Efficient Singlet Fission Research Articles

Hydrogen Migration-Triggered Diradicaloid Singlet-Fission Switch

We present a theoretical design of the singlet-fission (SF) interconversion between two hydrogen tautomers to attract attention to electronic devices such as switches in the SF field. We develop a tuned π-electron conjugation strategy based on single-hydrogen migration to introduce diradical character and yield low-lying E(T1) levels. Specifically, these objectives could be accomplished by moving one hydrogen from a dihydrogen-substituted pyrazine-fused ring to another unsubstituted pyrazine-fused ring in tetraazatetracenes. The predicted SF efficiency would be expected to exceed 120%. To guide future SF design development, one rule of thumb regarding the S0-state and T1-state emerges from our research: In the S0-state, single-hydrogen migration is crucial for effectively localized electrons, which are the key factor in the formation of diradicals. Conversely, single-hydrogen migration induces a large area of π-electron conjugation in the T1-state, which is completely applied to the electron-hole interaction in the S0 → T1 transition, thereby providing low-lying E(T1) levels. Furthermore, a series of hydrogen tautomers of tetraazaacenes have been proposed as diradicaloid SF switches to verify the reliability of the above rule of thumb. This study will not only help researchers in the photovoltaic field to obtain the desired E(T1) in the future but also broaden the application of hydrogen migration in photovoltaic switch research and supplement the SF database.

Evaluation of Fused Aromatic-Substituted Diketopyrrolopyrrole Derivatives for Singlet Fission Sensitizers.

Singlet fission (SF) is a spin-allowed carrier multiplication process that has potential to overcome the Shockley-Queisser limit of solar energy conversion efficiency for single-junction solar cells. It is of importance to prescreen appropriate SF candidates for both basic research and practical applications of SF. Besides common polycyclic aromatic hydrocarbons (PAHs), diketopyrrolopyrrole (DPP) derivatives also undergo efficient SF. A series of DPP derivatives with fused aromatic substituents were investigated considering their conjugation length, constitution, and the introduction of terminal substituents. A comparison of SF properties between nonfused and fused aromatic-substituted DPP derivatives was carried out. Detailed analysis focused on elucidating the relationship between the frontier molecular orbital energies, multiple diradical characters, and SF-relevant excited-state energy levels. Compared to nonfused aromatic-substituted DPP derivatives, fused aromatic-substituted DPP derivatives which contain three aromatic units (thiophene or furan) still share more appropriate energy levels for SF sensitizers. Changing the five-membered aromatic units with benzene ring and introducing -F, -OMe, and -COOH as terminal substituents are both effective ways to improve their performance as SF sensitizers. The results of this research help us to understand the SF properties of DPP derivatives deeply and are beneficial for the design of new DPP-based SF sensitizers.

Theoretical Study of Non-Markov Effects on Singlet Fission Dynamics of Model Pentacene Dimers Using the Second-Order Time-Convolutionless Quantum Master Equation Method

Non-Markov effects on singlet fission (SF) dynamics of model pentacene dimers are investigated using the second-order time-convolutionless quantum master equation method with the state-independent (SI)/dependent (SD) spectral density of vibronic coupling for each diabatic exciton state. It is found on the early time scale (∼80 fs) that, as compared to Markov dynamics, non-Markov dynamics using the SD spectral density causes a significant increase of the double-triplet (TT) population by 15%, though the SI one just causes a negligible variation (∼3%) of the TT population. The difference in the non-Markov effects between the cases of using SI and SD spectral densities is found to be related to the frequency giving the peak of spectral density for the charge-transfer (CT) state (which is concerned with amplification of the non-Markov population relaxation) and the amplitude of the spectral density for the TT state at the energy gap between high-lying CT-like (CT′) and TT-like (TT′) adiabatic states (which is concerned with acceleration of the non-Markov population relaxation from the initial (Frenkel exciton (FE)-like; FE′) to the final (TT′) adiabatic states). We also examine the dependence of non-Markov effects on the energy gap between FE′ and TT′ states using SD spectral densities. It turns out (i) that non-Markov effects depend on the relationship between the SF time scale and the detuning degree between the peak frequency of the primary contributing spectral density and the energy gap between the related adiabatic exciton states, and (ii) that they appear more significantly in the case of the detuning degree closer to the SF time scale. The obtained results reveal the conditions for inducing the non-Markov effects in realistic SF materials and thereby pave the way toward constructing design guidelines for highly efficient SF systems based on non-Markov dynamics.

