Achieving Efficient Triplet Exciton Utilization with Large ÎEST and

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Achieving Efficient Triplet Exciton Utilization with Large #E and NonObvious Delayed Fluorescence by Adjusting Excited State Energy Levels Lin Gan, Kuo Gao, Xinyi Cai, Dongcheng Chen, and Shi-Jian Su J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01961 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 1, 2018

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The Journal of Physical Chemistry Letters

Achieving Efficient Triplet Exciton Utilization with Large ∆ and Non-obvious Delayed Fluorescence by Adjusting Excited State Energy Levels Lin Gan, Kuo Gao, Xinyi Cai, Dongcheng Chen and Shi-Jian Su* State Key Laboratory of Lkeuminescent Materikeyi1als and Devices and Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Wushan Road 381, Tianhe District, Guangzhou 510640, Guangdong Province, P. R. China AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

ABSTRACT: Enhancing the rate of reverse intersystem crossing ( ) and the rate of radiative transition ( ) has been regarded as the key to improve molecular design strategy in the field of thermally activated delayed fluorescence (TADF) materials. Herein, two sky-blue donoracceptor (D-A) type TADF materials, namely CzDCNPy and tBuCzDCNPy, were designed following a strategy of controlling energy difference between charge transfer singlet state (1CT), local exciton triplet state (3LE), and charge transfer triplet state (3CT). Significantly different from most previously reported TADF materials, large values of and and nearly 100%

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exciton utilization efficiency were simultaneously achieved despite that non-obvious delayed fluorescence and large value of singlet–triplet energy difference (∆ ) were observed. This work presents a view that photo-induced delayed fluorescence and small ∆ are sufficient but not necessary for TADF materials. It also provides a reference that the high energy 3LE state plays a key role in the RISC process in electroluminescence.

TOC GRAPHICS

Thermally activated delayed fluorescence (TADF) materials have gotten much attention in the field of organic light-emitting diodes (OLEDs) due to its capability of efficient triplet exciton harvesting through reverse intersystem crossing (RISC) from the lowest triplet excited state (T1) to the lowest singlet excited state (S1).1-2 Reducing the exciton concentration to restrain efficiency “roll-off”3 and improve device lifetime by appropriate molecular design of TADF materials has been one of the most important topics in the field of TADF materials.4 As far as it is concerned, there are two strategies to reduce the exciton concentration: promoting RISC process and enhancing radiative transition.5


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Classically, rate constant of RISC process ( ) is considered to be increased with the narrowing of singlet–triplet energy difference (∆ ), while ∆ is proportional to the orbital overlap of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).6 In conventional molecular design of TADF materials, reducing ∆ by introducing electron donor (D) and electron accepter (A) units with a large twisting angle to separate the wave-function distributions of HOMO and LUMO by forming a strong intramolecular charge transfer (ICT) state is considered as the key to achieve TADF behavior and reduce the triplet exciton concentration.7-8 However, the oscillator strength ( )9 of the transition process between the ground state (S0) and S1 is directly proportional to the HOMOLUMO orbital overlap.10-14 Photoluminescence quantum yield (PLQY) and the rate constant of fluorescence radiation ( ) are limited by the low value of which seriously decrease the maximum efficiency of TADF-OLEDs.11 In addition, the rate constant of intersystem crossing ( ) from S1 to T1, which is generally 2-3 orders of magnitude larger than , is also inversely proportional to the HOMO-LUMO orbital overlap. Since the dynamic equilibrium between singlet excitons and triplet excitons are established by the value of , and , it is hard to reduce the exciton concentration by blindly reducing ∆ with an exorbitant value of .15 Furthermore, as shown in Scheme 1, the RISC process between the charge transfer triplet state (3CT) and charge transfer singlet state (1CT) is assumed to be very inefficient according to the forbidden spin–orbit coupling (SOC). Although the RISC process between 3CT and 1CT might be activated by hyperfine coupling (HFC) if the energy difference is small enough ( 5 ms at 77 K

UV-vis

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Normalized Intensity (a.u.)

