Linker-Dependent Singlet Fission in Tetracene Dimers

Simulating the full spin manifold of triplet-pair states in a series of covalently linked TIPS-pentacenes

Combined density functional theory and multireference configuration interaction methods have been used to elucidate singlet fission (SF) pathways and mechanisms in three regioisomers of side-on linked pentacene dimers. In addition to the optically bright singlets (S  1 and S  2 ) and singly excited triplets (T  1 and T  2 ), the full spin manifold of multiexcitonic triplet-pair states (  1 ME,  3 ME,  5 ME) has been considered. In the ortho- and para-regioisomers, the  1 ME and S  1 potentials intersect upon geometry relaxation of the S  1 excitation. In the meta-regioisomer, the crossing occurs upon delocalization of the optically bright excitation. The energetic accessibility of these conical intersections and the absence of low-lying charge-transfer states suggests a direct SF mechanism, assisted by charge-resonance effects in the  1 ME state. While the  5 ME state does not appear to play a role in the SF mechanism of the ortho- and para-regioisomers, its participation in the disentanglement of the triplet pair is conceivable in the meta-regioisomer.

Diverse quantum interference regimes in intramolecular singlet fission chromophores with thiophene-based linkers

An array of thiophene-based π-conjugated linkers in covalently linked pentacene dimers allow us to access diverse quantum interference (QI), modulating nonadiabatic coupling (NAC) in the singlet fission (SF) process. Simulations show that structural isomerism in terms of S atom orientation substantially alters NAC with relatively marginal impacts on energies. Extended curly arrow rules (ECARs) reveal sensitive dependence of QI on SF linker topologies and connectivity, categorizing regimes of constructive, destructive, and previously unrealized in SF research, shifted destructive QI. Drastic NAC changes in terms of S atom orientation are rationalized based on the nature of QI. Our results from nonequilibrium Green's function calculation using density functional theory corroborate the classification of QI regimes based on ECARs. Moreover, we found that the extent of charge resonance contribution in electronic states relevant to multiexciton formation and the appearance of optically allowed charge transfer excitation strongly depends on the operative QI regime. Notably, the magnitude of NAC effectively captures this influence. Our findings show that QI can rationalize and semi-quantitatively correlate with NAC for the multiexciton formation step in the SF process.

Singlet Fission in a New Series of Systematically Designed Through-space Coupled Tetracene Oligomers

Singlet fission (SF) holds great promise for current photovoltaic technologies, where tetracenes, with their relatively high triplet energies, play a major role for application in silicon-based solar cells. However, the SF efficiencies in tetracene dimers are low due to the unfavorable energetics of their singlet and triplet energy levels. In the solid state, tetracene exhibits high yields of triplet formation through SF, raising great interest about the underlying mechanisms. To address this discrepancy, we designed and prepared a novel molecular system based on a hexaphenylbenzene core decorated with 2 to 6 tetracene chromophores. The spatial arrangement of tetracene units, induced by steric hindrance in the central part, dictates through-space coupling, making it a relevant model for solid-state chromophore organization. We then revealed a remarkable increase in SF quantum yield with the number of tetracenes, reaching quantitative (196 %) triplet pair formation in hexamer. We observed a short-lived correlated triplet pair and limited magnetic effects, indicating ineffective triplet dissociation in these through-space coupled systems. These findings emphasize the crucial role of the number of chromophores involved and the interchromophore arrangement for the SF efficiency. The insights gained from this study will aid designing more efficient and technology-compatible SF systems for applications in photovoltaics.

Intramolecular Singlet Fission Coupled with Intermolecular Triplet Separation as a Strategy to Achieve High Triplet Yields in Fluorene-Based Small Molecules

In this work, detailed experimental proof and in-depth analysis of the singlet fission (SF) mechanism, operative in fluorene-based small molecules, are carried out by employing advanced time-resolved spectroscopies with nanosecond and femtosecond resolution. The investigation of the effect of solution concentration and solvent viscosity together with temperature and excitation wavelength demonstrates INTRAmolecular formation of the correlated triplet pair followed by INTERmolecular independent triplet separation via a "super-diffusional" triplet-triplet transfer process. This unconventional INTRA- to INTERmolecular SF may be considered an "ideal" mechanism. Indeed, intramolecular formation of the correlated triplet pair is here interestingly proved for small molecules rather than large multichromophoric systems, allowing easy synthesis and processability while maintaining good control over the SF process. On the other hand, the intermolecular triplet separation may be exploited to achieve high triplet quantum yields in these new SF small molecules.

Tetracene cyclophanes showing controlled intramolecular singlet fission by through-space orientations

Tetracene cyclophanes: a series of cyclic tetracene dimers bridged by two flexible ethylene glycol units demonstrated enhanced intramolecular singlet fission through through-space orientations by suppressing the H-type excited complex.

Intramolecular Triplet Diffusion Facilitates Triplet Dissociation in a Pentacene Hexamer

Triplet dynamics in singlet fission depend strongly on the strength of the electronic coupling. Covalent systems in solution offer precise control over such couplings. Nonetheless, efficient free triplet generation remains elusive in most systems, as the intermediate triplet pair 1 (T1 T1 ) is prone to triplet-triplet annihilation due to its spatial confinement. In the solid state, entropically driven triplet diffusion assists in the spatial separation of triplets, resulting in higher yields of free triplets. Control over electronic coupling in the solid state is, however, challenging given its sensitivity to molecular packing. We have thus developed a hexameric system (HexPnc) to enable solid-state-like triplet diffusion at the molecular scale. This system is realized by covalently tethering three pentacene dimers to a central subphthalocyanine scaffold. Transient absorption spectroscopy, complemented by theoretical structural optimizations and steady-state spectroscopy, reveals that triplet diffusion is indeed facilitated due to intramolecular cluster formation. The yield of free triplets in HexPnc is increased by a factor of up to 14 compared to the corresponding dimeric reference (DiPnc). Thus, HexPnc establishes crucial design aspects for achieving efficient triplet dissociation in strongly coupled systems by providing avenues for diffusive separation of 1 (T1 T1 ), while, concomitantly, retaining strong interchromophore coupling which preserves rapid formation of 1 (T1 T1 ).

