Intramolecular Noncovalent Interactions Facilitate Thermally Activated

Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 3260−3268

pubs.acs.org/JPCL

Intramolecular Noncovalent Interactions Facilitate Thermally Activated Delayed Fluorescence (TADF) Xian-Kai Chen,† Brandon W. Bakr,† Morgan Auffray,‡ Youichi Tsuchiya,‡ C. David Sherrill,† Chihaya Adachi,‡,§ and Jean-Luc Bredas*,†

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†

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States ‡ Center for Organic Photonics and Electronics Research (OPERA), Kyushu University,744 Motooka, Nishi, Fukuoka 819-0395, Japan § International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: In the conventional molecular design of thermally activated delayed fluorescence (TADF) organic emitters, simultaneously achieving a fast rate of reverse intersystem crossing (RISC) from the triplet to the singlet manifold and a fast rate of radiative decay is a challenging task. A number of recent experimental data, however, point to TADF emitters with intramolecular π−π interactions as a potential pathway to overcome the issue. Here, we report a comprehensive investigation of TADF emitters with intramolecular π···π or lone-pair···π noncovalent interactions. We uncover the impact of those intramolecular noncovalent interactions on the TADF properties. In particular, we find that folded geometries in TADF molecules can trigger lone-pair···π interactions, introduce a n → π* character of the relevant transitions, enhance the singlet−triplet spin−orbit coupling, and ultimately greatly facilitate the RISC process. This work provides a robust foundation for the molecular design of a novel class of highly efficient TADF emitters in which intramolecular noncovalent interactions play a critical function.

I

LUMO spatial separation leads to a vanishing electron exchange energy KHL and thus a vanishing ΔEST (since, in this framework, ΔEST = 2KHL). On the basis of this conventional moleculardesign strategy, numerous TADF emitters with very small ΔEST values were developed; they mainly correspond to either highly twisted molecules composed of electron-donor (D) moieties (on which the HOMO resides) and electron-acceptor (A) moieties (on which the LUMO resides) or D/A bimolecular complexes.2,3,12 Since in such (bi)molecular systems both S1 and T1 states have by design a pronounced charge-transfer (CT)excitation character, there appear two drawbacks: The first is that the spin−orbit couplings between such S1 and T1 states are limited since these states share the same CT-excitation character.13,14 The second is that, because of the HOMO/ LUMO spatial separation, the transition dipole moments |μS1−S0| between the S1 state and the ground state (S0) are very small. As a consequence, the radiative-decay rates, which depend on |μS1−S0|2, are low, a feature that is clearly detrimental to luminescence efficiency and OLED display operation.4 While the natures of the singlet and triplet states relevant to the TADF process actually turn out to be more complex than the simple HOMO−LUMO configurations initially considered,4,6 there is

n the OLED (organic light-emitting diode) community, much attention has been recently given to efficient harvesting of triplet excitons via a thermally assisted reverse intersystem crossing (RISC) process from the lowest triplet excited states to the lowest singlet excited states.1−8 This process gives rise to thermally activated delayed fluorescence (TADF), with electroluminescent internal quantum efficiencies (IQE) that can reach 100% in metal-f ree purely organic emitters.1 Such TADF emitters thus represent a promising pathway toward highperformance, low-cost, full-color or white OLEDs.5 The spin conversion between the lowest singlet (S1) and triplet (T1) states depends on the S1−T1 energy gap (ΔEST) and 1T1 their spin−orbit couplings (HSSO ); in the framework of firstorder perturbation theory, the mixing coefficient (cS1T1) between the S1 and T1 states can indeed be expressed as9 cS1T1 =

S1T1 HSO ΔEST

(1)

HSSO1T1

This relationship points to small ΔEST and large values in order to facilitate the T1 → S1 RISC process in TADF emitters. Early investigations on TADF systems were based on achieving a small ΔEST by separating spatially the wave functions related to the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).1,10,11 If the main electronic configuration describing both S1 and T1 states simply corresponds to a HOMO-to-LUMO transition, such a HOMO/ © 2019 American Chemical Society

Received: April 29, 2019 Accepted: May 29, 2019 Published: May 29, 2019 3260

DOI: 10.1021/acs.jpclett.9b01220 J. Phys. Chem. Lett. 2019, 10, 3260−3268

Letter

The Journal of Physical Chemistry Letters

Figure 1. Chemical structures of the molecules studied in the present work.

