A Simple Kinetic Model for Singlet Fission: A Role of Electronic and Entropic Contributions to Macroscopic Rates (original) (raw)
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Intermolecular Vibrations Drive Ultrafast Singlet Fission
arXiv: Chemical Physics, 2019
Singlet fission is a spin-allowed exciton multiplication process in organic semiconductors that converts one spin-singlet exciton to two triplet excitons. It offers the potential to enhance solar energy conversion by circumventing the Shockley-Queisser limit on efficiency. Recently, the mechanism of the primary singlet fission process in pentacene and its derivatives have been extensively investigated, however, the nature of the primary ultrafast process in singlet fission is still a matter of debate. Here, we study the singlet fission process in a pentacene film by employing a combination of transient-grating (TG) and two-dimensional (2D) electronic spectroscopy complemented by quantum chemical and nonadiabatic dynamics calculations. The high sensitivity of heterodyne detected TG spectroscopy enabled us to capture the vibrational coherence and to show that it mediates the transition from the singlet excited electronic state to the triplet-pair state. This coherent process is furthe...
Theory of Singlet Fission in Polyenes, Acene Crystals, and Covalently Linked Acene Dimers
The Journal of Physical Chemistry C, 2015
We report quadruple configuration interaction calculations within the extended Pariser−Parr−Pople Hamiltonian on the excited states of aggregates of polyenes, crystalline acenes, and covalently linked dimers of acene molecules. We determine the precise energy orderings and analyze the cluster wave functions in order to arrive at a comprehensive physical understanding of singlet fission in these diverse families of materials. Our computational approach allows us to retain a very large number of basis states and thereby obtain the correct relative energy orderings of one electron−one hole Frenkel and charge-transfer excitons versus intra-and intermolecular two electron−two hole triplet−triplet excited states. We show that from the energy orderings it is possible to understand the occurrence of singlet fission in polyene and acene crystals, as well as its near total absence in the covalently linked acene dimers. As in the acene crystals, singlet fission in the polyenes is a multichromophoric phenomenon, with the well-known 2 1 A − g playing no direct role. Intermolecular charge-transfer is essential for singlet fission in both acenes and polyenes, but because of subtle differences in the natures and orderings of the aggregate excited states, the mechanisms of singlet fission are slightly different in the two classes. We are thus able to give qualitative physical reasoning for the slower singlet fission in the polyenes, relative to that in crystalline pentacene. Our work also gives new insight into the complex exciton dynamics in tetracene crystals, which has been difficult to understand theoretically. Our large-scale many-body calculations provide us with the ability to understand the qualitative differences in the singlet fission yields and rates between different classes of π-conjugated materials.
Nature chemistry, 2017
The absorption of a photon usually creates a singlet exciton (S1) in molecular systems, but in some cases S1 may split into two triplets (2×T1) in a process called singlet fission. Singlet fission is believed to proceed through the correlated triplet-pair (1)(TT) state. Here, we probe the (1)(TT) state in crystalline hexacene using time-resolved photoemission and transient absorption spectroscopies. We find a distinctive (1)(TT) state, which decays to 2×T1 with a time constant of 270 fs. However, the decay of S1 and the formation of (1)(TT) occur on different timescales of 180 fs and <50 fs, respectively. Theoretical analysis suggests that, in addition to an incoherent S1→(1)(TT) rate process responsible for the 180 fs timescale, S1 may couple coherently to a vibronically excited (1)(TT) on ultrafast timescales (<50 fs). The coexistence of coherent and incoherent singlet fission may also reconcile different experimental observations in other acenes.