Catalyzing Singlet Fission by Transition Metals: Second versus Third Row Effects

The synthesis and characterization of platinum(II) and palladium(II) complexes bearing two (dimers Pt(Lpc)2Cl2 and Pd(Lpc)2Cl2), one (monomers Pt(Lpc)(Lref)Cl2 and Pd(Lpc)(Lref)Cl2), or no (reference compounds Pt(Lref)2Cl2 and Pd(Lref)2Cl2) pentacene-based pyridyl ligands are presented. Photophysical properties of the dimers are probed by means of steady-state and time-resolved transient absorption measurements in comparison to the monomer and model compounds. Our results document that despite enhanced spin-orbit coupling from the presence of heavy atoms, intramolecular singlet fission (iSF) is not challenged by intersystem crossing. iSF thus yields correlated triplet pairs and even uncorrelated triplet excited states upon decoherence. Importantly, significant separation of the two pentacenyl groups facilitates decoupling of the two chromophores. Furthermore, the mechanism of iSF is altered depending on the respective metal center, that is, Pt(II) versus Pd(II). The dimer based on Pt(II), Pt(Lpc)2Cl2, exhibits a direct pathway for the iSF and forms a correlated triplet pair with singlet-quintet spin-mixing within 10 ns in variable solvents. On the other hand, the dimer based on Pd(II), Pd(Lpc)2Cl2, leads to charge transfer mixing during the population of the correlated triplet pair that is dependent on solvent polarity. Moreover, Pd(Lpc)2Cl2 gives rise to a stable equilibrium between singlet and quintet correlated triplet pairs with lifetimes of up to 170 ns. Inherent differences in the size and polarizability, when contrasting platinum(II) with palladium(II), are the most likely rationale for the underlying trends.

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.

Multiexciton quintet state populations in a rigid pyrene-bridged parallel tetracene dimer

The multiexciton quintet state, 5TT, generated as a singlet fission intermediate in pairs of molecular chromophores, is a promising candidate as a qubit or qudit in future quantum information science schemes. In this work, we synthesize a pyrene-bridged parallel tetracene dimer, TPT, with an optimized interchromophore coupling strength to prevent the dissociation of 5TT to two decorrelated triplet (T1) states, which would contaminate the spin-state mixture. Long-lived and strongly spin-polarized pure 5TT state population is observed via transient absorption spectroscopy and transient/pulsed electron paramagnetic resonance spectroscopy, and its lifetime is estimated to be >35 µs, with the dephasing time (T2) for the 5TT-based qubit measured to be 726 ns at 10 K. Direct relaxation from 1TT to the ground state does diminish the overall excited state population, but the exclusive 5TT population at large enough persistent density for pulsed echo determination of spin coherence time is consistent with recent theoretical models that predict such behavior for strict parallel chromophore alignment and large exchange coupling.

Unraveling Triplet Formation Mechanisms in Acenothiophene Chromophores

The evolution of molecular platforms for singlet fission (SF) chromophores has fueled the quest for new compounds capable of generating triplets quantitatively at fast time scales. As the exploration of molecular motifs for SF has diversified, a key challenge has emerged in identifying when the criteria for SF have been satisfied. Here, we show how covalently bound molecular dimers uniquely provide a set of characteristic optical markers that can be used to distinguish triplet pair formation from processes that generate an individual triplet. These markers are contained within (i) triplet charge-transfer excited state absorption features, (ii) kinetic signatures of triplet-triplet annihilation processes, and (iii) the modulation of triplet formation rates using bridging moieties between chromophores. Our assignments are verified by time-resolved electron paramagnetic resonance (EPR) measurements, which directly identify triplet pairs by their electron spin and polarization patterns. We apply these diagnostic criteria to dimers of acenothiophene derivatives in solution that were recently reported to undergo efficient intermolecular SF in condensed media. While the electronic structure of these heteroatom-containing chromophores can be broadly tuned, the effect of their enhanced spin-orbit coupling and low-energy nonbonding orbitals on their SF dynamics has not been fully determined. We find that SF is fast and efficient in tetracenothiophene but that anthradithiophene exhibits fast intersystem crossing due to modifications of the singlet and triplet excited state energies upon functionalization of the heterocycle. We conclude that it is not sufficient to assign SF based on comparisons of the triplet formation kinetics between monomer and multichromophore systems.

Controlling Intramolecular Singlet Fission Dynamics via Torsional Modulation of Through-Bond versus Through-Space Couplings

Covalent dimers, particularly pentacenes, are the dominant platform for developing a mechanistic understanding of intramolecular singlet fission (iSF). Numerous studies have demonstrated that a photoexcited singlet state in these structures can rapidly and efficiently undergo exciton multiplication to form a correlated pair of triplets within a single molecule, with potential applications from photovoltaics to quantum information science. One of the most significant barriers limiting such dimers is the fast recombination of the triplet pair, which prevents spatial separation and the formation of long-lived triplet states. There is an ever-growing need to develop general synthetic strategies to control the evolution of triplets following iSF and enhance their lifetime. Here, we rationally tune the dihedral angle and interchromophore separation between pairs of pentacenes in a systematic series of bridging units to facilitate triplet separation. Through a combination of transient optical and spin-resonance techniques, we demonstrate that torsion within the linker provides a simple synthetic handle to tune the fine balance between through-bond and through-space interchromophore couplings that steer iSF. We show that the full iSF pathway from femtosecond to microsecond timescales is tuned through the static coupling set by molecular design and structural fluctuations that can be biased through steric control. Our approach highlights a straightforward design principle to generate paramagnetic spin pair states with higher yields.

Tetracene Dimers: A Platform for Intramolecular Down- and Up-conversion

Photon energy conversion can be accomplished in many different ways, including the two opposing manners, down-conversion (i.e., singlet fission, SF) and up-conversion (i.e., triplet-triplet annihilation up-conversion, TTA-UC). Both processes have the potential to help overcome the detailed balance limit of single-junction solar cells. Tetracene, in which the energies of the lowest singlet excited state and twice the triplet excited state are comparable, exhibits both down- and up-conversion. Here, we have designed meta-diethynylphenylene- and 1,3-diethynyladamantyl-linked tetracene dimers, which feature different electronic coupling, to characterize the interplay between intramolecular SF (intra-SF) and intramolecular TTA-UC (intra-TTA-UC) via steady-state and time-resolved absorption and fluorescence spectroscopy. Furthermore, we have used Pd-phthalocyanine as a sensitizer to enable intra-TTA-UC in the two dimers via indirect photoexcitation in the near-infrared part of the solar spectrum. The work is rounded off by temperature-dependent measurements, which outline key aspects of how thermal effects impact intra-SF and intra-TTA-UC in different dimers.

