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In this nanoparticle synthesis, the feed quantities of TEOS and PEG-silane greatly impact the formation of the external silica shell used to encapsulate upconversion materials, as well as the accessible upconverted photoluminescence.

Interchromophoric interactions such as Coulombic coupling and exchange interactions are crucial to the functional properties of numerous π-conjugated systems. Here, we use magnetic circular dichroism (MCD) spectroscopy to investigate interchromophoric interactions in singlet fission relevant pentacene dimers. Using a simple analytical model, we outline a general relationship between the geometry of pentacene dimers and their calculated MCD response. We analyze experimental MCD spectra of different covalently bridged pentacene dimers to reveal how the molecular structure of the "bridge" affects the magnitude of through-space Coulombic and through-bond exchange interactions in the system. Our results show that through-bond interactions are significant in dimers with conjugated molecules as bridging units and these interactions promote the overall electronic coupling in the system. Our generalized approach paves the way for the application of MCD in investigating interchromophoric interactions across a range of π-conjugated systems.

Two-photon photopolymerization delivers prints without support structures and minimizes layering artifacts in a broad range of materials. This volumetric printing approach scans a focused light source throughout the entire volume of a resin vat and takes advantage of the quadratic power dependence of two photon absorption to produce photopolymerization exclusively at the focal point. While this approach has advantages, the widespread adoption of two photon photopolymerization is hindered by the need for expensive ultrafast lasers and extremely slow print speeds. Here we present an analogous quadratic process, triplet-triplet-annihilation-driven 3D printing, that enables volumetric printing at a focal point driven by

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.

Triplet fusion upconversion (UC) allows for the generation of one high energy photon from two low energy input photons. This well-studied process has significant implications for producing high energy light beyond a material's surface. However, the deployment of UC materials has been stymied due to poor material solubility, high concentration requirements, and oxygen sensitivity, ultimately resulting in reduced light output. Toward this end, nanoencapsulation has been a popular motif to circumvent these challenges, but durability has remained elusive in organic solvents. Recently, a nanoencapsulation technique was engineered to tackle each of these challenges, whereupon an oleic acid nanodroplet containing upconversion materials was encapsulated with a silica shell. Ultimately, these nanocapsules (NCs) were durable enough to enable triplet fusion upconversion-facilitated volumetric three-dimensional (3D) printing. By encapsulating upconversion materials with silica and dispersing them in a 3D printing resin, photopatterning beyond the surface of the printing vat was made possible. Here, video protocols for the synthesis of upconversion NCs are presented for both small-scale and large-scale batches. The outlined protocols serve as a starting point for adapting this encapsulation scheme to multiple upconversion schemes for use in volumetric 3D printing applications.

Triplet-triplet annihilation upconversion (TTA-UC) is an unconventional photophysical process that yields high-energy photons from low-energy incident light and offers pathways for innovation across many technologies, including solar energy harvesting, photochemistry, and optogenetics. Within aromatic organic chromophores, TTA-UC is achieved through several consecutive energy conversion events that ultimately fuse two triplet excitons into a singlet exciton. In chromophores where the singlet exciton is roughly isoergic with two triplet excitons, the limiting step is the triplet-triplet annihilation pathway, where the kinetics and yield depend sensitively on the energies of the lowest singlet and triplet excited states. Herein we report up to 40-fold improvements in upconversion quantum yields using molecular engineering to selectively tailor the relative energies of the lowest singlet and triplet excited states, enhancing the yield of triplet-triplet annihilation and promoting radiative decay of the resulting singlet exciton. Using this general and effective strategy, we obtain upconversion yields with red emission that are among the highest reported, with remarkable chemical stability under ambient conditions.

A major benefit of intramolecular singlet fission (iSF) materials, in which through-bond interactions mediate triplet pair formation, is the ability to control the triplet formation dynamics through molecular engineering. One common design strategy is the use of molecular bridges to mediate interchromophore interactions, decreasing electronic coupling by increasing chromophore-chromophore separation. Here, we report how the judicious choice of aromatic bridges can enhance chromophore-chromophore electronic coupling. This molecular engineering strategy takes advantage of "bridge resonance", in which the frontier orbital energies are nearly degenerate with those of the covalently linked singlet fission chromophores, resulting in fast iSF even at large interchromophore separations. Using transient absorption spectroscopy, we investigate this bridge resonance effect in a series of pentacene and tetracene-bridged dimers, and we find that the rate of triplet formation is enhanced as the bridge orbitals approach resonance. This work highlights the important role of molecular connectivity in controlling the rate of iSF through chemical bonds and establishes critical design principles for future use of iSF materials in optoelectronic devices.