Molecular Design Principle for Efficient Singlet Fission Based on Diradical Characters and Exchange Integrals: Multiple Heteroatom Substitution Effect on Anthracenes

We theoretically investigate significant impacts of diradical characters and exchange integrals on the first singlet (S1) and triplet (T1) excitation energies related to singlet fission (SF), from which a series of C2h-symmetrically diazadibora-substituted anthracenes [(BN)2-anthracenes] are newly proposed as potential efficient SF chromophores. From the analytical expressions of electronic structures for a model system with two electrons in two orbitals, a tuning of exchange integrals represented by localized natural orbitals is shown to be effective to vary E(S1) and E(T1), in addition to the control of diradical characters, both of which are expected to result in SF isothermal or exothermic [E(S1) – 2E(T1) ≈ 0 or > 0]. The applicability of this strategy to realistic molecules is verified for multiple heteroatom substitution to anthracene, namely, (BN)2-anthracenes, by the complete active space configuration interaction method and the time-dependent density functional theory method. Our calculations show that (BN)2-anthracenes exhibit a wide range of diradical characters and enhanced exchange integrals, enabling flexible tunings of E(S1) and E(T1). Accordingly, several (BN)2-anthracenes are found to be promising candidates for efficient SF. Such a behavior of diradical character can be understood from resonance structures associated with the captodative effect. Moreover, a concrete strategy of tuning exchange integrals is demonstrated in terms of frontier molecular orbital distributions. The present results open up novel molecular design guidelines for efficient SF based on the tuning of exchange integrals in addition to diradical characters.

Singlet Fission in Self-assembled Amphipathic Tetracene Nanoparticles: Probing the Role of Charge-transfer State

Two tetracene derivatives with PEG group of different lengths (PhTc-PEG-1 with short PEG chain and PhTc-PEG-2 with long PEG chain) have been synthesized and fabricated into nanoparticles with re-precipitation method. Spherical nanoparticle is formed for these two compounds in water as revealed by scanning electron microscopy. Steady-state absorption and fluorescence spectra demonstrates that the interaction between the adjacent tetracene units in these two nanoparticles is relatively strong, but the interaction in PhTc-PEG-2 nanoparticle is weaker than that of PhTc-PEG-1 nanoparticle. Transient absorption experiment reveals that singlet fission (SF) can be conducted in these two nanoparticles via a real charge-transfer (CT) intermediate state resembled that in same tetracene core without PEG chain (PhTc). However, the SF yields of these nanoparticles are relatively low, suggesting that the formation of the real CT state is unfavorable for the occurrence of SF. Moreover, the weaker interaction in PhTc-PEG-2 nanoparticle leads to a relatively low SF yield compared to that of PhTc-PEG-1 nanoparticle, suggesting that the long PEG chain should be avoided when modifying SF chromophore for preparing colloidal nanoparticles with the goal of achieving efficient SF.

Competitive Triplet Formation and Recombination in Crystalline Films of Perylenediimide Derivatives: Implications for Singlet Fission

Developing photostable compounds that undergo quantitative singlet fission (SF) is a key challenge. As SF necessitates electron transfer between neighboring molecules, the SF rate is highly sensitive to intermolecular coupling in the solid state. We investigate SF in thin films for a series of perylenediimide (PDI) molecules. By adding different substituents at the imide positions, the packing of the molecules in the solid state can be changed. The relationship between SF parameters and the stacked geometry in PDI films is investigated, with two-electron direct coupling found to be the main SF mechanism. Time-resolved emission and transient absorption data show that all of the PDI films undergo SF although with different rates and yields varying from 35 to 200%. The results show that PDI1 and 2, which are stacked PDI pairs twisted out of alignment along the highest occupied molecular orbital to lowest unoccupied molecular orbital transition, exhibit faster and more efficient SF up to 200% yield. We demonstrate that both triplet formation and decay rates are highly sensitive to the ordering of the molecules within a film. The results of this study will assist in the design of optimized structures with a fast SF rate and low recombination rate that are required for useful light harvesting applications.