Efficient Intramolecular Singlet Fission in Spiro-Linked Heterodimers

We investigate intramolecular singlet fission (iSF) of spiro-linked azaacene heterodimers by time-resolved spectroscopy and quantum chemical calculations. Combining two different azaacenes through a nonconjugated linker using condensation chemistry furnishes azaacene heterodimers. Compared to their homodimers, iSF quantum yields are improved at an extended absorption range. The driving force of iSF, the energy difference ΔEiSF between the S1 state and the correlated triplet pair 1(TT), is tuned by the nature of the heterodimers. iSF is exothermic in all of the herein studied molecules. The overall quantum yield for triplet exciton formation reaches approximately 174%. This novel concept exploits large energy differences between singlet electronic states in combination with spatially fixed chromophores, which achieves efficient heterogeneous iSF, if the through-space interaction between the chromophores is minimal.

Framework-Topology-Controlled Singlet Fission in Metal-Organic Frameworks

Singlet fission (SF) has been explored as a viable route to improve photovoltaic performance by producing more excitons. Efficient SF is achieved through a high degree of interchromophoric coupling that facilitates electron superexchange to generate triplet pairs. However, strongly coupled chromophores often form excimers that can serve as an SF intermediate or a low-energy trap site. The succeeding decoherence process, however, requires an optimum electronic coupling to facilitate the isolation of triplet production from the initially prepared correlated triplet pair. Conformational flexibility and dielectric modulation can provide a means to tune the SF mechanism and efficiency by modulating the interchromophoric electronic interaction. Such a strategy cannot be easily adopted in densely stacked traditional organic solids. Here, we show that the assembly of the SF-active chromophores around well-defined pores of solution-stable metal-organic frameworks (MOFs) can be a great platform for a modular SF process. A series of three new MOFs, built out from 9,10-bis(ethynylenephenyl)anthracene-derived struts, show a topology-defined packing density and conformational flexibility of the anthracene core to dictate the SF mechanism. Various steady-state and transient spectroscopic data suggest that the initially prepared singlet population can prefer either an excimer-mediated SF or a direct SF (both through a virtual charge-transfer (CT) state). These solution-stable frameworks offer the tunability of the dielectric environment to facilitate the SF process by stabilizing the CT state. Given that MOFs are a great platform for various photophysical and photochemical developments, generating a large population of long-lived triplets can expand their utilities in various photon energy conversion schemes.

Singlet Fission-Based High-Resolution X-Ray Imaging Scintillation Screens

X-ray imaging technology is critical to numerous different walks of daily life, ranging from medical radiography and security screening all the way to high-energy physics. Although several organic chromophores are fabricated and tested as X-ray imaging scintillators, they generally show poor scintillation performance due to their weak X-ray absorption cross-section and inefficient exciton utilization efficiency. Here, a singlet fission-based high-performance organic X-ray imaging scintillator with near unity exciton utilization efficiency is presented. Interestingly, it is found that the X-ray sensitivity and imaging resolution of the singlet fission-based scintillator are dramatically improved by an efficient energy transfer from a thermally activated delayed fluorescence (TADF) sensitizer, in which both singlet and triplet excitons can be efficiently harnessed. The fabricated singlet fission-based scintillator exhibits a high X-ray imaging resolution of 27.5 line pairs per millimeter (lp mm^-1 ), which exceeds that of most commercial scintillators, demonstrating its high potential use in medical radiography and security inspection.

Engineering giant excitonic coupling in bioinspired, covalently bridged BODIPY dyads

Strong excitonic coupling in photosynthetic systems is believed to enable efficient light absorption and quantitative charge separation, motivating the development of artificial multi-chromophore arrays with equally strong or even stronger excitonic coupling. However, large excitonic coupling strengths have typically been accompanied by fast non-radiative recombination, limiting the potential of the arrays for solar energy conversion as well as other applications such as fluorescent labeling. Here, we report giant excitonic coupling leading to broad optical absorption in bioinspired BODIPY dyads that have high photostability, excited-state lifetimes at the nanosecond scale, and fluorescence quantum yields of nearly 50%. Through the synthesis, spectroscopic characterization, and computational modeling of a series of dyads with different linking moieties, we show that the strongest coupling is obtained with diethynylmaleimide linkers, for which the coupling occurs through space between BODIPY units with small separations and slipped co-facial orientations. Other linkers allow for broad tuning of both the relative through-bond and through-space coupling contributions and the overall strength of interpigment coupling, with a tradeoff observed in general between the strength of the two coupling mechanisms. These findings open the door to the synthesis of molecular systems that function effectively as light-harvesting antennas and as electron donors or acceptors for solar energy conversion.

Theoretical Study on Thermal Structural Fluctuation Effects of Intermolecular Configurations on Singlet Fission in Pentacene Crystal Models

Singlet fission (SF) occurs as a result of complex excited state relaxation dynamics in molecular aggregates, where a singlet exciton (FE) state is converted into a double-triplet exciton (TT) state through the interactions with several other degrees of freedom, such as nuclear motions. In this study, we combined quantum dynamics simulation based on the quantum master equation approach with all-atom-based classical molecular mechanics/molecular dynamics to examine the thermal structural fluctuation (i.e., static disorder) effects of intermolecular configuration on SF in pentacene crystal models. In particular, we considered two types of static-disordered models, in which excited states are assumed to interact with nuclear motions of intermolecular modes in the classical mechanical/statistical manner. We found that the introduction of static disorder effects leads to a faster decay of coherence between the FE and charge transfer (CT) states in the early stage of SF, contributing to the accelerations of several FE → TT relaxation pathways. Such acceleration in these models is shown to be attributed to fluctuations in the energies and electronic coupling of the CT states based on relative relaxation factor analysis. The present study is expected to contribute to further development of bottom-up materials design for efficient SF in condensed phases where the exitonic system interacts with nuclear motions in various coupling strengths.

