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The photophysical processes of singlet fission and triplet fusion have numerous emerging applications. They involve the separation of a photo-generated singlet exciton into two dark triplet excitons and the fusion of two dark triplet excitons into an emissive singlet exciton, respectively. The role of the excimer state and the nature of the triplet-pair state in these processes have been a matter of contention. Here we analyse the room temperature time-resolved emission of a neat liquid singlet fission chromophore and show that it exhibits three spectral components: two that correspond to the bright singlet and excimer states and a third component that becomes more prominent during triplet fusion. This spectrum is enhanced by magnetic fields, confirming its origins in the recombination of weakly coupled triplet pairs. It is thus attributed to a strongly coupled triplet pair state. These observations unite the view that there is an emissive intermediate in singlet fission and triplet fusion, distinct from the broad, unstructured excimer emission. (Figure presented.)

Exciton science sits at the intersection of chemical, optical and spin-based implementations of information processing, but using excitons to conduct logical operations remains relatively unexplored. Excitons encoding information could be read optically (photoexcitation-photoemission) or electrically (charge recombination-separation), travel through materials via exciton energy transfer, and interact with one another in stimuli-responsive molecular excitonic devices. Excitonic logic offers the potential to mediate electrical, optical and chemical information. Additionally, high-spin triplet and quintet (multi)excitons offer access to well defined spin states of relevance to magnetic field effects, classical spintronics and spin-based quantum information science. In this Roadmap, we propose a framework for developing excitonic computing based on singlet fission (SF) and triplet-triplet annihilation (TTA). Various molecular components capable of modulating SF/TTA for logical operations are suggested, including molecular photo-switching and multi-colour photoexcitation. We then outline a pathway for constructing excitonic logic devices, considering aspects of circuit assembly, logical operation synchronization, and exciton transport and amplification. Promising future directions and challenges are identified, and the potential for realizing excitonic computing in the near future is discussed.

Photochemical upconversion of green to blue light is demonstrated in thin films of nanostructured alumina stained with a metalloporphyrin sensitizer. The pores of the structure are filled with emitter molecules in a concentrated solution, allowing efficient upconversion within the solid-state scaffold. The photon generation quantum yield is measured to be 9.4%, which is nearly 40% of what is possible with a diphenylanthracene emitter. These results show that high-efficiency upconversion is possible with solid-state sensitization within a nanostructured thin-film architecture.

Two strategies for improving solar energy efficiencies, triplet fusion and singlet fission, rely on the details of triplet-triplet interactions. In triplet fusion, there are several steps, each of which is a possible loss mechanism. In solution, the parameters describing triplet fusion collisions are difficult to inspect. Here we show that these parameters can be determined by examining the magnetic field dependence of triplet fusion upconversion. We show that there is a reduction of the magnetic field effect for perylene triplet fusion as the system moves from the quadratic to linear annihilation regimes with an increase in laser power. Our data are modeled with a small set of parameters that characterize the triplet fusion dynamics. These parameters are cross-validated with molecular dynamics simulations. This approach can be applied to both solution and solid state materials, providing a tool for screening potential annihilators for photon upconversion.

Hybrid inorganic-organic semiconducting devices consisting of monolayer transition metal dichalcogenides (TMDs) represent a new frontier in advanced optoelectronics due to their high radiative efficiencies and capacity to form flexible p-n junctions with inherent device tunability. However, understanding how excitons and charges behave at the interface between TMDs and organic systems, a key requirement to advance the field, remains underexplored. Herein, a heterostructure consisting of a highly conjugated organic system, 9-(2-naphthyl)-10-[4-(1-naphthyl)phenyl]anthracene (ANNP), and monolayer molybdenum disulfide (MoS2) on quartz is elucidated via transient absorption and photoluminescence spectroscopies. Upon direct excitation of MoS2at 532 nm, hole transfer to ANNP of ∼5 ps and a charge separation time constant of ∼2.4 ns are observed. When the sample is excited at 400 nm (where both ANNP and MoS2absorb), a self-trapped exciton within ANNP is formed. The emission of the self-trapped exciton is long-lived compared to the exciton lifetime of ANNP, decaying within 20 ns. The trapping of the ANNP exciton is caused by structural deformities of the ANNP crystal lattice when grown on MoS2, which are removed by annealing the film. These observations highlight how exciton dissociation and charge transfer dominate at the interface of ANNP and MoS2whereas the exciton dynamics within ANNP are prone to the formation of trap states brought about by crystal defects within the film. These insights will aid in future developments of TMD-containing optoelectronics.

