Magnetic dipolar interaction between correlated triplets created by singlet fission in tetracene crystals

Polarity of Ordered Solvent Molecules in 9,9'-Bianthracene Single Crystals Selects between Singlet Fission or Symmetry-Breaking Charge Separation

Singlet exciton fission (SF) and symmetry-breaking charge separation (SB-CS) are both photophysical processes that can occur between two organic chromophores and are both of interest to improve solar energy conversion. Here, we tuned the photophysics of a 9,9'-bianthracene (BA) single crystal between SF and SB-CS using solvent intercalation to change the electric field within the crystal. Crystals of BA were grown in o-xylene, chlorobenzene, o-dichlorobenzene, and benzonitrile, as well as solvent-free from a melt. The crystals were studied by X-ray diffraction, steady-state optical spectroscopy, and transient absorption microscopy to elucidate the role of the intercalated solvent molecules. The crystals with no solvent in the structure undergo fast SF (<2 ps), while the crystals with intercalated moderately polar solvents o-xylene, chlorobenzene, and o-dichlorobenzene show evidence of charge-transfer-mediated SF. Finally, the crystals containing highly polar benzonitrile undergo SB-CS instead of SF. These results demonstrate that controlling solvation of BA in the crystal structure can tune its photophysics between SF and SB-CS.

Spin Mixing in Intramolecular Singlet Fission: A First-Principles-Based Quantum Dynamical Study

We investigate the dynamical interplay between the different triplet-pair spin states that are formed in the intramolecular singlet fission process in a series of pentacene-based dimers covalently bonded to a phenylene linker in ortho, meta, and para positions. Using first-principles calculations and a density matrix quantum dynamical approach we show that the spin dipole-dipole interaction leads to significant population of the quintet spin manifold in these regioisomers when the singlet, triplet and quintet triplet-pair states are quasidegenerate. Furthermore, we also show that the relative arrangement of the pentacene-like moieties has a profound impact on the dynamics of the spin-mixing process, affecting both the relative population of the different spin-states involved in the dynamics and the time scale of the process.

Electronic Couplings for Triplet-Triplet Annihilation Upconversion in Crystal Rubrene

Triplet-triplet annihilation photon upconversion (TTA-UC) is a process able to repackage two low-frequency photons into light of higher energy. This transformation is typically orchestrated by the electronic degrees of freedom within organic compounds possessing suitable singlet and triplet energies and electronic couplings. In this work, we propose a computational protocol for the assessment of electronic couplings crucial to TTA-UC in molecular materials and apply it to the study of crystal rubrene. Our methodology integrates sophisticated yet computationally affordable approaches to quantify couplings in singlet and triplet energy transfer, the binding of triplet pairs, and the fusion to the singlet exciton. Of particular significance is the role played by charge-transfer states along the b-axis of rubrene crystal, acting as both partial quenchers of singlet energy transfer and mediators of triplet fusion. Our calculations identify the π-stacking direction as holding notable triplet energy transfer couplings, consistent with the experimentally observed anisotropic exciton diffusion. Finally, we have characterized the impact of thermally induced structural distortions, revealing their key role in the viability of triplet fusion and singlet fission. We posit that our approaches are transferable to a broad spectrum of organic molecular materials, offering a feasible means to quantify electronic couplings.

Elucidating Singlet-Fission-Born Multiexciton Dynamics via Molecular Engineering: A Dilution Principle Extended to Quintet Triplet Pair

Multiexciton in singlet exciton fission represents a critical quantum state with significant implications for both solar cell applications and quantum information science. Two distinct fields of interest explore contrasting phenomena associated with the geminate triplet pair: one focusing on the persistence of long-lived correlation and the other emphasizing efficient decorrelation. Despite the pivotal nature of multiexciton processes, a comprehensive understanding of their dependence on the structural and spin properties of materials is currently lacking in experimental realizations. To address this gap in knowledge, molecular engineering was employed to modify the TIPS-tetracene structures, enabling an investigation of the structure-property relationships in spin-related multiexciton processes. In lieu of the time-resolved electron paramagnetic resonance technique, two time-resolved magneto-optical spectroscopies were implemented for quantitative analysis of spin-dependent multiexciton dynamics. The utilization of absorption and fluorescence signals as complementary optical readouts, in the presence of a magnetic field, provided crucial insights into geminate triplet pair dynamics. These insights encompassed the duration of multiexciton correlation and the involvement of the spin state in multiexciton decorrelation. Furthermore, simulations based on our kinetic models suggested a role for quintet dilution in multiexciton dynamics, surpassing the singlet dilution principle established by the Merrifield model. The integration of intricate model structures and time-resolved magneto-optical spectroscopies served to explicitly elucidate the interplay between structural and spin properties in multiexciton processes. This comprehensive approach not only contributes to the fundamental understanding of these processes but also aligns with and reinforces previous experimental studies of solid states and theoretical assessments.