Singlet fission preserves polarisation correlation of excitons

Singlet fission (SF) holds the promise to circumvent the photovoltaic efficiency limit to reach a power-conversion efficiency above 34%. SF of TIPS-pentacene (TIPS-Pn) has been investigated but its mechanism is yet to be well elucidated. Recently, we developed a nanoparticle (NP) system, in which doping of TIPS-Pn in a host matrix yields a range of average intermolecular distances, d, to study the dependence of SF in TIPS-Pn on d. At large d values, where the bimolecular SF process should be unfavourable, a relatively high SF quantum yield (Φ_SF) is still observed, which implies a deviation from a random distribution of TIPS-Pn throughout the NP. Here, using polarisation-sensitive femtosecond time-resolved spectroscopy and Monte Carlo simulations of exciton migration and SF, we quantify the level of clustering of TIPS-Pn in the host matrix, which is responsible for the higher than expected Φ_SF. The experimental data indicate a preservation of polarisation correlation by SF, which is uncommon because energy transfer in amorphous materials tends to result in depolarisation. We show that the preservation of polarisation correlation is due to SF upon exciton migration. Although exciton migration decorrelates polarisation, SF acts to remove decorrelated excitons to give an overall preservation of polarisation correlation.

Quantum interference effects elucidate triplet-pair formation dynamics in intramolecular singlet-fission molecules

Quantum interference (QI)-the constructive or destructive interference of conduction pathways through molecular orbitals-plays a fundamental role in enhancing or suppressing charge and spin transport in organic molecular electronics. Graphical models were developed to predict constructive versus destructive interference in polyaromatic hydrocarbons and have successfully estimated the large conductivity differences observed in single-molecule transport measurements. A major challenge lies in extending these models to excitonic (photoexcited) processes, which typically involve distinct orbitals with different symmetries. Here we investigate how QI models can be applied as bridging moieties in intramolecular singlet-fission compounds to predict relative rates of triplet pair formation. In a series of bridged intramolecular singlet-fission dimers, we found that destructive QI always leads to a slower triplet pair formation across different bridge lengths and geometries. A combined experimental and theoretical approach reveals the critical considerations of bridge topology and frontier molecular orbital energies in applying QI conductance principles to predict rates of multiexciton generation.

Regioisomeric Benzidine-Fullerenes: Tuning of the Diverse Hole-Distribution to Influence Charge Separation Patterns

Understanding the influence of molecular structure on charge distribution and charge separation (CS) provides essential guidance for optoelectronic materials design. Here we propose a regioisomeric strategy to tune the diverse hole-distribution, and probe the influence on CS patterns. Para-, meta- and ortho-substituted benzidine-fullerene, named 1 p, 1 m and 1 o are designed. Following CS, hole-delocalization occurs in 1 p, while hole-localization exists in 1 m and 1 o. The rates of charge separation (4.02×10¹¹  s^-1 ) and recombination (9.8×10⁹  s^-1 ) of 1 p is about 20 and 12 times faster than 1 m and 1 o, indicating that para-determined delocalization promotes ultrafast CS, while meta- and ortho-generated localization contributes to long-lived CS states. Computational analysis further implies that localization results from the destruction of electronic conjugation for 1 m, and limitation of conformational relaxation for 1 o. Given that the universality and simplicity of regional isomerism, this work opens up new thoughts for molecular design with tunable charge separation patterns.

Thermodynamic Control of Intramolecular Singlet Fission and Exciton Transport in Linear Tetracene Oligomers

We newly synthesized a series of homo- and hetero-tetracene (Tc) oligomers to propose a molecular design strategy for the efficient exciton transport in linear oligomers by promoting correlated triplet pair (TT) dissociation and controlling sequential exciton trapping process of individual doubled triplet excitons (T+T) by intramolecular singlet fission. First, entropic gain effects on the number of Tc units are examined by comparing Tc-homo-oligomers [(Tc)_n : n=2, 4, 6]. Then, a comparison of (Tc)_n and Tc-hetero-oligomer [TcF₃ -(Tc)₄ -TcF₃ ] reveals the vibronic coupling effect for entropic gain. Observed entropic effects on the T+T formation indicated that the exciton migration is rationalized by number of possible TT states increased both by increasing the number of Tc units and by the vibronic levels at the terminal TcF₃ units. Finally, we successfully observed high-yield exciton trapping process (trapped triplet yield: Φ_TrT =176 %).

Singlet fission dynamics modulated by molecular configuration in covalently linked pyrene dimers, Anti- and Syn-1,2-di(pyrenyl)benzene

Covalently linked dimers (CLDs) and their structural isomers have attracted much attention as potential materials for improving power conversion efficiencies through singlet fission (SF). Here, we designed and synthesized two covalently ortho-linked pyrene (Py) dimers, anti- and syn-1,2-di(pyrenyl)benzene (Anti-DPyB and Syn-DPyB, respectively), and investigated the effect of molecular configuration on SF dynamics using steady-state and time-resolved spectroscopies. Both Anti-DPyB and Syn-DPyB, which have different Py-stacking configurations, form excimers, which then relax to the correlated triplet pair ((T₁T₁)) state, indicating the occurrence of SF. Unlike previous studies where the excimer formation inhibited an SF process, the (T₁T₁)'s of Anti-DPyB and Syn-DPyB are formed through the excimer state. The dissociation of (T₁T₁)'s to 2T₁ in Anti-DPyB is more favorable than in Syn-DPyB. Our results showcase that the molecular configuration of a CLD plays an important role in SF dynamics.