Singlet fission (SF) is an exciton multiplication process with the potential to raise the efficiency limit of single junction solar cells from 33% to up to 45%. Most chromophores generally undergo SF as solid-state crystals. However, when such molecules are covalently coupled, the dimers can be used as model systems to study fundamental photophysical dynamics where a singlet exciton splits into two triplet excitons within individual molecules. Here we report the synthesis and photophysical characterization of singlet fission of a hexacene dimer. Comparing the hexacene dimer to analogous tetracene and pentacene dimers reveals that excess exoergicity slows down singlet fission, similar to what is observed in molecular crystals. Conversely, the lower triplet energy of hexacene results in an increase in the rate of triplet pair recombination, following the energy gap law for radiationless transitions. These results point to design rules for singlet fission chromophores: the energy gap between singlet and triplet pair should be minimal, and the gap between triplet pair and ground state should be large., We report the synthesis and photophysical characterization of highly exoergic singlet fission in a hexacene dimer revealing exciton dynamics that follow the energy gap law.

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.

Singlet fission is a photophysical process in which two triplet excitons are produced from one singlet exciton. Understanding the details of this process, especially the properties of intermediates between singlet excitons and free triplet excitons, is particularly important to its optimization and application. Herein, we explore the bound triplet pair, m(T1T1), an important intermediate state in singlet fission. We highlight a growing number of studies indicating there is an energetic stabilization of triplet-pair states relative to free triplets. This stabilization enables endothermic singlet fission processes and often results in long lifetimes of the bound triplet-pair state. However, triplets in close proximity demonstrate heightened recombination and reduced excited-state lifetimes. The ubiquity of bound triplet-pair states in recent reports indicates the importance of this state, the need to discern it from free triplets, and the usefulness of having carefully tuned interactions between triplets produced by singlet fission.

Triplet fusion upconversion, the conversion of two low-energy photons into one higher-energy photon via excitonic intermediates, has the potential to revolutionize fields as diverse as biological imaging, photovoltaics, and optogenetics. However, important hurdles to widespread application still exist; for example, the vast majority of demonstrations are in nonpolar solvents, limiting applications. Furthermore, the necessary high concentrations of dyes limit optical penetration depth. Efforts toward aqueous solutions utilizing micelles and other nanoencapsulants have been limited by poor efficiencies or scatter from the nanoparticles. Here, we demonstrate a facile micellular fabrication method that drives a high boiling point solvent into the core of a block copolymer micelle, greatly reducing molecular aggregation. We show that this simple preparation is scalable and provides benefits across five different colors of photon upconversion. We expect this simple, user-friendly, and high-performance system to aid a multitude of photon upconversion applications, in particular, for optogenetics, photodynamic therapy, and photochemistry.

Singlet fission has emerged as a key mechanism of exciton multiplication in organic chromophores, generating two triplet excitons from a single photon. Singlet fission is typically studied in crystalline films or in isolated dimers. Here, we investigate an intermediate regime where through-space interactions mediate singlet fission and triplet pair recombination within isolated polymer chains. Specifically, we investigate how appending pentacenes to a polynorbornene backbone can lead to macromolecules that take advantage of through-space π-π interactions for fast singlet fission and rapid triplet pair dissociation. Singlet fission in these systems is affected by molecular dynamics, and triplet-triplet recombination is a geminate process where the rate of recombination scales with molecular-weight. We find that these pendent pentacene polymers yield free triplets with lifetimes that surpass those of crystalline chromophores in both solution as isolated polymers and in thin films.

Interest in organic–inorganic hybrid perovskite (ABX3) LEDs has exploded over the past several years, yet significant gains in stability, efficiency, and brightness are required before commercialization is possible, particularly for blue devices. The perovskite composition has been shown to play a crucial role in its performance, yet to date nearly all existing reports focus on tuning the A-site composition. Here, we find that doping the B-site with manganese allows us to achieve bright, efficient, and stable LEDs regardless of A or X composition. By doping with Mn, we demonstrate ultrabright sky-blue, green, and red perovskite LEDs with a maximum brightness of 11800, 97000, and 1470 cd/m2 and quantum efficiencies of 0.58%, 3.2%, and 5.1%, respectively. Crucially, these devices show excellent operational stability, with the sky-blue devices lasting for 20 min and red devices over 5 h with strong spectral stability. Moreover, the green devices showed over 1% efficiency even at higher current densities, ∼20...