Dynamic Evolving Exothermicity Steers Ultrafast Formation of a Correlated Triplet Pair State

Singlet fission (SF) presents an attractive solution to overcome the Shockley-Queisser limit of single-junction solar cells. The conversion from an initial singlet state to final triplet is mediated by the correlated triplet pair state ¹(T₁T₁). Despite significant advancement on ¹(T₁T₁) properties and its role in SF, a comprehensive understanding of the energetic landscape during SF is still unclear. Here, we study an unconventional SF system with excited-state aromaticity, i.e., cyano-substituted dipyrrolonaphtheridinedione derivative (DPND-CN), using time-resolved spectroscopy as a function of the temperature. We demonstrate that the population transfer from S₁ to ¹(T₁T₁) is driven by a time-dependent exothermicity resulting from the coherent coupling between electronic and spin degrees of freedom. This is followed by thermal-activated dissociation of ¹(T₁T₁) to yield free triplets. Our results provide some new insight into the SF mechanism, which may guide the development of new efficient and stable SF materials for practical applications.

Singlet Fission in Colloidal Nanoparticles of Amphipathic Diketopyrrolopyrrole Derivatives: Probing the Role of the Charge Transfer State

To evaluate the role of the charge transfer (CT) state in the singlet fission (SF) process, we prepared three 3,6-bis(thiophen-2-yl)diketopyrrolopyrrole (TDPP) derivatives with zero (Ph₂TDPP), one (Ph₂TDPP-COOH), and two (Ph₂TDPP-(COOH)₂) carboxylic groups, respectively. Their colloidal nanoparticles were also prepared by a simple precipitation method. The SF dynamics and mechanism in these colloid nanoparticles were investigated by using steady-state/transient absorption and fluorescence spectroscopy. Steady-state absorption spectra reveal that the strength of the CT resonance interactions between the adjacent DPP units is increased gradually from Ph₂TDPP to Ph₂TDPP-COOH and then to Ph₂TDPP-(COOH)₂. Fluorescence and transient absorption spectra demonstrate that SF is proceeded via a CT-assisted superexchange mechanism in these three nanoparticles. Furthermore, SF rate and yield are enhanced gradually with the increase of the number of the carboxylic group, which may be attributed to the enhancement of the CT coupling strength. The result of this work not only provides a better understanding of the SF mechanism especially for the role of the CT state but also gives some new insights for the design of efficient SF materials based on DPP derivatives.

Spin-Enhanced Reverse Intersystem Crossing and Electroluminescence in Copper Acetate-Doped Thermally Activated Delayed Fluorescence Material

Thermally activated delayed fluorescence (TADF) materials are attractive for next-generation organic light-emitting diodes (OLEDs) because of their utilization of nonradiative triplets via reverse intersystem crossing (RISC), which requires not only a small singlet-triplet energy splitting but also the conservation of spin angular momentum. Here we use copper acetate as a spin sensitizer to facilitate RISC and thus enhance electroluminescence in TADF-exciplex OLEDs. Copper acetate is involved in the radiative decay process due to its coordination interaction with exciplex molecules having intermolecular charge-transfer characteristics, which causes significant changes in the photoluminescence intensity and lifetime. Meanwhile, magneto-photoluminescence reveals that the addition of copper acetate promotes spin conversion in the RISC process. It allows the enhancement of the electroluminescence (∼80%) from spin-sensitized OLEDs, accompanied by the suppression of magneto-electroluminescence upon the doping of copper acetate. These results illustrate that using a spin sensitizer may overcome the limitation of harvesting nonradiative triplets in organic luminescent materials and devices.