work clarifies the role of the intramolecular interactions in the TADF properties; importantly, it uncovers the positive impact on the TADF processes of intramolecular lone-pair···π interactions that are induced by the folded molecular geometries and allows us to start establishing relationships among molecular structure, intramolecular noncovalent interactions, and TADF properties. Geometries and Noncovalent Interactions. We first examined the optimized S0 geometries of the molecules of interest; see Figure 2a. Due to the ortho substitution of the carbazole with respect to boron, B-oTC has a folded structure, with the dihedral angle between the carbazole and the phenyl ring of the triarylboron moiety it substitutes reaching up to 90°; importantly, another phenyl of the triarylboron moiety nearly cofacially stacks with the carbazole, with a face-to-face distance in the range of 3−4 Å, which is conducive to intramolecular π···π interactions. In order to appreciate the differences with respect to emitters with the more traditional linear structure, we also investigated the B-pTC molecule, where the boron and nitrogen atoms appear in para positions around the central phenyl ring; there, the dihedral angle between the carbazole and the para-substituted phenyl of the triarylboron is significantly smaller (ca. 59°). In the cases of TRZ-oCz and TRZ-oBFCz, the ortho substitution again induces folded geometric structures. Interestingly, one of the N atoms of the triazine ring comes very close to the carbazole plane, at a distance of ca. 2.9 Å; see Figure 2a. Such a short distance between the N atom and the carbazole moiety is expected to trigger noncovalent interactions between the lone-pair electrons of this triazine N atom and the carbazole π electrons, i.e., there appear lone pair···π noncovalent interactions induced by the folded geometries. At this stage, it is useful to further characterize the steric hindrance in such folded, ortho-substituted molecular structures. In B-oTC, we examined the rotation (swing) barrier of the phenyl ring that cofacially stacks with the carbazole, and in TRZoCz the rotation barrier between the diphenyltriazine moiety and the phenyl bridge; see Figure 2b,c, respectively. The S0-state equilibrium geometries for B-oTC and TRZ-oCz correspond to a swing angle of 90° and a rotation angle of 43°, respectively. In

clearly value in probing alternative molecular-design strategies that, from the outset, provide a good balance between the singlet−triplet energy gaps, the S1−T1 spin−orbit couplings, and the S1−S0 transition dipoles.15−20 Recent experimental studies have indicated that TADF emitters with intramolecular noncovalent π···π interactions have the potential to combine small ΔEST values with substantial transition dipoles and achieve high luminescent efficiencies.17,21,22 For example, Lu and co-workers reported a blue TADF emitter (B-oTC, Figure 1) with an IQE value up to 95%.17 A crystal-structure analysis shows that one of the phenyl rings of the triarylboron moiety stacks nearly cofacially with the carbazole moiety, which points to possible intramolecular π···π interactions. Interestingly, this emitter does not show the luminescence concentration quenching behavior usually occurring in the emissive layer. Also, Swager, Baldo, and co-workers reported the phenothiazine−dimethylxanthene−diphenyltriazine (XPT, Figure 1) emitter, in which the phenothiazine donor and the diphenyltriazine acceptor cofacially pack.23 While TADF characteristics were observed, the IQE in devices based on XPT remains on the lower side, on the order of 33−50%.23 The promising properties shown by B-oTC have also been observed in other ortho-substituted TADF emitters, such as triphenyltriazine−benzofurocarbazole (TRZ-oBFCz)24,25 and triphenyltriazine−carbazole (TRZ-oCz),26 in which intramolecular lone-pair···π interactions could manifest; see Figure 1. These experimental results point to the importance of clarifying the role of such intramolecular noncovalent interactions in the TADF mechanism and of developing corresponding molecular structure−TADF characteristics relationships. Here, we report the results of quantum-chemical calculations carried out at the range-separated density-functional theory (DFT) level on TADF emitters with intramolecular noncovalent interactions; see the chemical structures in Figure 1. Functionalgroup partitioned symmetry-adapted perturbation theory (FSAPT) is then used to quantify the intramolecular noncovalent interactions in these molecules (our computational methodology is detailed in the Supporting Information). The present 3261


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Figure 2. (a) Optimized ground-state geometries of the molecules studied here. For the sake of clarity, the hydrogen atoms have been removed. (b) Rotation barrier (in kcal/mol) between the carbazole and the para-substituted phenyl of the triarylboron in B-pTC, and swing barrier of the phenyl that cofacially stacks with the carbazole in B-oTC. (c) Rotation barrier (in kcal/mol) between the diphenyltriazine and the phenyl bridge in TRZ-oCz. The pink dashed lines in (b) and (c) denote thermal energy at room temperature (kBT at RT = ∼0.6 kcal/mol). 3262


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Table 1. Electronic Interactions (kcal mol−1) between Selected Fragments in B-oTC and TRZ-oCz, As Predicted by the F-SAPT Approacha region #1 di-tert-butylcarbazole di-tert-butylcarbazole di-tert-butylcarbazole carbazole carbazole carbazole

region #2

Elst

Exch

B-oTC with Intramolecular π···π Interactions boron +1.37 +0.39 trimethylphenyl (above) +1.57 +10.51 trimethylphenyl (away) +1.06 +2.62 TRZ-oCz with Intramolecular Lone-Pair···π Interactions triazine +5.65 +4.67 phenyl (above) +0.18 +0.49 phenyl (away) +0.13 +0.003

