Acceptor-Type Singlet Fission Material Based on Strong Absorption Tetracyanothienoquinoid Skeleton

Open AccessCCS ChemistryCOMMUNICATIONS9 Mar 2024Acceptor-Type Singlet Fission Material Based on Strong Absorption Tetracyanothienoquinoid Skeleton Senhao Wang†, Xiang-Yang Liu†, Mengfan Zhang, Long Wang, Ganglong Cui, Hongbing Fu and Jiannian Yao Senhao Wang† Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024 , Xiang-Yang Liu† College of Chemistry and Material Science, Sichuan Normal University, Chengdu 610068 , Mengfan Zhang Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024 , Long Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024 , Ganglong Cui *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875 , Hongbing Fu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Beijing Key Laboratory for Optical Materials and Photonic Devices, Department of Chemistry, Capital Normal University, Beijing 100048 and Jiannian Yao Beijing National Laboratory for Molecules Science (BNLMS), Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 https://doi.org/10.31635/ccschem.024.202303814 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The practical efficiency of singlet fission (SF)-based photovoltaic devices is still far from satisfactory due to the limited scope of SF materials suitable for device application and the scarcity of schemes available for triplet utilization. Most SF materials identified to date are typically electron donors while acceptor-type SF materials remain largely unexplored. Basically, the combination of a conventional electron donor and SF-active electron acceptor could circumvent the competitive energy transfer channel and better play the unique advantages of the SF process, which might be an adequate alternative for practical application. In this work, we presented a new acceptor-type SF material based on a tetracyanothienoquinoid skeleton. Such a quinoid skeleton exhibited strong absorption, ultrafast SF process, and excellent stability. Using transient spectroscopy and multireference calculations (XDW-CASPT2), the SF dynamics were examined featuring the rapid generation and subsequent annihilation and/or partial dissociation of multiexciton states. Therefore, our results not only provide a robust acceptor-type SF material but also suggest an adequate donor–acceptor alternative for SF-based solar cells, which could pave the way for the practical application of such a potential process. Download figure Download PowerPoint Introduction Singlet fission (SF) is a carrier multiplication process by which organic semiconductors harvest one high-energy photon to form one singlet and then transform it into two triplet excitons.1–3 Such a process has attracted much attention due to its potential to effectively alleviate thermalization issues and surpass the Shockley–Queisser limit of single-junction solar cells.1–4 The successful fabrication of an SF-based photovoltaic device with external quantum efficiency over 100% proves its great potential in future high-efficiency optoelectronic conversion.5,6 Although great effort and progress in photophysical mechanisms and material design have been made, the power conversion efficiency of SF-based photovoltaic devices is still far from satisfactory in practical applications.1–6 The major barriers lie in the limited scope of SF materials suitable for device application and the scarcity of schemes available for triplet utilization. Most SF materials identified to date are typically electron donors, and thus, electron acceptors with very deep unoccupied states are required to implement effective multiple carriers harvesting. Moreover, the rapid singlet energy transfer from SF donor to electron acceptor is likely to inevitably compete with the premeditated multiexciton (ME) generation and subsequent multiple charge dissociation processes, which might undermine the SF advantages. This is not a trivial task since most efficient organic photovoltaic devices are based on bulk heterojunction structures, featuring nanoscopic phase-separated domains with high donor/acceptor interfacial area. Actually, the combination of a conventional electron donor and SF-active electron acceptor might be an adequate alternative to circumvent the competitive energy transfer