Recent synthetic studies on the organic molecules tetracene and pentacene have found certain dimers and oligomers to exhibit an intense absorption in the visible region of the spectrum which is not present in the monomer or many previously-studied dimers. In this article we combine experimental synthesis with electronic structure theory and spectral computation to show that this absorption arises from an otherwise dark charge-transfer excitation 'borrowing intensity' from an intense UV excitation. Further, by characterizing the role of relevant monomer molecular orbitals, we arrive at a design principle that allows us to predict the presence or absence of an additional absorption based on the bonding geometry of the dimer. We find this rule correctly explains the spectra of a wide range of acene derivatives and solves an unexplained structure-spectrum phenomenon first observed seventy years ago. These results pave the way for the design of highly absorbent chromophores with applications ranging from photovoltaics to liquid crystals., 29 pages, 63 pages with SI, 5 figures

Optical upconversion is a net process by which two low energy photons are converted into one higher energy photon. There is vast potential to exploit upconversion in applications ranging from solar energy and biological imaging to data storage and photocatalysis. Here, we link two upconverting chromophores together to synthesize a series of novel tetracene dimers for use as annihilators. When compared with the monomer annihilator, TIPS–tetracene, the dimers yield a strong enhancement in the triplet fusion process, also known as triplet–triplet annihilation, as demonstrated via a large increase in upconversion efficiency and an order of magnitude reduction of the threshold power for maximum yield. Along with the ongoing rapid improvements to sensitizer materials, the dimerization improvements demonstrated here open the way to a wide variety of emerging upconversion applications.

Three-dimensional (3D) printing has exploded in interest as new technologies have opened up a multitude of applications

Singlet fission, an exciton multiplication process, can augment existing solar cell technologies. We explore the current challenges facing the research field and how optical and magnetic resonance spectroscopies can help identify promising materials.

Singlet fission, in which two triplet excitons are generated from a single absorbed photon, is a key third-generation solar cell concept. Conservation of angular momentum requires that singlet fission populates correlated multiexciton states, which can subsequently dissociate to generate free triplets. However, little is known about electronic and spin correlations in these systems since, due to its typically short lifetime, the multiexciton state is challenging to isolate and study. Here, we use bridged pentacene dimers, which undergo intramolecular singlet fission while isolated in solution and in solid matrices, as a unimolecular model system that can trap long-lived multiexciton states. We combine transient absorption and time-resolved electron spin resonance spectroscopies to show that spin correlations in the multiexciton state persist for hundreds of nanoseconds. Furthermore, we confirm long-standing predictions that singlet fission produces triplet pair states of quintet character. We compare two different pentacene–bridge–pentacene chromophores, systematically tuning the coupling between the pentacenes to understand how differences in molecular structure affect the population and dissociation of multiexciton quintet states. Experiments show how molecular structure affects the interaction and dynamics of the triplet exciton pairs produced when an excited singlet exciton decays via singlet fission — a process that could be harnessed for optoelectronic applications.

Summary A recent surge of interest in singlet-fission (SF) dimers is inspired by their utility as model systems as well as by the inherent advantages of an intramolecular SF process. Here, we extend the scope of these materials to polypentacene, a model one-dimensional system designed to bridge studies of SF in constrained SF dimers and extended condensed phase systems. Using transient absorption spectroscopy, we studied a soluble polypentacene derivative and well-defined oligomers with 2–5 repeat units to elaborate the trend in triplet-pair dynamics with increasing lengths. We found that the SF rate constant increased monotonically with the number of repeat units. However, recombination in all cases was rapid (

Interest in materials that undergo singlet fission (SF) has been catalyzed by the potential to exceed the Shockley–Queisser limit of solar power conversion efficiency. In conventional materials, the mechanism of SF is an intermolecular process (xSF), which is mediated by charge transfer (CT) states and depends sensitively on crystal packing or molecular collisions. In contrast, recently reported covalently coupled pentacenes yield ∼2 triplets per photon absorbed in individual molecules: the hallmark of intramolecular singlet fission (iSF). However, the mechanism of iSF is unclear. Here, using multireference electronic structure calculations and transient absorption spectroscopy, we establish that iSF can occur via a direct coupling mechanism that is independent of CT states. We show that a near-degeneracy in electronic state energies induced by vibronic coupling to intramolecular modes of the covalent dimer allows for strong mixing between the correlated triplet pair state and the local excitonic state, despite weak direct coupling., We identify a specific, high-frequency, intramolecular vibrational mode that assists direct population transfer from the LE state to the ME state through an avoided crossing.

