Quanto-chemical insight into the singlet-triplet energy gap of diazine-based TADF blue emitters for OLED devices

doi:10.21203/rs.3.rs-2739288/v1

A series of molecules (1a-d, 2a-d, 3a-c) with donor-linker-acceptor-linker-donor (D-p-A-p-D) architecture type, having the dimethylacridine (DMA) as a strong electron-donor, a diazine as electron-accepting core and the phenyl as the π-linker, were theoretically studied as potential thermally activated delayed fluorescence (TADF) emitters in organic light-emitting diodes (OLEDs). The effect of the electron-accepting core (pyridazine, pyrimidine or pyrazine) with various linking positions of the phenyl bridge, on the energy gap between the lowest excited singlet and triplet states (ΔE_ST) of the modeled molecules is investigated using DFT and TD-DFT methods and the level B3LYP/6-31G(d,p) was selected to calculate their photophysical and optoelectronic properties. The present study revealed that i) the all designed molecules have strong exciton binding energies, ii) they emit light in the range 480-540 nm in toluene solvent from a charge transfer state, iii) they exhibit TADF properties because of their small energy gaps (ΔE_ST<0.3eV). Additionally, the obtained results also indicate that the singlet-triplet energy gap can be effectively fine-tuned by the simple modification of the linking position of the DMA units via the phenyl bridge. Moreover, the NPA analysis shows that the more electron-deficient diazine core induces smaller HOMO-LUMO overlap and smaller DE_ST energy gap. Finally, the molecules 1c, 2b, 2c and 3b, where the two phenyl bridges are linked at 4,5-, 4,6-, 2,3- and 2,5-positions, respectively of the pyrimidine core, with the lowest ΔE_ST values should be the more suitable for efficient reverse intersystem crossing (rISC) in the TADF molecules.

Diazine

TADF emitters

Reverse intersystem crossing (rISC)

Time-dependent density functional theory (TD-DFT)

NTO analysis

Over the past few decades, the rise of organic light-emitting diodes (OLEDs) technology was impressive [1-3]. OLEDs have attracted considerable research interest due to their potential relevance in solid state lighting and display applications [4-6]. According to quantum statistics, electro-excitation results in 1:3 ratio of singlet and triplet excitons. The theoretical luminous internal quantum efficiency (IQE) of common fluorescent emitters is limited to just singlet excitons and is only 25% in the devices, the radiative decay of triplet exciton being spin forbidden [7,8]. Theoretically, phosphorescent OLEDs can reach 100% IQE by using noble metals (such Iridium and Platinum) complexes as emitters [9,10] but this comes at a significant cost and with heavy metal pollution. In order to overcome this problem, a great deal of interest has been recently devoted to the elaboration of thermally activated delayed fluorescence (TADF) emitters [11-13] because they can also realize 100% IQE without the use of noble metals, TADF emitters are thus thought as the third generation OLED materials. The ability to produce OLEDs that compete and sometimes outperform phosphorescence-based OLEDs by harvesting both singlet and triplet excitons for light emission justifies the interest in this novel family of materials that Adachi and coworkers [14,15] have intensively promoted since 2011. In TADF emitters the energy gap (DE_ST) between the first singlet excited state (S1) and the first triplet excited state (T1) is extremely small [14]. It is noteworthy that such emitters can perform efficient reverse intersystem crossing (rISC) process by thermally up-converting the electrons from the triplet to the singlet excited states, which may enable full (100%) exciton usage in the optoelectronic electroluminescent devices [16]. Therefore, the design of stable and highly emissive TADF molecules becomes an intense research activity of the scientific community in this field [17,18]. We note that almost all of the reported organic TADF molecules are typically made of electron-donor (D) groups and electron-acceptor (A) groups with a large dihedral angle [19,20]. This reduces the overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) wave functions and lowers the electron exchange energy (K_HL), which in turn leads to a smaller energy gap between the excited singlet and triplet states (ΔE_ST=2K_HL) [21], and thus allowing to elaborate metal-free molecules suitable for OLED devices. The TADF molecules have used a great variety of donors and acceptors up until now [22-24], in particular Kido et al. [25,26] used pyrimidine as acceptor and dimethyl-acridine as donor to synthesize a series of TADF molecules exhibiting blue light emission (l_em=480 nm) with small ΔE_ST (<0.20eV) and high device performance when doped in bis-(2(diphenylphosphino)phenyl)ether oxide films. It would be helpful to know how the donor and acceptor units affect the singlet-triplet energy gap (DE_ST) in order to build and develop more efficient TADF emitters with small S1-T1 energy splitting.

In this work, a series of D-π-A-p-D systems with the dimethyl-acridine (DMA) as the donor moieties, the phenyl as the π-conjugated bridge and the three kinds of diazine core as the electron acceptor moieties with four different linking positions (scheme S1 of electronic supporting information file) of the phenyl bridge were designed and theoretically analysed. The chemical structures of the eleven molecules labelled as 1a-d, 2a-d and 3a-c, are shown in Scheme 1. The diazine ring was selected because it is a highly p-deficient acceptor that can be easily modified on four positions. The two DMA donor units contribute to increase the degree of intramolecular charge transfer in the studied molecules [27]. The influence of the electron-withdrawing ability and the incorporation of the two DMA units at different positions of the diazine core on the TADF efficiency are investigated. For this purpose, extremely important properties in the designing of OLEDs, such as the HOMO-LUMO energy gaps, the absorption and emission energies, the changes in molecular dipole moments, the electron affinities, the ionization potentials, the hole and electron reorganization energies [28], the exciton binding energies [29] and the DE_ST energy gaps of the studied molecules were calculated and discussed using DFT [30] and TD-DFT [31, 32] methods. Natural transition orbitals (NTOs) [33] analysis was also carried out to provide further insight into the nature of the excited states. Our investigation will offer a theoretical strategy for the molecular design of efficient diazine-based blue-color emitters exhibiting a thermally activated delayed fluorescence.