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0.0 600

Wavelength (nm)

Figure 2. UV-vis absorption spectra (solid point), steady-state PL spectra at room temperature (hollow point), steady-state PL spectra at 77 K (gray line) and phosphorescence spectra at 77 K (black line, detected with 5 ms time delay) of (a) CzDCNPy and (b) tBuCzDCNPy in dilute toluene solution.

As shown in Figure 2, low temperature PL spectra (LTPL) of both materials were measured in toluene solution at 77 K to characterize their energy levels of T1 and ∆ . Both T1 of CzDCNPy and tBuCzDCNPy estimated from the phosphorescence spectra (5 ms delay in LTPL) showed typical CT characteristics for their deletions of fine structure. As shown in Table S4,


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∆ of CzDCNPy and tBuCzDCNPy are 0.32 and 0.27 eV, respectively, which are quite large values for sky-blue and greenish-blue TADF materials30. It is obvious that the relative intensity of phosphorescence is so weak that it does very little contribution to the steady-state PL spectrum at 77 K, which means the ISC process is hindered to a large extent. The hindered ISC process is attributable to the large ∆ and large value of , the former reduces while the latter competes against the ISC process. PhDCNPy, CzPh and CzDCNPh were synthesized as auxiliary materials to characterize 3LE of both materials because it is hard to be measured directly. Different from the TADF materials with a large twisting angle, 3LE of CzDCNPy and tBuCzDCNPy depend on the π-conjugation length for their good planarity. Therefore, it can be considered that 3LE of both materials could be estimated by that of CzDCNPh which weaken the ICT state obviously but keep similar geometric structure and conjugation length (Figure S3). PhDCNPy and CzPh are not D-A structure molecules which could be considered as the isolated donor and accepter moieties of CzDCNPy and tBuCzDCNPy. According to the LTPL spectrum of CzDCNPh in Figure 3, its T1 exhibits a local excitation characteristic as expected (Figure S6). Thus, ∆ of CzDCNPy and tBuCzDCNPy are 0.12 and 0.02 eV, respectively, which are appropriate values to achieve RISC process. Furthermore, the phosphorescence spectra of both materials show palpable slope changes at around 450 nm. The phosphorescence band in the range of 400-450 nm with low intensity are considered to the direct radiative transition of 3LE generated from 1CT through SOC process, which does not convert into 3CT completely through IC process or generated from 3CT through reverse internal conversion (RIC) process. The coexistence of phosphorescence from both 3LE and 3CT indicate the VC process is efficient, which enable the RISC process between 3

CT-3LE-1CT.


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Figure 3. (a) Chemical structures of CzDCNPh, CzPh, PhDCNPy and AcRPh. (b) Phosphorescence spectra of CzDCNPh, CzPh, PhDCNPy and AcRPh measured at 77 K with 5 ms time delay. (c) RISC process kinetic model of the TADF molecules with large twisting angle. (d) RISC process kinetic model of the TADF molecules with small twisting angle.

In addition, PLQYs and PL spectra of both materials doped in DPEPO (bis[2(diphenylphosphino)phenyl] ether oxide) at a concentration of 20 wt.% were measured. DPEPO was chosen as the host material for its high triplet exciton energy level (~3.2 eV) and high rigidity31. Although the PL spectra of the doped films bring into correspondence with the PL spectra of the materials in dilute solution (Figure S7), the PLQYs of the doped films (44% for


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DPEPO: CzDCNPy and 47% for DPEPO: tBuCzDCNPy, respectively) show drastically decline compared with the PLQYs in solution. Time-resolved transient photoluminescence decay measurements were utilized to research the delayed fluorescence phenomena of the molecules in dilute solutions (10-5 M in toluene), solid state in doped films (20 wt.% in DPEPO) and neat films. Rate constants of different kinetic processes shown in Table 1 were calculated following the equations (S4) - (S9). As shown in Figure S9 a, both materials show conventional fluorescence behavior with non-obvious delayed fluorescence signal even if it has been excluding oxygen by bubbling argon for 10 min due to the large value which form competitive relationship with their ISC process. Besides, as shown in Figure S9 b, compared with the toluene solutions, materials in solid state show slight but observable delayed fluorescence. Reduced and PLQYs of the materials in solid state obviously weaken the competitive relationship between the radiative transition and the ISC process, which provide the conditions to generate triplet excitons and thus achieve slight but observable delayed fluorescence. Even so, significantly different from most previously reported TADF materials, the delayed fluorescence for the doped films still have a very little contribution to the overall PL intensity (

∅ ∅

< 0.005).