On the Cooperative Origin of Solvent-Enhanced Symmetry-Breaking Charge Transfer in a Covalently Bound Tetracene Dimer Leading to Singlet Fission

Singlet fission in covalently bound acene dimers in solution is driven by the interplay of excitonic and singlet correlated triplet ¹(TT) states with intermediate charge-transfer states, a process which depends sensitively on the solvent environment. We use high-level electronic structure methods to explore this singlet fission process in a linked tetracene dimer, with emphasis on the symmetry-breaking mechanism for the charge-transfer (CT) states induced by low-frequency antisymmetric vibrations and polar/polarizable solvents. A combination of the second-order algebraic diagrammatic construction (ADC(2)) and density functional theory/multireference configuration interaction (DFT/MRCI) methods are employed, along with a state-specific conductor-like screening model (COSMO) solvation model in the former case. This work quantifies, for the first time, an earlier mechanistic proposal [Alvertis et al., J. Am. Chem. Soc. 2019, 141, 17558] according to which solvent-induced symmetry breaking leads to a high-energy CT state which interacts with the correlated triplet state, resulting in singlet fission. An approximate assessment of the nonadiabatic interactions between the different electronic states underscores that the CT states are essential in facilitating the transition from the bright excitonic state to the ¹(TT) state leading to singlet fission. We show that several types of symmetry-breaking inter- and intra-fragment vibrations play a crucial role in a concerted mechanism with the solvent environment and with the symmetric inter-fragment torsion, which tunes the admixture of excitonic and CT states. This offers a new perspective on how solvent-induced symmetry-breaking CT can be understood and how it cooperates with intramolecular mechanisms in singlet fission.

A highly contorted push-pull naphthalenediimide dimer and evidence of intramolecular singlet exciton fission

Singlet fission is a process by which two molecular triplet excitons are generated subsequent to the absorption of one photon. Molecules that enable singlet fission have triplet state energy at least half of the bright singlet state energy. This stringent energy criteria have challenged chemists to device new molecular and supramolecular design principles to modulate the singlet-triplet energy gap and build singlet fission systems from a wide range of organic chromophores. Herein, we report for the first time intramolecular singlet fission in the seminal naphthalenediimide (NDI) scaffold constrained in a push-pull cyclophane architecture, while individually the NDI chromophore does not satisfy the energy criterion. The challenging synthesis of this highly contorted push-pull cyclophane is possible from the preorganized pincer-like precursor. The special architecture establishes the shortest co-facial NDI⋯NDI contacts (3.084 Å) realized to date. Using broadband femtosecond transient absorption, we find that the correlated T-T pair forms rapidly within 380 fs of photoexcitation. Electronic structure calculations at the level of state-averaged CASSCF (ne,mo)/XMCQDPT2 support the existence of the multi-excitonic T-T pair state, thereby confirming the first example of singlet exciton fission in a NDI scaffold.

Direct Exciton Harvesting from a Bound Triplet Pair

Singlet fission is commonly defined as the generation of two triplet excitons from a single absorbed photon. However, ambiguities within this definition arise due to the complexity of the various double triplet states that exist in SF chromophores and the corresponding interconversion processes. To clarify this process, singlet fission is frequently depicted as sequential two-step conversion in which a singlet exciton decays into a bound triplet-pair biexciton state that dissociates into two "free" triplet excitons. However, this model discounts the potential for direct harvesting from the coupled biexciton state. Here, it is demonstrated that individual triplet excitons can be extracted directly from a bound triplet pair. It is demonstrated that due to the requirement for geminate triplet-triplet annihilation in intramolecular singlet fission compounds, unique spectral and kinetic signatures can be used to quantify triplet-pair harvesting yields. An internal quantum efficiency for triplet exciton transfer from the triplet pair of >50%, limited only by the solubility of the compounds is achieved. The harvesting process is not dependent on the net multiplicity of the triplet-pair state, suggesting that an explicit, independent dissociation step is not a requirement for using triplet pairs to do chemical or electrical work.

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.

Steering the multiexciton generation in slip-stacked perylene dye array via exciton coupling

Dye arrays from dimers up to larger oligomers constitute the functional units of natural light harvesting systems as well as organic photonic and photovoltaic materials. Whilst in the past decades many photophysical studies were devoted to molecular dimers for deriving structure-property relationship to unravel the design principles for ideal optoelectronic materials, they fail to accomplish the subsequent processes of charge carrier generation or the detachment of two triplet species in singlet fission (SF). Here, we present a slip-stacked perylene bisimide trimer, which constitutes a bridge between hitherto studied dimer and solid-state materials, to investigate SF mechanisms. This work showcases multiple pathways towards the multiexciton state through direct or excimer-mediated mechanisms by depending upon interchromophoric interaction. These results suggest the comprehensive role of the exciton coupling, exciton delocalization, and excimer state to facilitate the SF process. In this regard, our observations expand the fundamental understanding the structure-property relationship in dye arrays.

Understanding structure-property relationship of dye arrays is of great importance for designing organic photonic and photovoltaic materials. Here, authors present a slip-stacked perylene bisimide array as a model system to investigate singlet fission mechanisms by depending upon interchromophoric interaction.

Photophysical Properties and Electronic Structure of Hydroporphyrin Dyads Exhibiting Strong Through-Space and Through-Bond Electronic Interactions

Electronic interactions between tetrapyrroles are utilized in natural photosynthetic systems to tune the light-harvesting and energy-/charge-transfer processes in these assemblies. Such interactions also can be employed to tailor the electronic properties of tetrapyrrolic dyads and larger arrays for use in materials science and biomedical research. Here, we have utilized static and time-resolved optical spectroscopy to characterize the optical absorption and emission properties of a set of chlorin and bacteriochlorin dyads with varying degrees of through-bond (TB) and through-space (TS) interactions between the constituent macrocycles. The dyads consist of two chlorins or two bacteriochlorins joined by a linker that utilizes a triple-double-triple-bond (enediyne) motif in which the double-bond portion is an ester-substituted ethylene or o-phenylene unit. The photophysical studies are coupled with density functional theory (DFT) calculations to probe the ground-state molecular orbital (MO) characteristics of the dyads and time-dependent DFT calculations (TDDFT) to elucidate excited-state properties. The latter include electronic characteristics of the singlet excited-state manifold and the absorption transitions to these states from the electronic ground state. A comparison of the MO and calculated spectral properties of each dyad with the linker present versus disrupted (by eliminating the double-bond portion) gives insight into the relative contributions of TB versus TS interactions to the electronic properties of the dyads. The results show that the TB and TS contributions are additive (constructively interfere), which is not always the case for molecular dyads. Most of the dyads have shorter lifetimes of the lowest singlet excited state compared to the parent monomer, which derives from increased S₁ → S₀ internal conversion. The enhancement is greater for the dyads in benzonitrile than in toluene. The studies provide insights into the nature of the electronic interactions between the constituents in the tetrapyrrole arrays and how these interactions dictate the spectral properties and excited-state decay characteristics.