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All the ground state and excited state computations were performed using the Gaussian 09 software package [34]. In the gas phase, the equilibrium geometries of the studied molecules were optimized in their ground and excited states without any symmetry constraint. The ground-state (S0) geometry optimizations were performed by density functional theory (DFT) [30] at the popular Becke three-parameters Lee-Yang-Parr hybrid functional (B3LYP) [35] computational level in conjunction with dual-polarized 6-31G(d,p) atomic basis set. The geometries of the first excited singlet (S1) and triplet (T1) states were optimized by time-dependent DFT (TD-DFT) [31] using the same functional. The harmonic vibrational frequencies have been calculated for the optimized geometries to confirm that all frequencies are positive and belong to minima in the respective potential energy surface. The electronic emission spectra were calculated in toluene solvent using an extended atomic basis set at the TD-B3LYP/6-311++G(d,p) level of theory. The solvent effect was simulated using the conductor-like polarizable continuum model (C-PCM) [36]. For the calculation of singlet and triplet excitation energy levels and the singlet-triplet energy gap (ΔE_ST) values, the TD-B3LYP/6-31G(d,p) method was used. It has been noted that B3LYP functional typically provides results that are reasonably good for most of the organic molecules [37-40]. The hole and electron natural transition orbitals (NTOs) have been calculated for the S1 and T1 excited states. The orbital overlap integral (S_h/e) between hole and electron density distribution which is used to identify the electron excitation mode, the centroid distance (D) [41,42] between each hole-electron orbitals pair which is a good indicator of charge transfer length of a considered electron transition mode and the charge transfer index (Δr) [43] which is the sum of the weighted centroid distances of each pair of natural orbital transitions, were calculated by Multiwfn program [44].

The efficiency of an OLED device is determined by injection and transport of charge carriers [45]. The internal reorganization energy, usually used as the main property to assess the charge transport rate [46], is calculated by the following equations:

respectively. Low reorganization energy is beneficial to charge transport. According to Marcus theory [47], the reorganization energy should be low enough to obtain the higher hole/electron mobility. On the other hand, to evaluate the energy barrier for hole and electron injections, it is necessary to estimate the vertical ionization potential (IP) and the vertical electron affinity (EA), which are determined using Eqs 2a-b. For OLED materials, a smaller IP value means the easier hole injection ability, whereas a larger EA value will facilitate the electron injection [48].

The exciton binding energy (EBE) is a crucial factor in determining the optoelectronic properties of a material. In particular, it affects the electroluminescence quantum efficiency of OLEDs [49,50]. Since the hole carries positive charge while the electron is of course, negatively charged, there is Coulomb attractive energy between them. Consequently, excitons are created by Coulomb interaction when electrons and holes are injected into the active organic layer. The excitons are formed, and when they dissociate in the form of radiation, OLEDs emit light. As a result, the coulombic interaction between holes and electrons can be evaluated using the exciton binding energy [51]. For an effective OLED material, larger exciton binding energies are required to increase the probability of electron-hole pair recombination, which in turn relates to the optical gap (E_opt) and transfer gap (E_T), as shown in Eq. 3 presented below [52].

where E_opt is the optical gap, which is determined as the S0 to S1 excitation energy and E_T is the transfer gap which is the energy difference between HOMO and LUMO.

The singlet-triplet energy gap, ΔE_ST,is a fundamental parameter in determining the efficiency of OLED materials since it affects the rate of reverse intersystem crossing [53]. The reverse intersystem crossing (RISC) rate constant (k_RISC) is related to the ΔE_ST energy gap via the Marcus formula [54] :

where k_Bis the Boltzamann constant. It is clear from Eq. 4 that at a given absolute temperature T, k_RISC constant increases with small ΔE_ST, enhancing the emission quantum yields. A small ΔE_ST is one of the material design principles for TADF molecules as well as the requirements for efficient blue TADF emitters.

Geometries and FMO analysis in S0 ground state

The optimized ground state geometries of the eleven molecules 1a-d, 2a-d and 3a-c indicate large torsion angles between the acceptor and the donor fragments. The dihedral angles j between the DMA unit and the connected phenyl are evaluated to be approximately 82° for 1a-1d, 80° for 2a-2d and 87° for 3a-3c. As depicted in Figure S1, the electron density distribution of the frontier molecular orbitals (FMOs) for all derivatives 1a-d, 2a-d and 3a-c in the S0 state shows a p-character. It is noted that the lowest unoccupied molecular orbital (LUMO) is located mainly on the diphenyl-diazine core, whereas the highest occupied molecular orbital (HOMO) concentrated on the dimethyl-acridine donor fragment. As a result, with just a very small spatial orbital overlap, the HOMO and LUMO are almost completely separated which is demonstrated by the compositions (%) of the HOMOs and LUMOs (Table S1). In Table 1, the low calculated values of donor/acceptor compositions (D/A) ratio of LUMO in the S0 state are also provided to indicate the effective HOMO-LUMO separation in the all designed molecules. This great spatial separation made possible by the orthogonal geometry between the dimethylacridine donor units and the biphenyl-diazine accepting core, enabling thermally activated delayed fluorescence.

As depicted in Figure 1, the calculated electrostatic potential maps indicate clearly that the negative region (red color) is mainly localized on the two nitrogen atoms of the diazine core. From Table 1, the LUMO energy levels of 1a-d, 2a-d and 3a-c are varied from -1.69 to -2.10 eV, while the HOMO energies vary between -1.69 and -2.10 eV. Furthermore, the studied molecules display a large energy gap DE_H-L ranging from 2.83 to 3.35 eV, which allows to expect blue light emission [25,26].