Table 1. Photophysical data of the investigated molecules in dilute solutions and doped films (20 wt.% in DPEPO) at room temperature.

∅ Compound

∅"#

∅ $#

%"#

% $#

&

(%) (%)

(%)

(ns)

(µs)

(107 s-1) (107 s-1) (105 s-1) (105 s-1)

92

92

-

8.22

-

11.19

0.97

-

-

tBuCzDCNPya 95

95

-

9.68

-

9.81

0.52

-

-

CzDCNPyb

43.94 0.06

3.31

4.21

1.30

4.76

CzDCNPya

44

13.27 0.35


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tBuCzDCNPyb 47

46.85 0.15

17.15 4.33

2.73

3.08

1.71

2.52

a

Measured in dilute solutions. bMeasured in doped films. , & , , represent the rate constant of radiative, non-radiative, intersystem crossing and reverse intersystem crossing, respectively; ∅, ∅"# , ∅ $# , %"# and % $# represent total PLQY, quantum yield of the prompt component, quantum yield of the delayed component, lifetimes of the prompt and delayed components, respectively.

As shown in Table 1, the values of are still at a high level (> 107 s-1) in solid state. Meanwhile, it is noteworthy that the values of are reduced to the same order of magnitude with (>105 s-1). The similar values of and rebuild the dynamic equilibrium between singlet excitons and triplet excitons which lead to not only a high exciton utilization efficiency but also non-obvious delayed fluorescence in PL decay measurement. As shown in Figure 3 c, D-A type TADF materials with large twisting angle usually have a high value of 3LE, which is close to the 3LE of the isolated donor unit32, due to the restricted conjugation. For example, 9,9-dimethyl-10-phenyl-9,10-dihydroacridine (AcRPh) is one of the most common donor unit utilized in D-A type TADF materials with large twisting angle and has a high value of 3

LE (~3.2 eV, Figure 3 b) which is much higher than the value of 1CT of most TADF materials.

Meanwhile, ∆ of this kind of materials are usually small with a relationship that 3LE >> 1CT > 3CT due to the separated wave-function distributions of HOMOs and LUMOs. Although ∆ of those materials are small, the rate of the RISC process mainly depends on both ∆ and ∆ because SOC between 1CT and 3CT is forbidden. Efficient RISC process of this kind of materials cannot be ensured. In contrast, TADF materials with small twisting angle, such as CzDCNPy and tBuCzDCNPy, have tiny ∆ that RISC process could be achieved efficiently by SOC between 3LE and 1CT. More importantly, of those materials are generally large because of the high oscillator strength caused by the incompletely


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separated wave-function distributions of HOMOs and LUMOs. In addition, due to the high value of ∆ , of the compounds with small twisting angle is much smaller than that of the materials with large twisting angle, which is propitious to increase triplet exciton utilization. It is important to control the energy difference between 1CT-3LE-3CT that keeping the value of ∆ large but the values of ∆ and ∆ small. Thus, it can be considered that the non-obvious delayed fluorescence of the materials is mainly caused by the low triplet exciton concentration (small ) and competitive radiative transition process (large ). Generally speaking, small ∆ and obvious delayed fluorescence have been regarded as a criterion to judge whether materials have the ability to achieve TADF or not. However, both large ∆ and non-obvious delayed fluorescence were observed as mentioned above, which are considered as the result of low triplet exciton concentration in PL measurement. Thus, it is still hard to prove that the materials designed following this strategy have the ability for efficient triplet exciton utilization. Fortunately, the generation ratio of singlet excitons to triplet excitons is 1:3 in electroluminescence (EL) due to the excitons are generated by the recombination of injected holes and electrons. The capability of triplet exciton utilization could be investigated by the OLED device efficiency and time-resolved transient EL decay measurement. Therefore, as shown in Figure 5, EL properties of CzDCNPy and tBuCzDCNPy were characterized in a device structure of ITO/TAPC (30 nm)/mCP (10 nm)/emitting layer (30 nm)/DPEPO (10 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al, where TAPC, mCP and TmPyPB represent 1,1-bis(4-(di-ptolylamino)phenyl)cyclohexane, 1,3-di(9H-carbazol-9-yl)benzene and 1,3,5-tri[(3-pyridyl)-phen3-yl]benzene, respectively. DPEPO was chosen as the host material for the devices based on CzDCNPy and tBuCzDCNPy in the emitting layer. Meanwhile, to prove the importance of 3LE


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in the RISC process and triplet exciton utilization, mCP was also chosen as the host materials for the device based on tBuCzDCNPy. All data of the devices are shown in Table S5.

