Singlet Fission Rate Research Articles

Slowed Singlet Exciton Fission Enhances Triplet Exciton Transport in Select Perylenediimide Crystals.

Singlet fission (SF) materials used in light-harvesting devices must not only efficiently produce spin-triplet excitons but also transport them to an energy acceptor. N,N'-Bis(2-phenylethyl)-3,4,9,10-perylenedicarboximide (EP-PDI) is a promising SF chromophore due to its photostability, large extinction coefficient, and high triplet yield, but the energy transport mechanisms in EP-PDI solids are minimally understood. Herein, we use transient absorption microscopy to directly characterize exciton transport in EP-PDI crystals. We find evidence for singlet-mediated transport in which pairs of triplet excitons undergo triplet fusion (TF), producing spin-singlet excitons that rapidly diffuse. This interchange of singlet and triplet excitons shuttles triplets as far as 205 nm within the first 500 ps after photoexcitation. This enhanced transport comes at a cost, however, as it necessitates favoring triplet recombination and thus requires fine-tuning of SF dynamics to balance triplet yields with triplet transport lengths. Through numerical modeling, we predict tuning the ratio of SF and TF rate constants, kSF/kTF, to between 1.9 and 3.8 allows for an optimized triplet transport length (425-563 nm) with minimal loss (7-10%) in triplet yield. Interestingly, by adjusting the size of EP-PDI crystals, we find that we can subtly tune their crystal structure and thereby alter their SF and TF rates. By slowing SF within small EP-PDI crystals, we are able to boost their triplet transport length by ∼20%. Although counterintuitive, our work suggests slowing SF by introducing moderate structural distortions can be preferential when optimizing triplet exciton transport, provided singlet exciton transport is not significantly hindered.

Long-Lived Triplets from Singlet Fission in Pentacene-Decorated Helical Supramolecular Polymers.

Singlet fission (SF), which involves the conversion of a singlet excited state into two triplet excitons, holds great potential to boost the efficiency of photovoltaics. However, losses due to triplet-triplet annihilation hamper the efficient harvesting of SF-generated triplet excitons, which limits an effective implementation in solar energy conversion schemes. A fundamental understanding of the underlying structure-property relationships is thus crucial to define design principles for cutting-edge SF materials, yet it remains elusive. Herein, we harness helical supramolecular polymers decorated with pentacene side groups to elucidate intermolecular SF dynamics in solution and promote the formation of long-lived mobile triplets. By leveraging the hydrogen bonding-driven assembly of benzene-1,3,5-tricarboxamide (BTA) cores into one-dimensional scaffolds, we direct the organization of appended pentacene motifs into long-range ordered helical frameworks. Dynamic interactions between weakly coupled SF pendants mediate singlet conversion within hundreds of picoseconds, affording triplet quantum yields well above 100%. Moreover, analysis of triplet dynamics with a Monte Carlo simulation model reveals that triplet diffusion along the supramolecular fibers is favored over annihilation, resulting in independent triplets exhibiting considerably slow decay on the time scale of tens of microseconds. The molecular packing within the assembly is tuned by subtle changes in monomer design to increase the rate and efficiency of SF while ensuring exceptionally long-lived mobile triplets, allowing to maintain triplet quantum yields exceeding 100% for at least 100 ns. This work opens new opportunities to exploit self-assembled supramolecular polymers as functional templates to achieve long-lived SF-generated triplets.

Configuration Interaction in Frontier Molecular Orbital Basis for Screening the Spin-Correlated, Spatially Separated Triplet Pair State 1(T···T) Formation.

In the theoretical screening of Singlet Fission rates in molecular aggregates, often the frontier molecular orbital model for dimers is employed. However, the dimer approach fails to account for recent experimental findings that suggest singlet fission progresses through a further intermediate state featuring two spatially separated, spin-correlated triplets, specifically a 1(T···T) state. We address this limitation by generalizing the often used frontier molecular orbital model for singlet fission by incorporation of both separated Charge Transfer (C···T) and 1(T···T) states as well as mixed triplet-charge transfer states, delivering analytic expressions for the diabatic matrix elements. Applying the methodology to the perylene diimide trimer, we examine the packing dependence of competing formation pathways of the 1(T···T) state by evaluation of diabatic matrix elements.

Simultaneous Intra- and Intermolecular Singlet Fission in Bipentacene Macrocycle Aggregates.

