larger than 0.1, absorption wavelengths (λ), and dominant electronic excitations. As shown in Figure 2(a), the absorption peak at 596nm arises from the transition to the D1 state of TTM-1Cz. While most of high-lying states are heavily spin-contaminated, this lowest excited state is almost free from spin-contamination and should be well described by both X-TDDFT and U-TDDFT (detailed discussions of spin-contamination can be found in Sec. 2 of SI). Compared with X-TDDFT, U-TDDFT slightly overestimates the vertical excitation energy (VEE) and blue shifts the absorption peak by about 27nm. As illustrated in Table S5 in SI, the D1 state can be perfectly described by the 171 → 172 excitation, where the 171-th orbital is the HDMO and the 172-th orbital is the SOMO. In other words, the D1 state belongs to the type (I) exciton of Scheme B in Figure 1. Such a finding is inconsistent with the assumption of Peng et al. 12 in which a nonphysical SOMO to SUMO transition was proposed. In Figure 2(c) and 2(d), the hole NTO of this state mainly locates at the carbazole moiety (see Figure S3 in SI), while the particle NTO distributes on three chlorine-substituted benzene rings, revealing that D1 is a charge-transfer state. In order to study the emission spectra of TTM-1Cz, the molecular structures of the D1 state in different environments were optimized using U-TDDFT by Gaussian 09, 33 where the solvent effects were considered via the polarizable continuum model (PCM). 34 At the optimized structures, the emission wavelengths, and oscillator strengths of D1 are obtained, 6


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(a)

(b)

(c)

(d)

Figure 2: Absorption spectra of TTM-1Cz in chloroform solution calculated by X-TDDFT (a) and U-TDDFT (b) with the PBE0 functional and natural transition orbitals (NTO) of the first excited state (c,d). The experimental absorption spectra was taken from Ref. 12 for comparison. (a) Absorption spectra from X-TDDFT/PBE0; (b)Absorption spectra from U-TDDFT/PBE0; (c) The hole NTO; (d) The particle NTO.

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and the radiation rates can be estimated via the Einstein equation kr =

2 f (∆E)2 . 35,36 3

In Table 1, we present the main geometric parameters, emission wavelengths, oscillator strengths, the radiation rates kr , as well as the available experimental data for comparison. Table 1: Main dihedral angles (in degree) of the D1 and D0 (values in parenthesis) states, emission spectra (λe in nm), oscillator strengths (f ) and radiation rates (kr in s−1 ) of TTM-1Cz in different environments. Tas phase Toluene Chloroform a b c

C4-N21-C22-C24a C26-C28-C30-34Ca λe f 86.0 (49.8) 39.4 (48.9) 774 0.0177 73.6 (50.4) 39.8 (48.9) 726 0.0364 62.1 (50.4) 41.0 (49.1) 700 0.1020

kr 2.0 × 106 4.6 × 106 1.4 × 107

λe (Expt.) 680b 660c

The atomic label can be found in Figure S3 in SI. Ref. 37. The radiation lifetime of 25ns corresponds to the radiation rate of 4.0 × 107 s−1 . Ref. 12. We shall first compare the molecular geometries of TTM-1Cz at its ground state D0

and the first excited state D1 . As shown in Table 1, the major difference between the D0 and D1 structures lies in the dihedral angles C4-N21-C22-C24 and C26-C28-C30-C34. Especially, both dihedral angles are almost the same at the D0 structures in three different environments, but they differ greatly at the D1 structures, suggesting strong influence of solvent effects on the first excited state of TTM-1Cz. Besides, a large Stokes shift can be observed by comparing with the absorption peaks, which also demonstrates the large geometric relaxation after TTM-1Cz being excited into the D1 state. In their experimental study, Peng et al. 12 have measured the photo-induced fluorescence of TTM-1Cz in the toluene solution, and the radiation rate of the peak at 680nm is 4.0 × 107 s−1 . For comparison, the theoretical radiation rate of the emission wavelength at 726nm is 4.6 × 106 s−1 in the toluene solution, and the one at 700nm is 1.4×107 s−1 in chloroform. Although the absolute positions of the theoretical emission peaks in two different environments are respectively red-shifted by 46nm and 40nm compared with the experimental ones, the wavelength separation of 26nm agrees well with the experimental interval of 20nm in the two solutions. 12,37 The shorter emission wavelength and larger oscillator strength in the chloroform than in toluene are caused by the relatively smaller dihedral angle C4-N21-C22-C24 in the former 8