Figure 4. (a) OLED device diagram and chemical structures of the materials utilized for device fabrication. (b) The proposed model of triplet exciton utilization processed in the OLED devices with difference host materials.

The current density-voltage-luminance (J-V-L) and current efficiency (CE)-luminance characteristics of the devices are shown in Figure 5. Maximum external quantum efficiencies (EQEmax) of 12.7 and 14.1% were achieved for the devices based on DPEPO: CzDCNPy and


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DPEPO: tBuCzDCNPy, corresponding to maximum exciton utilization efficiencies (EUEmax, EUE = EQE/ (PLQY ▪ '()* ), '()* represents out-coupling coefficient (~0.3)) of 96 and 100%, respectively. Not only high EUEmax, but also time-resolved transient EL decay measurements prove that the devices based on DPEPO: CzDCNPy and DPEPO: tBuCzDCNPy have efficient

DPEPO: CzDCNPy DPEPO: tBuCzDCNPy mCP: tBuCzDCNPy

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triplet exciton utilization ability.

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1000


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0.4 0.2

1

10 0.0

400

500 600 700 Wavelength (nm)

800

0

200,

400,

600,

800,

Tim(∝s) ∝s) Time

Figure 5. (a) J-V-L and (b) current efficiency-luminance characteristics of the devices. (c) EL spectra of the devices at the luminance of 10 cd m-2. (d) Time-resolved transient electroluminescence decay spectra of the devices.

In contrast, the device based on mCP: tBuCzDCNPy exhibits very poor performance with EQEmax of 4.35%, EUEmax of 30.8% and evidently reduced delayed component in time-resolved transient EL decay measurement. As shown in Figure 5 b, it is hard to explain the poor triplet


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exciton utilization if the RISC process can take place between 3CT and 1CT directly due to the high enough triplet energy level of mCP compared with 3CT of tBuCzDCNPy. In contrast, assuming 3LE participates and plays the key role in the RISC process, the triplet energy level of mCP is matching 3LE of tBuCzDCNPy, which is not high enough to avoid the reverse triplet energy transfer from the emitter to the host. The current work not only confirms the feasibility of designing molecule by aligning excited energy levels but also provides a reference for the point of view that the high energy 3LE plays a key role in the RISC process of electroluminescence. By controlling the energy difference between 1CT-3LE-3CT states, CzDCNPy and tBuCzDCNPy show high values of and with low value of simultaneously, leading to non-obvious delayed fluorescence in PL decay measurement for both solution and neat/doped films. Nevertheless, over 12% and 14% EQEmax were still achieved for the OLED devices based on CzDCNPy and tBuCzDCNPy with nearly 100% EUEmax. More importantly, it has been proved that photo-induced delayed fluorescence and small ∆ are sufficient but not necessary for TADF materials to achieve high EUE. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (S.J.S.) Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors greatly appreciate the financial support from the National Key R&D Program of China (2016YFB0401004), the National Natural Science Foundation of China (51625301,


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U1601651, 51573059 and 91233116), 973 Project (2015CB655003), and Guangdong Provincial Department of Science and Technology (2016B090906003 and 2016TX03C175). ASSOCIATED CONTENT Supporting Information General information, synthesis of materials, thermal and electrochemical properties of materials, density functional theory calculations and simulation, photophysical properties of CzDCNPh, PL spectra of doped films and verification of aggregation-caused quenching phenomenon, timeresolved transient photoluminescence decay spectra, OLED data, photophysical properties of tBuCzDCNPy in solid states and computation geometry data are provided in Supporting Information. REFERENCES (1)

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