Charge injection and transport properties

The reorganization energies for electron (λ_e) and hole (λ_h) of the studied molecules were predicted from the single point energy calculations at the B3LYP/6-31G(d,p) level based on the B3LYP/6-31G(d,p) optimized neutral, cationic, and anionic geometries. The computed values of reorganization energies are summarized in Table 1. The nature of diazine core and the donor linking position seem to have significant effect on the reorganization energy for both electrons (0.29-0.61eV) and holes (0.12-0.21eV). The obtained results given in Table 1 show that the λ_h value is the lowest when the two phenyl bridges are linked at the 3,5-positions or 3,6-positions of pyridazine (2a and 3a) and 2,5-positions of pyrazine (3c), whereas the λ_e value is the lowest when the two phenyl units are linked at the 4,6-positions of pyrimidine (2b). Furthermore, the reorganization energies for hole transport (λ_h) of all the studied compounds are comparatively smaller than the reorganization energies for electron transport (λ_e), which reveals that the hole transport performance of these compounds is better than the electron transport ability. Moreover, the calculated λ_h values of 1a-3c are smaller than that of N,N’-diphenyl-N,N’-bis(3-methlphenyl)-(1,1’-biphenyl)-4,4’-diamine (TPD), which is a typical hole transport material (λ_h= 0.292 eV) [55], showing relevance of the studied derivatives as p-type material for organic light emitting devices. According to Table 1, the IP value is the lowest and the EA value is the highest when the two phenyl units are linked at 2,5-positions of pyrazine (3c). Thus, the molecule 3c would have a greater capacity for both electron or hole injection.

Exciton binding energies

The computed values of exciton binding energies of the investigated molecules are given in Table 2. The all investigated molecules 1a-d, 2a-d, 3a-c have appreciable exciton binding energies ranging from 0.39 to 0.54 eV, due to the effective attraction in the electron-hole pair, suggesting that when used in OLEDs, the excitons are formed regardless of the diazine kind and regardless of the linking positions of the DMA units.

Singlet-triplet energy gap

The S1 and T1 energy levels of the studied materials were calculated using TD-DFT at B3LYP/6-31G(d,p) level of theory (See Table 2). The eleven studied molecules exhibit high S1 energy levels above 2.47 eV, indicating that they are blue materials and their T1 energy levels are calculated to be in the range 2.38-2.82 eV. The calculated ΔE_ST gaps for the investigated molecules range from 4.2 to 135.1 eV indicating that they exhibit TADF properties and demonstrating that the singlet-triplet energy gap is significantly controlled by adjusting the linking position of the two dimethyl-acridine electron-donor units with the diphenyldiazine acceptor core. From Table 2, it turns out that, among the designed molecules, 1c (where the phenyl bridges are linked at the 4,5-positions of pyrimidine), 2b and 2c (where the phenyl bridges are linked at the 4,6- and 2,3-positions of pyrimidine) and 3b (where the phenyl bridges are linked at the 2,5-positions of pyrimidine) are characterized by the smaller DE_ST values which suggests more efficient reverse intersystem crossing and consequently results in thermally activated delayed fluorescence.

Emission spectra

The values of the variation of dipole moment with respect to ground state for the first singlet excited S₁state (Dm_S1-S0) of the studied molecules are listed in Table 2. It can be seen that all molecules exhibit significant change in dipole moment between fundamental and first excited states which is equivalent to important geometric distortion in the S1 state [56]. The large Dm_S1-S0 values suggest that the emission of these compounds originates from a state which is much polar than the ground-state [57]. Further, the large values of relaxation energies (λ_S1₋_S0~2.0 eV) in the singlet excited state obtained for the all studied compounds and listed in Table 2, suggest significant geometrical deformations in the S1 state and indicate that compounds 2c and 3c with lowest λ_S1₋_S0values possess slower radiative rates and thus extended excited state lifetimes. Indeed, the rate of the non-radiative internal conversion from the S1 to S0 is directly correlated with the relaxation energy [58], which quantifies the strength of the electron-vibration interactions in the S1 state. According to the Einstein spontaneous emission relationship: [59], typically large λ_S1-S0 values correspond to large rates for the internal conversion from the S1 to S0 state. On the other hand, from the values of the emission wavelength from the singlet state listed in Table 2, it can be seen that the all molecules show light emission in the region from 480 nm to 540 nm in toluene solvent and can be considered as potential TADF blue emitters.

Aromaticity of the diazine core

In order to examine the effect of aromatic diazine core on the optoelectronic properties of the designed D-p-diazine-p-D framework molecules, we have calculated the topological HOMA (harmonic oscillator model for aromaticity) aromaticity indices [60] of the diazine ring. The HOMA indices are computed using Multiwfn program [44] and the obtained results are presented in Table 3. The obtained HOMA values indicate that among the modeled molecules, 1c, 2b, 2c and 3b are characterized by the more aromatic diazine core which is consistent with their lowest values of the singlet-triplet ΔE_ST energy gap. We conclude that the TADF properties of the studied molecules are as much affected by the aromaticity of the diazine core whether it is pyridazine, pyrimidine or pyrazine as by the linking positions of the donor groups.

Natural Population Analysis (NPA)

As part of the natural bonding orbital (NBO) method [61], the net natural charges of the two nitrogen atoms of the diazine core calculated at B3LYP/6-31G(d,p) and HF/6-31G(d,p) [62] levels of theory are given in Table 3. The negative charge values obtained clearly validate the electron-withdrawing capacity of the diazine ring. It is clear that the nitrogen charge is affected both by the linking positions of the donor groups and by the diazine kind, it varies in the following order : pyridazine < pyrazine < pyrimidine. The Table 3 shows that compounds 1c, 2b, 2c and 3b are characterized by the more electron-deficient diazine core which is consistent with their lowest values of the singlet-triplet energy gap.