Ind

Disp

Ttotal

−0.16 −1.65 −0.52

−0.50 −12.90 −2.73

+1.10 −2.47 +0.43

−1.27 −0.23 −0.05

−5.51 −2.48 −0.35

+3.54 −2.04 −0.27

a

Each row corresponds to the total interaction energies between a pair of interacting fragments and its decomposition in terms of electrostatics (Elst), exchange−repulsion (Exch), induction/polarization (Ind), and London dispersion (Disp).

Figure 3. Illustration of the fragmentation schemes used for the F-SAPT calculations of the intramolecular interactions in B-oTC (left) and TRZ-oCz (right).

to the π···π (instantaneous dipole···induced dipole) interactions between the two regions. Importantly, the trimethylphenyl moiety that lies above the di-tert-butylcarbazole dominates the overall intramolecular interaction energy due to favorable π···π interactions with the di-tert-butylcarbazole, with a substantial dispersion interaction of −12.9 kcal mol−1 and an exchange− repulsion contribution of +10.5 kcal mol−1; globally, the interaction between the di-tert-butylcarbazole and this trimethylphenyl stabilizes B-oTC by −2.5 kcal mol−1. In TRZ-oCz, we examined the intramolecular interactions between the carbazole (referred to as region #1 marked by the black circle in Figure 3) and the two phenyls linked by a triazine (referred to as region #2); the bridge phenyl ring is treated as the linker. Region #2 was further divided into a triazine (marked by the green circle), the phenyl (marked by the red circle) that is above the carbazole, and the phenyl (marked by the blue circle) that is away from the carbazole; see Figure 3. The F-SAPT results indicate that the overall interaction between the two regions in fact destabilizes TRZ-oCz by +1.2 kcal mol−1, despite having a significant stabilizing dispersion interaction of −8.3 kcal mol−1. This overall interaction is dominated by the destabilizing electrostatic (+6.0 kcal mol−1) and exchange−repulsion (+5.2 kcal mol−1) interactions between the carbazole and the triazine. The destabilizing electrostatic interactions are due to the proximity of nitrogen atoms with a negative net charge on the carbazole and triazine fragments; this is supported by a natural bond orbital (NBO) analysis that shows that the carbazole nitrogen has a charge of −0.5 e− and the nearest nitrogen in the triazine has a charge of −0.6 e−. The large exchange−repulsion interaction between carbazole and triazine is mainly induced by the Pauli exclusion between the π electrons of the carbazole and

B-oTC, the phenyl rotation in such a congested structure turns out to be very difficult; for example, the activated swing motion due to thermal energy at room temperature (kBT ∼ 0.6 kcal/ mol) happens within a small angle range from 70° to 100°; see Figure 2b. The results are similar for TRZ-oCz; at room temperature, the dihedral angle between the diphenyltriazine moiety and the phenyl bridge can evolve between 30° and 60°; see Figure 2c. In contrast, in B-pTC, the dihedral angle between the carbazole and the para-substituted phenyl of the triarylboron can easily rotate from 50° to 130° at room temperature (see Figure 2b). Since the folded geometrical structures of B-oTC and TRZoCz can lead to significant intramolecular noncovalent π···π and lone-pair···π interactions, respectively, we have turned to the recently developed intramolecular F-SAPT approach27,28 to specify their nature and quantify them. The results are listed in Table 1. In B-oTC, we probed the intramolecular interactions between the di-tert-butylcarbazole (referred to as region #1 marked by the black circle) in Figure 3 and two trimethylphenyl groups linked by the boron atom (referred to as region #2). The phenyl connecting the two interacting regions is treated as the linker; region #2 was further divided into the boron atom (marked by the green circle in Figure 3), the trimethylphenyl (marked by the red circle) that is above the di-tertbutylcarbazole, and the trimethylphenyl (marked by the blue circle) that is away from the di-tert-butylcarbazole. Our F-SAPT results (Table 1) show that the overall interaction between regions #1 and #2 slightly stabilizes B-oTC by −0.9 kcal mol−1. The overall interaction is dominated by exchange−repulsion (+13.5 kcal mol−1) due to the proximity of the electron clouds of the two regions, and London dispersion (−16.1 kcal mol−1) due 3263


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Figure 4. Highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) at the optimized ground-state geometries for B-oTC, TRZ-oCz, and XPT. The region marked by the red dashed square in the TRZ-oCz HOMO represents the lone-pair orbital of the interacting triazine nitrogen atom.

