Engineering singlet and triplet excitons of TADF emitters by different host‐guest interactions

Understanding the host‐guest interactions for thermally activated delayed fluorescence (TADF) emitters is critical because the interactions between the host matrices and TADF emitters enable precise control on the optoelectronic performance, whereas technologically manipulating the singlet and triplet excitons by using different kinds of host‐guest interactions remains elusive. Here, we report a comprehensive picture that rationalizes host‐guest interaction‐modulated exciton recombination by using time‐resolved spectroscopy. We found that the early‐time relaxation is accelerated in polar polymer because dipole‐dipole interaction facilitates the stabilization of the 1CT state. However, an opposite trend is observed in longer delay time, and faster decay in the less polar polymer is ascribed to the π‐π interaction that plays the dominant role in the later stage of the excited state. Our findings highlight the technological engineering singlet and triplet excitons using different kinds of host‐guest interactions based on their electronic characteristics.


INTRODUCTION
The electroluminescence that results from the recombination of electrons and holes in organic materials was first discovered in 1953 by applying a cellulose film doped with acridine orange [1] and was further developed in 1963 using anthracene crystals. [2]According to spin statistics, only 25% of singlet excitons yield electroluminescence, and the other 75% of generated excitons are dark triplets. [3,4][7][8] To promote RISC rates, the most common strategy is to minimize the spatial overlap of the high occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), resulting in a small energy gap (ΔE ST ) between This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.© 2023 The Authors.Aggregate published by South China University of Technology; AIE Institute and John Wiley & Sons Australia, Ltd.
Previous efforts have been made to understand the photophysical properties of TADF emitters in solid states.Ginsberg group found that nonbonding interaction via π-π stacking can promote nonradiative decay and mediate the TADF molecule configuration. [17]In addition, they demonstrated that the small-molecule dopants camphoric anhydride (CA), which are used to adjust host polarity, experience an ultrafast reorientation with the same time scale as SSS of emitter. [46]The dielectric spectroscopic study conducted by Zhang group revealed that the dielectric relaxation time of host molecules is comparable to the SSS time for TADF emitter. [47]Reineke group reported the redshift of emission spectra in all investigated concentrations arises from the SSS and that aggregation is not the origin of varying emissive performance. [48]To date, researchers mainly interrogated the origin of SSS and the temporal behavior of TADF emitters in various hosts, [17,18,[46][47][48][49][50][51][52][53] which are all based on the singlet excitons with 1 CT character.Despite this progress, the complete picture of host-guest interactions and the impact of the matrix on singlet and triplet excitons needs further investigation.
The CT character of TADF emitters is widely investigated by probing the photophysical properties in solid films.The S 1 states of TADF are usually 1 CT states, while some of the T 1 states have significant local-excitation ( 3 LE) character.Unlike the 1 CT states in TADF emitters, the dynamics of the 3 LE state and the strategy to manipulate the exciton relaxation pathway remains largely unknown.In this work, we focus on a blue TADF emitter, 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)−3,6-dimethyl-9H-carbazole (Cz-TRZ). [9]The S 1 and T 1 states of Cz-TRZ are the 1 CT state and 3 LE state, respectively, as proven by steady-state photoluminescence spectroscopy and theoretical calculations.Femtosecond and nanosecond transient absorption (fs-TA and ns-TA) measurements were performed on Cz-TRZ in polymethyl methacrylate (PMMA, ε = 3.41) [50] and polystyrene (PS, ε = 2.44) [54] to reveal the 1 CT and 3 LE relaxation dynamics, respectively.We found that the highly polar PMMA lead to a faster decay of the 1 CT state because of the SSS, which can be rationalized by the dipole-dipole interaction.On the other hand, ns-TA shows that the relaxation of 3 LE is slower in highly polar PMMA, because the dipole-dipole interaction is no longer dominant in 3 LE state, and the π-π interaction between PS and emitters is stronger than the interaction of heteroatoms and emitters in PMMA.These findings proved that the singlet and triplet excitons recombination can be engineered by tuning the physical and chemical properties of glassy hosts.