Mechanism of Singlet Fission in Carotenoids from a Polyene Model System

Singlet fission (SF) is the process of formation of multiple excitons (triplet) from a locally excited singlet state. The mechanism of SF in polyacenes has been shown to proceed via a charge transfer intermediate state. However, carotenoids are not understood in the context of SF. This is possibly due to the complicated multireference nature of the low-lying excited states of carotenoids and the presence of a dark 2¹A_g state below the optically bright ¹B_u state. In this work, we show that the dark A_g state in polyenes and/or carotenoids, along with the charge transfer states, plays a pivotal role in the SF process. We notice that the relative importance of these states varies with a change in geometry and the overall presence of multiple pathways is crucial to the success of the SF process in carotenoid aggregates and disordered geometries.

Modulating singlet fission through interchromophoric rotation

Singlet fission (SF) is a spin-allowed, exciton-multiplying phenomenon that can be utilized to improve the efficiency of organic solar cells. It is well-understood that SF is sensitive to the local crystal morphology and an appropriately balanced coupling is essential to facilitate efficient SF. In this study, we show how the interchromophoric rotation selectively modulates the interaction between the monomer frontier molecular orbitals, promoting both fast and exothermal SF. We evaluate the effective electronic coupling for SF (V_SF), the square of which is proportional to the SF rate, and the effective energies of the Frenkel exciton (FE/S₁S₀) and triplet pair exciton (TT) in a terrylene dimer model. Optimal interplanar rotation of the chromophoric moieties in slip-stacked arrangements pulls the effective energy of the TT state below that of the FE state. Consequently, SF is favored over competing pathways such as excimer formation, thereby enhancing the overall triplet yield. This work represents a step towards improvising the molecular design guidelines for SF and understanding the importance of interchromophoric rotation over the conventional slip-stacked arrangements for achieving favorable intermolecular electronic coupling towards efficient SF.

Environmental effects on the singlet fission phenomenon: a model Hamiltonian-based study

In the screening of compounds for singlet fission, the relative energies of the constitutive units are decisive to fulfil the thermodynamic rules. From a model Hamiltonian constructed on the local spin states of an active chromophore and its environment, it is suggested that embedding greatly influences the energy differences of the active monomer spin states. Even in the absence of charge transfer, the field generated by a singlet environment produces an increase of the [E(S₁) - E(S₀)]/[E(T₁) - E(S₀)] critical ratio by up to 6% as compared to the one of a free chromophore. Besides, variations are observed when the intimate electronic structure of the singlet environment is modified. This propensity towards singlet fission is even more pronounced (10%) when the environment is switched to the triplet state. Finally, the embedding is likely to reverse the spin state ordering in the limit of vanishing atomic orbital overlaps. Despite its simplicity, the model stresses the importance of the environment spin nature in the quest for singlet fission candidates, and more generally in spectroscopy analysis.

Optimizing the Thermodynamics and Kinetics of the Triplet-Pair Dissociation in Donor-Acceptor Copolymers for Intramolecular Singlet Fission

Singlet fission (SF) is a two-step process in which a singlet splits into two triplets throughout the so-called correlated triplet-pair (1TT) state. Intramolecular SF (iSF) materials, in particular, have attracted growing interest as they can be easily implemented in single-junction solar cells and boost their power conversion efficiency. Still, the potential of iSF materials such as polymers and oligomers for photovoltaic applications has been partially hindered by their ability to go beyond the 1TT intermediate and generate free triplets, whose mechanism remains poorly understood. In this work, the main aspects governing the 1TT dissociation in donor-acceptor copolymers and the key features that optimize this process are exposed. First, we show that both thermodynamics and kinetics play a crucial role in the intramolecular triplet-pair separation and second, we uncover the inherent flexibility of the donor unit as the fundamental ingredient to optimize them simultaneously. Overall, these results provide a better understanding of the intramolecular 1TT dissociation process and establish a new paradigm for the development of novel iSF active materials.

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.

Altering singlet fission pathways in perylene-dimers; perylene-diimide versus perylene-monoimide

We used a systematic approach to shed light on the inherent differences in perylenes, namely monoimides versus diimides, including coplanarity and dipole moment, and their impact on singlet fission (SF) by designing, synthesizing, and probing a full fledged series of phenylene- and naphthalene-linked dimers. Next to changing the functionality of the perylene core, we probed the effect of the spacers and their varying degrees of rotational freedom, molecular electrostatic potentials, and intramolecular interactions on the SF-mechanism and -efficiencies. An arsenal of spectroscopic techniques revealed that for perylene-monoimides, a strong charge-transfer mixing with the singlet and triplet excited states restricts SF and yields low triplet quantum yields. This is accompanied by an up-conversion channel that includes geminate triplet-triplet recombination. Using perylene-diimides alters the SF-mechanism by populating a charge-separated-state intermediate, which either favors or shuts-down SF. Napthylene-spacers bring about higher triplet quantum yields and overall better SF-performance for all perylene-monoimides and perylene-diimides. The key to better SF-performance is rotational freedom because it facilitates the overall excited-state polarization and amplifies intramolecular interactions between chromophores.

Exploring a new class of singlet fission fluorene derivatives with high-energy triplets

In this study, we report strong experimental evidence for singlet fission (SF) in a new class of fluorene-based molecules, exhibiting two-branched donor-acceptor structures. The time-resolved spectroscopic results disclose ultrafast formation of a double triplet state (occurring in few picoseconds) and efficient triplet exciton separation (up to 145% triplet yield). The solvent polarity effect and the role of intramolecular charge transfer (ICT) on the SF mechanism have been thoroughly investigated with several advanced spectroscopies. We found that a stronger push-pull character favors SF, as long as the ICT does not act as a trap by opening a competitive pathway. Within the context of other widely-known SF chromophores, the unconventional property of generating high-energy triplet excitons (ca. 2 eV) via SF makes these materials outstanding candidates as photosensitizers for photovoltaic devices.

