Unveiling the Molecular Symmetry Dependence of Exciton

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C: Energy Conversion and Storage; Energy and Charge Transport

Unveiling the Molecular Symmetry Dependence of Exciton Dissociation Processes in Small-Molecular Heterojunctions Xian Wang, Bin Kan, Zhuoran Kuang, Hongwei Song, Guankui Long, Qianjin Guo, Yongsheng Chen, and Andong Xia J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08537 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 9, 2018

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The Journal of Physical Chemistry

Unveiling the Molecular Symmetry Dependence of Exciton Dissociation Processes in Small-Molecular Heterojunctions Xian Wang,†,∥ Bin Kan,‡ Zhuoran Kuang,†,∥ Hongwei Song,†,∥ Guankui Long,‡ Qianjin Guo,†,∥ Yongsheng Chen,*,‡ Andong Xia,*,†,∥ †Beijing

National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of

Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. ∥University

‡State

of Chinese Academy of Sciences, Beijing 100049, China.

Key Laboratory and Institute of Elemento-Organic Chemistry and Centre for Nanoscale

Science and Technology, Institute of Polymer Chemistry and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), College of Chemistry, Nankai University, Tianjin 300071, China.

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ABSTRACT. Understanding molecular symmetry dependence of exciton dissociation could facilitate optimizing the overall power conversion efficiencies (PCEs) in solution-processed small molecule solar cells. In this work, a series of high performance oligomer molecules DRCNnT (n = 4−8) containing different number of thiophene units are systematically investigated to clarify the dependence of molecular symmetry on exciton dissociation dynamics. Femtosecond transient absorption spectroscopy are employed to track the evolution of photo-generated electron-hole pairs across the DRCNnT:PC71BM interfaces, faster intermolecular charge transfer is observed in the odd-numbered (axisymmetric) molecules relative to the even-numbered (centrosymmetric) molecules. It is found that those molecules with axisymmetric structures exhibit larger local dipole (Δµge) and better interpenetrating morphology than the centrosymmetric even-numbered molecules. Furthermore, charge separation efficiency up to ~80% are observed, which are facilitated by large degree of polarization lying in the axisymmetric excitons through decreasing overall Coulomb binding energy and finally preventing geminate recombination. The present results indicate that odd−even charge motion differs by molecular structure-symmetries, could be the key issue for illustrating the distinct odd−even trend in PCEs of small molecule based organic photovoltaics.

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INTRODUCTION Solution-processed small-molecule-based organic photovoltaic cells (SM-OPVs) are highly promising from the perspective of materials science due to their potentials in developing welldefined, large-scale, higher mobility, and open circuit voltage solar panels.1-3 Recently, SM-OPVs have made great strides in their use as the donor component in fullerene-based and non-fullerene bulk heterojunction (BHJ) solar cells. Impressive power conversion efficiencies over 14% have been achieved for both single-junction SM-solar cells and tandem devices.4-8 In view of the advantages of small molecules, they are good candidates for understanding the relationships between molecular structure and solar cell performance. Optical absorption in an inorganic semiconductor results in immediate creation of free charge carriers, but it also leads to the formation of Frankel type excitons in organic semiconductors, which are spatially localized electron-hole pairs.9,10 To dissociate these excitons, heterojunction between electron-donor (D) and electron-acceptor (A) materials is widely designed, in which the energetic offsets drive the charge transfer (CT).9,11 The hole and electron are still subject to their mutual Coulomb interaction, on the order of or larger than 0.1 eV.12 Understanding what factors further separate these CT excitons into long-range charge separation (CS) in the D/A interface is an important issue that determines the overall PCEs of these BHJ systems.13,14 Optimization of the D/A interfaces can increase the electronic coupling between the excitonic and CT/CS states, thereby enhancing the photocurrent. Several ultrafast spectroscopic studies in polymer/fullerene systems have revealed that the charge separation process within 300 fs requires no excess energy beyond that needed to overcome the Coulomb interaction.15-18 Namely, delocalized π-electron states in the fine-ordered regions of the fullerene acceptor material are generated on the fs time scale. These results point toward the importance of charge delocalization that are strongly