Singlet fission (SF) is a process where a singlet state splits into two triplet states, which is essential for enhancing optoelectronic devices. Macrocyclic structures allow for precise control of chromophore orientation and facilitate singlet fission in solutions. However, the behavior of these structures in thin films, crucial for solid-state device optimization, remains underexplored. This study examines the aggregation and singlet fission processes of bipentacene macrocycles (BPc) in thin films using molecular dynamics simulations and electronic structure calculations. Findings indicate that BPc aggregates more rapidly with less chloroform, aligning parallel to the substrate. Intramolecular singlet fission (iSF) rates are rarely changed during evaporation, but the efficiency of intermolecular singlet fission (xSF) improves due to the increase in packing domains, suggesting that orderly crystal domains are not necessary for device efficiency. This opens avenues for varied device designs and traditional solution-based methods for optimal device development.

Structural Disorder at the Edges of Rubrene Crystals Enhances Singlet Fission.

Materials that undergo singlet fission are of interest for their use in light-harvesting, photocatalysis, and quantum information science, but their ability to undergo fission can be sensitive to local variations in molecular packing. Herein we employ transient absorption microscopy, molecular dynamics simulations, and electronic structure calculations to interrogate how structures found at the edges of orthorhombic rubrene crystals impact singlet fission. Within a micrometer-scale spatial region at the edges of rubrene crystals, we find that the rate of singlet fission increases nearly 4-fold. This observation is consistent with formation of a region at crystal edges with reduced order that accelerates singlet fission by disrupting the symmetry found in rubrene's orthorhombic crystal structure. Our work demonstrates that structural distortions of singlet fission materials can be used to control fission in time and in space, potentially offering a means of controlling this process in light harvesting and quantum information applications.

Re-Thinking Dimer Design Principles with Indolonaphthyridine Intramolecular Singlet Fission.

Singlet fission is a phenomenon that could significantly improve the efficiency of photovoltaic devices. Indolonaphthyridine thiophene (INDT) is a photostable singlet fission material that could potentially be utilised in singlet fission-based photovoltaic devices. This study investigates the intramolecular singlet fission (i-SF) mechanism of INDT dimers linked via para-phenyl, meta-phenyl and fluorene bridging groups. Using ultra-fast spectroscopy the highest rate of singlet fission is found in the para-phenyl linked dimer. Quantum calculations show the para-phenyl linker encourages enhanced monomer electronic coupling. Increased rates of singlet fission were also observed in the higher polarity o-dichlorobenzene, relative to toluene, indicating that charge-transfer states have a role in mediating the process. The mechanistic picture of polarisable singlet fission materials, such as INDT, extends beyond the traditional mechanistic landscape.

Organic solar cells and their applications Shah, Razon Oda, F. Hazuan

Enhancing the power conversion efficiency (PCE) is the ma- jor task in the development of solar cells. In 1961, Willi- am Shockley and Hans J. Queisser pointed out that the highest PCE for a single-junction solar cell is limited to 31% due to the thermalization and transmission losses. Singlet fission (SF) is an appealing carrier-multiplication approach to break the Shockley-Queisser limit, since it converts one high- energy singlet exciton into two low-energy triplet excitons. Thus, one absorbed photon generates two electron–hole pairs [1]. SF phenomenon was first observed in anthracene single crystals in 1965. It attracted attention again in 2006. In organic semiconductors, a singlet exciton (S1) forms after the ab- sorption of a photon. If the energy of this singlet exciton is greater than twice of that for triplet exciton (T1), namely E(S1) > 2E(T1), then spin-allowed fission (S1 → 2T 1) could occur. The general kinetic model for SF process is S0 is the ground state, S1 is the excited singlet state, 1(TT) is the intermediate state of a correlated triplet pair, and T1 + T1 are two independent triplet states (Figure 1(a)). The application needs materials with high fission efficiency, suitable bandgap for capturing high-energy photons, appropriate triplet energy and high chemical stability. Owing to the existence of competitive pathways (i.e. S1 to S0 relaxation in picosecond–nanosecond timescale; excimer formation in femtosecond timescale; charge transfer (CT) process in femtosecond timescale, SF needs to occur in pico- second or sub-picosecond timescale. Thus, SF requires strong intermolecular coupling. The intermolecular coupling can be realized via Van der Waals force, weak non-bonded interactions and so on. The coupling distance not only affects SF rate and triplet yield, but also influences the lifetime and diffusion length of triplet excitons [2].

Inverse Design of Tetracene Polymorphs with Enhanced Singlet Fission Performance by Property-Based Genetic Algorithm Optimization.