Quantifying Exciton Transport in Singlet Fission Diblock Copolymers

Singlet fission (SF) is a mechanism of exciton multiplication in organic chromophores, which has potential to drive highly efficient optoelectronic devices. Creating effective device architectures that operate by SF critically depends on electronic interactions across multiple length scales─from individual molecules to interchromophore interactions that facilitate multiexciton dephasing and exciton diffusion toward donor-acceptor interfaces. Therefore, it is imperative to understand the underpinnings of multiexciton transport and interfacial energy transfer in multichromophore systems. Interestingly, block copolymers (BCPs) can be designed to control multiscale interactions by tailoring the nature of the building blocks, yet SF dynamics are not well understood in these macromolecules. Here, we designed diblock copolymers comprising an inherent energy cleft at the interface between a block with pendent pentacene chromophores and an additional block with pendent tetracene chromophores. The singlet and triplet energy offset between the two blocks creates a driving force for exciton transport along the BCP chain in dilute solution. Using time-resolved optical spectroscopy, we have quantified the yields of key energy transfer steps, including both singlet and triplet energy transfer processes across the pentacene-tetracene interface. From this modular BCP architecture, we correlate the energy transfer time scales and relative yields with the length of each block. The ability to quantify these energy transfer processes provides valuable insights into exciton transport at critical length scales between bulk crystalline systems and small-molecule dimers─an area that has been underexplored.

Excimer Formation Inhibits the Intramolecular Singlet Fission Dynamics: Systematic Tilting of Pentacene Dimers by Linking Positions

The role of excimer formation in inhibiting or enhancing the efficiency of the intramolecular singlet fission (iSF) process has been a subject of recent debate. Here, we investigated the effect of excimer formation on iSF dynamics by modifying its configuration by connecting pentacenes at various positions. Hence, pentacene dimers having slip-stacked (2,2' BP, J-type), oblique (2,6' BP), and facial (6,6' BP, H-type) configurations were synthesized by covalently linking pentacenes at positions 2,2', 2,6', and 6,6', respectively, with an ethynyl bridge, and their ultrafast excited-state relaxation dynamics were characterized. Femtosecond time-resolved transient absorption spectra revealed that the efficiency of iSF dynamics decreased from slip-stacked (182%) to oblique configuration (97%),whereas in the 6,6' BP with facial configuration, strong electronic coupling led to the formation of excimers that decayed nonradiatively without formation of correlated triplet pairs. These studies reveal the formation of excimers by strong intrapentacene electronic coupling upon ultrafast excitation, preventing the efficient iSF process.

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.

Singlet fission and triplet pair recombination in bipentacenes with a twist

We investigate triplet pair dynamics in pentacene dimers that have varying degrees of coplanarity (pentacene-pentacene twist angle). The fine-tuning of the twist angle was achieved by alternating connectivity at the 1-position or 2-positions of pentacene. This mix-and-match connectivity leads to tunable twist angles between the two covalently linked pentacenes. These twisted dimers allow us to investigate the subtle effects that the dihedral angle between the covalently linked pentacenes imparts on singlet fission and triplet pair recombination dynamics. We observe that as the dihedral angle between the two bonded pentacenes is increased, the rates of singlet fission decrease, while the accompanying decrease in triplet recombination rates is stark. Temperature-dependent transient optical studies combined with theoretical calculations show that the triplet pair recombination proceeds primarily through a direct multiexciton internal conversion process. Calculations further show that the significant decrease in recombination rates can be directly attributed to a corresponding decrease in the magnitude of the nonadiabatic coupling between the singlet multiexcitonic state and the ground state. These results highlight the importance of the twist angle in designing systems that exhibit rapid singlet fission, while maintaining long triplet pair lifetimes in pentacene dimers.

Structure Set in Stone: Designing Rigid Linkers to Control the Efficiency of Intramolecular Singlet Fission

Research on materials facilitating efficient singlet fission (SF) is driven by a possible reduction of thermalization losses in organic photovoltaic devices. Intramolecular SF (iSF) is in this context of special interest, as the targeted modification of either chromophores or linkers enables gradual variations of molecular properties. In this combined synthetic, spectroscopic, and computational work, we present and investigate nine novel spiro-linked azaarene dimers, which undergo efficient iSF with triplet yields up to 199%. Additional molecular braces enhance the rigidity of these tailor-made dimers (TMDs), resulting in great agreement between crystal structures and predicted optimal geometries for iSF in solution. Regardless of the employed chromophores and linkages, the dynamics of all nine TMDs are perfectly described by a unified kinetic model. Most notably, an increase in the orbital overlap of the π-systems by decreasing the twist angle between the two chromophores does not only increase the rate of formation of the correlated triplet pair but also further promotes its decorrelation. This new structure-function relationship represents a promising strategy toward TMDs with high triplet lifetimes to be utilized in optoelectronic devices.

Chemical Tuning of Exciton versus Charge-Transfer Excited States in Conformationally Restricted Arylene Cages

Covalent assemblies of conjugated organic chromophores provide the opportunity to engineer new excited states with novel properties. In this work, a newly developed triple-stranded cage architecture, in which meta-substituted aromatic caps serve as covalent linking groups that attach to both top and bottom of the conjugated molecule walls, is used to tune the properties of thiophene oligomer assemblies. Benzene-capped and triazine-capped 5,5'-(2,2-bithiophene)-containing arylene cages are synthesized and characterized using steady-state and time-resolved spectroscopic methods. The conformational freedom and electronic states are analyzed using time-dependent density functional theory. The benzene cap acts as a passive spacer whose electronic states do not mix with those of the chromophore walls. The excited state properties are dominated by through-space interactions between the chromophore subunits, generating a neutral Frenkel H-type exciton state. This excitonic state undergoes intersystem crossing on a 200 ps time scale while the fluorescence output is suppressed by a factor of 2 due to a decreased radiative rate. Switching to a triazine cap enables electron transfer from the chromophore-linker after the initial excitation to the exciton state, leading to the formation of a charge-transfer state within 10 ps. This state can avoid intersystem crossing and exhibits red-shifted fluorescence with enhanced quantum yield. The ability to interchange structural modules with different electronic properties while retaining the overall cage morphology provides a new approach for tuning the properties of discrete chromophore assemblies.