(Invited) High-Yield Singlet Fission of Tetracene Derivatives Utilizing Organic-Inorganic Hybrid Systems for Light Energy Conversion

Singlet fission (SF) is highly expected to improve the solar energy conversion properties by generating two excitons from one photon absorption. Among various organic compounds, acene derivatives such as tetracene and pentacene are promising for occurrence of efficient SF. Pentacene derivatives sufficiently meet the energy-level matching condition between a singlet and two triplet states: [E(S1) ≥ 2E(T1)]. Although tetracene is well-kwown as a good candidate for SF, the number of high-yield and long-lived triplet excited states through SF is extremely limited because of the relative acceleration of the reverse triplet-triplet annihilation (TTA) considering the above-mentioned energy matching condition. Consequently, the precise control of orientation and distance between two neighboring Tc units is definitely required for long-lived individual triplets through SF but not correlated triplet pair. Because the longer lifetime of the individual triplet states is useful for efficient light energy conversion.Therefore, we first demonstrate the quantitative sequential process from intramolecular SF to intermolecular two-electron transfers using 2,2′-biphenyl-bridged tetracene dimer (Tet-BP-Tet). The obtained high-yield and long-lived triplet excited states of Tet-BP-Tet by intramolecular SF (quantum yield of individual triplet states: ΦT = 175 ± 5% and lifetime of individual triplet states: τT = 0.29 ms) resulted in the quantitative intermolecular two-electron transfer process with chloranil in benzonitrile (electron transfer yield: ΦET = 173 ± 5%).1 Then, we also propose a novel supramolecular strategy utilizing the mixed self-assembled monolayers (SAMs) with two different chain lengths. Specifically, mixed tetracene-SAMs on gold nanoclusters, which are prepared by a tetracene-modified hetero-disulfide with two different chain lengths, attain high-yield SF (Φ SF: ~90%) and individual triplet yields (Φ Τ: ~160%).2 References Nakamura, S.; Sakai, H.; Nagashima, H.; Kobori, Y.; Tkachenko, N. V.; Hasobe, T. ACS Energy Lett. 2019, 4, 26-31.Saegusa, T.; Sakai, H.; Nagashima, H.; Kobori, Y.; Tkachenko, N. V.; Hasobe, T. J. Am. Chem. Soc. 2019, 141, 14720-14727.

Ultrafast Electron Transfer Before Singlet Fission and Slow Triplet State Electron Transfer in Pentacene Single Crystal/C60 Heterostructure

The singlet fission (SF) process converts one high-energy singlet exciton to two low-energy triplet excitons after absorbing one photon. Organic photovoltaic devices based on the SF process have shown great potential in solar energy conversion to exceed Shockley-Queisser limit. The key to SF photovoltaic devices requires efficient electron transfer (ET) from triplet exciton after SF, which is yet to be thoroughly investigated. Here, we performed thorough photophysical studies in 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene)/C60 heterostructures using TIPS-pentacene microsize single crystal as a well-defined model system. We show the SF process in TIPS-pentacene single crystal occurs by a two-step process, with triplet pair intermediates forming in 75 fs and then dissociating to non-interacting triplets in 1.6 ps. The SF process in single crystal is comparable to that in polycrystalline film. Importantly, we observe a considerable fraction of singlet excitons is quenched by ultrafast (<75 fs) interfacial ET prior to fission and no ET from triplet excitons in 1.5 ns time window. We confirm that the absence of ET is not limited by exciton diffusion but due to very slow (≫1.5 ns) interfacial ET from triplet exciton. The observations contradict expected singlet and triplet ET behaviors based on a simple two-state Marcus ET model and suggest long-range interfacial ET from delocalized photoexcitation. The ultrafast ET from singlet exciton before SF and slow ET from triplet exciton call for reconsideration and careful design of efficient SF photovoltaic devices.

Effect of Vibronic Coupling on Correlated Triplet Pair Formation in the Singlet Fission Process of Linked Tetracene Dimers.