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dependent on the aggregate size.19-22 It has been found that for most high-performance organic semiconductors, their conjugated frameworks always contain symmetric π-building blocks.23-25 Several studies have highlighted the significance of molecular symmetry in affecting the electronic performance through mediation of solid-state packing, solubility, and charge distribution.23,26,27 The morphology of the active layer can be a determining factor behind OPV performance. For small molecules, the important role of monomer symmetry-induced methodology effect by altering the odd−even chain lengths is suggested by both experimental and theoretical studies.27,28 However, no comprehensive photophysical studies exists to date that address the effect of molecular symmetry in the view of excited state dynamics on the excitonic states. Additionally, it remains unclear what kind of symmetry is best for achieving high-performances. In an effort to achieve high PCEs of such solution-processed OPVs, the creation of a molecular platform from the viewpoints of excited state dynamics for elucidating the relationship between structural symmetry and OPV performance is desirable.


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Figure 1. Chemical Structures and steady-state absorption and fluorescence spectra for DRCNnT measured in toluene. (a) Chemical Structures of the DRCNnT (n = 4−8) Molecules. (b) Normalized UV/Vis absorption spectra and fluorescence spectra of DRCNnT (n = 4−8) in toluene. The emission spectra were obtained at 500 nm excitation. Here, we present an extensive investigation of the excited-state dynamics of a series of linear acceptor-donor-acceptor (A−D−A) type molecules, named DRCNnT (n = 4-8, see Figure 1a), which have the similar oligothiophene electron-donating backbones but different conjugation lengths and spatial symmetry.27 By looking into the photophysics of a series of symmetric DRCNnT (n = 4−8) with the same A−D−A conjugated framework, we hope to seek an understanding of: (i) how the excited-state dynamic evolves as a function of the thiophene chain length and (ii) how the spatial symmetry affects the excited state dynamics. As a result, we performed the detailed absorption spectra measurements on these compounds (DRCNnT, n = 4−8) both in solid films and dilute solutions, to build a concise model to explore the charge transfer mechanism with and without morphology effects. The optimized DRCNnT:PC71BM devices show odd−even alternation with increasing thiophene chain lengths in the PCEs.27 The two molecules DRCN5T and DRCN7T have axisymmetric structures and therefore larger dipole moment changes (Δμge) than the centrosymmetric DRCN4T, DRCN6T, and DRCN8T. It is expected that a large dipole moment change (Δμge) in going from the ground state (μg) to the excited state (μe) could decrease the Coulomb binding energy of the exciton, which makes electron transfer from the donors to the acceptors easier.12 This notion is supported by Yu’s work that posited strong intramolecular local dipole plays a vital role in assisting the separation of excitons along the aggregates, then enriches the generation free charges, which is reflected in the enhancement of short-circuit current (Jsc).12,29,30 However, the influence of the molecular geometry on the excited