Revealing the Contest between Triplet-Triplet Exchange and Triplet-Triplet Energy Transfer Coupling in Correlated Triplet Pair States in Singlet Fission

Understanding the separation of the correlated triplet pair state ¹(TT) intermediate is critical for leveraging singlet fission to improve solar cell efficiency. This separation mechanism is dominated by two key interactions: (i) the exchange interaction (K) between the triplets which leads to the spin splitting of the biexciton state into ¹(TT),³(TT) and ⁵(TT) states, and (ii) the triplet-triplet energy transfer integral (t) which enables the formation of the spatially separated (but still spin entangled) state ¹(T···T). We develop a simple ab initio technique to compute both the biexciton exchange (K) and biexciton transfer coupling. Our key findings reveal new conditions for successful correlated triplet pair state dissociation. The biexciton exchange interaction needs to be ferromagnetic or negligible to the triplet energy transfer for favorable dissociation. We also explore the effect of chromophore packing to reveal geometries where these conditions are achieved for tetracene.

Excimer formation dynamics in the isolated tetracene dimer

The understanding of excimer formation and its interplay with the singlet-correlated triplet pair state ¹(TT) is of high significance for the development of efficient organic electronics. Here, we study the photoinduced dynamics of the tetracene dimer in the gas phase by time-resolved photoionisation and photoion imaging experiments as well as nonadiabatic dynamics simulations in order to obtain mechanistic insight into the excimer formation dynamics. The experiments are performed using a picosecond laser system for excitation into the S₂ state and reveal a biexponential time dependence. The time constants, obtained as a function of excess energy, lie in the range between ≈10 ps and 100 ps and are assigned to the relaxation of the excimer on the S₁ surface and to its deactivation to the ground state. Simulations of the quantum-classical photodynamics are carried out in the frame of the semi-empirical CISD and TD-lc-DFTB methods. Both theoretical approaches reveal a dominating relaxation pathway that is characterised by the formation of a perfectly stacked excimer. TD-lc-DFTB simulations have also uncovered a second relaxation channel into a less stable dimer conformation in the S₁ state. Both methods have consistently shown that the electronic and geometric relaxation to the excimer state is completed in less than 10 ps. The inclusion of doubly excited states in the CISD dynamics and their diabatisation further allowed to observe a transient population of the ¹(TT) state, which, however, gets depopulated on a timescale of 8 ps, leading finally to the trapping in the excimer minimum.

The understanding of excimer formation and its interplay with the singlet-correlated triplet pair state ¹(TT) is of high significance for the development of efficient organic electronics.

Pentacene-Bridge Interactions in an Axially Chiral Binaphthyl Pentacene Dimer

Molecular chirality can be exploited as a sensitive reporter of the nature of intra- and interchromophore interactions in π-conjugated systems. In this report, we designed an intramolecular singlet fission (iSF)-based pentacene dimer with an axially chiral binaphthyl bridge (2,2'-(2,2'-dimethoxy-[1,1'-binaphthalene]-3,3'-diyl) n-octyl-di-isopropyl silylethynyl dipentacene, BNBP) to utilize its chiroptical response as a marker of iSF chromophore-bridge-chromophore (SFC-β-SFC) interactions. The axial chirality of the bridge enforces significant one-handed excitonic coupling of the pentacene monomer units; as such, BNBP exhibits significant chiroptical response in the ground and excited states. We analyzed the chiroptical response of BNBP using the exciton coupling method and quadratic response density functional theory calculations to reveal that higher energy singlet transitions in BNBP involve significant delocalization of the electronic density on the bridging binaphthyl group. Our results highlight the promising application of chiroptical techniques to investigate the nature of SFC-β-SFC interactions that impact singlet fission dynamics.

Pushing the Limits of the Donor-Acceptor Copolymer Strategy for Intramolecular Singlet Fission

Donor-acceptor (D-A) copolymers have shown great potential for intramolecular singlet fission (iSF). Nonetheless, very few design principles exist for optimizing these systems for iSF, with very little knowledge about how to engineer them for this purpose. In recent work, a fundamental trade-off between the main electronic ingredients required for iSF capable D-A coplanar copolymers was revealed. Still, further investigations are needed to understand these limitations and learn how to bypass them. In this work, we propose to induce torsion as an effective way to circumvent the limits of the coplanar approach. We disclose the potential of noncoplanar copolymers with inherently low triplet energies that encompass all the characteristics required for iSF beyond the limiting values associated with fully coplanar systems. Our findings shed some light on the electronic structure aspects of D-A copolymers for iSF and offer a new avenue for the rational design of novel promising candidates.

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).

The Role of the Core Attachment Positioning in Triggering Intramolecular Singlet Exciton Fission in Perylene Diimide Tetramers

Previous studies have proposed that the presence of a flexible π-bridge linker is crucial in activating intramolecular singlet exciton fission (iSEF) in multichromophoric systems. In this study, we report the photophysical properties of three analogous perylene diimide (PDI) dendritic tetramers showing flexible/twisted π-bridged structures with α- and β-substitutions and a rigid/planar structure with a β-fused ring (βC) connection to a benzodithiophene-thiophene (BDT-Th) core. The rigidity and enhanced planarity of βC lead to significant intramolecular charge transfer and triplet formation via an intersystem crossing pathway. Steady-state spectroscopic measurements reveal similar absorption and emission spectra for the α-tetramer and the parent PDI monomer. However, their fluorescence quantum yield is significantly different. The negligible fluorescence yield of the α-tetramer (0.04%) is associated with a competitive nonradiative decay pathway. Indeed, for this twisted compound in a high polar environment, a fast and efficient iSEF with a triplet quantum yield of 124% is observed. Our results show that the α-single-bond connections in the α compound are capable of interrupting the coupling among the PDI units, favoring iSEF. We propose that the formation of the double triplet (¹[TT]) state is through a superposition of singlet states known as [S₁S₀]_[TT]CT, which has been suggested previously for pentacene derivatives. Using steady-state and time-resolved spectroscopic experiments, we demonstrate that the conformational flexibility of the linker itself is necessary but not sufficient to allow iSEF. For the case of the other twisted tetramer, β, the strong π-π co-facial interactions between the adjacent PDI units in its structure lead to excimer formation. These excimer states trap the singlet excitons preventing the formation of the ¹[TT] state, thus inhibiting iSEF.