the manifestation of the interaction between the π orbital (φπCz) of the carbazole and the lone-pair (here, taken as the pz) orbital (φpz,N) of this nitrogen atom. The HOMO wave function (φHOMO) can thus be expressed as a superposition among the lone-pair orbital (φpz,N), the π orbital (φπCz) of the carbazole, and the π orbital (φπPh) of the phenyl bridge, as follows:

the lone-pair electrons of the nearest triazine nitrogen atom. Although the interaction between carbazole and triazine destabilizes the molecule, it is the congested nature of the geometric structure induced by the ortho substitution that locks in a small interaction distance (ca. 2.9 Å; see Figure 2a) between these moieties. Frontier Molecular Orbitals. We now turn to the impact that the intramolecular noncovalent interactions have on the frontier molecular orbitals. Figure 4 displays the HOMO and LUMO wave functions at the S0-state geometries for B-oTC, TRZ-oCz, and XPT. In B-oTC, the LUMO is essentially localized on the triarylboron acceptor; interestingly, the HOMO wave function is not only distributed over the carbazole donor, but also somewhat spreads to the triarylboron phenyl that stacks on top of the carbazole. The intramolecular π···π interactions thus lead to some degree of spatial overlap between the HOMO and LUMO wave functions. In the case of XPT, in contrast to BoTC, the HOMO and LUMO wave functions are completely separated, without hardly any spatial overlap (see Figure 4); the reason is the large distance (ca. 4.7 Å) between the phenothiazine and the diphenyltriazine planes, which precludes any significant electronic coupling between the two moieties. In TRZ-oCz, which displays intramolecular lone-pair···π interactions, while the HOMO wave function is mainly distributed over the carbazole and the phenyl bridge linking the diphenyltriazine with the carbazole, it also extends to the triazine N atom nearest to the carbazole (see Figure 4); this is

ΦHOMO = c p ,Nφp ,N + cπCzφπ + cπPhφπ z

Cz

z

2

2

(2)

Ph

2

The expansion coefficients cpz,N , cπCz , and cπPh are calculated to be ca. 0.4%, 90.7%, and 8.4%, respectively. The LUMO wave function localizes on the diphenyltriazine (dpt) and the phenyl bridge and can be expressed as ΦLUMO = cπdptφπ + cπPhφπ dpt

Ph

(3)

where φπdpt denotes the π orbital of the diphenyltriazine contributed by the px/y orbital of the triazine N atom nearest to the carbazole; the expansion coefficients cπdpt2 and cπPh2 are ca. 96.7% and 2.7%, respectively. Thus, the HOMO and LUMO wave functions show spatial overlap on the phenyl bridge. Also, note that the px/y orbitals of the triazine nitrogen that contribute to the LUMO are spatially perpendicular to the pz orbital that contributes to the HOMO; thus, this triazine N atom does not induce any spatial overlap between the HOMO and LUMO wave functions and does not participate in the electron exchange energy KHL. 3264


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Figure 5. (a) Natural transition orbitals (NTOs; h, hole; e, electron) describing the S1 and T1 states for B-oTC, XPT, and TRZ-oCz. (b) Evolution of ΔEST and the S1−T1 spin−orbit coupling in TRZ-oCz as a function of the dihedral angle θ between the diphenyltriazine and the phenyl bridge (all other geometry parameters remaining fixed at the optimal S0 geometry).

Description of the Singlet and Triplet Excited States and the Relevant TADF Processes. (i) TADF Emitters with Intramolecular π···π Interactions. With these HOMO−LUMO characteristics in mind, we now discuss the relaxation energies and electronic structure in the singlet (S1) and triplet (T1) excited states and the relevant TADF processes. In B-oTC, because of the congested geometric structure that locks rings in place, the S1 state shows only small geometrical deformations, resulting in a relaxation energy (λS1−S0) of ca. 0.25 eV (see the schematic diagram of the potential energy surfaces for the relevant electronic states in Figure S1 in the Supporting Information). We recall that the relaxation energy (that

quantifies the strength of the electron-vibration interactions) in the S1 state is directly related to the rate of the S1 → S0 nonradiative internal conversion, with large [small] λS1 values normally leading to large [small] rates for the S1 → S0 internal conversion.29 The S1 state mainly displays a CT-excitation character (see the NTO orbitals in Figure 5a); however, the NTO-hole wave function slightly extends to the phenyl bridge as a result of the HOMO/LUMO spatial overlap and brings in some local-excitation (LE) character in the S1 state; this leads to a S1−S0 transition dipole moment of 0.67 D and a radiativedecay rate (evaluated via the rate formula of spontaneous radiative emission; see eq S1 in the Supporting Information)30 3265