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state dynamics, especially charge separation at the D/A interface, is still missing. Thus, in this work we attempted to explore how molecular symmetry controls the rate of ultrafast CT and charge separation efficiejncy by using femtosecond transient electronic absorption spectroscopy. Based on our studies, we found that both the rates of CT and CS are very sensitive to the molecular symmetry. A possible charge transfer mechanism was further proposed for the strong correlation between molecular symmetry and charge transfer dynamics. MATERIALS AND METHODS Sample Preparation. Materials used here including the films, their synthetic details as well as the spectroscopic characterization have been reported elsewhere.27,31 [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) was purchased from American Dye Source, Inc. and used as received. DRCNnT:PC71BM blends were spun from chloroform with 10 mg ml-1 for 1:0.8 blends (DRCNnT:PC71BM mass ratio) in argon-protected glovebox. Monolayer and heterojunction films were prepared on thin glass substrates, which were cleaned by sonication in acetone and isopropyl alcohol and exposure to O2 plasma for 15 min each. Substrates were subsequently placed in an argon-filled glovebox and spun at 1,500 r.p.m. for 60 s. UV−Vis Absorption and Emission Spectral Measurements. All steady-state spectra of samples were measured using a UV−vis spectrometer (Model U-3010, Hitachi, Japan) and a fluorescence spectrometer (F-4600, Hitachi, Japan) in 1 × 1 cm quartz cuvettes. Femtosecond Pump−Probe Absorption Spectroscopy. Transient absorption spectra were acquired using a 500 Hz regenerative Ti:sapphire amplifier (Coherent Legend Elite, 1 mJ, 50 fs, 800 nm), split by a 90/10 beam splitter to generate pump and probe beams, details are descripted in our previous works.32,33 An optical parametric amplifier (TOPAS, Light Conversion, Lithuania) was tuned to 600 nm as pump beam and chopped at 250 Hz. The white light was generated from


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a 2 mm thick water cell. To avoid isotropic signals, the angle between the polarizations of pump and probe beams was set to the magic angle (54.7°). The pump intensity was about 45 nJ/pulse for film samples and about 70 nJ/pulse for solution samples (spot size of ca. 130 μm at sample), and the probe beam size at film sample ~100 μm. No significant sample degradation was observed throughout the TA experiments. Repeat measurements were done by changing several different spots for each sample to ensure the reliable results. Global analysis of the dispersion-corrected data was done using Glotaran software package (please see details in the Supporting Information).34 Computational Details. All computational calculations were performed with the Gaussian 09 software using density functional theory (DFT) method.35 The structures of single DRCNnT molecules were optimized at the level of B3LYP/6-31G*. The frequency analysis was followed to assure that the optimized structures were stable states. Furthermore, anti-para π- π stacking dimer was out of consideration in our calculations. Time-dependent DFT (TD-DFT) calculation for the S0→Sn transitions using the same functional and basis set were then performed based on the optimized structures at ground states. RESULTS AND DISCUSSION Steady-State and Transient Absorption Measurements in Dilute Solutions. As shown in Figure 1b, all DRCNnT compounds in dilute solutions exhibit intense absorption with band maxima at ~510 nm, which is independent of the thiophene conjugation length and molecular symmetry. The main absorption is assigned to the intramolecular charge transfer (ICT) band. The fluorescence of these chromophores (Figure 1b) are modest with large Stokes shifts (Table S1), reaching 4371 cm-1 and 5425 cm-1 for DRCN5T in toluene and THF, respectively. The large Stokes shifts with high sensitivity to the solvent’s polarity indicate a dominant ICT character in the


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emission state due to a charge that shifted away from the central thiophene units towards the electron acceptor units at both ends. With the assistance of femtosecond transient absorption (fsTA) spectroscopy, the evolution of the photophysics (such as solvation, vibrational cooling, and conformational change) of DRCNnT compounds in toluene is successfully traced in the time range of 0.1-1000 ps. As shown in the transient absorption spectra (for example, Figure S1a, DRCN5T), the positive signals represent the excited-state absorption (ESA) whereas the negative signals with fixed band shape ranging from 400−580 nm represent ground state bleaching (GSB). The remainder of the broad negative signals with significant band peak redshifts over time are attributed to stimulated emission (SE). At 1000 ps, the ΔODs of GSB and SE do not yet decrease to zero. By tracking the ns-TA spectrum at 200 ns, a new broad ESA band appearing above 600 nm is identified to triplet states through kinetic comparison (the triplet lifetime becomes longer in the degassed solution, Figure S1b). A sequential model (Figure S1c) is applied to globally analyze the fs-TA data of these compounds in toluene (the fitting results are shown in Figure S1d and S1e) using the Glotaran software package.34 A rough odd−even alternation (Figure S1f) over structures in the time constants of solvation and followed conformational planarization are clearly observed (the fitting quality is seen in Figure S1g and S1h). In our previous study on another similar quadrupolar molecules, such an odd−even effect on the excited state relaxation process as caused by the backbone lengths is even strong in polar solvent. Here, in weak quadrupolar toluene, the relaxation time constants (listed in Table S2) for all DRCNnT (n = 4−8) compounds are very close to each other regardless of their symmetrical differences, indicating that toluene with an weak quadrupole moment ( = 7.92) is sufficient to induce a moderate asymmetric distribution of the excitation over the two arms of each molecule.36,37 After symmetry breaking, two terminal acceptor groups will have different basicities. Consequently, solvation effects and electron-vibration


