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• W2763352905 abstract "In the latest issue of Nature Chemistry, Stern et al. combine ultrafast transient absorption, sliding-window Fourier transform, and time-resolved emission spectroscopy to describe a comprehensive mechanism for endothermic singlet fission in a tetracene derivative. In the latest issue of Nature Chemistry, Stern et al. combine ultrafast transient absorption, sliding-window Fourier transform, and time-resolved emission spectroscopy to describe a comprehensive mechanism for endothermic singlet fission in a tetracene derivative. Singlet fission (SF) is the spin-allowed conversion of a singlet exciton into two spin-correlated triplet excitons, and solar cells that operate by this mechanism have the potential to overcome the Shockley-Queisser limit of power conversion efficiency.1Smith M.B. Michl J. Chem. Rev. 2010; 110: 6891-6936Crossref PubMed Scopus (1380) Google Scholar For each absorbed photon in a chromophore, this process generates two triplet excitons. The process by which the singlet state splits into two triplets involves a series of intermediate states that have been the subject of many studies. The information gained from numerous ultrafast spectroscopic measurements has been the cornerstone of evolving material design principles and the synthesis of new chromophores to establish fundamental structure-property relationships. However, fundamental information is lacking on the excited-state dynamics of SF chromophores, especially in cases where the energetics of the singlet state is below the energy of twice the triplet energy (E[S1] > 2E[T1], endothermic SF). Only recently have advancements in spectroscopic techniques started to shed light on the key states that lie between the singlet and the individual triplets.2Breen I. Tempelaar R. Bizimana L.A. Kloss B. Reichman D.R. Turner D.B. J. Am. Chem. Soc. 2017; 139: 11745-11751Crossref PubMed Scopus (89) Google Scholar Thus, the holistic picture deduced by Stern et al. in the latest issue of Nature Chemistry unveils important features of the excited-state dynamics and the interplay between electronic and vibrational degrees of freedom.3Stern H.L. Cheminal A. Yost S.R. Broch K. Bayliss S.L. Chen K. Tabachnyk M. Thorley K. Greenham N. Hodgkiss J.M. et al.Nat. Chem. 2017; https://doi.org/10.1038/nchem.2856Crossref PubMed Scopus (154) Google Scholar SF has been studied since the 1960s, but it has remained a laborious challenge to achieve a comprehensive understanding of the entire process, particularly in endothermic SF systems.1Smith M.B. Michl J. Chem. Rev. 2010; 110: 6891-6936Crossref PubMed Scopus (1380) Google Scholar Although the SF process was first studied as a photophysical curiosity, its potential relevance to devices was eventually perceived by Dexter. In fact, Dexter was the first to suggest the use of SF to enhance solar power conversion efficiency. Indeed, in 1979 he proposed that tetracene could serve as a sensitizer for silicon4Dexter D.L. J. Lumin. 1979; 18: 779Crossref Scopus (103) Google Scholar because it undergoes SF and produces triplets with sufficiently high triplet energy to transfer to silicon, which has a bandgap ∼1.1 eV. The potential to use tetracene as a sensitizer for silicon has made it a subject of extensive studies over the ensuing decades.5Tayebjee M.J.Y. Clady R.G.C.R. Schmidt T.W. Phys. Chem. Chem. Phys. 2013; 15: 14797-14805Crossref PubMed Scopus (94) Google Scholar These materials have also provided a number of surprising and seemingly contradictory results. One example is the confounding observation that, despite its endothermic nature, SF is temperature independent in tetracene.6Wilson M.W.B. Rao A. Johnson K. Gélinas S. di Pietro R. Clark J. Friend R.H. J. Am. Chem. Soc. 2013; 135: 16680-16688Crossref PubMed Scopus (171) Google Scholar A similar observation was reported in the paper by Stern et al. and was incorporated into their mechanistic understanding (vide infra). Another is the fact that different techniques have found different timescales for triplet-pair formation. For example, Zhu and coworkers reported the ultrafast population of triplet-pair states on sub-picosecond timescales, whereas most other studies have observed only slower dynamics.7Chan W.