Stable microbubbles can be prepared by evaporation of a liquid core inside an expandable polymeric shell. To control the expansion temperature and the size of resulting microbubbles, we prepared polymeric microcapsules that contain multiple liquids in their core. One-step suspension polymerization allowed for encapsulation of at least two different liquids with near quantitative yield. The liquids may be miscible or immiscible, which directly affects the vapor pressure inside the capsule according to either Raoult's or Dalton's law, respectively. In the case of miscible liquids, e.g., perfluorohexane and perfluoropentane, both the vapor pressure and the expansion temperature vary within a range bounded by the properties of the neat liquids. For immiscible liquids, e.g. perfluoropentane and isopentane, the total vapor pressure is above this range as it equals to a sum of the individual pressures. Due to increased vapor pressure, encapsulation of two immiscible liquids significantly lowers the expansion temperature below the corresponding temperatures observed during expansion of microcapsules with neat perfluoropentane and isopentane cores. The microbubble diameter and shell thickness were controlled within ca. 10–50 μm and 0.1–3 μm, respectively, by varying the fractions and vapor pressure of the core fluids. We also showed that the effect of core composition on the expansion temperature was more significant than the effect of the shell thickness. Furthermore, one of the encapsulated materials may carry additional functions including imaging contrast, catalysis, and density compensation. To that end, these designer microcapsules may find applications in the fields of drug delivery, acoustic imaging, drilling, and self-healing materials.

We describe a facile route to well-defined, solution-processable pentacene oligomers (2 to 7) and homopolymer using Suzuki–Miyaura cross-coupling reactions. Because this synthetic strategy leads to regioisomers, regiopure syn- and anti-trimers were also synthesized, revealing minimal changes in solution properties but significant changes in the solid state arising from differing levels of crystallinity. The materials were characterized by steady state absorption spectroscopy and cyclic voltammetry to study their electronic structure. The steady state absorption spectra exhibit a new high-energy transition in the oligomers, which intensifies as a function of oligomer length, thus increasing the range of absorption to include the entire visible spectrum. Density functional theory calculations indicate that the new peak results directly from the oligomerization. Solid state UV–vis suggests that while the monomer is amorphous, bricklayer packing in the higher oligomers significantly alters the solid state abs...

In the latest issue of Nature Chemistry , Stern et al. combine ultrafast transient absorption, sliding-window Fourier transform, and time-resolved emission spectroscopy to describe a comprehensive mechanism for endothermic singlet fission in a tetracene derivative.

Accurate extreme ultraviolet spectra of open N-shell neodymium (Nd) ions were recorded at the electron beam ion trap facility of the National Institute of Standards and Technology. The measurements were performed for nominal electron beam energies in the range of 0.90 keV to 2.31 keV. The measured spectra were then compared with the spectra simulated by a collisional-radiative model utilizing atomic data produced with a fully relativistic atomic structure code. Consequently, 59 lines from Br-like to Ni-like Nd ions were unambiguously identified, most of which were newly assigned in this study. The wavelengths of 9 known lines from Ni-, Cu- and Zn-like Nd ions were in excellent agreement with previous measurements.

Singlet fission—that is, the generation of two triplets from a lone singlet state—has recently resurfaced as a promising process for the generation of multiexcitons in organic systems. Although advances in this area have led to the discovery of modular classes of chromophores, controlling the fate of the multiexciton states has been a major challenge; for example, promoting fast multiexciton generation while maintaining long triplet lifetimes. Unravelling the dynamical evolution of the spin- and energy conversion processes from the transition of singlet excitons to correlated triplet pairs and individual triplet excitons is necessary to design materials that are optimized for translational technologies. Here, we engineer molecules featuring a discrete energy gradient that promotes the migration of strongly coupled triplet pairs to a spatially separated, weakly coupled state that readily dissociates into free triplets. This ’energy cleft’ concept allows us to combine the amplification and migration processes within a single molecule, with rapid dissociation of tightly bound triplet pairs into individual triplets that exhibit lifetimes of ~20 µs. Although they are synthetically tunable, organic molecules that undergo singlet fission (the generation of two excitons from one photon) have not demonstrated the excited-state properties necessary to improve optoelectronic devices. Now, a general ‘energy cleft’ molecular design scheme has been demonstrated that enables rapid generation and long lifetimes of multiple triplet excitons that are for device applications.