Tetracene-based singlet fission (SF) materials show application prospects as triplet sensitizers in organic optoelectronics. SF involves internal conversion from photoexcited singlet states 1(S1S0) to correlated triplet pair states 1(T1T1). We derive an expression for the internal conversion rate on the basis of the Fermi golden rule with an artificial Lorentzian broadening. The internal conversion rate depends on the interstate vibronic couplings (VCs) and energy difference (ΔESF) between 1(S1S0) and 1(T1T1). Therefore, understanding the interplay between interstate VCs and ΔESF is necessary to reveal how the structure-property relationship affects the SF efficiency. Here, we propose a method to quantitatively analyze interstate VCs between 1(S1S0) and 1(T1T1). We apply this method to SF in ortho-, meta-, and para-bis(ethynyltetracenyl)benzene and identify an effect of interstate VCs on the 1(T1T1) formation rate. The interstate VCs of the meta dimer are remarkably weak, which reasonably explains the experimentally obtained slow 1(T1T1) formation rate. The weak VCs result from a very small overlap density between 1(S1S0) and 1(T1T1) of the meta dimer. Furthermore, we investigate structure-dependence of the 1(T1T1) formation rate of the para dimer and find that the para dimer shows large VCs and small ΔESF when the rotational angle between the two tetracene units is large, which leads to the faster 1(T1T1) formation rate than those of the ortho and meta dimers. The rotation of the tetracene units is the origin of the experimentally observed fast 1(T1T1) formation rate of the para dimer.

Efficient Multiexciton State Generation in Charge-Transfer-Coupled Perylene Bisimide Dimers via Structural Control.

The singlet fission (SF) process is generally defined as the conversion of one singlet exciton (S1) into two triplet excitons (2·T1), which has the potential to overcome thermalization losses in the field of photovoltaic devices. Among the applicable compounds for SF-based photovoltaic devices, perylene bisimide (PBI) is one of the best candidates because of its electronic tunability and photostability. However, the strategy for efficient SF in PBIs remains ambiguous because of numerous competing relaxation pathways in PBI-based molecular materials. In this regard, for the first time, we observed the SF mechanism in PBI dimers by controlling the intrinsic factor (exciton coupling) and the external environment (solvent polarity and viscosity). Time-resolved spectroscopic measurements and quantum chemical simulations reveal that efficient SF occurs through the charge-transfer-assisted mechanism, entailing a large structural fluctuation. Our findings not only highlight the SF mechanism in PBI dimers but also suggest the factors responsible for an efficient SF process, which are important considerations in the design of molecular materials for photovoltaic devices.

Singlet Fission in Quaterrylenediimide Thin Films

Singlet fission (SF) creates two triplet excitons following absorption of a photon by two electronically interacting chromophores. Quaterrylene-3,4:13,14-bis(dicarboximide) (QDI) is a strongly absorbing chromophore that readily fulfills the energetic requirements for SF, E(S1) > 2E(T1), and thus should undergo rapid and efficient SF. SF was studied in thin films of the QDI derivative N,N-bis(2,6-diisopropylphenyl)-QDI (ArQDI), which undergoes SF in <300 fs to form the correlated triplet pair state, 1(T1T1), which dissociates with a (7.3 ± 1.2 ns)−1 rate constant. The observed triplet yield for a thin film that has been solvent-vapor annealed with CH2Cl2 is 135 ± 20% instead of 200%, which is typically expected of chromophores that undergo ultrafast formation of the 1(T1T1) state. The lower SF yield in ArQDI results from the failure of the 1(T1T1) state to dissociate before returning to the ground state. In contrast to other molecules, like hexacene, which have low triplet energies, the SF rate in ArQDI is not limited by a multiphonon relaxation bottleneck, largely due to the fact that the S–T energy gap in the film is substantially smaller than that measured for monomeric ArQDI. The ability to maintain a favorable S–T energy gap in a film is a design consideration when chromophores are considered for use to enhance solar cell performance.

Organizing Crystalline Functionalized Pentacene Using Periodicity of Poly(Vinyl Alcohol).

Nanoparticles of acenes exhibit highly efficient intermolecular singlet fission (SF). Recent reports indicate that altering the morphology of 6,13-bis-(triisopropylsilylethynyl)pentacene (TIPS-Pn) nanoparticles has a profound influence on their SF dynamics. Here, we show that poly(vinyl alcohol) (PVA) induces a phase transition in preformed TIPS-Pn nanoparticles. These nanoparticles are amorphous when initially formed but crystalline after addition of PVA. Surface characterization indicates that a diffuse PVA layer surrounds the nanoparticles. We propose that a periodic interaction between the hydroxyl groups of PVA and TIPS groups of TIPS-Pn on the nanoparticle surface induces a large-scale structural rearrangement to yield crystalline TIPS-Pn. Such reorganization in preformed organic nanoparticles is unprecedented, and we believe that this is the first report of such an effect induced by polymer adsorption. Transient absorption spectroscopic results reveal that SF within these nanoparticles is accelerated by an order of magnitude upon structural rearrangement.