The efficiency of solar cells may be improved by using singlet fission (SF), in which one singlet exciton splits into two triplet excitons. SF occurs in molecular crystals. A molecule may crystallize in more than one form, a phenomenon known as polymorphism. Crystal structure may affect SF performance. In the common form of tetracene, SF is experimentally known to be slightly endoergic. A second, metastable polymorph of tetracene has been found to exhibit better SF performance. Here, we conduct inverse design of the crystal packing of tetracene using a genetic algorithm (GA) with a fitness function tailored to simultaneously optimize the SF rate and the lattice energy. The property-based GA successfully generates more structures predicted to have higher SF rates and provides insight into packing motifs associated with improved SF performance. We find a putative polymorph predicted to have superior SF performance to the two forms of tetracene, whose structures have been determined experimentally. The putative structure has a lattice energy within 1.5 kJ/mol of the most stable common form of tetracene.

Influence of core-twisted structure on singlet fission in perylenediimide film

To reveal the relationship between molecular packing and singlet fission (SF) property is necessary for developing efficient SF materials, yet it remains unclear. Herein, we investigated SF in two perylenediimide (PDI) films (PDI-C4 and ClPDI-C4) with different molecular arrangements. Crystal structure analysis reveals that PDI-C4 with no substituent at the bay position has a planar structure and shows a close intermolecular packing with a sufficient π-π overlap. Compared to PDI-C4, ClPDI-C4, with four chlorine groups at the bay position, exhibits a twisted core structure that prevents a close intermolecular packing and shows a small π-π overlap. This leads to a smaller intermolecular interaction in ClPDI-C4 film with respect to PDI-C4 as revealed by their steady-state absorption spectra. Transient absorption spectra demonstrate that ClPDI-C4 shows a relatively slower SF rate compared to PDI-C4, but with a longer triplet lifetime. This may be caused by the looser molecular packing and the more endothermic SF in ClPDI-C4 film. This indicates that the introduction of substituents onto the bay position of PDI may be a good choice to tune the PDI packing for achieving long-lived triplets.

Modulating Singlet Fission by Scanning through Vibronic Resonances in Pentacene-Based Blends

Vibronic coupling has been proposed to play a decisive role in promoting ultrafast singlet fission (SF), the conversion of a singlet exciton into two triplet excitons. Its inherent complexity is challenging to explore, both from a theoretical and an experimental point of view, due to the variety of potentially relevant vibrational modes. Here, we report a study on blends of the prototypical SF chromophore pentacene in which we engineer the polarizability of the molecular environment to scan the energy of the excited singlet state (S1) continuously over a narrow energy range, covering vibrational sublevels of the triplet-pair state (1(TT)). Using femtosecond transient absorption spectroscopy, we probe the dependence of the SF rate on energetic resonance between vibronic states and, by comparison with simulation, identify vibrational modes near 1150 cm-1 as key in facilitating ultrafast SF in pentacene.

Substituent Effect on Vibrationally Resolved Absorption Spectra and Exciton Dynamics of Dipyrrolonaphthyridinedione Aggregates.

Dipyrrolonaphthyridinedione (DPND) thin films exhibit interesting photophysical properties and singlet fission (SF) processes. A recent experimental work found that the alkyl substitution in the DPND skeleton has the remarkable influence on the characteristics of electronic absorption spectra and SF rates. Here, we theoretically elucidate the microscopic mechanism of the substituent effect on the optical properties and exciton dynamics of materials by combining the electronic structure calculations and the quantum dynamics simulations. The results show that the alkyl substituent has a minor effect on the single molecular properties but dramatically changes those of DPND aggregates via varying the intermolecular interactions. The aggregates of DPND with and without alkyl side chains exhibit the more likely characters of H-type aggregations. In the former (DPND6), the weak degree of mixing of intramolecular localized excited (LE) states and intermolecular charge transfer (CT) states makes the low-energy absorption band possess the predominant optical absorption, while in the latter (DPND), the CT and LE states are close in energy, together with their strong interaction, resulting in the substantial state-mixing, so that its two low-energy absorption bands have nearly equal oscillator strengths and a wide energy spacing of more than 0.5 eV. The simulation of exciton dynamics elucidates that the photoinitiated states in both aggregates cannot generate the free charge carrier because of the lack of enough driving forces. However, the population exchanges between LE and CT states in DPND aggregates are much faster than in DPND6 aggregates, indicating the different SF behaviors, consistent with the experimental observation.