channel and better play the SF advantages in practical application. However, in contrast to the more extensively studied electron donor systems exemplified by acene derivatives,7–15 the acceptor-type SF materials remain largely unexplored.1,16–18 In this work, we presented a new acceptor-type SF material based on a tetracyanothienoquinoid {CNBDT = 2,6-bis(dicyanomethylene)-2,6-dihydrobenzo-[1,2-b:4,5-b']dithiophene} skeleton (Scheme 1). This quinoid skeleton exhibited strong absorption with an extinction coefficient (εmax) of ∼105 M−1cm−1, ultrafast SF with a rate (kSF) of (0.5 ps)−1, and excellent stability. Based on transient spectroscopy and multireference calculations (XDW-CASPT2),19 the SF dynamics were examined featuring the rapid formation and subsequent annihilation and/or partial dissociation of ME states. Accordingly, our work demonstrates that the current CNBDT skeleton could serve as a potential acceptor-type SF candidate system for device exploration and attempt. Scheme 1 | Acceptor-type SF material based on tetracyanothienoquinoid system. SF, singlet fission. Download figure Download PowerPoint Results and Discussion Molecular system and steady-state characterizations Materials with quinoidal characteristics recently emerged as a distinctive class of organic semiconductors with excellent optoelectronic properties and device performance.20–32 The innate quinoidal character not only endows these skeletons with diradical properties to enable them to fulfill the SF energetics but also gives these compounds the relatively larger electronic transition dipolar moments to achieve high extinction coefficients, which make them suitable for potential SF candidates.33–39 Herein, we selected a bis(dicyanomethylene)-substituted benzodithiophene, CNBDT skeleton as a model system on account of its facile synthesis and excellent optoelectronic properties.40–42 Molecular synthetic routes are provided in the Supporting Information. Single-crystal X-ray diffraction data revealed that the CNBDT molecules exhibited a slip-stack compact packing arrangement with a close intermolecular π–π distance of 3.29 Å (Figure 1), which were conducive to SF occurrence and exciton diffusion processes. Next, the steady-state absorption spectra for CNBDT were measured in both dilute solution and thin film (Figure 2a). In a dilute solution, the spectra showed a strong absorption peak maximum at 563 and 595 nm assigned to the optically allowed third singlet excited state (S3) (the first and second singlet excited states, S1 and S2, were optically forbidden; for detailed discussion, see the theoretical calculation section). In a vapor-deposited thin film, the spectra exhibited a significantly redshifted absorption peak around 700 nm, indicating the strong intermolecular interaction in the characteristic J-type aggregate, as shown in the single crystal structure (Figure 1). Moreover, on account of the quinoidal structure nature, the CNBDT skeleton was proved to have a very high molar extinction coefficient of 9.1 ± 0.7 × 104 M−1 cm−1 (Figure 2b).33 High molar extinction coefficients of SF materials are known to effectively control and minimize the active layer thickness conducive to practical device fabrication.2,33 Subsequently, we studied the electrochemical properties of the CNBDT skeleton using cyclic voltammetry (Figure 2c). The current system exhibited two reversible oxidation peaks that indicated a step-by-step oxidation process. Based on the electrochemical data, the lowest unoccupied molecular orbital (LUMO) energy level was estimated to be −4.23 eV for the CNBDT molecule. Such a deep LUMO energy level provided clear-cut evidence for the electron acceptor-type character, which might not only have facilitated exciton separation in the solar cells but also improved chemical stability. The stability test results confirmed that the CNBDT molecule exhibited excellent stability given that the thin film maintained over 90% of its original absorbance exposed to air and light for more than 30 days (Figure 3a; for more details, see section 4 of the Supporting Information). The half-life (t1/2) was estimated to be 142 days for the CNBDT molecule (Figure 3b). In addition, both the solution and thin film of CNBDT showed extremely weak fluorescence