Dynamics of Excitons in Conjugated Molecules and Organic Semiconductor Systems

The exciton, an excited electron-hole pair bound by Coulomb attraction, plays a key role in photophysics of organic molecules and drives practically important phenomena such as photoinduced mechanical motions of a molecule, photochemical conversions, energy transfer, generation of free charge carriers, etc. Its behavior in extended π-conjugated molecules and disordered organic films is very different and very rich compared with exciton behavior in inorganic semiconductor crystals. Due to the high degree of variability of organic systems themselves, the exciton not only exerts changes on molecules that carry it but undergoes its own changes during all phases of its lifetime, that is, birth, conversion and transport, and decay. The goal of this review is to give a systematic and comprehensive view on exciton behavior in π-conjugated molecules and molecular assemblies at all phases of exciton evolution with emphasis on rates typical for this dynamic picture and various consequences of the above dynamics. To uncover the rich variety of exciton behavior, details of exciton formation, exciton transport, exciton energy conversion, direct and reverse intersystem crossing, and radiative and nonradiative decay are considered in different systems, where these processes lead to or are influenced by static and dynamic disorder, charge distribution symmetry breaking, photoinduced reactions, electron and proton transfer, structural rearrangements, exciton coupling with vibrations and intermediate particles, and exciton dissociation and annihilation as well.

Spin Statistics for Triplet-Triplet Annihilation Upconversion: Exchange Coupling, Intermolecular Orientation, and Reverse Intersystem Crossing

Triplet-triplet annihilation upconversion (TTA-UC) has great potential to significantly improve the light harvesting capabilities of photovoltaic cells and is also sought after for biomedical applications. Many factors combine to influence the overall efficiency of TTA-UC, the most fundamental of which is the spin statistical factor, η, that gives the probability that a bright singlet state is formed from a pair of annihilating triplet states. The value of η is also critical in determining the contribution of TTA to the overall efficiency of organic light-emitting diodes. Using solid rubrene as a model system, we reiterate why experimentally measured magnetic field effects prove that annihilating triplets first form weakly exchange-coupled triplet-pair states. This is contrary to conventional discussions of TTA-UC that implicitly assume strong exchange coupling, and we show that it has profound implications for the spin statistical factor η. For example, variations in intermolecular orientation tune η from to through spin mixing of the triplet-pair wave functions. Because the fate of spin-1 triplet-pair states is particularly crucial in determining η, we investigate it in rubrene using pump-push-probe spectroscopy and find additional evidence for the recently reported high-level reverse intersystem crossing channel. We incorporate all of these factors into an updated model framework with which to understand the spin statistics of TTA-UC and use it to rationalize the differences in reported values of η among different common annihilator systems. We suggest that harnessing high-level reverse intersystem crossing channels in new annihilator molecules may be a highly promising strategy to exceed any spin statistical limit.

Evaluating the Electronic Structure of Coexisting Excitonic and Multiexcitonic States in Periodic Systems: Significance for Singlet Fission

Singlet fission (SF) in organic molecular solids is an example of a process that is challenging to describe with the most common electronic structure approaches. It involves optically bright singlet excited states delocalized over many molecules, which could be efficiently treated by density functional theory, and multiexcitonic localized states that have to be studied with wavefunction methods, usually with small clusters considering their expensive computational costs. In this work, we propose a methodology to combine multiconfigurational wavefunction calculations with reduced Hamiltonian to investigate the electronic structure of large clusters or fully periodic systems. The method is applied to the prototypical SF materials tetracene and pentacene. The results allow one to study how states of different natures (excitonic, charge-transfer, and multiexcitonic) coexist and are contaminated by their couplings in large or periodic systems. Novel insights are therefore possible. For example, because the excitonic bands are relatively broad with respect to the multiexcitonic states, there are limited regions of the crystal momentum space where the transition between the two is more likely.

Free-triplet generation with improved efficiency in tetracene oligomers through spatially separated triplet pair states

Singlet fission (SF) can potentially boost the efficiency of solar energy conversion by converting a singlet exciton (S₁) into two free triplets (T₁ + T₁) through an intermediate state of a correlated triplet pair (TT). Although efficient TT generation has been recently realized in many intramolecular SF materials, their potential applications have been hindered by the poor efficiency of TT dissociation. Here we demonstrate that this can be overcome by employing a spatially separated ¹(T…T) state with weak intertriplet coupling in tetracene oligomers with three or more chromophores. By using transient magneto-optical spectroscopic methods, we show that free-triplet generation can be markedly enhanced through the SF pathway that involves the spatially separated ¹(T…T) state rather than the pathway mediated by the spatially adjacent TT state, leading to a marked improvement in free-triplet generation with an efficiency increase from 21% for the dimer to 85% (124%) for the trimer (tetramer).