Theoretical calculations
The Cz-TRZ shown in Figure 1A consists of a carbazolebased group (3,6-dimethylcarbazole) as an electron-donor and cyaphenine as an electron-acceptor.Density functional density (DFT) [55] was applied to calculate the optimized ground-state geometry, as shown in Figure S1a.The fron-tier molecular orbitals (FMOs) based on the optimized S 0 geometry are shown in Figure S1b.To qualify the electronic transition from S 0 to S 1 /T 1 , the electron-hole analysis [56] was performed based on the optimized ground-state geometry (see Figure 1B).It is observed that the S 0 to S 1 transition of Cz-TRZ is a typical 1 CT character that mainly consists of HOMO to LUMO transition, whereas the transition from S 0 to T 1 is a 3 LE character.This will be further confirmed by steady-state spectroscopy in the following studies.

Steady-state spectroscopy
The steady-state absorption and emission spectra of Cz-TRZ (Figure 1C) were measured in different polar polymers PS and PMMA (the structures are shown in Figure 1A).Two absorption peaks (approximately 330 and 340 nm) are located on the blue side, which are independent of the host polarity.However, the absorption peak centered at ∼360 nm for Cz-TRZ show hypsochromic shift as the host polarity increases because the lone electron pairs of nitrogen atoms can be particularly stabilized by the polar host. [57,58]The spectra in solvents are akin to those in polymer hosts, as shown in Figure S2a.The peaks at 330 and 340 nm can be ascribed to the 1 LE transition, and the peak on the red side (∼360 nm) is assigned to the 1 CT transition.The emission spectra in all hosts, based on either solvents or the polymer hosts, exhibit the bathochromic shift, which is due to the 1 CT from the carbazole-based moiety to cyaphenine.The results herein agree with the theoretical calculations.One could observe that the fluorescence spectra in solutions are much broader than those in the polymer hosts (see Figure 1C and Figure S2a), indicating that the SSS behavior is weaker than that of the liquid-state solvation due to the more restrictive solid environment. [49,50]The phosphorescence spectrum in frozen toluene (TOL) at 77 K was obtained previously and is also provided in Figure S2b. [9]Interestingly, the phosphorescence spectrum of Cz-TRZ is well resolved and shows vibrational characteristics, suggesting that the triplet state is the 3 LE state, following the above theoretical calculations.Such different electronic characteristics of singlets and triplets ( 1 CT vs 3 LE) for Cz-TRZ imply that the relaxation pathways of singlet and triplet excitons can be modulated because the 3 LE is hardly affected by dipole-dipole interaction.To address this question, we performed ultrafast spectroscopy studies on Cz-TRZ embedded in solid films.