Advances in organic chemistry have enabled the synthesis of almost any molecule imaginable for high performance organic electronics. In this field of many possibilities, the question becomes not “what can we make” but “how do we guide the design of materials that can be made?” In this review, we address how single molecule conductance measurements can inform the rational design of organic electronics. We also survey recent developments in singlet fission and highlight areas where work can be done to make this process efficient and applicable.

We report a novel self-assembly pathway from a bis(imidazolyl) diphenyl–diacetylene (DPDA) compound as a realization of self-templated photopolymerization with high polymerization degrees. The work takes advantage of a cylindrical self-assembly that strengthens the preorganization of the diphenyl–diacetylene moiety at the single molecular level. On this basis, photopolymerization of DPDA can be conducted smoothly to form high-molecular-weight polydiphenyl diacetylene. Such a cylindrical self-assembly is highly dependent on molecular structure, and control studies show that only oligomers can be formed on random self-assemblies from a monoimidazolyl or nonimidazolyl diphenyl–diacetylene compound. Moreover, the cylindrical self-assembly based systems bear aggregation-induced emission enhancement characteristics and are solution processable. The leading thin-film could afford a selectively tunable function in luminescent micropatterns.

Singlet fission (SF) has the potential to significantly enhance the photocurrent in single-junction solar cells and thus raise the power conversion efficiency from the Shockley-Queisser limit of 33% to 44%. Until now, quantitative SF yield at room temperature has been observed only in crystalline solids or aggregates of oligoacenes. Here, we employ transient absorption spectroscopy, ultrafast photoluminescence spectroscopy, and triplet photosensitization to demonstrate intramolecular singlet fission (iSF) with triplet yields approaching 200% per absorbed photon in a series of bipentacenes. Crucially, in dilute solution of these systems, SF does not depend on intermolecular interactions. Instead, SF is an intrinsic property of the molecules, with both the fission rate and resulting triplet lifetime determined by the degree of electronic coupling between covalently linked pentacene molecules. We found that the triplet pair lifetime can be as short as 0.5 ns but can be extended up to 270 ns.

Quantitative singlet fission has been observed for a variety of acene derivatives such as tetracene and pentacene, and efforts to extend the library of singlet fission compounds is of current interest. Preliminary calculations suggest anthradithiophenes exhibit significant exothermicity between the first optically-allowed singlet state, S1, and 2 × T1 with an energy difference of >5000 cm−1. Given the fulfillment of this ingredient for singlet fission, here we investigate the singlet fission capability of a difluorinated anthradithiophene dimer (2ADT) covalently linked by a (dimethylsilyl)ethane bridge and derivatized by triisobutylsilylethynyl (TIBS) groups. Photophysical characterization of 2ADT and the single functionalized ADT monomer were carried out in toluene and acetone solution via absorption and fluorescence spectroscopy, and their photo-initiated dynamics were investigated with time-resolved fluorescence (TRF) and transient absorption (TA) spectroscopy. In accordance with computational predictions, two conformers of 2ADT were observed via fluorescence spectroscopy and were assigned to structures with the ADT cores trans or cis to one another about the covalent bridge. The two conformers exhibited markedly different excited state deactivation mechanisms, with the minor trans population being representative of the ADT monomer showing primarily radiative decay, while the dominant cis population underwent relaxation into an excimer geometry before internally converting to the ground state. The excimer formation kinetics were found to be solvent dependent, yielding time constants of ∼1.75 ns in toluene, and ∼600 ps in acetone. While the difference in rates elicits a role for the solvent in stabilizing the excimer structure, the rate is still decidedly long compared to most singlet fission rates of analogous dimers, suggesting that the excimer is neither a kinetic nor a thermodynamic trap, yet singlet fission was still not observed. The result highlights the sensitivity of the electronic coupling element between the singlet and correlated triplet pair states, to the dimer conformation in dictating singlet fission efficiency even when the energetic requirements are met.

We explore the synthesis and photophysics of nanostructured block copolymers that mimic light-harvesting complexes. We find that the combination of a polar and electron-rich boron dipyrromethene (BODIPY) block with a nonpolar electron-poor perylene diimide (PDI) block yields a polymer that self-assembles into ordered "nanoworms". Numerical simulations are used to determine optimal compositions to achieve robust self-assembly. Photoluminescence spectroscopy is used to probe the rich exciton dynamics in these systems. Using controls, such as homopolymers and random copolymers, we analyze the mechanisms of the photoluminescence from these polymers. This understanding allows us to probe in detail the photophysics of the block copolymers, including the effects of their self-assembly into nanostructures on their excited-state properties. Similar to natural systems, ordered nanostructures result in properties that are starkly different than the properties of free polymers in solution, such as enhanced rates of electronic energy transfer and elimination of excitonic emission from disordered PDI trap states.