Pitfalls of quantifying intersystem crossing rates in singlet-fission chromophore solutions

Singlet fission (SF), a process that produces two triplet excitons from one singlet exciton, has attracted recent interest for its potential to circumvent the detailed-balance efficiency limit of single-junction solar cells. For the potential of SF to be fully realized, accurate assignment and quantification of SF is necessary. Intersystem crossing (ISC) is another process of singlet to triplet conversion that is important to distinguish from SF to avoid either over- or under-estimation of SF triplet production. Here, we quantify an upper bound on the rate of ISC in two commonly studied SF chromophores, TIPS–pentacene and TIPS–tetracene, by using transient absorption spectroscopy of solutions of varying concentrations in toluene. We show that SF in solutions of these acenes has previously been misidentified as ISC, and vice versa. By determining a bimolecular SF rate constant in concentrated solutions in which SF dominates over ISC, we distinguish triplet formation due to SF from triplet formation due to ISC and show that the characteristic time scale of ISC must be longer than 325 ns in TIPS–pentacene, while it must be longer than 118 ns in TIPS–tetracene. We additionally note that no excimer formation is observed in the relatively dilute (up to 8 mM) solutions studied here, indicating that previous excimer formation observed at much higher concentrations may be partially due to aggregate formation. This work highlights that an accurate quantification of ISC is crucial as it leads to accurate determination of SF rate constants and yields.

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.

State-specific solvation for restricted active space spin-flip (RAS-SF) wave functions based on the polarizable continuum formalism.

The restricted active space spin-flip (RAS-SF) formalism is a particular form of single-reference configuration interaction that can describe some forms of strong correlation at a relatively low cost and which has recently been formulated for the description of charge-transfer excited states. Here, we introduce both equilibrium and nonequilibrium versions of a state-specific solvation correction for vertical transition energies computed using RAS-SF wave functions, based on the framework of a polarizable continuum model (PCM). Ground-state polarization is described using the solvent's static dielectric constant and in the nonequilibrium solvation approach that polarization is modified upon vertical excitation using the solvent's optical dielectric constant. Benchmark calculations are reported for well-studied models of photo-induced charge transfer, including naphthalene dimer, C2H4⋯C2F4, pentacene dimer, and perylene diimide (PDI) dimer, several of which are important in organic photovoltaic applications. For the PDI dimer, we demonstrate that the charge-transfer character of the excited states is enhanced in the presence of a low-dielectric medium (static dielectric constant ɛ0 = 3) as compared to a gas-phase calculation (ɛ0 = 1). This stabilizes mechanistic traps for singlet fission and helps to explain experimental singlet fission rates. We also examine the effects of nonequilibrium solvation on charge-separated states in an intramolecular singlet fission chromophore, where we demonstrate that the energetic ordering of the states changes as a function of solvent polarity. The RAS-SF + PCM methodology that is reported here provides a framework to study charge-separated states in solution and in photovoltaic materials.

The Role of Bridge-State Intermediates in Singlet Fission for Donor-Bridge-Acceptor Systems: A Semianalytical Approach to Bridge-Tuning of the Donor-Acceptor Fission Coupling.

We describe a semianalytical/computational framework to explore structure-function relationships for singlet fission in Donor (D)-Bridge (B)-Acceptor (A) molecular architectures. The aim of introducing a bridging linker between the D and A molecules is to tune, by modifying the bridge structure, the electronic pathways that lead to fission and to D-A-separated correlated triplets. We identify different bridge-mediation regimes for the effective singlet-fission coupling in the coherent tunneling limit and show how to derive the dominant fission pathways in each regime. We describe the dependence of these regimes on D-B-A many-electron state energetics and on D-B (A-B) one-electron and two-electron matrix elements. This semianalytical approach can be used to guide computational and experimental searches for D-B-A systems with tuned singlet fission rates. We use this approach to interpret the bridge-resonance effect of singlet fission that has been observed in recent experiments.

Structure and photophysics of rubrene–tetracene blends

The application potential of singlet fission (SF), describing the spontaneous conversion of an excited singlet into two triplets, underlines the necessity to independently control SF rates, energetics and the optical band gap. Heterofission, whereby the singlet splits into triplets on chemically distinct chromophores, is a promising approach to control the above-mentioned parameters, but its details are not yet fully understood. Here, we investigate the photophysics of blends of two prototypical SF chromophores, tetracene (TET) and rubrene (RUB) using time-resolved photoluminescence spectroscopy and time-correlated single photon counting to explore the potential for heterofission in combinations of endothermic SF chromophores.

Molecular rotational conformation controls the rate of singlet fission and triplet decay in pentacene dimers.