Slow charge transfer from pentacene triplet states at the Marcus optimum.

Singlet fission promises to surpass the Shockley-Queisser limit for single-junction solar cell efficiency through the production of two electron-hole pairs per incident photon. However, this promise has not been fulfilled because singlet fission produces two low-energy triplet excitons that have been unexpectedly difficult to dissociate into free charges. To understand this phenomenon, we study charge separation from triplet excitons in polycrystalline pentacene using an electrochemical series of 12 different guest electron-acceptor molecules with varied reduction potentials. We observe separate optima in the charge yield as a function of driving force for singlet and triplet excitons, including inverted regimes for the dissociation of both states. Molecular acceptors can thus provide a strategic advantage to singlet fission solar cells by suppressing singlet dissociation at optimal driving forces for triplet dissociation. However, even at the optimal driving force, the rate constant for charge transfer from the triplet state is surprisingly small, ~107 s-1, presenting a previously unidentified obstacle to the design of efficient singlet fission solar cells.

Singlet Fission in Tetraaza-TIPS-Pentacene Oligomers: From fs Excitation to μs Triplet Decay via the Biexcitonic State

Generating two long-living low-energy excitations after absorption of a single high-energy photon has stoked interest in singlet fission (SF) to enhance solar energy conversion in photovoltaics. To this end, survival of the triplet states is critical. This process is investigated in diethynylbenzene-linked tetraaza-triisopropylsilylethynyl-pentacene dimers, for which SF is energetically feasible and facilitated by the close distances between the azapentacenes. The ortho and meta connectivities are explored and compared with the tetraazapentacene molecule and the (1,3,5) trimer. Efficient SF (potential ΦT ≥ 160%) is demonstrated in all oligomers by quantitative kinetic analysis of broadband transient absorption and fluorescence signals. Together with dynamics of the starting singlet, the triplet pair, and the final free triplet state, our results show an intermediate component with spectral properties compatible with a biexcitonic state. Long-living triplets represent only a fraction of the high number of transient triplet pair intermediates, which undergo triplet-triplet annihilation as well as fusion between neighboring pentacenes. Therefore, our work provides new insight into the SF in covalent dimers and paves the way for the application of these materials for carrier multiplication.

Singlet Fission in Crystalline Organic Materials: Recent Insights and Future Directions.

Singlet fission (SF) involves the conversion of one excited singlet state into two lower excited triplet states and has received considerable renewed attention over the past decade. This Perspective highlights recent developments and emerging concepts of SF in solid-state crystalline materials. Recent experiments showed the crucial role of vibrational modes in speeding up SF, and theoretical modeling has started to define an optimal energetic landscape and intermolecular orientation of chromophores for highly efficient singlet fission. A critical analysis of these developments leads to directions for future research to eventually find singlet fission chromophores with excellent optoelectronic properties.

Singlet-Fission-Induced Enhancement of Third-Order Nonlinear Optical Properties of Pentacene Dimers.

Using quantum chemical calculations and exciton dynamics simulations, we investigate the static second hyperpolarizability γ [the third-order nonlinear optical (NLO) property at the molecular scale] of slip-stacked pentacene dimer models in the correlated-triplet-pair [1(TT)] state created from the singlet excited state in the singlet fission (SF) process. It is found that the SF induces significant (∼20 times at maximum) enhancement of γ/monomer in the 1(TT) state as compared to that in the singlet ground state. The origin of the remarkable enhancement of γ/monomer is revealed by analyzing the γ density distribution and the intermolecular orbital interaction. Furthermore, we clarify molecular packings suitable for highly efficient SF and largely enhanced γ values of a novel class of SF-induced NLO systems, which have promising potential to surpass the conventional NLO systems.

Remote Functionalization through Symmetric or Asymmetric Substitutions Control the Pathway of Intermolecular Singlet Fission.