-L. Ligges M. Zhu X.Y. Nat. Chem. 2012; 4: 840-845Crossref PubMed Scopus (261) Google Scholar Stern et al. observed both sub-picosecond and slower dynamics for triplet-pair production and suggested a possible mechanism to explain this complex behavior. Overall, this study represents a step toward reconciling many of these seemingly contradictory observations into a unified model of endothermic SF. To achieve such a comprehensive understanding of the process, Stern et al. studied a tetracene derivative (TIPS-tetracene) because it undergoes endothermic SF. Two factors enabled the authors to unveil complex singlet decay dynamics in this system. First, they employed broadband transient absorption spectroscopy with sufficiently fast time resolution of 16 fs, and second, the compound under investigation, TIPS-tetracene, exhibited sharp singlet and triplet absorption features.3Stern H.L. Cheminal A. Yost S.R. Broch K. Bayliss S.L. Chen K. Tabachnyk M. Thorley K. Greenham N. Hodgkiss J.M. et al.Nat. Chem. 2017; https://doi.org/10.1038/nchem.2856Crossref PubMed Scopus (154) Google Scholar This combination of factors led the authors to resolve the ultrafast singlet decay, in addition to a slower singlet decay component. Both decays were accompanied by a concomitant rise in triplet-pair photoinduced absorption. Furthermore, the high time resolution of the excitation pulse enabled impulsive excitation of the sample, resulting in vibrational features in the transient absorption spectra. The authors used these vibrational features to gain insights into the mechanism behind the complex kinetics observed. When TIPS-tetracene was measured in dilute solution, where SF did not occur, a Fourier transform across the spectrum revealed the vibrational modes unique to the singlet excited state. Many of these vibrational modes were absent in the ground-state resonance Raman spectrum. Subsequently, the authors investigated the triplet pair populated in films and found that oscillations persisted, although in different modes than for the singlet exciton. This preservation of vibrational coherence after conversion from singlet to triplet pair suggests that vibrational modes are important in coupling the singlet- and triplet-pair states.8Musser A.J. Liebel M. Schnedermann C. Wende T. Kehoe T.B. Rao A. Kukura P. Nat. Phys. 2015; 11: 352Crossref Scopus (254) Google Scholar Using a novel sliding-window Fourier transform method, the authors confirmed these findings by following the vibrational modes present in the photoexcited TIPS-tetracene as a function of time. This measurement revealed a decrease in the singlet-associated 315 cm−1 mode and a concurrent rise in the triplet-associated 760 cm−1 mode on a timescale similar to that of the population conversion observed in transient absorption spectroscopy. These measurements were coupled with variable temperature studies, which revealed temperature-independent triplet-pair formation but activated triplet-pair separation. When the triplet pair is lower in energy than the singlet, temperature-independent SF can be rationalized. Similarly, if there is some binding energy between two triplets, this stabilizing interaction explains why triplet-pair dissociation is thermally activated. Although empirical evidence for some energetic stabilization for triplet pairs continues to gather, the exact origin of this interaction is not yet fully clear.2Breen I. Tempelaar R. Bizimana L.A. Kloss B. Reichman D.R. Turner D.B. J. Am. Chem. Soc. 2017; 139: 11745-11751Crossref PubMed Scopus (89) Google Scholar, 9Pun A.B. Sanders S.N. Kumarasamy E. Sfeir M.Y. Congreve D.N. Campos L.M. Adv. Mater. 2017; https://doi.org/10.1002/adma.201701416Crossref PubMed Scopus (55) Google Scholar All of the experimental studies could be combined to describe a comprehensive mechanism (Figure 1). Directly after light absorption, strong coupling of vibrational and electronic degrees of freedom (which has been suggested theoretically for unsubstituted tetracene10Morrison A.F. Herbert J.M. J. Phys. Chem. Lett. 2017; 8: 1442-1448Crossref PubMed Scopus (82) Google Scholar) promotes an equilibrium between the singlet and triplet pairs. The secondary singlet decay and triplet-pair rise are tentatively assigned to the damping of vibrational modes, which enables this equilibration and leaves all population in the lower-energy triplet-pair state. From the triplet-pair state, despite the entropy gain afforded by dissociation,7Chan W.-L. Ligges M. Zhu X.Y. Nat. Chem. 2012; 4: 840-845Crossref PubMed Scopus (261) Google Scholar triplet separation is thermally activated. Finally, the authors found that the more crystalline film featured much longer-lived bound triplet-pair states. This result was rationalized by the poor pi overlap in the crystal, which led to slow triplet hopping. Therefore, faster triplet-pair separation might be promoted by more favorable orientations for triplet hopping in more disordered samples that adhere less strictly to the prevailing crystal structure present in crystalline domains. Altogether, the results presented here advance the mechanistic understanding of SF in endothermic systems. Although it remains a major challenge to incorporate SF chromophores in high-performance next-generation devices, the fundamental understanding of the process sets the foundation for molecular engineering to synthesize families of materials that advance our understanding of this fascinating photophysical process." @default.
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• W2763352905 title "A Birds-Eye View of the Uphill Landscape in Endothermic Singlet Fission" @default.
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