emission (ΦF > 0.01, Supporting Information Figure S4), indicating that there were other more efficient nonradiative pathways dominating the excited state deactivation. The high molar extinction coefficient and excellent stability made it more interesting for us to exploit the feasibility of the current quinoidal system as a potential accept-type SF material. Figure 1 | Structural characterizations of the current quinoidal CNBDT system: Solid-packing arrangements within single-crystal structure. CNBDT, 2,6-bis(dicyanomethylene)-2,6-dihydrobenzo-[1,2-b:4,5-b']dithiophene. Download figure Download PowerPoint Excited state energy landscape and capture of ME species Then theoretical calculations based on the multireference XDW-CASPT2(12,9) method19 were carried out to investigate excited state electronic structure and property focusing on obscure dark states and potential ME state (Figure 4 and Table 1; for details see section 6 of the Supporting Information).1,38,39,43,44 Based on the optimized ground state (11Ag) geometry, we explored the electronic structures of the relevant excited states, that is, 21Ag, 31Ag, and 11Bu singlet states, as well as the first triplet 13Bu state. The S1 state turned out to be a symmetry-forbidden 21Ag dark state featuring a dominated H–1 → L single excitation transition (the two highest-occupied and two lowest-lying unoccupied orbitals are denoted as H–1, H, and L, L+1). Above the S1 state, was the S2 state, surprisingly found to be another dark state in which the characteristic double excitation involved the simultaneous promotion of two electrons from the occupied to unoccupied orbitals featuring H → L transition, with an electronic configuration of (222220200) (Table 1). This state was, therefore, referred to as an ME (31Ag) state. Further, we observed that the strongly allowed electronic transition led to the optically bright S3 (11Bu) state (f = 0.76), with H → L singly excited contribution, which represented the usual entry point into the excited singlet manifold by light absorption. The low-lying triplet state, T1 (13Bu), made the quinoidal system meet the SF energetics either from the S3 (11Bu) bright state or the S2 (31Ag) ME/S1 (21Ag) dark states, ascribed to the innate quinoidal character. More importantly, given the quasi-degeneracy of the S3 (11Bu) and 31Ag ME states in energy, the conversion process from bright to ME state was generally accepted to have occurred on a sub-picosecond time scale or shorter (Figure 4). Such a rapid process likely outperformed other excited state decay pathways, possibly including the S3 (11Bu) → S1 (21Ag) process, which might be responsible for fluorescence quenching in the current CNBDT system, and was consistent with the spectroscopic results discussed next. Table 1 | Excited State Energy Diagram and Electronic Configuration of the CNBDT Skeleton State Configurationa Weight Evb Eac S3 (11Bu) (22222ud00) 0.33 2.24 2.14 (2222ud200) 0.14 S2, ME (31Ag) (222220200) 0.24 2.02 1.95 (2u2222d00) 0.15 (22222ud00) 0.14 S1 (21Ag) (2222u2d00) 0.58 1.44 1.27 (222220200) 0.11 T1 (13Bu) (22222uu00) 0.80 — 0.46 aElectronic configuration describing the occupation of the 12 electrons in the 9 molecular orbitals occupied by: 2, two electrons; u: single electron with "up" spin; d, single electron with "down" spin; 0, empty with no electron. bEv, vertical excitation energy calculated at the Frank–Condon region, measured as electron volts (eV). cEa, adiabatic excitation energy calculated at the energy-minima structures (eV). CNBDT, 2,6-bis(dicyanomethylene)-2,6-dihydrobenzo-[1,2-b:4,5-b']dithiophene; FC, Frank–Condon; ME, mutiexciton. Figure 2 | Steady-state characterizations of the quinoidal CNBDT system: (a) Normalized absorption curves from a dilute solution (black line) and thin film (red line). (b) Molar extinction coefficient plot (595 nm). (c) Cyclic voltammograms in the CH2Cl2 solution. CNBDT, 2,6-bis(dicyanomethylene)-2,6-dihydrobenzo-[1,2-b:4,5-b']dithiophene. Download figure Download PowerPoint Figure 3 | Stability testing. (a) Changes of the UV–vis absorption spectra of CNBDT thin films under ambient light and air conditions. (b) Linear regression and extrapolation to 50% absorption from the plot of absorption at maximum absorption obtains a half-life