NTO analysis in S1 and T1 excited states

The natural transition orbitals (NTOs) describing the S1 and T1 excited states of the designed molecules are examined in order to better characterize the nature of the electronic transitions and to learn more about the charge transfer properties within these molecules. Figure 2 displays the hole-particle pairs of NTOs for the first singlet and triplet excited states. The highest occupied NTO (HONTO) is represented by the hole, whereas the lowest unoccupied NTO (LUNTO) is represented by the particle. The weight (ν) of the corresponding NTOs pair is also given. According to Figure 2, the NTOs for the lowest singlet excitation of the all investigated molecules showed good agreement with their HOMO and LUMO distributions (Figure S1). For the S1 and T1 excited states in the eleven emitters, the hole is mainly localized on the dimethyl-acridine donor moiety, whereas the particle is distributed on the diphenyl-diazine fragment. There is practically no overlap between holes and particles, which denotes that the S1 and T1 states have charge transfer (CT) type excitations. On the other hand, according to the obtained results, by adjusting the linking position of the two donor moieties for the same diazine kind (1a,1b, 2a and 3a for pyridazine, 1c, 2b, 2c and 3b for pyrimidine, 1d, 2d and 3c for pyrazine) lead to the decreasing of S1 and T1 energy levels (see Table 2). Furthermore, the hole and particle overlap integral (S_h/e) [41], the hole-electron centroid distance (D) [42] of the singlet and triplet states are calculated for a quantitative comparison and listed in Table 4 along with the (Dr) [43] index values. The integral of overlap of hole-electron distribution is a measure of spatial separation of hole and electron and it indicates how much the hole and the particle overlap. A smaller value of S_h/e index denotes a greater hole-electron separation, which increases the possibility of the rISC process [63]. The distance D between centroid of hole and electron is a measure of CT length. In general, longer centroid distance of the orbitals indicates more CT transition component. We have also calculated the charge transfer index (Dr) [43] to assess the degree of charge transfer. The degree of charge transfer is greater the higher the value of Dr. From the obtained results (Table 4), the S_h/e values are closer to zero implying that the CT character is dominant in the studied molecules. Additionally, all of the molecules have large Dr indices (larger than the threshold value 2.0 Å) [43] leading to strong CT character type of the electronic excitation. It is clear from Table 4 that in general the decrease in ΔE_ST values (from 135 to 4.2 eV) is consistent with the decrease in S_h/e values (from 0.114 in S1 and 0.326 in T1 for 1a to 0.027 in S1 and 0.029 in T1 for 3c) which increases the possibility of the triplet's up-conversion in these compounds.

We have examined the optoelectronic properties for a series of diazine-based compounds with donor-linker-acceptor-linker-donor (D-p-A-p-D) frameworks using DFT-B3LYP and TD-DFT-B3LYP calculations. In the D-p-A-p-D type molecules, the phenyl unit was used as p-linker and the three kinds of diazine (pyridazine, pyrimidine and pyrazine) were chosen as the electron acceptor core with various linking positions of the dimethyl-acridine as electron donors. Such architecture allows to spatially separate the HOMO and LUMO orbitals and leads to intramolecular charge transfer, which reduces the overlap of the frontier orbitals and lowers the electron exchange energy (K_HL). This in turn leads to a smaller energy gap between the excited singlet and triplet states (ΔE_ST), and thus allowing to elaborate metal-free molecules that can be employed as TADF materials for OLEDs. Our theoretical calculations show that the reorganization energies for hole transport are lower than reorganization energies for electron transport, which indicate that the performance of these compounds for hole transport is superior to that of electron transport. The NTO analysis indicates that the electronic excitations result in charge transfer for all the molecules. The present study revealed that the all designed compounds are characterized by high exciton binding energies, they emit blue light in toluene solvent and because of their small energy gaps (ΔE_ST<0.3eV) they exhibit TADF properties. However, the theoretical calculations prove that the simple modification of adjusting the linking position of the electron-donor DMA units via the phenyl bridge can effectively fine-tune the singlet-triplet energy gap in the molecules. The results indicate that the aromaticity and the withdrawing nature exert the main effects on the DE_ST energy gap. The more efficient rISC between charge transfer-type singlet and triplet excited states are obtained for the most aromatic and electron-deficient diazine rings. Among the studied molecules, 1c (where the phenyl bridges are linked at the 4,5-positions of pyrimidine), 2b and 2c (where the phenyl bridges are linked at the 4,6- and 2,3-positions of pyrimidine) and 3b (where the phenyl bridges are linked at the 2,5-positions of pyrimidine) with highly p-deficient diazine as an electron-accepting core, exhibit the lowest values of ΔE_ST and are more suitable for applications in the development of organic optoelectronic devices with blue electroluminescence. We hope that this theoretical study will be beneficial for the molecular design of novel and efficient D-p-diazine-p-D type TADF emitters.

Acknowledgments

The authors gratefully acknowledge the University of Tlemcen for technical support and the Ministry of Higher Education and Scientific Research for the PRFU project (approval No. B00L01UN130120220006).

Conflict of interest: No potential conflict of interest declared by the authors.