Time-resolved spectroscopy
Fs-TA spectroscopy was performed on Cz-TRZ doped in PS and PMMA films.Upon 350-nm excitation, the TA spectra of Cz-TRZ in PS and PMMA (Figure 2A and B) are dominated by excited-state absorption (ESA) with two absorption bands located at ∼510 nm and 600-750 nm.There is only monotonous decay in intensity, and no other spectral evolution can be observed during the delay time increases.The ESA bands at 510 nm and 600-750 nm are assigned to the transition to higher excited states S m and S n (m ≠ n) states because there is no clear correlation between the kinetics at 510 and 730 nm (Figure S3).To elucidate the earlytime dynamics, the kinetic traces at 730 nm were extracted as shown in Figure 2C, and the fitted results are shown in Table 1.
Several key features highlight the different exciton relaxations in the initial hundred picoseconds for Cz-TRZ in PS and PMMA.First, a sum of two exponentials with lifetimes of τ 2 = 90.8ps and τ 3 = 2.3 ns is sufficient to fit the kinetics of 730 nm in PS.These two processes correspond to the excited-state molecular nuclear reorganization and excited-state depletion of the TADF emitter. [46]However, an initial rapid decay with a time constant τ 1 = 2.2 ps that is absent in PS can be observed in PMMA, which reflects SSS stabilization because the S 1 state possesses predominantly 1 CT character and thus can be affected by the local dielectric environment. [46,47,54]In addition, a small but resolvable ESA blueshift at 655 nm also indicates the SSS process; as a comparison, the ESA bands of Cz-TRZ in PS are unchanged (see Figure S4).The second key feature is that the intramolecular dynamics in PMMA leading to the stabilization of 1 CT state is faster than that in PS (τ 2 = 90.8ps in PS and τ 2 = 70.7 ps in PMMA).Although the side chains of PS and PMMA also nonbonding interact with the TADF emitter and may affect the excited-state dynamics, [17] the dipole-dipole interaction dominates the initial hundreds of picosecond decay processes based on our experimental results.To rule out the pump power dependence of TA dynamics in solid films, Cz-TRZ in PMMA was selected as an example to conduct fs-TA with different pump powers (see Figure S5), and the TA kinetics show no pump power dependence.The global analysis on the fs-TA data of Cz-TRZ in solid films are also provided in Figure S6 and Table S1, which show similar time constants as we obtained from the kinetic traces fitting (Figure 2C), implying reliable fitting results.
To connect the observations above to the liquid-state dynamics, we conducted fs-TA spectroscopy of Cz-TRZ in TOL and tetrahydrofuran (THF) under nitrogen protection (Figure S7).The overall excited-state dynamics in solutions are similar to those in high-polar host PMMA, especially the first relaxation process due to SSS also occurring on characteristic liquid-state solvation with picosecond time constants (Table S2).The global analysis results on Cz-TRZ in THF and TOL are shown in Figure S8 and Table S3, which display similar time constants as the results from Table S2.In addition, the relaxation processes are faster when changing the solvent from low-polar TOL to high-polar THF because stronger solvation in high-polar THF accelerates the 1 CT stabilization. [32,33]olecular reorientation in the excited state is one of the important factors that can affect excited-state deactivation dynamics.In previous work, researchers demonstrated that small polar molecules, such as CA (molecular weight M = 182), can rotationally reorient in solid films during the first several picoseconds. [46]However, whether the TADF emitter herein can freely orientate in polymer films needs further investigation.To address this question, transient absorption anisotropy (TAA) spectroscopy was performed on the emitter to monitor the dipole moment change during the excited-state decay processes.The TAA signal r(t) is calculated as: [59] where I VV and I VH are the fs-TA signals with the probe pulse parallel and vertical to the pump pulse, respectively.
To extract the molecular reorientation information, we show the TAA kinetic traces probed at 730 nm and displayed in Figure 3A.The kinetic at 730 nm is pure ESA (see Figure 2), which can avoid the effect of spectral overlap.There is no change in anisotropy intensity between 1 ps and 1 ns.In contrast, a significant decrease in TAA anisotropy is observed in solvents (shown in Figure 3B), and the TAA dynamics can be fitted by monoexponential decay with time constants larger than 100 ps.Note that the molecular bond rotation and charge transfer from donor to acceptor in the excited state may change the molecular dipole moment orientation, which will further affect the TAA dynamics.However, the calculation results in Figure 3C  From the fs-TA and TAA measurements, we demonstrate that PMMA plays an active role in stabilizing the 1 CT excitons of Cz-TRZ, and the dynamics is dominated by the dipole-dipole interaction, which is also demonstrated by the liquid-state dynamics.To further reveal the impact on the relaxation dynamics of triplet excitons with 3 LE character, we also performed ns-TA spectroscopy to gain insight into the triplet state.Figure 2D,E shows the ns-TA data map of Cz-TRZ obtained in PS and PMMA.With 350-nm excitation, the triplet state spectra consist of a broadly positive band across the entire spectral region.It is observed that there is no other spectral evolution except the rapid decay in approximately 20 ns.The extracted kinetic traces at 730 nm were fitted by a biexponential decay model, and the fitting results are displayed in Table 1.To assign the first process obtained from ns-TA, we collected the quantum yields and the timeresolved fluorescence as shown in Figure S9, and the detailed photophysical data in PS and PMMA films are provided in Table S4.The time constants of intersystem crossing (ISC) in PMMA and PS are about 4.3 and 4.2 ns, respectively, which are close to the time constants of the first process as shown in Table 1.Therefore, the first process obtained from ns-TA can be assigned to the ISC process.Besides, we found that the late-stage relaxation in PMMA is slower than that in PS, which is the opposite of the trend in fs-TA.This is attributed to the stronger host-guest interaction in PS than in PMMA because the aromatic PS side chains have more substantial π-π interactions with the emitters than the interaction between PMMA and emitters, thus creating additional decay pathways.Ginsberg and coworkers [17] demonstrated that the lower photoluminescence quantum yield of TADF molecules in PS film compared to PMMA is ascribed to the π-π interactions of the side benzene groups of PS leading to a stronger quenching effect than from heteroatoms in PMMA.
Overall, the 1 CT excitons of Cz-TRZ in polymer hosts is both enhanced and accelerated in the presence of highly polar PMMA, and the characteristic early time dynamics in PMMA remarkably resemble those observed in TOL and THF, suggesting that PMMA dynamics solid-solvated the 1 CT state of Cz-TRZ.By comparing the TAA dynamics, we confirmed that the molecular reorientation in polymers is completely restricted, while this reorientation involved the entire 1 CT stabilization processes in solutions.In ns-TA, an opposite trend is found that the late-time decay in PMMA is slower than that in PS.This is because in fs-TA, the lowest singlet exciton is a 1 CT state, so the dipole-dipole interaction between the polymer and TADF emitters dominates the relaxation dynamics.On the other hand, the triplet exciton with 3 LE character is populated after the ISC process of ∼4 ns, and the π-π interactions between PS and emitters are more substantial in triplet state than the interaction between PMMA and emitters.Thus, we can manipulate the recombination pathways of excitons using different kinds of host-guest interactions based on the electronic character of singlets and triplets.The proposed mechanism (shown in Figure 4) can sustain a comprehensive interpretation of the results obtained from fs-and ns-TA.
To examine the reliability of our conclusion, another TADF emitter, 9-(4-(4,6-diphenyl-1,3,5-triazin-2yl)phenyl)−1,3,6,8-tetramethyl-9H-carbazole (Cz-TRZ′, Figure 5A), [9] was chosen to interrogate the fs-and ns-TA dynamics (see Part S3 in Supplementary Information) in the same hosts as Cz-TRZ.The molecular structure of Cz-TRZ′ is slightly different from that of Cz-TRZ, and there are two methyl groups located at the o-position of the nitrogen atom, such a steric hindrance effect making a vertical conformation of Cz-TRZ′.This means that the electronic coupling between the donor and acceptor is weaker in Cz-TRZ′ relative to that in Cz-TRZ, leading to a stronger CT character of Cz-TRZ′.By conducting steady-state spectroscopy and theoretical calculations (Figure 5B and C), we found that the singlet and triplet states of Cz-TRZ′ are 1 CT and 3 LE characters, respectively.The early-time dynamics of Cz-TRZ′ obtained from fs-TA in PMMA is faster than in PS (Figure 5D, Table S6), while the late-stage obtained from ns-TA in PMMA is slower (Figure 5E, Table S8).Although the D-A electronic coupling in Cz-TRZ′ is weaker, the experimental results agree well with those of Cz-TRZ, which further verify that the singlet and triplet excitons can be manipulated by using different kinds of host-guest interactions.