Evidence for conical intersection dynamics mediating ultrafast singlet exciton fission
Singlet exciton fission is the process in organic semiconductors through which a spin-singlet exciton converts into a pair of spin-triplet excitons residing on diierent chromophores, entangled in an overall spin-zero state. For some systems, singlet fission has been shown to occur on the 100 fs timescale and with a 200% quantum yield, but the mechanism of this process remains uncertain. Here we study a model singlet fission system, TIPS-pentacene, using ultrafast vibronic spectroscopy. We observe that vibrational coherence in the initially photogenerated singlet state is transferred to the triplet state and show that this behaviour is eeectively identical to ultrafast internal conversion for polyenes in solution. This similarity in vibronic dynamics suggests that both multi-molecular singlet fission and single-molecular internal conversion are mediated by the same underlying relaxation processes, based on strong coupling between nuclear and electronic degrees of freedom. In its most eecient form this leads to a conical intersection between the coupled electronic states. S inglet exciton fission has long commanded interest as an exceptionally fast channel to generate triplet excitons in organic materials. At the heart of the process is the coupling of the final triplet pair to the initially photoexcited singlet state, which ensures conservation of spin 1. In systems where singlet fission is exergonic, it can be rapid and highly efficient. For instance, thin films of pentacene and TIPS-pentacene exhibit triplet formation with a time constant of 80 fs and quantum yields of 200% (refs 2,3). Current interest in this phenomenon is driven by its potential to circumvent the Shockley–Queisser limit for single-junction solar cells. By converting high-energy photons into two low-energy excited states, singlet fission offers a means to overcome thermalization losses. Devices based on pentacene, a fission sensitizer, have demonstrated external quantum efficiencies of 129%, the highest for any photovoltaic technology to date 4. Current theoretical descriptions of singlet fission are framed by the kinetic model proposed by Johnson and Merrifield in 1970 (ref. 5), which established the role of a triplet pair (TT) coupled into an overall singlet state as an intermediate to triplet formation, but the details of the ultrafast fission process are subject to debate. Most theoretical studies of the canonical system, pentacene, focus on the low-lying electronic states and their composition by TT, intermolecular charge-transfer and monomolecular singlet configurations 6–9. The relative strengths of the direct electronic couplings between these states then determine the overall fission mechanism, proposals for which can be roughly divided into two classes. In the 'direct' model, the TT state is formed by electronic coupling between the initial singlet and the TT manifolds, and singlet-to-triplet conversion is accomplished through an avoided crossing or a conical intersection 9–11. Alternatively, the 'mediated' model proposes that the initial singlet couples much more strongly to virtual charge-transfer configurations 6,7 and, in some descriptions, may even directly populate charge-transfer states 1,8. These charge-transfer configurations, whether real or virtual, in turn have strong coupling to TT and enable singlet fission. The limit of very strong coupling between the singlet and this intermediate state results in the 'coherent' model, where the photoexcited singlet state and TT are linked by electronic coherence 12. A recent survey of acene derivatives 3 suggests that two of these mechanisms, direct and virtual mediated coupling, may contribute to determining the rate of singlet fission in the acenes, with the weight of mediated coupling dependent on the contribution of charge-transfer character to the first excited state. In contrast to the rigorous theoretical debate, there is little experimental data available that helps our understanding of the mechanism behind singlet fission. Standard experimental techniques such as transient absorption or photoluminescence spectroscopy enable direct monitoring of singlet and triplet exciton populations and the kinetics of their interconversion 2,13–17. These methods, however, are generally insensitive to the nature of the coupling between states. More sophisticated spectroscopic approaches are therefore required to build a clear picture of such underlying dynamics. At the same time, there is a growing awareness of the central role of vibronic coupling in a range of ultrafast photophysical processes, from photosynthesis to charge separation in organic heterojunctions 18–21. Many ultrafast (<200 fs) photophysical processes, such as internal conversion in isolated molecules, have been studied experimentally and theoretically in detail and are generally accepted to involve conical intersections between the electronic states induced by strong coupling between vibrational and electronic degrees of freedom 22,23. The ultrafast timescales for fission point to the importance of understanding the associated nuclear dynamics that mediate the conversion of singlets to triplets, but no experimental information on the underlying vibronic coupling is at present available. To probe the involved nuclear dynamics and thereby provide new insight into the mechanism of ultrafast singlet fission, we approach the problem with a recently developed experimental technique that uses impulsively generated vibrational coherence as a probe of vibronic coupling 24. We study thin films of TIPS-pentacene (Fig. 1a), a solution-processable pentacene derivative that undergoes efficient fission on sub-100-fs