Excitation spectra of protonated and deuteronated anthracene are obtained by triple-resonance dissociation spectroscopy. Very cold cations, protonated/deuteronated exclusively at the 9-position, are generated from two-colour two-photon threshold ionisation of 9-dihydroanthracenyl radicals (C14H11). The excitation spectra reveal rich structure, not resolved in previous studies, that is assigned based on anharmonic and Herzberg-Teller coupling calculations. This work reveals that the excitation of protonated anthracene induces a symmetry-breaking intramolecular charge-transfer process along a Marcus-Hush coordinate, where the positively charged hole hops from the central bridging sp2 carbon, onto one of the aromatic rings. Signatures of this charge-transfer event are observed in the excitation spectrum, through active Herzberg-Teller progressions.

Gating logical operations through high-lying electronic excited states presents opportunities for developing ultrafast, subnanometer computational devices. A lack of molecular systems with sufficiently long-lived higher excited states has hindered practical realization of such devices, but recent studies have reported intriguing photophysics from high-lying excited states of perylene. In this work, we use femtosecond spectroscopy supported by quantum chemical calculations to identify and quantify the relaxation dynamics of monomeric perylene's higher electronic excited states. The 21B2u state is accessed through single-photon absorption at 250 nm, while the optically dark 21Ag state is excited via the 11B3u state. Population of either state results in subpicosecond relaxation to the 11B3u state, and we quantify 21Ag and 21B2u state lifetimes of 340 and 530 fs, respectively. These lifetimes are significantly longer than the singlet fission time constant from the perylene 21B2u state, suggesting that the higher electronic states of perylene may be useful for gating logical operations.

Polycyclic aromatic hydrocarbons (PAHs) are intermediates in the formation of soot particles and interstellar grains. However, their formation mechanisms in combustion and interstellar environments are not fully understood. The production of tricyclic PAHs and, in particular, the conversion of a PAH containing a five-membered ring to one with a six-membered ring are of interest to explain PAH abundances in combustion processes. In the present work, resonant ionization mass spectrometry in conjunction with isotopic labeling is used to investigate the formation of the phenalenyl radical from acenaphthylene and methane in an electrical discharge. We show that in this environment the CH cycloaddition mechanism converts a five-membered ring to a six-membered ring. This mechanism can occur in tandem with other PAH formation mechanisms such as hydrogen abstraction/acetylene addition (HACA) to produce larger PAHs in flames and the interstellar medium.

The electronic wave function of molecules is 3N-dimensional and inseparable in the coordinates of the N electrons. Whereas molecular orbitals are often invoked to visualize the electronic structure, they are nonunique, with the same 3N-dimensional wave function being represented by an infinite number of 3-D, one-electron functions (orbitals). Furthermore, multireference wave functions cannot be described by an antisymmetrized product of a single set of occupied orbitals. What is required is a way to visualize the full dimensionality of the wave function, including the effects of correlation, as a 3N-dimensional being would be able to do. In the past 5 years, we have been developing a way to analyze and visualize highly dimensional wave functions by focusing on the structure of the repeating unit demanded by fermionic behavior. This 3N-dimensional repeating unit, the wave function "tile", can be projected onto the three dimensions of each electron, in turn, to reveal the complete electronic structure. It is found that the tile reproduces canonical chemical motifs such as core-electrons, single bonds and lone pairs. Multiple bonds emerge as the "banana" bonds favored by Pauling. As a function of the reaction coordinate, electron motions are visualized that correspond to the curly arrow notation of organic chemists. Excited states can also be inspected. Analyzing a wave function in terms of fermionic tiling allows for insight not facilitated by the inspection of orbitals or configuration interaction vectors: The wave function tiles of resonance structures reveal that electron correlation in benzene pushes opposing spin electrons to occupy alternate Kekulé structures, and in C2, the emerging structure supports the notion of a triply bonded structure with a weak, fourth bonding contribution.

Many applications, including solar energy conversion, can be enhanced by the upconversion of low energy photons. A flexible approach towards achieving this is by sensitized triplet-triplet annihilation, or triplet fusion. Despite high efficiencies being achieved for many compositions in solution, this has not yet been translated to the solid state. In this perspective, we provide an overview of the various approachesto obtain triplet fusion upconversion in solid state and discuss the challenges inherent to its implementation. Finally, we outlay our view on target areas to successfully envision solid state upconversion in the future.