Photogeneration of Long-Lived Triplet States through Singlet Fission in Lycopene H-Aggregates

Molecular triplet excitons produced through singlet fission (SF) usually have shorter triplet lifetimes due to exciton-exciton recombination and relaxation pathways, thereby resulting in complex device architectures for SF-boosted solar cells. Using broadband transient absorption spectroscopy, we here show that the photoexcitation of nanostructured lycopene H-aggregates at room temperature produces free triplets with an unprecedented 35-fold enhancement in the lifetime compared to those localized on the monomer backbone. The observed rise of a spectrally blue-shifted correlated T-T pair state in ∼19 ps with distinct vibronic features provides the basis for SF-induced triplet generation.

Unconventional singlet fission materials

Singlet fission (SF) is a photophysical downconversion pathway, in which a singlet excitation transforms into two triplet excited states. As such, it constitutes an exciton multiplication generation process, which is currently at the focal point for future integration into solar energy conversion devices. Beyond this, various other exciting applications were proposed, including quantum cryptography or organic light emitting diodes. Also, the mechanistic understanding evolved rapidly during the last year. Unfortunately, the number of suitable SF-chromophores is still limited. This is per se problematic, considering the wide range of envisaged applicability. With that in mind, we emphasize uncommon SF-scaffolds and outline requirements as well as strategies to expand the chromophore pool of SF-materials.

Using temperature dependent fluorescence to evaluate singlet fission pathways in tetracene single crystals

The temperature-dependent fluorescence spectrum, decay rate, and spin quantum beats are examined in single tetracene crystals to gain insight into the mechanism of singlet fission. Over the temperature range of 250 K-500 K, the vibronic lineshape of the emission indicates that the singlet exciton becomes localized at 400 K. The fission process is insensitive to this localization and exhibits Arrhenius behavior with an activation energy of 550 ± 50 cm^-1. The damping rate of the triplet pair spin quantum beats in the delayed fluorescence also exhibits an Arrhenius temperature dependence with an activation energy of 165 ± 70 cm^-1. All the data for T > 250 K are consistent with direct production of a spatially separated ¹(T⋯T) state via a thermally activated process, analogous to spontaneous parametric downconversion of photons. For temperatures in the range of 20 K-250 K, the singlet exciton continues to undergo a rapid decay on the order of 200 ps, leaving a red-shifted emission that decays on the order of 100 ns. At very long times (≈1 µs), a delayed fluorescence component corresponding to the original S₁ state can still be resolved, unlike in polycrystalline films. A kinetic analysis shows that the redshifted emission seen at lower temperatures cannot be an intermediate in the triplet production. When considered in the context of other results, our data suggest that the production of triplets in tetracene for temperatures below 250 K is a complex process that is sensitive to the presence of structural defects.

Anisotropic Geminate and Non-Geminate Recombination of Triplet Excitons in Singlet Fission of Single Crystalline Hexacene

Singlet fission is believed to improve the efficiency of solar energy conversion by breaking up the Shockley-Queisser thermodynamic limit. Understanding of triplet excitons generated by singlet fission is essential for solar energy exploitation. Here we employed transient absorption microscopy to examine dynamical behaviors of triplet excitons. We observed anisotropic recombination of triplet excitons in hexacene single crystals. The triplet exciton relaxations from singlet fission proceed in both geminate and non-geminate recombination. For the geminate recombination, the different rates were attributed to the significant difference in their related energy change based on the Redfield quantum dissipation theory. The process is mainly governed by the electron-phonon interaction in hexacene. On the other hand, the non-geminate recombination is of bimolecular origin through energy transfer. In the triplet-triplet bimolecular process, the rates along the two different optical axes in the a-b crystalline plane differ by a factor of 4. This anisotropy in the triplet-triplet recombination rates was attributed to the interference in the coupling probability of dipole-dipole interactions in the different geometric configurations of hexacene single crystals. Our experimental findings provide new insight into future design of singlet fission materials with desirable triplet exciton exploitations.