CONCLUSIONS
It is well known that environmental polarity can modulate the recombination pathways of CT states due to the solvation effect, while the LE state will not be affected.Previous photophysical properties research on TADF emitters embedded in solid hosts mostly focused on the molecular 1 CT character.However, the triplet state that participates in the ISC and RISC can be 3 LE character, and the important electronic character modulated excited-state dynamics remains largely unknown.For the first time, we report an effective strategy to manipulate excited-state exciton relaxation on TADF emitters with 1 CT singlets and 3 LE triplets in polymer hosts.
With the use of time-resolved absorption spectroscopy, we obtained a comprehensive picture that rationalizes hostguest interaction-modulated exciton recombination.We first showed that the TADF emitter Cz-TRZ has a 1 CT singlet state and a 3 LE triplet state, allowing us to manipulate the singlet and triplet dynamics using different host-guest interactions.By tracking the excited-state dynamics of TADF emitters embedded in PS and PMMA with fs-and ns-TA measurements, we found that the relaxation processes of the singlet excitons in polar PMMA are intrinsically analogous to liquid-state solvation.Furthermore, it is demonstrated that the molecular reorientation in polymer hosts is strictly suppressed.However, an opposite trend is observed in triplet states, in which the π-π interaction between PS and emitters is more substantial than the interaction between PMMA and emitters; thus, the dipole-dipole interaction is not the dominant factor.Spectroscopic results on another molecule Cz-TRZ′ further supported our conclusions.
Understanding the molecular mechanism behind hostguest interactions is important because designing and choosing new host materials for making OLEDs has a considerable impact on device behavior.The host materials, not only through dipole-dipole interaction but also through other host-guest interactions, influence the exciton recombination dynamics.The former factor stabilizes the CT energy level, while the latter accelerates the nonradiative pathways, both of which can influence the ISC and RISC, which are key processes that alter the performance of the TADF emitters.The results herein provide valuable insights for manipulating the TADF device with glassy matrices, which can be implemented by introducing codopants with polar side chains or conjugate π-planar structures.Beyond the applications, the fundamental mechanism in this work will also provide valuable new insights for photophysical studies of other optoelectronic materials.