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coupling are stronger with the polar arm. Although the excited state symmetry breaking occurs in toluene, the differences in dipole-quadrupole interactions between the two arms for DRCN5T and DRCN7T are still clearly observed to have had shorter solvation time and faster conformational stabilization relative to the even-numbered ones. This indicates that the axial symmetric chromophores bear high polarizability, enhancing remarkable ICT character during the excited state relaxation. On the contrary, the nonpolar central symmetric molecules show poor interaction with nearby solvent molecules upon excitation, and thus contribute to slow structural relaxation from the initial twisted excited state to the final relaxed planar state.

Figure 2. (a) Normalized absorption spectra of neat DRCNnT (n = 4−8) films as cast from chloroform. (b) Normalized absorption spectra of DRCNnT:PC71BM (n = 4−8, 1:0.8, wt/wt) blend films. Steady-State and Transient Absorption Measurements in Solid Films. We now turned to investigate the excited state dynamics of these odd-even oligothiophenes in the solid states and check whether the molecular geometry (odd or even) would influence the charge separation. For


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the solid films, as shown in Figure 2a, the electronic absorption spectra of all compounds (DRCNnT, n = 4−8) consist of a broad band peaking around ~600 nm. The distinct red-shifted absorption (Table S3) compared with their corresponding absorptions in solution is ascribed in part to the π−π* transition of conjugated portion and intramolecular charge transfer as well as stronger intermolecular π−π interaction in the solid. Notably, the neat DRCN5T−DRCN8T films exhibit similarly pronounced low-energy shoulder except for the DRCN4T film, indicating a higher degree of disorder in DRCN4T film and more effective molecular packing in DRCN5T−DRCN8T films. With respect to their pristine neat films, narrower absorption and moderate blueshifts (data shown in Table S3) are observed especially in the blends of DRCNnT:PC71BM (n = 4-8, Figure 2b). These changes are attributed to the disruptions of the crystallized domains by aggregated PC71BM clusters.38 It is worth noting that DRCN5T:PC71BM and DRCN7T:PC71BM blends exhibit larger blueshifts, which indicate that the crystallized oligothiophene domains is strongly disrupted after mixing with PC71BM. This is further evidenced by the transmission electron microscopy (TEM) as shown in Figure S2. This kind of miscible morphology could enhance the charge separation as discussed below. The TEM bright-field images (Figure S2) show that the morphologies of DRCN5T:PC71BM and DRCN7T:PC71BM blend films are characterized by a well-developed interpenetrating network which is favorable for exciton dissociation. Large well-ordered domains are identified to be a highway that accelerate the separation process of strongly bound CT to loosely bound CS at the interfaces, which finally lead to abundant bimolecular recombination with lifetimes beyond nanoseconds.38 But conversely, the films of the even-numbered molecules coarsen significantly with larger domains over 40 nm, which is beyond the hole or electron diffusion length after exciton dissociation (reported as 36 nm for the hole in the as-cast DRCN5T:PC71BM device),38 facilitating the formation of strongly