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Table 2. Adiabatic Excitation Energies (Ea(S1), Ea(T1), and Ea(T2)) of the S1, T1, and T2 States, Transition Dipole Moments (|μS1−S0|), Energy Gaps (ΔEST) between Ea(S1) and Ea(T1), Geometrical Relaxation Energies (λT1−S1) between T1 and S1 States, and 1T1 Spin−Orbit Couplings (HSSO ), As Calculated at the TDA Tuned-ωB97XD/6-31G(d,p) Level of Theory, RISC Rates (kRISC) from T1 to S1 Evaluated via the Marcus Electron-Transfer Rate Equation, Radiative Decay Rates (kR) of the S1 States Evaluated via the Rate Formula of Spontaneous Radiative Emission, and Maximum External Quantum Efficiencies (EQE) of the OLED Devices Reported in the Related Experimental Studies system

Ea(S1)/eV

B-oTC B-pTC XPT

2.66 2.90 2.21

TRZ-oCz TRZ-oBFCz

2.66 2.58

Ea(T1)/eV

Ea(T2)/eV

ΔEST/eV

λT1−S1/eV

HSO1 1/cm−1 ST

TADF Molecules with Intramolecular π···π Interactions 2.60 3.19 0.06 0.06 0.14 2.75 3.41 0.15 0.41 0.27 2.21 2.82 0.00 0.00 0.003 TADF Molecules with Intramolecular Lone-Pair···π Interactions 2.63 3.34 0.03 0.08 0.19 2.55 3.20 0.03 0.03 0.11

of 1.4 × 106 s−1; see Table 2. The T1 state of B-oTC also shows a hybrid-excitation character similar to that of the S1 state; see Figure 5a. This CT-LE hybridization of the S1 and T1 states does not prevent achieving a small ΔEST value, which we calculate to be 0.06 eV, in excellent agreement with the experimental value, 1T1 ca. 0.05 eV. Moreover, the spin−orbit coupling HSSO falls in a S1T1 −1 reasonable range, 0.14 cm (we note that the HSO values in purely organic TADF molecules are usually on the order of 0.01 ̵ 1 cm−1).13 With the S1 and T1 states having similar electronic characteristics, the difference in their equilibrium geometries is small, which results in a very small relaxation energy λT1−S1, 0.06 eV. The combination of these properties ultimately leads to a fast rate of reverse intersystem crossing (evaluated via the Marcus electron-transfer rate expression; see eq S2 in the Supporting Information)31 from T1 to S1, on the order of 1.8 × 106 s−1. In addition, we note that the T2 state is not expected to play an important role in the RISC process here because of the large energy gap (0.59 eV) between the T1 and T2 states; see Table 2. In contrast, in B-pTC (which we recall has a linear structure), due to the easy rotation between the carbazole and the parasubstituted phenyl of the triarylboron, the dihedral angle at the optimized S1-state geometry (ca. 90°) is much larger than that at the S0-state geometry (ca. 60°). Such a geometrical deformation leads to a larger relaxation energy between the two states (ca. 0.31 eV), which is expected to induce a faster rate of S1 → S0 internal conversion. Moreover, such a strongly twisted S1-state geometry leads to a very substantial CT-excitation character, with a smaller electron/hole density overlap (ca. 0.1) than that for B-oTC (ca. 0.3); see Figure S2 in the Supporting Information. As a consequence, the transition dipole moment reduces to 0.08 D and the radiative-decay rate comes down to 2.6 × 104 s−1; see Table 2. However, the T1 state has a strong hybrid CT-LE character (see Figure S2 in the Supporting Information), which leads to a larger ΔEST (0.15 eV) and a much larger reorganization energy λT1−S1 (0.41 eV). In spite of a 1T1 value (0.27 cm−1), the RISC rate somewhat enhanced HSSO strongly reduces in B-pTC, down to 1.4 × 104 s−1, i.e., 2 orders of magnitude lower than in B-oTC. In XPT, the large distance (ca. 4.7 Å) between the phenothiazine and diphenyltriazine moieties induces a rather loose molecular structure, which translates into a large λS1−S0 relaxation energy (ca. 0.48 eV), which accelerates the internal conversion process. The complete separation of the HOMO and LUMO wave functions leads to a nearly purely CT-excitation character of both S1 and T1 states (see Figure 5a), a feature that

kRISC/s−1

|μS1−S0|/D

kR/s−1

EQE/% (exp)