Materials
The synthesis and characterization of Cz-TRZ and Cz-TRZ′ were reported previously, [9] the PS and PMMA were bought from Macklin Inc., the TOL, THF and dichloromethane were bought from Sinopharm Chemical Reagent Co., Ltd., all materials were used without further purification.The polymer (50 mg/mL) and TADF emitters (0.5 mg/mL) were dissolved in dichloromethane separately, then the TADF solutions were filtered through 0.2 μm pore size syringe filter to avoid the possible aggregates.The polymer solutions and TADF solutions were mixed (v/v = 1:1) with overnight stirring.The mixed solution was dropped cast onto a clean 2 × 2 cm quartz coverslip, then dried slowly with dichloromethane atmosphere protection.The dried films (w/w = 1%) were encapsulated in a sealed glove box with nitrogen protection (H 2 O < 0.01 ppm, O 2 < 20 ppm).The solutions used in this work were prepared with a concentration of 10 −5 mol/L, and the solutions were filtered through 0.2 μm pore size before spectroscopic measurements.

Computational studies
All computational calculations were based on density functional theory (DFT)/time-dependent DFT (TD-DFT) and performed with the Gaussian 16, Revision A.03. [55] Optimization of the ground-state and excited-state geometries were obtained using the B3LYP [60] functional, at the basis set 6-31G** level.The CAM-B3LYP [61] along with basis set 6-31G** was employed to calculate the vertical excitation energy.The electron−hole distributions were carried out on the Multiwfn program [56] based on the output from Gaussian calculations.

Steady-state spectroscopy
The steady-state spectra, including UV−vis absorption and photoluminescence, were recorded using Agilent Cary 60 UV-vis and Agilent Cary Eclipse fluorescence spectrometers, respectively.The time-resolved fluorescence decay profiles were recorded by a time-correlated single photon counting spectrometer (FluoTime 300, PicoQuant, Germany), excited at 375-nm picosecond laser (PDL 820, PicoQuant diode laser).The quantum yields were acquired on a Quantaurus-QY Plus UV-NIR absolute photoluminescence quantum yield spectrometer (C13534, HAMAMATSU, Japan) with excitation at 350 nm.