Singlet fission (SF), a promising mechanism of multiple exciton generation, has only recently been engineered as a fast, efficient, intramolecular process (iSF). The challenge now lies in designing and optimizing iSF materials that can be practically applied in high-performance optoelectronic devices. However, most of the reported iSF systems, such as those based on donor–acceptor polymers or pentacene, have low triplet energies, which limits their applications. Tetracene-based materials can overcome significant challenges, as the tetracene triplet state is practically useful, ≈1.2 eV. Here, the synthesis and excited state dynamics of a conjugated tetracene homopolymer are studied. This polymer undergoes ultrafast iSF in solution, generating high-energy triplets on a sub-picosecond time scale. Magnetic-field-dependent photocurrent measurements of polytetracene-based devices demonstrate the first example of iSF-generated triplet extraction in devices, exhibiting the potential of iSF materials for use in next-generation devices.

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

In this issue of Chem, Basel et al. describe solvent-induced changes in photophysics for a non-conjugated pentacene dimer. The authors clearly demonstrate dynamic relaxation of the excited singlet exciton and find that singlet fission rates can be enhanced in more polar media and are particularly rapid in benzonitrile.

We have synthesized a series of asymmetric pentacene-tetracene heterodimers with a variable-length conjugated bridge that undergo fast and efficient intramolecular singlet fission (iSF). These compounds have distinct singlet and triplet energies, which allow us to study the spatial dynamics of excitons during the iSF process, including the significant role of exciton correlations in promoting triplet pair generation and recombination. We demonstrate that the primary photoexcitations in conjugated dimers are delocalized singlets that enable fast and efficient iSF. However, in these asymmetric dimers, the singlet becomes more localized on the lower energy unit as the length of the bridge is increased, slowing down iSF relative to analogous symmetric dimers. We resolve the recombination kinetics of the inequivalent triplets produced via iSF, and find that they primarily decay via concerted processes. By identifying different decay channels, including delayed fluorescence via triplet-triplet annihilation, we can separate transient species corresponding to both correlated triplet pairs and uncorrelated triplets. Recombination of the triplet pair proceeds rapidly despite our experimental and theoretical demonstration that individual triplets are highly localized and unable to be transported across the conjugated linker. In this class of compounds, the rate of formation and yield of uncorrelated triplets increases with bridge length. Overall, these constrained, asymmetric systems provide a unique platform to isolate and study transient species essential for singlet fission, which are otherwise difficult to observe in symmetric dimers or condensed phases.

Using ultrafast transient absorption and emission spectroscopy, we probe the spatial dynamics of excitons during intramolecular singlet fission in asymmetric oligoacene heterodimers. Exciton-exciton correlations are crucial for promoting both triplet pair formation and recombination.

We investigate singlet fission (SF) in heterodimers comprising a pentacene unit covalently bonded to another acene as we systematically vary the singlet and triplet pair energies. We find that these energies control the SF process, where dimers undergo SF provided that the resulting triplet pair energy is similar or lower in energy than the singlet state. In these systems the singlet energy is determined by the lower-energy chromophore, and the rate of SF is found to be relatively independent of the driving force. However, triplet pair recombination in these heterodimers follows the energy gap law. The ability to tune the energies of these materials provides a key strategy to study and design new SF materials-an important process for third-generation photovoltaics.

The third generation of photovoltaic technology aims to reduce the fabrication cost and improve the power conversion efficiency (PCE) of solar cells. Singlet fission (SF), an efficient multiple exciton generation (MEG) process in organic semiconductors, is one promising way to surpass the Shockley-Queisser limit of conventional single-junction solar cells. Traditionally, this MEG process has been observed as an intermolecular process in organic materials. The implementation of intermolecular SF in photovoltaic devices has achieved an external quantum efficiency of over 100% and demonstrated significant promise for boosting the PCE of third generation solar cells. More recently, efficient intramolecular SF has been reported. Intramolecular SF materials are modular and have the potential to overcome certain design constraints that intermolecular SF materials possess, which may allow for more facile integration into devices.