Authors’ contributions

L. KARA ZAITRI: Investigation, Writing-review and editing

S. M. MEKELLECHE: Conceptualization, Methodology, Writing-review and editing

Data availability N/A

Compliance with ethical standards

Ethics approval: N/A

Consent to participate: N/A

Consent for publication : N/A

Code availability: N/A

ORCID

Lamia KARA ZAITRI : https://orcid.org/0000-0002-3237-8468

Sidi Mohamed MEKELLECHE: https://orcid.org/0000-0002-5806-3333

Sun M, Li T, Xie M, Zhou H, Sun Q, Liu D, Pan Y, Zhang S, Yang W, Xue S (2023) Highly efficient deep-blue electrofluorescence with optimized excited state composition and "hot-exciton" channel. Dyes and Pigments 210:111002. https://doi.org/10.1016/j.dyepig.2022.111002
Yu S, Hu J, Zhang H,Zhao G, Li B, Xia Y, Chen Y (2022) Recent Progress in AC-Driven Organic and Perovskite Electroluminescent Devices. ACS photonics 9(6):1852-1874. https://doi.org/10.1021/acsphotonics.2c00360
Chen WC, Lee CS, Tong QX (2015) Blue-emitting organic electro fluorescence materials: progress and prospective. J Mater Chem C 3:10957-10963. https://doi.org/10.1039/C5TC02420J
Yoshida K, Matsushima T, Nakanotani H, Adachi C (2016) Quantification of temperature rise in unipolar organic conductors during short voltage-pulse excitation using electrical testing methods. Org Electron 31:191-197. https://doi.org/10.1016/j.orgel.2016.01.033
Wu W, Liu Y, Zhu D (2010) π-Conjugated molecules with fused rings for organic field-effect transistors: design, synthesis and applications. Chem Soc Rev 39:1489-1502. https://doi.org/10.1039/B813123F
Wu Y, Zhang Z, Yue S, Huang R, Du H, Zhao Y (2015) High performance top-emitting organic light-emitting diodes for super video graphics array monochromatic microdisplays application. Chin J Chem 33:897-901. https://doi.org/10.1002/cjoc.201500303
Uoyama H, Goushi K, Shizu K, Nomura H, Adachi C (2012) Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 492: 234-238. https://doi:10.1038/nature11687
Friend RH, Gymer RW, Holmes AB (1999) Electroluminescence in conjugated polymers. Nature 397:121-128.
Liu Y, Li C, Ren Z, Yan S, Bryce MR (2018) All-organic thermally activated delayed fluorescence materials for organic light-emitting diodes. Nat Rev Mater 3(4):18020. http://doi:10.1038/natrevmats.2018.20
Ahn DH, Kim SW, Lee H, Ko IJ, Karthik D, Lee JY, Kwon JH (2019) Highly efficient blue thermally activated delayed fluorescence emitters based on symmetrical and rigid oxygen-bridged boron acceptors. Nat Photonics 13:540-546. https://doi.org/10.1038/s41566-019-0415-5
Wada Y, Nakagawa H, Matsumoto S, Wakisaka Y, Kaji H (2020) Organic light emitters exhibiting very fast reverse intersystem crossing. Nat Photonics 14: 643-649. https://doi.org/10.1038/s41566-020-0667-0
Jeon SK, Lee HL, Yook KS, Lee JY (2019) Recent progress of the lifetime of organic light-emitting diodes based on thermally activated delayed fluorescent material. Adv Mater 31:1803524. https://doi.org/10.1002/adma.201803524
Lee HL, Oh CS, Lee KH, Lee JY, Hong WP (2021) Lifetime-extending 3-(4-Phenylbenzo [4,5]Thieno[3,2-d]Pyrimidin-2-yl)Benzonitrile acceptor for thermally activated delayed fluorescence emitters. ACS Appl Mater Interfaces 13:2908-2918. https://doi.org/10.1021/acsami.0c17819
Endo A, Sato K, Yoshimura K, Kai T, Kawada A, Miyazaki H, Adachi C (2011) Efficient up-conversion of triplet excitons into a singlet state and its application for organic light emitting diodes. Appl Phys Lett 98:083302. http://doi.org/10.1063/1.3558906
Mamada M, Maedera S, Oda S, Nquyen TB, Nakanotani H, Hatakeyama T, Adachi C (2023) A very low lasing threshold of DABNA derivatives with DFB structures. Mater Chem Front 7: 259-266. https://doi.org/10.1039/D2QM01040B
Lee SY, Yasuda T, Nomura H, Adachi C (2012) High-efficiency organic light-emitting diodes utilizing thermally activated delayed fluorescence from triazine-based donor–acceptor hybrid molecules. Applied Physics Letters 101:093306. http://dx.doi.org/10.1063/1.4749285
Bui TT, Goubard F, Ibrahim-Ouali M, Gigmes D, Dumur F (2018) Recent advances on organic blue thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes (OLEDs). Beilstein J Org Chem 14:282-308. https://doi:10.3762/bjoc.14.18
Anupriya K R, Thomas J, Nagar MR, Shahnawaz, Jou JH (2021) Imidazo[1,2-a]pyridine based deep-blue emitter: effect of donor on the optoelectronic properties. J Mater Sci: Mater Electron 32:26838-26850. https://doi: 10.1007/s10854-021-07060-5
Li Y, Liu JY, Zhao YD, Cao YC (2017) Recent advancements of high efficient donor-acceptor type blue small molecule applied for OLEDs. Mater Today 20:258-266. https://doi.org/10.1016/j.mattod.2016.12.003
Ye J, Chen Z, Fung MK, Zheng C, Ou X, Zhang X (2013) Carbazole/sulfone hybrid D-p-A-structured bipolar fluorophores for high-efficiency blue-violet electroluminescence. Chem Mater 25:2630-2637. https://doi: 10.1021/cm400945h