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solution. The charge transfer excitation induces a large charge redistribution that results in a strong electrostatic interaction between the excited TTM-1Cz and the polar solvent. Therefore, the geometric relaxation of the excited TTM-1Cz in chloroform is hampered, which gives rise to a smaller Stokes shift of emission spectra than that in toluene. Moreover, larger C4-N21-C22-C24 angle leads to smaller HDMO-SOMO overlap that reduces the oscillator strength of the D1 → D0 transition in toluene. As a result, the emission rate in chloroform is larger than that in toluene. Since geometry relaxation is important in emission process, the twisting of C4-N21-C22-C24 should be prevented to increase the emission strength and the blue-shift of emission wavelength (vide post). Formation of emissive exciton in the real device setup. Figure 3(a) sketches the energy levels of each material layer in the real device setup as well as the electron-hole hoping paths to illustrate the exciton-formation mechanism within the TTM-1Cz based OLED. In the electroluminescence device by Peng and co-works, 12 NPB, CBP:TTM-1Cz, and TPBi are served as the hole transport layer (HTL), the emissive layer, and the electron transport layer (ETL), respectively, where NPB is N,N’-di-1-naphthyl-N,N’-diphenylbenzidine, CBP is 4,4-bis(carbazol-9-yl)biphenyl and TBPi is 1,3,5-tri(phenyl-2-benzimidazolyl)-benzene. The orbital energies of HDMO, SOMO, and LUMO of TTM-1Cz are -6.0eV, -3.2eV and -1.3eV, respectively (alpha orbital’s energy from UKS/PBE0), whereas the energy levels of NPB, CBP, and TPBi are taken from Refs. 16 and 38. The electron-hole injection into CBP is energetically allowed due to the proper electronic potential barriers. After holes and electrons are injected into CBP, the hole-electron pair may transmit from CBP to TTM-1Cz via the Förster-Dexter energy transfer (ET) 39,40 or the charge transfer (CT) processes. However, the Förster-Dexter ET is negligible due to the small spectral overlap between TTM-1Cz and CBP, as evidenced by the fluorescence and phosphorescence spectra of CBP peaked at 360nm and 469nm 41 that are far from the first absorption peak of TTM-1Cz, and therefore it should be difficult to arrive at the D1 state directly via the ET process. 9



(a)

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(b)

Figure 3: (a) Hoping of holes and electrons and formation of excitons in the electroluminescence device in Ref. 12 (the energy levels are in eV); (b) Several possible charge-transfer pathways.

Table 2: Charge-transfer integrals, reorganization energies and charge-transfer rate constants of three charge-transfer processes from CBP to TTM-1Cz. Transmission path

V (eV) ∆G (eV) Electron transmission CBP(LUMO) → TTM-1Cz(SOMO) 0.0048 −0.30 CBP(LUMO) → TTM-1Cz(LUMO) 0.0163 1.60 Hole transmission CBP(HOMO) → TTM-1Cz(HOMO) 0.0008 0.00

10


λ (eV)

k (s−1 )

0.38 0.91

5.4 × 1011 4.3 × 10−17

0.18

4.0 × 109

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The D1 state may be produced by the CT process. In this case, electron and hole jump from CBP to TTM-1Cz to form the emissive exciton directly. The reaction rate constant of the CT process can be estimated using Marcus theory. 42,43 Table 2 presents the chargetransfer integrals V , Gibbs free energy changes ∆G, reorganization energies λ as well as the CT rate constants of electron transmission (kET ) and hole transmission (kHT ). The charge transfer integral is calculated via frozen orbital density function theory (FODFT) method. 44 The ∆G is estimated from the difference of the energy levels of CBP and TTM-1Cz in Figure 3(a). The reorganization energy λ is calculated by E[CBP+/− ]+E[TTM-1Cz]+/− −E[CBP]− E[TTM-1Cz+/− ]+/− . Here E[CBP+/− ] is the energy of the neutral CBP calculated at the minimal structure of the charged CBP. E[TTM-1Cz]+/− is energy of the charged TTM-1Cz at the minimal structure of the neutral TTM-1Cz. E[CBP] is the energy of the neutral CBP on its minimal structure. E[TTM-1Cz+/− ]+/− is the energy of the charged TTM-1Cz on its the minimal structure. The lowest triplet state is used for the TTM-1Cz cation. As shown in Table 2, the electron hopping from the LUMO of CBP to the SOMO of TTM1Cz is exergonic with the Gibbs free energy about -0.30eV, and the corresponding reorganization energy is 0.38eV. The calculated electron-transfer rate constant is 5.4×1011 s−1 , which is two order of magnitude larger than the hole-injection rate from CBP(HOMO) to TTM1Cz(HOMO). Moreover, the electron hopping from CBP(LUMO) to TTM-1Cz(LUMO) is endergic with a large ∆G of 1.6eV that gives rise to a tiny electron-transfer rate constant. Considering these three CT rate constants, the path A in Figure 3b is more favorable than the paths B and C, that is, the electron is first injected into TTM-1Cz to form an anion, then the hole hops into the TTM-1Cz(HDMO) to form an emissive type (I) exciton in Figure 1. The generation of excitons of types (II) and (III) in Figure 1 can hence be neglected. Such a conclusion provides strong support for the assumed electrical injection pathway by Ai et al. 16 in which two similar radical molecules exhibit 100% IQE in the OLED device. Molecular design. A full-color display requires red, green and blue emission materials. Contrary to red OLEDs, blue and green OLEDs (especially the efficient and stable blue 11