Manifestation of specific features of T-exciton migration in magnetic field effects on TT-annihilation in molecular crystals: Analysis of low-field resonances

The manifestation of specific features of T-exciton migration in the shape of low field resonances (LFRs) in the magnetic field effects on the TT-annihilation in molecular crystals is studied in detail. The LFRs are shown to be caused by avoided crossing of spin-levels of T-excitons in magnetic fields nearly parallel to the axis of the zero field splitting interaction tensor. Simple and accurate formulas for the shape of the LFR-line are derived within the hopping model of T-exciton migration. With these formulas, we demonstrate that the LFR-line shape is fairly sensitive to the anisotropy of T-exciton migration, in particular, in quasi-one-dimensional (quasi-1D) and quasi-two-dimensional (quasi-2D) limits of exciton migration. The analysis of the shape is shown to allow for obtaining the magnitude of the small rate of jumps out of 1D and 2D spaces of fast migration in the cases quasi-1D and quasi-2D migration, respectively. In addition, this analysis enables one to obtain the spin relaxation rate of T-excitons.

Triplet-Pair States in Organic Semiconductors

Entanglement of states is one of the most surprising and counterintuitive consequences of quantum mechanics, with potent applications in cryptography and computing. In organic semiconductor materials, one particularly significant manifestation is the spin-entangled triplet-pair state, which consists of a pair of localized triplet excitons coupled into an overall spin-0, -1, or -2 configuration. The most widely analyzed of these is the spin-0 pair, denoted ¹(TT), which was initially invoked in the 1960s to explain delayed fluorescence in acene films. It is considered an essential gateway state for triplet-triplet annihilation and the reverse process, singlet fission, enabling interconversion between one singlet and two triplet excitons without any change in overall spin. This state has returned to the forefront of organic materials research in recent years, thanks both to its central role in the resurgent field of singlet fission and to its implication in a host of exotic new photophysical behaviors. Here we review the properties of triplet-pair states, from first principles to recent experimental results.

Tuning Spin Dynamics in Crystalline Tetracene

Tetracene is an archetypal material undergoing singlet fission-the generation of a pair of triplet excitons from one singlet exciton. Here, using time-resolved electron spin resonance, we show how the spin dynamics in tetracene crystals are influenced by temperature and morphology. Upon cooling from 300 to 200 K, we observe a switch between singlet fission and intersystem crossing generated triplets, manifesting as an inversion in transient spin polarization. We extract a spin dephasing time of approximately 40 ns for fission-generated triplets at room temperature, nearly 100 times shorter than the dephasing time that we measure for triplets localized on isolated tetracene molecules. These results highlight the importance of morphology and thermal activation in singlet fission systems.

Triplet Pair States in Singlet Fission

This account aims at providing an understanding of singlet fission, i.e., the photophysical process of a singlet state ( S₁) splitting into two triplet states (2 × T₁) in molecular chromophores. Since its discovery 50 years ago, the field of singlet fission has enjoyed rapid expansion in the past 8 years. However, there have been lingering confusion and debates on the nature of the all-important triplet pair intermediate states and the definition of singlet fission rates. Here we clarify the confusion from both theoretical and experimental perspectives. We distinguish the triplet pair state that maintains electronic coherence between the two constituent triplets, ¹(TT), from one which does not, ¹(T···T). Only the rate of formation of ¹(T···T) is defined as that of singlet fission. We present distinct experimental evidence for ¹(TT), whose formation may occur via incoherent and/or vibronic coherent mechanisms. We discuss the challenges in treating singlet fission beyond the dimer approximation, in understanding the often neglected roles of delocalization on singlet fission rates, and in realizing the much lauded goal of increasing solar energy conversion efficiencies with singlet fission chromophores.

Electrical Detection of Singlet Fission in Single Crystal Tetracene Transistors

We present the electrical detection of singlet fission in tetracene by using a field-effect transistor (FET). Singlet fission is a photoinduced spin-dependent process, yielding two triplet excitons from the absorption of a single photon. In this study, we engineered a more deterministic platform composed of an organic single crystal FET rather than amorphous or polycrystalline FETs to elucidate spin-dependent processes under magnetic fields. Despite the unipolar operation and relatively high mobility of single crystal tetracene FETs, we were able to manipulate spin dependent processes to detect magnetoconductance (MC) at room temperature by illuminating the FETs and tuning the bias voltage to adjust majority charge carrier density and trap occupancy. In considering the crystalline direction and magnetic field interactions in tetracene, we show the MC response observed in tetracene FETs to be the result of the singlet fission process.