Goushi K, Yoshida K, Sato K, Adachi C (2012) Organic light emitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion. Nat. Photonics 6: 253-258. https://doi.org/10.1038/nphoton.2012.31
Skaisgiris R, Serevicius T, Dodonova J, Banevicius D, Kazlauskas K, Tumkevicius S, Jursenas S (2022) Tuning of HOMO-LUMO localization for achieving thermally activated delayed fluorescence. J Lumin 241:118473. https://doi.org/10.1016/j.jlumin.2021.118473
Verbitskiy EV, Kvashnin YA, Bogdanov PI, Medvedeva MV, Svalova TS, Kozitsina AN, Samsonova LG, Degtyarenko KM, Grigoryev DV, Kurtcevich AE (2021) The effect of molecular structure on the efficiency of 1,4-diazine based D-p-A push-pull systems for non-doped OLED applications. Dye Pigment 187:109124. https://doi.org/10.1016/j.dyepig.2020.109124
Kim HM, Choi JM, Lee JY (2016) Blue thermally activated delayed fluorescent emitters having a bicarbazole donor moiety. RSC Adv 6:64133-64139. https://doi.org/10.1039/C6RA13240E
Komatsu R, Sasabe H, Seino Y, Nakao k, Kido J (2016) Light-Blue Thermally Activated Delayed Fluorescent Emitters Realizing a High External Quantum Efficiency of 25% and Unprecedented Low Drive Voltages in OLEDs. J Mater Chem C 4:2274-2278. https://doi.org/10.1039/C5TC04057D
Komatsu R, Sasabe H, Kido J (2018) Recent progress of pyrimidine derivatives for high-performance organic light-emitting devices. J Photon Ener 8(3):032108. https://doi: 10.1117/1.JPE.8.032108
Park IS, Lee SY, Adachi C, Yasuda T (2016) Full-Color Delayed Fluorescence Materials Based on Wedge-Shaped Phthalonitriles and Dicyanopyrazines: Systematic Design, Tunable Photophysical Properties, and OLED Performance. Adv Funct Mater 26:(11)1813-1821. https://doi.org/10.1002/adfm.201505106
Hebbali R, Mekelleche SM, Kara Zaitri L (2022) Computational study of optoelectronic properties of oxadiazole-based compounds for organic light emitting diodes. Molecular Physics 120:14. https://doi.org/10.1080/00268976.2022.2103467
Kraner S, Scholz R, Plasser F, Koerner C, Leo K (2015) Exciton size and binding energy limitations in one-dimensional organic materials. J Chem Phys 143:244905. https://doi.org/10.1063/1.4938527
Parr RG, Yang W (1989) Density-functional theory of atoms and molecules. Oxford University press, Oxford.
Casida, M.E. (1995) Time-Dependent Density Functional Response Theory for Molecules. World Scientific. http://dx.doi.org/10.1142/9789812830586-0005
Casida ME, Jamorski C, Casida KC, Salahub DR (1998) Molecular excitation energies to high-lying bound states from time-dependent density-functional response theory of the time-dependent local density approximation ionization threshold. J Chem Phys 108: 4439. https://doi.org/10.1063/1.475855
Martin RL (2003) Natural transition orbitals. J Chem Phys 118:4775-4777. https://doi.org/10.1063/1.1558471
Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Scalmani G, Barone V, Mennucci B, Petersson GA, Nakatsuji H, Caricato M, Li X, Hratchian HP, Izmaylov AF, Bloino J, Zheng G, Sonnenberg JL, Hada M, Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao O, Nakai H, Vreven T, Montgomery JR JA, Peralta JE, Ogliaro F, Bearpark M, Heyd JJ, Brothers E, Kudin KN, Staroverov VN, Keith T, Kobayashi R, Normand J, Raghavachari K, Rendell A, Burant JC, Iyengar SS, Tomasi J, Cossi M, Rega N, Millam JM, Klene M, Knox JE, Cross JB, Bakken V, Adamo C, Jaramillo J, Gomperts R, Stratmann RE, Yazyev O, Austin AJ, Cammi R, Pomelli C, Ochterski JW, Martin RL, Morokuma K, Zakrzewski VG, Voth GA, Salvador P, Dannenberg JJ, Dapprich S, Daniels AD, Farkas O, Foresman JB, Ortiz JV, Cioslowski J, Fox DJ, Gaussian 09, Revision A.02, 2013, Gaussian Inc. Wallingford CT
Becke AD (1993) A new mixing of Hartree-Fock and local density-functional theories, J Chem Phys 98:372-377. https://doi.org/ 10.1063/1.464304
Tomasi J, Mennucci B, Cammi R (2005) Quantum Mechanical Continuum Solvation Models. Chem Rev 105:2999-3094. https://doi.org/10.1021/cr9904009
Cai M, Auffray M, Zhang D, Zhang Y, Nagata R, Lin Z, Tang X, Chan CY, Lee YT, Huang T, Song X, Tsuchiya Y, Adachi C, Duan L (2021) Enhancing spin-orbital coupling in deep-blue/blue TADF emitters by minimizing the distance from the heteroatoms in donors to acceptors. Chem Eng J 420 : 127591. https://doi.org/10.1016/j.cej.2020.127591
Paras, Dhiman A, Ramachandran CN (2022) Effect of aromatic linkers on thermally activated delayed fluorescence of selected organic molecules. Chem Phys Lett 801:139711. https://doi.org/10.1016/j.cplett.2022.139711
Bella AP, Solomon RV, Vedha SA, Merlin JP (2019) Enhanced luminescence efciency of structurally tailored new coumarin‑based heterocyclic organic materials: a DFT/TD‑DFT study. Theoretical Chemistry Accounts 138:53. https://doi.org/10.1007/s00214-019-2448-8