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Coulombic bounded CT states with resulting lifetimes of geminate recombination within nanoseconds. Next, to discern the influence of Δμge as caused by molecular symmetry on the charge generation dynamics, transient absorption studies at 300 K on a time resolution of ~100 fs were performed. The TA spectra of DRCN7T: PC71BM blend film (Figure 3a, using DRCN7T as the example) are characterized by a broad GSB and part of ESA in the visible region. However, all signals are more intense in the blend because of better film quality than the neat film (Figure S3a). The generation of excitons were mainly located from 500 to 700 nm, which is consistent with the main peak in the steady state UV–vis absorption. Afterward, all transient features decay entirely to zero around 1 ns, pointing to a full recovery of the ground-state population. The GSB bleach signals in the blend film (Figure 3a) persist to the time scale much longer than exciton lifetimes in neat film (kinetic comparison shown in Figure S3b), suggesting a competing charge separation process. Here, the strong ESA signals at the very beginning are attributed to photo-induced excitons (EX), followed by CT and CS states till the end of the measurement. It should be stressed that among this series compounds, the axial symmetric compounds exhibit high polarizability in light of the conclusion based on the abovementioned TA tests in solution, so the high initial “pseudo” charge transfer (PCT, refer to intramolecular charge transfer within DRCNnT molecule, which is distinct from intermolecular CT state between different molecules in films) state population will be observed in the odd-numbered (axisymmetric) molecule films. Through the ordered aggregate, highly delocalized excitons migrate rapidly to the interfaces, providing the driving force for the PCT state and further evolving to the charge separated state.


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Figure 3. (a) Transient absorption spectra at different time delays for DRCN7T:PC71BM blend film with 600 nm excitation. The artifact signals between 725 nm and 745 nm were removed. (b) Proposed exciton dissociation mechanism. (c) Species-associated difference spectra (SADS, normalized) of DRCN7T:PC71BM blend films obtained by target analysis. (d) Concentrations of the individual species as functions of time obtained by target analysis. (e) Distinct odd−even trends observed in Δμge and charge separation yields. (f) Graphical representation of the proposed explanation of the dipolar effect as shown for the excited states of DRCN7T. Based on these results, we then proposed a model for the excited state dynamics during the charge separation process in these blend films (Figure 3b). Loosely speaking, the lowest D+-A(CT) state corresponds to the situation in which the hole delocalized on the HOMO level of a DRCNnT molecule and the electron on the LUMO level of an adjacent PC71BM molecule, while the electron-hole pairs are still strongly Coulombic bounded. After the CT state dissociated, the charge carriers could perform a few ultrafast hops, allowing their separation before the CS states thermally relax and charge carriers enter the normal polaron hopping regime. In particular, the


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formation of the CT/CS states are shown to be extremely fast (in the hundred-femtosecond regime); as a result, many internal relaxation processes (such as structural relaxations) are much slower. Limited to our time window, longer-lifetime transients (such as traps and triplet states) are not involved in our simplified model. Analysis based on this model are employed for global fitting (refer to the detailed discussion in the Supplementary Materials, the fitting parameters are collected in Table S4) to yield species-associated difference spectra (SADS, Figure 3c) and corresponding concentrations over time (Figure 3d). In Figure 3c, the signatures of the CT/CS signals partially match with the spectrum of the initial exciton (EX) signal but differ substantially at ~450 nm, due to the both charges and electro absorptions (EA, see details in Figure S5). In this blend film, when the molecule was photo-excited to form a polarized exciton with partial electron-hole separation, the positive charge will be mainly localized in the central thiophene units and the negative charge in the RCN units at both ends. Due to the high intramolecular polarization (the calculated µg, µe and Δµge of DRCN7T are depicted in Figure S4), the reduced Coulombic potential among electronhole pairs assists the following intermolecular electron to easily transfer from the RCN units to an adjacent PC71BM molecule. As a result, the distance between electrons and holes is already slightly large, which helps to minimize the electron-hole binding energy and leads to impressive GSB peak

r redshifts (this Stark shift derives from a microscopic electric field E at the heterojunctions and the resulting EA signature appears), as shown in the SADS (Figure 3c). The Stark shift causes a change in the absorption spectra of the surrounding molecules, which normally leads to a redshift in absorption due to lowering of the optical absorption gap.16,20 For all TA measurements of DRCNnT:PC71BM blends, the fitted data shown in Table S4 reveals that the formation of the CS state for DRCN5T/DRCN7T is obviously faster than the same processes for DRCN4T/DRCN6T/DRCN8T, in which an odd-even trend is found in the CS yields