Three pentacene dimers have been synthesized to investigate the effect of molecular rotation and rotational conformations on singlet fission (SF). In all three dimers, the pentacene units are linked by a 1,4-diethynylphenylene spacer that provides almost unimpeded rotational freedom between the pentacene- and phenylene-subunits in the parent dimer. Substituents on the phenylene spacer add varying degrees of steric hindrance that restricts both the rotation and the equilibrium distribution of different conformers; the less restricted conformers exhibit faster SF and more rapid subsequent triplet-pair recombination. Furthermore, the rotational conformers have small shifts in their absorption spectra and this feature has been used to selectively excite different conformers and study the resulting SF. Femtosecond transient absorption studies at 100 K reveal that the same dimer can have orders of magnitude faster SF in a strongly coupled conformer compared to a more weakly coupled one. Measurements in polystyrene further show that the SF rate is nearly independent of viscosity whereas the triplet pair lifetime is considerably longer in a high viscosity medium. The results provide insight into design criteria for maintaining high initial SF rate while suppressing triplet recombination in intramolecular singlet fission.

Permanent Dipole Moments Enhance Electronic Coupling and Singlet Fission in Pentacene.

Singlet fission (SF), the photophysical process in which one singlet exciton is transformed into two triplets, depends inter alia on the coupling of electronic states. Here, we use fluorination and the resulting changes in partial charge distribution across the chromophore backbone as a particularly powerful tool to control this parameter in pentacene. We find that the introduction of a permanent dipole moment leads to an enhanced coupling of Frenkel exciton and charge transfer states and to an increased SF rate which we probed using ultrafast transient absorption spectroscopy. These findings are contrasted with H-aggregate formation and a significantly reduced triplet-pair state lifetime in a fluorinated pentacene for which the different partial charge distribution leads to a negligible dipole moment.

Theoretical Study on Singlet Fission Dynamics in Slip-Stack-like Pentacene Ring-Shaped Aggregate Models.

We investigate the singlet fission (SF) dynamics of a slip-stack-like pentacene ring-shaped aggregate model, which is constructed by rotating each pentacene unit around its longitudinal axis in an H-aggregate ring. The aggregate size (N) and rotation angle (α) dependences of SF rates and double triplet (TT) yields are clarified using the quantum master equation method. It is found that there exist optimal ranges of the rotation angle α for each N, yielding efficient SF with high SF rates and TT yields. For example, in an 8-mer model, SF rates at α = 23 and 43° are 18.9 and 38.6 times as high as that at α = 30°, respectively, and the TT yields are as high as 0.871, 0.988, and 0.882 at α = 23, 30, and 43°, respectively. Analysis of the relative relaxation factors shows that the many-to-many relaxation paths from adiabatic Frenkel exciton (FE)-like states to TT-like states are opened by tuning α at relevant aggregate sizes, causing fast and high-TT-yield SF, and efficient SF occurs at α = 40° for medium N (7 ≤ N ≤ 10) or at α = 30° for large N (>10). This mechanism is interpreted by the second-order perturbation theory for electronic couplings. Namely, the inequality in the energies of charge-transfer states [CA and AC states, where the cation (C) and anion (A) are located at two neighboring sites in anticlockwise and clockwise directions, respectively] and the change in the amplitude and sign of the couplings between the FE, CT, and TT states are found to cause quantum superposition of the FE and TT states, which contribute to the high TT yield and SF rate. The present results contribute to a deeper understanding of SF dynamics in ring-shaped aggregates as well as to the development of their new design guidelines.

Exploring Unusual Effects of the Ring Substituents in the Type II Reaction with TD-DFT and DFT.

The type II photoelimination reactions are important in biological systems. The different roles played by ring substituents are often not obvious from quantum yields for product formation and the rate constant for hydrogen abstraction is quite low (<107 s-1 ), but Hautala (Tetrahedron Lett. 1977, 18, 2499-2502) reported unusual effects of the nitro substituent in aryl ketones. They reported the first examples of such and discuss for another role played by the nitro group. They determined an upper limit of less than 2×10-5 to the quantum yield for formation of nitro substituent. They hypothesized the carbonyl localized state may be populated with high quantum efficiency, rapid deactivation occurs to populate the lower lying nitro localized state. We focus on the case where the theoretical studies are of great importance and indispensable, such as resolving the debate on the stepwise and concerted mechanism of hydrogen abstraction. Our proposed new mechanisms for 2-ethoxy-1-phenylethanone are based on the energy, HOMO-LUMO, dipole moments, etc. We provide modifications to experimentally proposed mechanisms or completely new mechanism with Hammett, synchronicity, singlet fission and relative rate equations to explain successfully the previous result. At the end, we show why an unusual effect of the nitro substituent exists in it.