of 142 days. CNBDT, 2,6-bis(dicyanomethylene)-2,6-dihydrobenzo-[1,2-b:4,5-b']dithiophene. Download figure Download PowerPoint Photophysical characterizations and SF dynamics In order to unravel the SF dynamics in the current quinoidal system, we performed femtosecond transient absorption (TA) measurements both in dilute solution and thin film. In dilute solution, upon 480 nm excitation, the TA spectra featured a strong ground state bleaching (GSB) signal near 595 nm and broad excited state absorption (ESA) in the near-infrared (NIR) region at the initial 0.1 ps, assigned to the initially populated S3 bright state (Figure 5a). Additionally, the spectral line shape of absorption signals exhibited an obvious change and a new ESA band arose around 500–700 nm overlapping significantly with the GSB signal, indicating that a new transient species appeared. The kinetic decay curves also presented such a rapid spectral evolution process within a 2 ps time scale (Figure 5b). Subsequently, the spectral line shape remained constant and the corresponding signals gradually recovered to the ground line without further evolution at the long time delays. The TA measurements in the long wavelength NIR region also showed a similar spectral evolution process ( Supporting Information Figure S9). Figure 4 | Excited state electronic structures and ME generation process in the current CNBDT system based on multireference theoretical calculations (XDW-CASPT2). CNBDT, 2,6-bis(dicyanomethylene)-2,6-dihydrobenzo-[1,2-b:4,5-b']dithiophene; ME, mutiexciton. Download figure Download PowerPoint Figure 5 | SF dynamics in dilute solution. (a) TA spectra, (b) selected kinetics, and (c) species-associated spectra based on global analysis from TA measurements in dilute solution. SF, singlet fission; TA, transient absorption. Download figure Download PowerPoint The global fitting analysis was carried out using a two-state model for the TA data; the corresponding species-associated spectra and kinetics are shown in Figure 5c. Combined with the triplet sensitization experiment (for details, see section 7.4 of the Supporting Information), we found that the second transient species had a distinctive triplet-like spectral signature (Figure 5c). Given the rapid formation rate of such species, we assign this to the intramolecular triplet pairs, namely the 31Ag ME state elaborated in the above the theoretical calculation section. That is, in dilute solution, the excited state dynamics of CNBDT molecule represented a rapid intramolecular SF process in which 31Ag ME state could be populated from the optically excited S3 bright state with a time constant of 0.5 ± 0.2 ps and then underwent triplet–triplet annihilation recovery to the ground state, with a lifetime of 33.7 ± 3.2 ps. Next, we turned to thin film measurements to decipher the aggregate state SF dynamics for the CNBDT system (Figure 6). Although an apparent difference was observed in absorption signatures due to the aggregation effect mainly featuring the significantly redshifted GSB signals, the TA data presented a spectral evolution similar to the case in dilute solution at the initial stage, including (1) the rapid decay of the initial ESA band around 800 nm, (2) the appearance of a new ESA band around 550–900 nm overlapping seriously with GSB signals and leading to the marked attenuation of the latter (Figure 6a,b). That is, the rapid decay of S3-ESA around 800 nm was concurrent with the rise of 31Ag ME state around 550–900 nm illustrating the occurrence of ultrafast SF process in the thin film, albeit the obvious molecular aggregation, ascribed to the quasi-degeneracy energy levels of the 11Bu and 31Ag ME states, as confirmed in the theoretical calculation section. Thus, the conversion process from 11Bu bright to 31Ag ME state was generally accepted to have occurred on a time scale of sub-picosecond or shorter and could outperform other excited state decay pathways. At the longer time delays (>25 ps), following the decay of the 31Ag ME state, a new spectral species appeared around 550–900 nm, similar to the absorption signature of the ME state but featured more obvious positive ESA signals. Such transient species could persist to several nanoseconds of our TA apparatus. However, we did not detect