Coupled double triplet state in singlet fission

The highly unusual state, 1(TT), is a coupled, double triplet state that has recently garnered significant attention. This multiexcitonic state can be formed by a quantum transition from a single-photon bright state in a variety of organic semiconducting materials. 1(TT)'s transient nature and similarity to independent triplets, however, has led to significant difficulties in characterization and prediction of its properties. Recent progress describing 1(TT) from theory and experiment are breaking through these difficulties, and have greatly advanced our comprehension of this state. Starting from the early description of 1(TT) in polyenes, this perspective discusses formation mechanisms, spectroscopic signatures, and the scope of intertriplet interactions. When employing singlet fission to generate charge carriers in a solar cell, 1(TT) has a central role. Due to the variety of coupling strengths between triplet states in 1(TT) amongst different chromophores, two different strategies are discussed to enable efficient charge carrier extraction. Continued growth in our understanding of 1(TT) may lead to control over complex quantum states for intriguing applications beyond high-efficiency, organic solar cells.

Transport of Spin-Entangled Triplet Excitons Generated by Singlet Fission

Singlet fission provides a promising route for overcoming the Shockley-Queisser limit in solar cells using organic materials. Despite singlet fission dynamics having been extensively investigated, the transport of the various intermediates in relation to the singlet and triplet states is largely unknown. Here we employ temperature-dependent ultrafast transient absorption microscopy to image the transport of singlet fission intermediates in single crystals of tetracene. These measurements suggest a mobile singlet fission intermediate state at low temperatures, with a diffusion constant of 36 cm²s^-1 at 5 K, approaching that for the free singlet excitons, which we attribute to the spin-entangled correlated triplet pair state ¹[TT]. These results indicate that ¹[TT] could transport with a similar mechanism as the bright singlet excitons, which has important implications in designing materials for singlet fission and spintronic applications.

Exciton Transport in Singlet Fission Materials: A New Hare and Tortoise Story

Singlet fission is promising for redistributing the solar spectrum to overcome the Shockley-Queisser limit for single-junction solar cells using molecular materials. Despite recent experimental and theoretical advances in understanding the underlying mechanisms, how exciton transport is coupled to singlet fission dynamics is much less explored. In this Perspective, we examine exciton transport in singlet fission materials, highlighting the use of transient absorption microscopy (TAM) to track the population of different states in both spatial and temporal domains. In contrast to the conventional picture where singlet and triplet excitons migrate independently, TAM measurements of acene single crystals reveal cooperative transport between fast-moving singlet and slow-moving triplet excitons. Such cooperative transport is unique to singlet fission materials and allows hundreds of nanometers triplet migration on the nanosecond time scale, beneficial for solar cell applications. The transport of triplet pair intermediates and general criteria for achieving cooperative singlet-triplet transport are also discussed.

Striking the right balance of intermolecular coupling for high-efficiency singlet fission

Singlet fission is a process that splits collective excitations, or excitons, into two with unity efficiency. This exciton splitting process, unique to molecular photophysics, has the potential to considerably improve the efficiency of optoelectronic devices through more efficient light harvesting. While the first step of singlet fission has been characterized in great detail, subsequent steps critical to achieving overall highly-efficient singlet-to-triplet conversion are only just beginning to become well understood. One of the most elementary suggestions, which has yet to be tested, is that an appropriately balanced coupling is necessary to ensure overall highly efficient singlet fission; that is, the coupling needs to be strong enough so that the first step is fast and efficient, yet weak enough to ensure the independent behavior of the resultant triplets. In this work, we show how high overall singlet-to-triplet conversion efficiencies can be achieved in singlet fission by ensuring that the triplets comprising the triplet pair behave as independently as possible. We show that side chain sterics govern local packing in amorphous pentacene derivative nanoparticles, and that this in turn controls both the rate at which triplet pairs form and the rate at which they decay. We show how compact side chains and stronger couplings promote a triplet pair that effectively couples to the ground state, whereas bulkier side chains promote a triplet pair that appears more like two independent and long-lived triplet excitations. Our results show that the triplet pair is not emissive, that its decay is best viewed as internal conversion rather than triplet-triplet annihilation, and perhaps most critically that, in contrast to a number of recent suggestions, the triplets comprising the initially formed triplet pair cannot be considered independently. This work represents a significant step toward better understanding intermediates in singlet fission, and how molecular packing and couplings govern overall triplet yields.