The Journal of Physical Chemistry C, 2017
We have applied our functional mode framework for singlet fission to pentacene, a prototypical organic material for multiple exciton generation. It was found that singlet fission in pentacene occurs predominantly through a coherent process mediated by a virtual charge-transfer (CT) intermediate, which lies slightly above the photoexcited S 1 S 0 state. This energetic near-degeneracy facilitates a substantial vibronic superposition, leading to a rapid transition rate of 25.1 ps −1. By contrast, the direct S 1 S 0 → T 1 T 1 path constitutes a much more sluggish route with a rate of 2.6 ps −1 , largely due to the weak diabatic coupling between participant states. These data collectively afford an experimentally consistent rate of 27.7 ps −1 for the entire singlet fission process. The presence of this lowlying CT intermediate suggests that enhanced electronic coupling between S 1 S 0 and T 1 T 1 states may collude with coherent vibrational mixing to expedite the formation of triplet pairs. The knowledge gleaned from our investigations heralds a new approach to charge transfer-mediated singlet fission, a rapidly growing research field that holds great promise to circumvent the Shockley−Queisser thermodynamic limit for solar energy conversion.
Tuneable Singlet Exciton Fission and Triplet–Triplet Annihilation in an Orthogonal Pentacene Dimer
1 wileyonlinelibrary.com proceed on ultrafast (≈100 fs) time scales, allowing it to out-compete other decay channels and achieve high effi ciencies. [ 3 ] The essential condition for effi cient SEF is the energetic alignment of the singlet and triplet states, such that 2 E (T 1) ≤ E (S 1). A recent combined theoretical and experimental study of SEF rates in a range of acene solids has demonstrated that the rate of SEF is also greatly affected by the strength of intermolecular coupling within the fi lm. [ 4 ] In the canonical system, pentacene, triplet pair formation is exo-thermic and the intermolecular coupling is strong, resulting in SEF with an 80 fs time constant and nearly 200% yield. [ 5 ] Though most experimental studies of SEF have involved crystalline, polycrystalline or amorphous solids, the most basic unit capable of SEF is a pair of chromophores. Indeed, it was recently demonstrated in concentrated solutions of TIPS-pentacene that singlet fi ssion can proceed at high efficiency through bimolecular diffusional interactions. [ 6 ] However , early attempts to directly control the interaction between chromophores through the use of covalent dimers have not been as successful. The most notable systems in this regard are tetracene and 1,3-diphenylisobenzofuran. These materials are found to exhibit effi cient SEF in the solid state, but their covalent dimers achieved triplet yields of only a few percent. In both of these studies, [ 7 ] the two SEF chromophores were joined by a range of linkers to modify the strength of the electronic coupling between them, with the aim of tuning the rate and effi ciency of SEF. The impact was subtle, and it thus remains unclear why covalent dimers have proved ineffi cient to date. Current models suggest that dimers should be asymmetric or contain signifi cant cofacial interaction between chromophores to attain high triplet yields. [ 2,8 ] Interestingly, a recent study of pentacene dimers separated by a phenyl spacer unit achieved triplet yields above 100% in spite of using the same symmetric bonding motifs of the earlier tetracene dimers. [ 9 ] In this work, we report highly effi cient intramolecular SEF in a new type of covalent dimer, with triplet yields of up to 192 ± 3%. The molecule used in this study, 13,13′-bis(mesityl)-6,6′-dipentacenyl (DP-Mes, Figure 1 a), consists of two pen-tacenes directly bonded through a single C C bond with two bulky mesityl groups at the meso-positions. The geometry of the dimer, with two nearly orthogonal pentacene cores, is unlike Fast and highly effi cient intramolecular singlet exciton fi ssion in a pentacene dimer, consisting of two covalently attached, nearly orthogonal pentacene units is reported. Fission to triplet excitons from this ground state geometry occurs within 1 ps in isolated molecules in solution and dispersed solid matrices. The process exhibits a sensitivity to environmental polarity and competes with geometric relaxation in the singlet state, while subsequent triplet decay is strongly dependent on conformational freedom. The near orthogonal arrangement of the pentacene units is unlike any structure currently proposed for effi cient singlet exciton fi ssion and may lead to new molecular design rules.