1.8 × 106 1.4 × 104 ̵

0.67 0.08 0.07

1.4 × 106 2.6 × 104 8.0 × 103

19.117 ̵ 1023

8.4 × 106 6.0 × 106

0.85 1.11

2.2 × 106 3.5 × 106

9.326 2025

resembles the case usually found in D/A bimolecular complexes. As a consequence, the calculated radiative-decay rate decreases to ca. 8.0 × 103 s−1, which is consistent with the photoluminescence quantum yield in solution being as low as 7.7%;23 S1T1 also, the ΔEST, λT1−S1, and HSO parameters all nearly vanish, which rationalizes the poor TADF efficiency reported by Swager and co-workers.23 (ii) TADF Emitters with Intramolecular Lone-Pair···π Interactions. In TRZ-oCz, the S1 and T1 electronic configurations overwhelmingly consist of the HOMO → LUMO transition (>95%). While, accordingly (see our earlier discussion of the HOMO and LUMO wave functions), the two states predominantly show a CT-excitation character, they also clearly include a n → π* excitation character (see Figure 5a), a feature that is not present in the linear TRZ-pCz isomer.15 The implication is that the n → π* excitation character in TRZ-oCz is in fact induced by the folded geometry. Such an n → π* excitation character favors spin−orbit coupling, which reaches 0.19 cm−1. Also, recalling that the px/y orbital of the triazine N atom closest to the carbazole contributes to the LUMO (π*) and is perpendicular to its lone-pair pz (n) orbital that contributes to the HOMO, this n → π* transition does not increase the ΔEST value, which remains very small, 0.03 eV. In combination with the λT1−S1 value of 0.08 eV and the 0.19 cm−1 spin−orbit coupling, the RISC rate turns out to be very fast, ca. 8.4 × 106 s−1. Simultaneously, the radiative-decay rate remains as high as 2.2 × 106 s−1, given the HOMO−LUMO spatial overlap at the phenyl bridge. To get a better grasp on how the intramolecular lone-pair···π interactions in TRZ-oCz impact the RISC process, we have 1T1 parameters evolve with the examined how the ΔEST and HSSO dihedral angle θ between the diphenyltriazine moiety and the phenyl bridge; see Figure 5b. Changing θ from 0° to 90° leads to an increase in the N1···carbazole distance, which weakens the interaction between the N1 lone-pair pz orbital and the carbazole π orbital. As the lone-pair···π interactions weaken, the contribution of the n → π* transition to the S1 and T1 states 1T1 value decreases. For example, when θ goes reduces, and the HSSO from 43° (the optimal value in the S0 state) to 60°, while ΔEST 1T1 reduces by ∼25%, the HSSO value is cut in four, from 0.18 to 0.04 cm−1; see Figure 5b. This result implies that, provided the reorganization energy λT1−S1 remains constant, the RISC rate decreases by about 1 order of magnitude. (We note that, when θ 1T1 increases again due to the appearance of goes beyond 60°, HSSO lone-pair···π interactions with the triazine N2 atom.) 3266


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Foundation through grant CHE-1566192. The work performed at Kyushu University was supported by the Japan Science and Technology Agency (JST), ERATO, Adachi Molecular Exciton Engineering Project, under JST ERATO Grant Number JPMJER1305, Japan, and JSPS KAKENHI Grant Number 17H01232.

In summary, we have investigated the geometric and electronic properties of TADF emitters that display specific intramolecular π···π or lone-pair···π noncovalent interactions. We have shown that in such TADF emitters, a fast rate of reverse intersystem crossing (RISC) from the triplet T1 to the singlet S1 excited state can coexist with a fast rate of S1−S0 radiative decay, which overcomes one of the major drawbacks of the conventional design of linear, highly twisted TADF emitters. Moreover, in such blue TADF emitters, fast RISC rates contribute to reduce the lifetimes of the triplet excitons and thus to limit triplet−triplet and triplet−polaron annihilations, which is helpful to improve the stability of blue OLED devices.32,33 Several conclusions can be highlighted: (i) An ortho substitution of the donor and acceptor moieties leads to sterically congested, folded geometrical structures. Such folded geometries can result in specific intramolecular noncovalent interactions, either of the π···π type (as in the B-oTC emitter) or the lone-pair···π type (as in TRZ-oCz). The locked character of these structures results in limited geometric deformations in the excited states, small relaxation energies, and slow internal conversion. (ii) The intramolecular π···π interactions in B-oTC lead to HOMO−LUMO spatial overlaps, hybridization of the charge-transfer (CT) and local-excitation characters in the S1 and T1 states, and significant S1 → S0 transition dipole moment and spin−orbit coupling. (iii) The folded geometry in TRZ-oCz triggers lone-pair···π interactions introducing an n → π* character of the relevant transitions, thereby enhancing the S1−T1 spin− orbit coupling, and ultimately greatly facilitates the RISC process. Overall, our work provides a solid foundation for the molecular design of a novel class of highly efficient TADF emitters in which intramolecular noncovalent interactions, especially lone-pair···π interactions, play a critical role.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b01220. Theoretical and computational methodologies, schematic diagram of the potential energy surfaces, and natural transition orbitals (PDF)