Panneerselvam M, Kathiravan A, Solomon RV, Jaccob M (2017) The role of π-linkers in tuning the optoelectronic properties of triphenylamine derivatives for solar cell applications:A DFT/TDDFT study. Phys Chem Chem Phys 19(8) : 6153-6163. https://doi.org/10.1039/C6CP07768D
Dapprich S, Frenking G (1995) Investigation of Donor-Acceptor Interactions: A Charge Decomposition Analysis Using Fragment Molecular Orbitals. J Phys Chem 99:9352-9362. https://doi.org/10.1021/j100023a009
Moore B, Sun H, GovindN, Kowalski K, Autschbach J (2015) Charge-Transfer Versus Charge-Transfer-Like Excitations Revisited. J Chem Theory Comput 11:3305-3320. https://doi.org/10.1021/acs.jctc.5b00335
Guido CA, Cortona P, Mennucci B, Adamo C (2013) On the Metric of Charge Transfer Molecular Excitations: A Simple Chemical Descriptor. J Chem Theory Comput 9:3118-3126. https://doi.org/10.1021/ct400337e
Lu T, Chen F (2012) Multiwfn : A multifunctional wavefunction analyzer. J Comput Chem 33:580-592. https://doi.org/10.1002/jcc.22885
Tang X, Zhao X, Zhu H, Tu L, Ma C, Wang Y, Ye S, Xiong Z (2021) Efficient tuning of the conversion from ISC to high-level RISC via adjusting the triplet energies of charge-transporting layers in rubrene-doped OLEDs. J Mater Chem C 9 : 2775-2783. https://doi.org/10.1039/D0TC05898J
Stark MS (1997) Epoxidation of Alkenes by Peroxyl Radicals in the Gas Phase: Structure-Activity Relationships. J Phys Chem A 101:8296-8301. https://doi.org/10.1021/jp972054+
Marcus RA (1993) Electron transfer reactions in chemistry. Theory and experiment. Rev Mod Phys 65:599-630. https://doi.org/10.1103/RevModPhys.65.599
Shang X, Wan N, Han D, Zhang G (2014) A theoretical study on injection, transport, absorption and phosphorescence properties of heteroleptic iridium(III) complexes with different ancillary ligands. Photochem. Photobiol. Sci. 13:574-582. https://doi.org/10.1039/C3PP50394A
Kraner S, Scholz R, Plasser F, Koerner C, Leo K (2017) Exciton size and binding energy limitations in one-dimensional organic materials. J Chem Phys 143:244905. https://doi.org/10.1063/1.4938527
Seo JH, Jin Y, Brzezinski JZ, Walker B, Nguyen TQ (2009) Exciton Binding Energies in Conjugated Polyelectrolyte Films. Chem Phys Chem 10: 1023-1027. https://doi.org/10.1002/cphc.200800751
Chen L, Zhu L, Shuai Z (2006) Singlet-triplet splittings and their relevance to the spin-dependent exciton formation in light-emitting polymers: an EOM/CCSD study. J Phys Chem A 110:13349-13354. https://doi.org/10.1021/jp0652998
Bawendi MG, Steigerwald ML, Brus LE (1990) The Quantum mechanics of larger semiconductor clusters (“Quantum Dots”). Annu Rev Phys Chem 41:477-496.
Yersin H (2019) Highly Efficient OLEDs-Materials Based on Thermally Activated Delayed Fluorescence. Wiley-VCH: Weinheim, Germany.
Niwa A, Kobayashi T, Nagase T, Goushi K, Adachi C, Naito H (2014) Temperature dependence of photoluminescence properties in a thermally activated delayed fluorescence emitter. Appl Phys Lett 104:213303. https://doi.org/10.1063/1.4878397
Malagoli M, Brédas JL (2000) Density functional theory study of the geometric structure and energetics of triphenylamine-based hole-transporting molecules. Chem Phys Lett 327:13-17. https://doi.org/10.1016/S0009-2614(00)00757-0
Gawale Y, Sekar N (2018) Investigating the excited state optical properties and origin of large stokes shift in Benz[c,d]indole N-Heteroarene BF2 dyes with ab initio tools. J Photochem Photobiol B 178:472-480. https://doi.org/10.1016/j.jphotobiol.2017.12.006
Mathapati GB, Patil O, Basavaraj S, Gounalli S, Hanagodimat SM (2019) Estimation of ground and excited state dipole moments of newly synthesized coumarin molecule by solvatochromic shift method and Gaussian software. Int J sci Res Phys Appl Sci 7:38-43. https://doi.org/10.26438/ijsrpas/v7i2.3843
Niu Y, Peng Q, Deng C, Gao X, Shuai Z (2010) Theory of Excited State Decays and Optical Spectra: Application to Polyatomic Molecules. J Phys Chem A 114:7817-7831. https://doi.org/10.1021/jp101568f
Einstein A (1917) On the quantum theory of radiation. Phys Z 18:121-128.
Kruszewski J, Krygowski T M (1972) Definition of aromaticity basing on the harmonic oscillator model. Tetrahedron Lett 13:3839-3842. https://doi.org/10.1016/S0040-4039(01) 94175-9
Reed A E, Weinhold F (1991) Natural Bond Orbital Analysis of Internal Rotation Barriers and Related Phenomena. Isr J Chem 31:277-285. https://doi.org/10.1002/ijch.199100032
Valatin JG (1961) Generalized Hartree-Fock Method. Phys Rev 122:1012. https://doi.org/10.1103/PhysRev.122.1012
Zhang Q, Li B, Huang S, Nomura H, Tanaka H, Adachi C (2014) Efficient blue organic light-emitting diodes employing thermally activated delayed fluorescence. Nat photon 8: 326-332. https://doi.org/10.1038/nphoton.2014.12