Fs-and Ns-TA measurements
The fs-TA spectra were measured using a home-built femtosecond pump-probe set-up.The laser pulse (800 nm, 35 fs pulse width, 1 kHz repetition rate) was generated by a regeneratively amplified Ti:sapphire laser (Coherent Astrella-Tunalbe-USP, USA).The output of the pulse is then divided into two beams with a beam splitter.For the pump beam, the TOPAS Prime (Light Conversion) was used to generate the pulse with central wavelength of 350 nm (0.14 μJ).The probe beam was delayed with a computer-controlled optical delay line and then focused on a thin sappire plate to generate the white light supercontinuum which split into two beams by using a broadband 50/50 beam splitter as the signal and reference beams (450-800 nm).The focused pump and probe pulses were overlapped into a sample cuvette or film sample.The mutual polarization between the pump and probe beams was set to the magic angle (54.7 • ) by placing a half-wave plate in the pump beam.There is no photodegrading after fs-TA experiments by checking the steady-state absorption spectra.
The ns-TA spectra were measured by a commercial spectrometer (Time-Tech Spectra).The generation of the pump beam is the same as that in fs-TA.The probe beam was generated from a supercontinuum laser (LEUKOS-DISCO, French) with the spectral region from 350 to 1800 nm, the repetition rate is 2 kHz, pulse width is 700 ps-1 ns.There is no photodegrading after ns-TA experiments by checking the steady-state absorption spectra.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no competing financial interest.

D ATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are availablefrom the corresponding author upon reasonable request.

E T H I C S S TAT E M E N T
There are no ethical issues in this work.

F I G U R E 2
The molecular structures of Cz-TRZ, PS, and PMMA.(B) The electron-hole distributions of Cz-TRZ in the S 1 and T 1 states.The blue and green denote the hole and electron, respectively.(C) The steady-state absorption and fluorescence spectra of Cz-TRZ in PS and PMMA.The fs-TA spectroscopy of Cz-TRZ in (A) PS, and (B) PMMA, the unit of color bar is OD.(C) The 730-nm kinetic comparison of Cz-TRZ in PS and PMMA.The ns-TA spectroscopy of Cz-TRZ in (D) PS and (E) PMMA, the unit of color bar is OD.(F) The 730-nm kinetic comparison of Cz-TRZ in PS and PMMA.

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I G U R E 3 The TAA kinetic traces probed at 730 nm of Cz-TRZ in (A) polymer hosts and (B) solution.The fitting results in solutions are shown in the figures.(C) The calculated dipole moment orientation of Cz-TRZ in optimized S 0 and S 1 geometries.F I G U R E 4 Schematic diagram of the relaxation routes followed by Cz-TRZ after excitation in polymer hosts.The main host-guest interactions in each process are shown in the schematic diagram.

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I G U R E 5 (A) The molecular structure of Cz-TRZ′.(B) The steady-state absorption and fluorescence spectra of Cz-TRZ′ in PS and PMMA.(C) The electron-hole distribution of Cz-TRZ′ in S 1 and T 1 states.(D) Comparison of fs-TA kinetics and fittings probed at 730 nm of Cz-TRZ′ in PS and PMMA.(E) Comparison of ns-TA kinetics and fittings probed at 730 nm of Cz-TRZ′ in PS and and PMMA.

A
C K N O W L E D G M E N T S This work was financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (grant number: XDB0450202), National Natural Science Fundation of China (grant numbers: 22203085 and 22273095), and Chinese Academy of Sciences (grant number: YSBR-007).
The fitted results of fs-and ns-TA kinetic traces at 730 nm for Cz-TRZ in different hosts.
TA B L E 1