any long-lived signals at the microsecond timescale by nanosecond laser flash photophysics measurements, indicating that this end species could not be free triplets from the effective decoupling separation of the ME state. Therefore, we prudently assign it to the relaxed species of ME state, denoted as ME*. The TA measurements in the long wavelength NIR region also showed a similar spectral evolution process, especially the characteristic spectral change of the ME and ME* states ( Supporting Information Figure S12). Figure 6 | SF dynamics in thin film. (a) TA spectra, (b) selected kinetics, and (c) species-associated spectra based on global analysis from TA measurements in thin film. SF, singlet fission; TA, transient absorption. Download figure Download PowerPoint The TA data were best fit to a three-state branched kinetic model, considering the direct decay of the intermediate ME state (Figure 6c). That is, the excited state dynamics of CNBDT in thin film elucidated rapid intramolecular SF and subsequent ME evolution process. After photo-excitation, the initial bright S3 (11Bu) populates the 31Ag ME state at an ultrafast rate of 1.0 ± 0.2 ps. Then a portion of ME species underwent triplet–triplet annihilation recovery to the ground state with a lifetime of 25.6 ± 3.8 ps, while the rest could happen the partial decoupling separation with a rate of 79.1 ± 8.8 ps to form a more stabilized ME* state. These stabilized ME* excitons finally recovered to ground state with a lifetime of 6.8 ± 1.5 ns. We presumed that the excessively strong intermolecular interaction in the thin film, as confirmed by the remarkably redshifted absorption, might restrain and even preclude the ME state from further decoupling and separating into totally free long-lived triplets. Future work points to fabricating the current CNBDT into practical photovoltaic devices and adjusting the aggregate state molecular arrangements via a systematic side chain modification engineering to achieve an effective triplet dissociation.18,37,45–47 Conclusion Based on TA spectroscopy and multireference calculations, we present a new acceptor-type SF material based on a quinoidal system consisting of a CNBDT skeleton. Such a quinoid skeleton exhibits strong absorption with an extinction coefficient of ∼105 M−1cm−1, an ultrafast SF process with a rate of (0.5 ps)−1, and excellent stability. Our work demonstrates that the current CNBDT skeleton could serve as a potential acceptor-type SF candidate system for device exploration and attempt. Moreover, the results elucidate that the combination of conventional donor and SF-active acceptor might be an adequate alternative for SF-based solar cells, which could pave the way for the practical application of such a potential process. Supporting Information Supporting Information is available and includes molecular synthesis, theoretical calculation details, and supplementary materials for TA measurements. Conflict of Interest There is no conflict of interest to report. Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC; grant no. 22005210) and by the Fundamental Research Program of Shanxi Province, China (grant nos. 202203021224004 and 20210302124469). References 1. Smith M. B.; Michl J.Singlet Fission.Chem. Rev.2010, 110, 6891–6936. Google Scholar 2. Rao A.; Friend R. H.Harnessing Singlet Exciton Fission to Break the Shockley–Queisser Limit.Nat. Rev. Mater.2017, 2, 17063. Google Scholar 3. Xia J. L.; Sanders S. N.; Cheng W.; Low J. Z.; Liu J. P.; Campos L. M.; Sun T. L.Singlet Fission: Progress and Prospects in Solar Cells.Adv. Mater.2017, 29, 1601652. Google Scholar 4. Hanna M. C.; Nozik A. 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Crossref, Google Scholar Previous articleNext article FiguresReferencesRelatedDetails Issue AssignmentNot Yet AssignedSupporting Information Copyright & Permissions© 2024 Chinese Chemical SocietyKeywordssinglet fissionacceptorquinoidal skeletonmutiexciton statemultireference calculationtransient spectroscopyAcknowledgmentsThis work was supported by the National Natural Science Foundation of China (NSFC; grant no. 22005210) and by the Fundamental Research Program of Shanxi Province, China (grant nos. 202203021224004 and 20210302124469). Downloaded 557 times PDF downloadLoading ...