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REFERENCES

(1) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 2012, 492, 234−238. (2) Tao, Y.; Yuan, K.; Chen, T.; Xu, P.; Li, H.; Chen, R.; Zheng, C.; Zhang, L.; Huang, W. Thermally Activated Delayed Fluorescence Materials Towards the Breakthrough of Organoelectronics. Adv. Mater. 2014, 26, 7931−7958. (3) Wong, M. Y.; Zysman-Colman, E. Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes. Adv. Mater. 2017, 29, 1605444. (4) Chen, X.-K.; Kim, D.; Brédas, J.-L. Thermally Activated Delayed Fluorescence (TADF) Path toward Efficient Electroluminescence in Purely Organic Materials: Molecular Level Insight. Acc. Chem. Res. 2018, 51, 2215−2224. (5) Adachi, C. Third-generation organic electroluminescence materials. Jpn. J. Appl. Phys. 2014, 53, 060101. (6) Olivier, Y.; Sancho-Garcia, J. C.; Muccioli, L.; D’Avino, G.; Beljonne, D. Computational Design of Thermally Activated Delayed Fluorescence Materials: The Challenges Ahead. J. Phys. Chem. Lett. 2018, 9, 6149−6163. (7) Dias, F. B.; Penfold, T. J.; Monkman, A. P. Photophysics of thermally activated delayed fluorescence molecules. Methods Appl. Fluoresc. 2017, 5, 012001. (8) Liu, Y.; Li, C.; Ren, Z.; Yan, S.; Bryce, M. R. All-organic thermally activated delayed fluorescence materials for organic light-emitting diodes. Nature Reviews Materials 2018, 3, 18020. (9) Lower, S. K.; El-Sayed, M. A. The Triplet State and Molecular Electronic Processes in Organic Molecules. Chem. Rev. 1966, 66, 199− 241. (10) Zhang, Q.; Li, B.; Huang, S.; Nomura, H.; Tanaka, H.; Adachi, C. Efficient blue organic light-emitting diodes employing thermally activated delayed fluorescence. Nat. Photonics 2014, 8, 326. (11) Goushi, K.; Yoshida, K.; Sato, K.; Adachi, C. Organic lightemitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion. Nat. Photonics 2012, 6, 253−258. (12) Sarma, M.; Wong, K.-T. Exciplex: An Intermolecular ChargeTransfer Approach for TADF. ACS Appl. Mater. Interfaces 2018, 10, 19279−19304. (13) Samanta, P. K.; Kim, D.; Coropceanu, V.; Brédas, J.-L. UpConversion Intersystem Crossing Rates in Organic Emitters for Thermally Activated Delayed Fluorescence: Impact of the Nature of Singlet vs Triplet Excited States. J. Am. Chem. Soc. 2017, 139, 4042− 4051. (14) Ma, H.; Peng, Q.; An, Z.; Huang, W.; Shuai, Z. Efficient and Long-Lived Room-Temperature Organic Phosphorescence: Theoretical Descriptors for Molecular Designs. J. Am. Chem. Soc. 2019, 141, 1010−1015. (15) Chen, X.-K.; Tsuchiya, Y.; Ishikawa, Y.; Zhong, C.; Adachi, C.; Brédas, J.-L. A New Design Strategy for Efficient Thermally Activated Delayed Fluorescence Organic Emitters: From Twisted to Planar Structures. Adv. Mater. 2017, 29, 1702767−1702774. (16) Hatakeyama, T.; Shiren, K.; Nakajima, K.; Nomura, S.; Nakatsuka, S.; Kinoshita, K.; Ni, J.; Ono, Y.; Ikuta, T. Ultrapure Blue Thermally Activated Delayed Fluorescence Molecules: Efficient HOMO−LUMO Separation by the Multiple Resonance Effect. Adv. Mater. 2016, 28, 2777−2781. (17) Chen, X.-L.; Jia, J.-H.; Yu, R.; Liao, J.-Z.; Yang, M.-X.; Lu, C.-Z. Combining Charge-Transfer Pathways to Achieve Unique Thermally Activated Delayed Fluorescence Emitters for High-Performance

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

C. David Sherrill: 0000-0002-5570-7666 Chihaya Adachi: 0000-0001-6117-9604 Jean-Luc Bredas: 0000-0001-7278-4471 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X.K.C. and J.L.B. acknowledge funding of their TADF research from the Georgia Institute of Technology and the Department of Energy (DE-EE0008205); they are also grateful to Kyulux for generous support of their activities. B.W.B. and C.D.S. acknowledge financial support from the U.S. National Science 3267