Maximizing Singlet Fission by Intermolecular Packing
The Journal of Physical Chemistry Letters, 2014
A novel nonadiabatic molecular dynamics scheme is applied to study the singlet fission (SF) process in pentacene dimers as a function of longitudinal and lateral displacements of the molecular backbones. Detailed twodimensional mappings of both instantaneous and long-term triplet yields are obtained, characterizing the advantageous and unfavorable stacking arrangements, which can be achieved by chemical substitutions to the bare pentacene molecule. We show that the SF rate can be increased by more than an order of magnitude through tuning the intermolecular packing, most notably when going from cofacial to the slipped stacked arrangements encountered in some pentacene derivatives. The simulations indicate that the SF process is driven by thermal electron−phonon fluctuations at ambient and high temperatures, expected in solar cell applications. Although charge-transfer states are key to construct continuous channels for SF, a large charge-transfer character of the photoexcited state is found to be not essential for efficient SF. The reported time domain study mimics directly numerous laser experiments and provides novel guidelines for designing efficient photovoltaic systems exploiting the SF process with optimum intermolecular packing.
Solution-based intramolecular singlet fission in cross-conjugated pentacene dimers
Nanoscale, 2016
We show unambiguous and compelling evidence by means of pump-probe experiments, which are complemented by calculations using ab initio multireference perturbation theory, for intramolecular singlet fission (SF) within two synthetically tailored pentacene dimers with cross-conjugation, namely XC1 and XC2. The two pentacene dimers differ in terms of electronic interactions as evidenced by perturbation of the ground state absorption spectra stemming from stronger through-bond contributions in XC1 as confirmed by theory. Multiwavelength analysis, on one hand, and global analysis, on the other hand, confirm that the rapid singlet excited state decay and triplet excited state growth relate to SF. SF rate constants and quantum yields increase with solvent polarity. For example, XC2 reveals triplet quantum yields and rate constants as high as 162 ± 10% and (0.7 ± 0.1) × 10 12 s −1 , respectively, in room temperature solutions. † Electronic supplementary information (ESI) available. See
The Journal of Physical Chemistry C, 2014
Singlet fission (SF) is a spin-allowed process by which a singlet excited state splits into a pair of triplet states. This process can potentially increase the efficiency of organic solar cells by a factor of 1.5. In this article, we study the dynamics of SF in different molecular aggregates of perylenediimide (PDI) derivatives, pentacene, and 1,3diphenylisobenzofuran (DPB). To compute the SF rate, we have adopted a Markovian density matrix propagation approach to model SF in a molecular dimer. This approach allows accounting for both the coherent and incoherent processes that mediate the triplet formation. Our calculations show that SF can be much faster in PDI derivatives than in pentacene and DPB. Our analysis also indicates that SF is principally mediated by a superexchange mechanism that involves charge transfer states as virtual intermediates. In addition, because of the existence of different pathways for the formation of the triplet states, signatures of quantum interference are clearly observed.
Intra- to Intermolecular Singlet Fission
The Journal of Physical Chemistry C, 2015
Singlet fission, the splitting of one singlet into two triplets, can potentially increase the efficiency of optoelectronic devices beyond conventional limits. Among the singlet fission molecules discovered to date, two mechanisms have emerged: intra-or intermolecular singlet fission. Here we show a combined intra-to intermolecular singlet fission mechanism in the model system of diphenyl-dicyano-oligoene (DPDC). Excitation of DPDC to the first optically bright state leads to the ultrafast formation of an intramolecular triplet pair, which decays in 40 ps in the solution phase but can also split competitively in 30 ps into two long-lived triplets (2×T 1) on adjacent molecules in solid films. These findings suggest a design principle for efficient singlet fission: the independent tuning of singlet−triplet pair coupling and triplet pair splitting from intra-and intermolecular interactions, respectively.