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(shown in the left coordinate of Figure 3e, see the calculation method in the Supplementary Materials). DFT calculations (Table S5) further confirms the molecular symmetry (odd or even) can significantly influence the polarization of the ground and excited states, thus the dipole moment changes upon the electronic transition from the ground to excited states.39 The order of magnitude of the dipole moment change Δμge via a HOMO−LUMO transition (shown in the right coordinate of Figure 3e) suggests that axial symmetric molecules could potentially have a more polarized exciton, with the largest effective electron−hole displacement compared to the excitons of other even oligomers in this series. As the exciton split, the resulting hole−electron separation could initially have different separation distances according to the excess energy and local environment.29 In addition, the size of the exciton depends on the strength of the electronic coupling and disruptions to the coupling due to disorder.25 As a result of the shorter conjugation length and poor film quality, DRCN4T-based device exhibits such low CS yield as well as its extremely poor device performance. A large Δμge indeed facilitates the EX-to-CS and CT-to-CS processes, resulting in large hole−electron separation distances (Figure 3f), while a small Δμge effectively stabilizes the interacting hole−electron pairs in the CT state at a lower energy than in the CS state. Apparently, to split the excitons in the conjugated molecules, a larger local Δμge is desirable, which can be rationalized by stabilizing the CS state after charge transfer to PC71BM, thus decreasing the overall Coulomb binding energy and enhancing the CT/CS rate. CONCLUSIONS In summary, a series of thiophene-based donor molecules is studied to clarify the dependence of molecular symmetry on exciton dissociation dynamics. We found that the charge separations are faster and the CS yields are higher for such kinds of donor materials with axial symmetric molecules than those molecules with central symmetry. Upon excitation, the odd-numbered


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DRCNnT molecules have relative larger values of Δμge, which correlates to a large degree of polarizability. Due to the formation of polarized exciton, the “pseudo” charge transfer states are promptly stabilized at the well-ordered domain. As a result of the approximately free barrier of charge delocalization for electron transfer to fullerene acceptors, the corresponding binding energy of e-h species is decreased at the heterojunction interface. Therefore, the distance between electron and hole for the odd-numbered chromophores is much larger than for the even-numbered ones; whereas the value of Δμge for even-numbered molecules is close to 0, and the degree of polarized excited state is quite small; thus, further dissociation to loosely bound electron-hole pairs at the interface for even-numbered molecules is difficult and geminate recombination is then facilitated. All of the results mentioned above carry important implications for organic solar cells in that the most critical aspect of designing OPV systems should be optimized over nanoscale morphology through structural symmetry mediation on monomers to achieve optimal aggregates for the efficient formation of coherent delocalized states. The observed odd−even effect is generally considered to be a microscopic factor on film aggregates, which resulted in the obvious differences in the initial charge separation paths and dynamics from different symmetric molecules. We expect our transient absorption spectral analyses will further attract interest in detailed theoretical and experimental studies to shed light on the validity of this insight that molecular symmetry control mediation of the charge separation processes in the organic photovoltaics, and further improve device performances towards commercialization. ASSOCIATED CONTENT Supporting Information


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Additional results, such as detailed TA measurements for DRCNnT (n = 4−8) in toluene and Transmission electron microscopy (TEM) images of DRCNnT (n = 4−8) blend films, and transient absorption spectra for DRCN7T neat film and kinetic comparison (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by NSFCs (Nos. 21673252, 21333012, 21672211, and 21773252) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12020200).

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Unveiling the Molecular Symmetry Dependence of Exciton

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