Singlet fission (SF) produces two coupled triplet excitons from a high energy singlet excitation. The mechanism of SF in a variety of phenyl (-Ph) substituted pentacene is systematically studied through both ab initio and density functional theory calculations. Two classes of substitution to pentacene are considered, namely, symmetric configuration with four Ph groups (TPP) and an asymmetric configuration with two Ph groups (DPP). The positions of the singlet and triplet states are determined by calibrating the active space through state averaged complete active space self-consistent field (SA-CASSCF) calculations. The SF rates are computed based on restricted active space with single and double spin flip wave functions (RAS-SF and RAS-2SF), which are analyzed based on different intermolecular π-stacking patterns of TPP and DPP. The contribution of charge transfer (CT) state near the multiexciton (ME) state plays a significant role for SF efficiency. The role of excimer formation is supportive for ME generation [J. Am. Chem. Soc. 2016, 138, 617], and hence it is critically studied. The ME generation in TPP is a slower process and occurs through an excimer-mediated path with a large coupling between the first singlet excited state and ME state. On the other hand, DPP exhibits a relatively faster SF rate through the formation of a ME state via low-lying CT state, especially the slip-stacked dimers. The present computation elegantly demonstrates the crucial role of functional group substitution in the structure of SF active molecules in determining the efficiency of fission dynamics.

Preparation and redox properties of fluorinated 1,3-diphenylisobenzofurans

Eleven fluorinated 1,3-diphenylisobenzofurans, materials that are promising to perform efficient singlet fission, have been synthesized and their electrochemical reduction and oxidation in non-aqueous dimethylformamide examined. The redox properties are not changed continuously with increasing number of fluorine substituents but there are some qualitative turns in behavior for specific types of substitution revealing a discontinuity requiring deeper investigation. The DFT calculations confirm the trends observed experimentally by correlation of experimental and calculated data. The differences between the geometries of the neutral species and their radical ions were computed and rationalized.

Heavy-atom effects on intramolecular singlet fission in a conjugated polymer.

A chief aim in singlet fission research is to develop new materials concepts for more efficient singlet fission. The typical approaches such as tuning π-overlap and charge-transfer interactions, enhancing delocalization, altering diradical character, or extending the conjugation length have profound effects simultaneously on the singlet and triplet energetics and the couplings between them. While these strategies have resulted in a handful of high-efficiency materials, the complex interplay of these factors makes systematic materials development challenging, and it would be useful to be able to selectively manipulate the properties and dynamics of just part of the singlet fission pathway. Here, we investigate the potential of heteroatom substitution as just such a selective tool. We explore the influence of heavy atoms within the main backbone of polythienylenevinylene and its selenophene and tellurophene derivatives. We find no significant effects on the prompt <300 fs intramolecular singlet fission dynamics but a clear heavy-atom effect on longer time scales.

Substituent effects on energetics and crystal morphology modulate singlet fission in 9,10-bis(phenylethynyl)anthracenes.

Singlet fission (SF) converts a singlet exciton into two triplet excitons in two or more electronically coupled organic chromophores, which may then be used to increase solar cell efficiency. Many known SF chromophores are unsuitable for device applications due to chemical instability or low triplet state energies. The results described here show that efficient SF occurs in derivatives of 9,10-bis(phenylethynyl)anthracene (BPEA), which is a highly robust and tunable chromophore. Fluoro and methoxy substituents at the 4- and 4'-positions of the BPEA phenyl groups control the intermolecular packing in the crystal structure, which alters the interchromophore electronic coupling, while also changing the SF energetics. The lowest excited singlet state (S1) energy of 4,4'-difluoro-BPEA is higher than that of BPEA so that the increased thermodynamic favorability of SF results in a (16 ± 2 ps)-1 SF rate and a 180% ± 16% triplet yield, which is about an order of magnitude faster than BPEA with a comparable triplet yield. By contrast, 4-fluoro-4'-methoxy-BPEA and 4,4'-dimethoxy-BPEA have slower SF rates, (90 ± 20 ps)-1 and (120 ± 10 ps)-1, and lower triplet yields, (110 ± 4)% and (168 ± 7)%, respectively, than 4,4'-difluoro-BPEA. These differences are attributed to changes in the crystal structure controlling interchromophore electronic coupling as well as SF energetics in these polycrystalline solids.