OLEDs 45–48 ) are scarce. As we have discussed above, the emission of TTM-1Cz can be mainly attributed to the electronic transition from D1 to D0 . Therefore, it will be interesting to screen the TTM-1Cz derivatives with tuned color of emission light by varying the HDMOSOMO gap. It is convenient to change the HDMO level by substituting the hydrogen atoms in the carbazole group using electron-withdrawing groups while the stability of material will not be affected. Moreover, the substitution may increase the steric hindrance between the carbazole and 1,3-dichlorobenzene groups and restrict the twisting of the dihedral angle C4N21-C22-C24, and thus may give rise to smaller Stokes shifts as well as stronger emissions than TTM-1Cz. The cyan (−CN), sulfo (−SO3 H), carboxyl (−COOH) and aldehyde (−CHO) groups that have strong electron-withdrawing ability have been selected to replace the H atoms on the 1st and 2nd sites (see in Figure S4 in SI). The molecular geometries at the ground and the first excited states are optimized in the chloroform solution. The calculated absorption and emission wavelengths, oscillator strengths and radiation rates are presented in Table S6 in SI. We found that the absorption spectra of all the derivatives have a notable blue-shift compared with the reference spectra of TTM-1Cz, supporting our assumption that substitutions with electron-withdrawing groups could reduce the absorption wavelength. Moreover, the emission spectra of the −CN and −SO3 H substituted TTM-1Cz derivatives have fallen into the green light region (λe = 560nm and λe = 557nm) while the −CHO and −COOH substituted ones emit yellow (λe = 577nm) and orange light (λe = 584nm), respectively. In addition, the radiation rates of these four derivatives become larger than that of TTM-1Cz, indicating that such a substitution could indeed enhance emission strength. Based on the geometry parameters of the TTM-1Cz derivatives (see Table S7 of SI), we may analyze the reason for the enhancement of emission strengths. As expected, the dihedral angles C4-N21-C22-C24 in the optimized D1 structures of all derivatives are in the range of 49◦ -52◦ , being much smaller than the corresponding angle of 62.6◦ of TTM-1Cz. Therefore, the transition dipole moment between the D0 and D1 states is so greatly increased that the 12


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radiation rate is evidently enhanced. Another interesting finding is that all the derivatives are more rigid than TTM-1Cz, and hence are less influenced by environments as verified by the similarity of structures in gas phase and in chloroform solution, being more preferable in real applications. Considering the emission wavelength, the −CN and −SO3 H substituted derivatives are recommended to be synthesized as a potential green light OLED materials in future studies. Conclusion and discussion. The radical molecule TTM-1Cz has been systematically studied to understand the luminescence mechanism of this emitter in OLED. Our results indicate that the D1 state is dominated by the HDMO→SOMO excitation and is a typical charge transfer state. At the optimized D1 geometries by U-TDDFT in different environments, the luminescence mechanism of TTM-1Cz is examined, and the twisting of the dihedral angle C4-N21-C22-C24 is found to strongly affect the emission wavelength and the radiation rate. The Marcus theory is used to estimate the charge transfer rate in order to understand the electron-hole transmit path in the real device setup. Since the electron injection is much faster than the hole injection, the emissive exciton should be produced by a two-step charge transfer process, in which an electron hops first from CBP’s LUMO to TTM-1Cz’s SOMO, and then a hole transmits from HOMO of CBP to HOMO of TTM-1Cz in the second step. This result helps us to rationalize the 100% IQE in a series of newly synthesized radical molecules in OLED. 13,16 Based on the gained understandings of emission mechanism, some electron-withdrawing groups are proposed to substitute the hydrogen atoms on two different sites of the carbazole, which can enlarge the HOMO-SOMO gap and thus blue shifts the emission spectra. By using four different electron-withdrawing groups to decorate TTM1Cz, the cyan (-CN) and sulfo (−SO3 H) groups are found to perform the best and to tune the emission wavelength into the green region. More importantly, the substitution can also increase the steric effect between the carbazole and 1,3-dichlorobenzene groups, resulting in more rigid structures and hence more enhanced emission strength.

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Acknowledgement This work is supported by National Key R&D Program of China (grant No. 2017YFB0203404). The authors thank to the financial support from National Natural Science foundation of China (NSFC, grant No. 21673174, 21673175 and 21873077). B. Suo thanks to Prof. Shiwei Yin in Shaanxi Normal University and Prof. Qian Peng in Institute of Chemistry, Chinese Academic of Science for valuable discussions.

Supporting Information Available The following files are available free of charge. • Additional computational details and results including 1) comparison of X-TDDFT and U-TDDFT results; 2) Absorption spectra calculated by X-TDDFT with different exchange-correlation functionals; 3) discussion of spin-contamination; 4) Details to calculate charge-transfer rates; 5) Spectra of TTM-1Cz derivatives; 6) Cartesian coordinates of the optimized structure of molecules.

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