Letter

The Journal of Physical Chemistry Letters Solution-Processed, Non-doped Blue OLEDs. Angew. Chem., Int. Ed. 2017, 56, 15006−15009. (18) Pershin, A.; Hall, D.; Lemaur, V.; Sancho-Garcia, J.-C.; Muccioli, L.; Zysman-Colman, E.; Beljonne, D.; Olivier, Y. Highly emissive excitons with reduced exchange energy in thermally activated delayed fluorescent molecules. Nat. Commun. 2019, 10, 597. (19) Pan, Y.; Li, W.; Zhang, S.; Yao, L.; Gu, C.; Xu, H.; Yang, B.; Ma, Y. High Yields of Singlet Excitons in Organic Electroluminescence through Two Paths of Cold and Hot Excitons. Adv. Opt. Mater. 2014, 2, 510−515. (20) Li, W.; Pan, Y.; Xiao, R.; Peng, Q.; Zhang, S.; Ma, D.; Li, F.; Shen, F.; Wang, Y.; Yang, B.; Ma, Y. Employing ∼ 100% Excitons in OLEDs by Utilizing a Fluorescent Molecule with Hybridized Local and ChargeTransfer Excited State. Adv. Funct. Mater. 2014, 24, 1609−1614. (21) Lee, Y. H.; Park, S.; Oh, J.; Shin, J. W.; Jung, J.; Yoo, S.; Lee, M. H. Rigidity-Induced Delayed Fluorescence by Ortho Donor-Appended Triarylboron Compounds: Record-High Efficiency in Pure Blue Fluorescent Organic Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2017, 9, 24035−24042. (22) Spuling, E.; Sharma, N.; Samuel, I. D. W.; Zysman-Colman, E.; Bräse, S. Deep) blue through-space conjugated TADF emitters based on [2.2]paracyclophanes. Chem. Commun. 2018, 54, 9278−9281. (23) Tsujimoto, H.; Ha, D.-G.; Markopoulos, G.; Chae, H. S.; Baldo, M. A.; Swager, T. M. Thermally Activated Delayed Fluorescence and Aggregation Induced Emission with Through-Space Charge Transfer. J. Am. Chem. Soc. 2017, 139, 4894−4900. (24) Zhang, X.; Fuentes-Hernandez, C.; Zhang, Y.; Cooper, M. W.; Barlow, S.; Marder, S. R.; Kippelen, B. High performance blue-emitting organic light-emitting diodes from thermally activated delayed fluorescence: A guest/host ratio study. J. Appl. Phys. 2018, 124, 055501. (25) Lee, D. R.; Choi, J. M.; Lee, C. W.; Lee, J. Y. Ideal Molecular Design of Blue Thermally Activated Delayed Fluorescent Emitter for High Efficiency, Small Singlet−Triplet Energy Splitting, Low Efficiency Roll-Off, and Long Lifetime. ACS Appl. Mater. Interfaces 2016, 8, 23190−23196. (26) Cha, J.-R.; Lee, C. W.; Lee, J. Y.; Gong, M.-S. Design of ortholinkage carbazole-triazine structure for high-efficiency blue thermally activated delayed fluorescent emitters. Dyes Pigm. 2016, 134, 562−568. (27) Parrish, R. M.; Parker, T. M.; Sherrill, C. D. Chemical Assignment of Symmetry-Adapted Perturbation Theory Interaction Energy Components: The Functional-Group SAPT Partition. J. Chem. Theory Comput. 2014, 10, 4417−4431. (28) Parrish, R. M.; Gonthier, J. F.; Corminbœuf, C.; Sherrill, C. D. Communication: Practical intramolecular symmetry adapted perturbation theory via Hartree-Fock embedding. J. Chem. Phys. 2015, 143, 051103. (29) Niu, Y.; Peng, Q.; Deng, C.; Gao, X.; Shuai, Z. Theory of Excited State Decays and Optical Spectra: Application to Polyatomic Molecules. J. Phys. Chem. A 2010, 114, 7817−7831. (30) Einstein, A. Quantum theory of radiation. Phys. Z. 1917, 18, 121−128. (31) Brédas, J.-L.; Beljonne, D.; Coropceanu, V.; Cornil, J. ChargeTransfer and Energy-Transfer Processes in π-Conjugated Oligomers and Polymers: A Molecular Picture. Chem. Rev. 2004, 104, 4971−5004. (32) Chan, C.-Y.; Tanaka, M.; Nakanotani, H.; Adachi, C. Efficient and stable sky-blue delayed fluorescence organic light-emitting diodes with CIEy below 0.4. Nat. Commun. 2018, 9, 5036. (33) Furukawa, T.; Nakanotani, H.; Inoue, M.; Adachi, C. Dual enhancement of electroluminescence efficiency and operational stability by rapid upconversion of triplet excitons in OLEDs. Sci. Rep. 2015, 5, 8429.

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Intramolecular Noncovalent Interactions Facilitate Thermally Activated

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