Table 1. The donor/acceptor ratio (D/A) of LUMO, the HOMO and LUMO energies (in eV), the |HOMO-LUMO| gap (DE_H-L/eV), the electron reorganization energies (l_e/eV), the hole reorganization energies (l_h/eV), the vertical ionization potentials (IP/eV) and the vertical electronic affinities (EA/eV) for the studied compounds in S0 states.

	D/A	E_LUMO	E_HOMO	DE_H-L	l_e	l_h	IP	EA
1a	0.0117	-1.80	-5.03	3.23	0.61	0.20	5.92	0.75
1b	0.0107	-1.98	-5.18	3.21	0.57	0.20	6.01	0.91
1c	0.0125	-1.83	-5.09	3.26	0.58	0.20	5.94	0.79
1d	0.0129	-1.69	-5.04	3.35	0.55	0.20	5.86	0.69
2a	0.0107	-1.97	-4.92	2.95	0.46	0.12	5.89	0.87
2b	0.0118	-2.06	-5.10	3.04	0.29	0.20	5.84	0.92
2c	0.0118	-1.91	-4.98	3.07	0.37	0.19	5.81	0.78
2d	0.0120	-1.88	-5.08	3.21	0.34	0.19	5.84	0.68
3a	0.0121	-1.97	-4.92	2.95	0.35	0.12	5.81	0.92
3b	0.0122	-1.88	-4.85	2.97	0.41	0.13	5.81	0.88
3c	0.0115	-2.10	-4.93	2.83	0.35	0.12	5.80	1.01

Table 2. The exciton binding energies (EBE/eV), the excitation energies of S1 (E_S1/meV) and T1 (E_T1/meV) states, the singlet-triplet energy gaps (DE_ST/meV), the variation of dipole moments with respect to ground state (Dm_S1-S0/a.u.) for the first singlet excited S1 state, the geometric relaxation energy in S1 state (l_S1-S0/eV) and the emission wavelengths (l_em/nm) in toluene solvent for the studied molecules.

	EBE	E_S1	E_T1	DE_ST	Dm_S1-S0	l_S1-S0	l_em
1a	0.41	2824.2	2689.1	135.1	12.945	1.995	530
1b	0.47	2740.9	2654.2	86.7	12.285	1.988	540
1c	0.54	2724.8	2716.5	8.3	11.126	1.997	510
1d	0.52	2826.2	2.8168	9.4	11.155	2.062	490
2a	0.39	2555.5	2547.2	8.3	14.169	1.972	540
2b	0.48	2562.4	2569.0	6.6	10.635	2.224	520
2c	0.41	2664.5	2658.7	5.8	13.273	1.969	540
2d	0.48	2731.5	2723.5	8.0	11.641	2.119	480
3a	0.47	2473.0	2466.9	6.1	13.437	2.054	510
3b	0.41	2555.9	2551.7	4.2	12.894	2.015	520
3c	0.45	2380.3	2375.1	5.2	13.880	1.965	540

Table 3. The topological HOMA indices of the diazine core and the net natural charges of diazine’s nitrogen atoms for the investigated molecules.

	HOMA Index	NPA of the two nitrogen atoms in the diazine core
	HOMA Index	B3LYP/6-31G(d,p)		B3LYP/6-31++G(d,p)		HF/6-31G(d,p)		HF/6-31++G(d,p)
1a	0.942	-0.219	-0.227	-0.228	-0.220	-0.236	-0.248	-0.248	-0.235
1b	0.935	-0.226	-0.226	-0.227	-0.227	-0.246	-0.246	-0.244	-0.244
1c	0.955	-0.471	-0.478	-0.467	-0.474	-0.559	-0.566	-0.553	-0.563
1d	0.942	-0.419	-0.420	-0.411	-0.412	-0.454	-0.455	-0.446	-0.446
2a	0.961	-0.238	-0.213	-0.241	-0.215	-0.223	-0.267	-0.270	-0.229
2b	0.977	-0.491	-0.491	-0.485	-0.489	-0.586	-0.586	-0.581	-0.581
2c	0.986	-0.487	-0.490	-0.488	-0.480	-0.585	-0.579	-0.584	-0.580
2d	0.968	-0.408	-0.437	-0.406	-0.429	-0.433	-0.480	-0.424	-0.473
3a	0.958	-0.222	-0.222	-0.231	-0.226	-0.243	-0.243	-0.249	-0.244
3b	0.975	-0.474	-0.475	-0.472	-0.469	-0.568	-0.562	-0.560	-0.558
3c	0.963	-0.424	-0.424	-0.421	-0.418	-0.457	-0.457	-0.451	-0.448

Table 4. The charge transfer index (Δr/Å), the orbital overlap integral between holes and particles (S_h/e/a.u.), the hole and particle centroid distance (D/Å) as well as the weight (ν) of the corresponding NTOs pair for the studied molecules.

	S1				T1
	Dr	S_h/e	D	nNTO	Dr	S_h/e	D	nNTO
1a	7.403	0.114	6.852	0.999	2.316	0.326	1.080	0.999
1b	7.004	0.112	6.495	0.999	2.162	0.326	1.300	0.989
1c	6.388	0.035	5.887	0.999	6.384	0.037	5.851	0.999
1d	6.822	0.035	5.903	0.999	6.794	0.041	5.839	0.998
2a	8.353	0.023	7.499	0.999	8.008	0.058	7.073	0.984
2b	7.182	0.035	5.631	0.983	7.182	0.032	6.674	0.999
2c	8.179	0.029	7.025	0.998	8.088	0.034	6.547	0.984
2d	5.653	0.033	6.161	0.989	5.638	0.036	5.158	0.914
3a	8.077	0.029	7.113	0.999	8.069	0.031	7.073	0.999
3b	7.696	0.032	6.826	0.999	7.691	0.034	6.773	0.999
3c	8.089	0.027	7.351	0.999	8.083	0.029	7.319	0.999

Scheme 1 is available in the Supplementary Files section.

No competing interests reported.

ESIMDKARA.docx
scheme1.png
Scheme 1. Chemical structures of the investigated molecules 1a-d, 2a-dand 3a-c.

Quanto-chemical insight into the singlet-triplet energy gap of diazine-based TADF blue emitters for OLED devices

Status:

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Abstract

Figures

Introduction

Computational Methodology

Results And Discussion

Conclusion

Declarations

References

Tables

Scheme

Additional Declarations

Supplementary Files

Status:

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