Vibronic exciton theory of singlet fission. I. Linear absorption and the anatomy of the correlated triplet pair state

Recent time-resolved spectroscopic experiments have indicated that vibronic coupling plays a vital role in facilitating the process of singlet fission. In this work, which forms the first article of a series, we set out to unravel the mechanisms underlying singlet fission through a vibronic exciton theory. We formulate a model in which both electronic and vibrational degrees of freedom are treated microscopically and non-perturbatively. Using pentacene as a prototypical material for singlet fission, we subject our theory to comparison with measurements on polarization-resolved absorption of single crystals, and employ our model to characterize the excited states underlying the absorption band. Special attention is given to the convergence of photophysical observables with respect to the basis size employed, through which we determine the optimal basis for more expensive calculations to be presented in subsequent work. We furthermore evaluate the energetic separation between the optically prepared singlet excited state and the correlated triplet pair state, as well as provide a real-space characterization of the latter, both of which are of key importance in the discussion of fission dynamics. We discuss our results in the context of recent experimental studies.

The entangled triplet pair state in acene and heteroacene materials

Entanglement of states is one of the most surprising and counter-intuitive consequences of quantum mechanics, with potent applications in cryptography and computing. In organic materials, one particularly significant manifestation is the spin-entangled triplet-pair state, which mediates the spin-conserving fission of one spin-0 singlet exciton into two spin-1 triplet excitons. Despite long theoretical and experimental exploration, the nature of the triplet-pair state and inter-triplet interactions have proved elusive. Here we use a range of organic semiconductors that undergo singlet exciton fission to reveal the photophysical properties of entangled triplet-pair states. We find that the triplet pair is bound with respect to free triplets with an energy that is largely material independent (∼30 meV). During its lifetime, the component triplets behave cooperatively as a singlet and emit light through a Herzberg-Teller-type mechanism, resulting in vibronically structured photoluminescence. In photovoltaic blends, charge transfer can occur from the bound triplet pairs with >100% photon-to-charge conversion efficiency.

Distinct properties of the triplet pair state from singlet fission

Singlet fission, the conversion of a singlet exciton (S1) to two triplets (2 × T1), may increase the solar energy conversion efficiency beyond the Shockley-Queisser limit. This process is believed to involve the correlated triplet pair state 1(TT). Despite extensive research, the nature of the 1(TT) state and its spectroscopic signature remain actively debated. We use an end-connected pentacene dimer (BP0) as a model system and show evidence for a tightly bound 1(TT) state. It is characterized in the near-infrared (IR) region (~1.0 eV) by a distinct excited-state absorption (ESA) spectral feature, which closely resembles that of the S1 state; both show vibronic progressions of the aromatic ring breathing mode. We assign these near-IR spectra to 1(TT)→Sn and S1→Sn' transitions; Sn and Sn' likely come from the antisymmetric and symmetric linear combinations, respectively, of the S2 state localized on each pentacene unit in the dimer molecule. The 1(TT)→Sn transition is an indicator of the intertriplet electronic coupling strength, because inserting a phenylene spacer or twisting the dihedral angle between the two pentacene chromophores decreases the intertriplet electronic coupling and diminishes this ESA peak. In addition to spectroscopic signature, the tightly bound 1(TT) state also shows chemical reactivity that is distinctively different from that of an individual T1 state. Using an electron-accepting iron oxide molecular cluster [Fe8O4] linked to the pentacene or pentacene dimer (BP0), we show that electron transfer to the cluster occurs efficiently from an individual T1 in pentacene but not from the tightly bound 1(TT) state. Thus, reducing intertriplet electronic coupling in 1(TT) via molecular design might be necessary for the efficient harvesting of triplets from intramolecular singlet fission.

Optical properties of acene molecules and pentacene crystal from the many-body Green's function method

Using many-body Green's function theory, we compare the excitation of several acene molecules at geometries optimized by different approaches.