Sheng et al. (2021) report an increase in the power conversion efficiency of a methylammonium lead iodide perovskite solar cell, which they have attributed to the benefits of in situ perovskite-sensitized upconversion by triplet-triplet annihilation in the organic semiconductor rubrene. In the following, we will discuss why in the device structure presented here upconversion inherently cannot be the underlying cause of the observed effects. Rather, the process of upconversion can fundamentally only lead to a reduction of the device efficiency if the same material is used as the active layer in the photovoltaic devices and as the triplet sensitizer. Therefore, the improved photovoltaic performance must have a different root cause. We follow up with an alternative interpretation for the performance increase observed by the authors.

The excited-state dynamics of 6,13-bis(triisopropylsilylethynyl)pentacene is investigated to determine the role of excimer and aggregate formation in singlet fission in high-concentration solutions. Photoluminescence spectra were measured by excitation with the evanescent wave in total internal reflection, in order to avoid reabsorption effects. The spectra over nearly two magnitudes of concentration were nearly identical, with no evidence for excimer emission. Time-correlated single-photon counting measurements confirm that the fluorescence lifetime shortens with concentration. The observed rate constant grows at high concentrations, and this effect is modeled in terms of the hard-sphere radial distribution function. NMR measurements confirm that aggregation takes place with a binding constant of between 0.14 and 0.43 M-1. Transient absorption measurements are consistent with a diffusive encounter mechanism for singlet fission, with hints of more rapid singlet fission in aggregates at the highest concentration measured. These data show that excimers do not play the role of an emissive intermediate in exothermic singlet fission in solution and that, while aggregation occurs at higher concentrations, the mechanism of singlet fission remains dominated by diffusive encounters.

Molecular photoswitches capable of generating precise pH changes will allow pH-dependent processes to be controlled remotely and noninvasively with light. We introduce a series of new merocyanine photoswitches, which deliver reversible bulk pH changes up to 3.2 pH units (pH 6.5 to pH 3.3) upon irradiation with 450 nm light, displaying tunable and predictable timescales for thermal recovery. We present models to show that the key parameters for optimizing the bulk pH changes are measurable: the solubility of the photoswitch, the acidity of the merocyanine form, the thermal equilibrium position between the spiropyran and the merocyanine isomers, and the increased acidity under visible light irradiation. Using ultrafast transient absorption spectroscopy, we determined the quantum yields for the ring-closing reaction and found that the lifetimes of the transient cis-merocyanine isomers ranged from 30 to 550 ns. Quantum yields did not appear to be a limitation for bulk pH switching. The models we present use experimentally determined parameters and are, in principle, able to predict the change in pH obtained for any related merocyanine photoacid.

The economic value of a photovoltaic installation depends upon both its lifespan and power conversion efficiency. Progress toward the latter includes mechanisms to circumvent the Shockley-Queisser limit, such as tandem designs and multiple exciton generation (MEG). Here we explain how both silicon tandem and MEG-enhanced silicon cell architectures result in lower cell operating temperatures, increasing the device lifetime compared to standard c-Si cells. Also demonstrated are further advantages from MEG enhanced silicon cells: (i) the device architecture can completely circumvent the need for current-matching; and (ii) upon degradation, tetracene, a candidate singlet fission (a form of MEG) material, is transparent to the solar spectrum. The combination of (i) and (ii) mean that the primary silicon device will continue to operate with reasonable efficiency even if the singlet fission layer degrades. The lifespan advantages of singlet fission enhanced silicon cells, from a module perspective, are compared favorably alongside the highly regarded perovskite/silicon tandem and conventional c-Si modules.

A chemical system is proposed that is capable of amplifying small optical inputs into large changes in internal composition, based on a feedback interaction between switchable fluorescence and visible-light photoswitching. This system would demonstrate bifurcating reaction kinetics under irradiation and reach one of two stable photostationary states depending on the initial composition of the system. This behavior would allow the system to act as a chemical realization of the flip-flop circuit, the fundamental element in sequential logic and binary memory storage. We use detailed numerical modeling to demonstrate the feasibility of the proposed behavior based on known molecular phenomena and comment on some of the conditions required to realize this system.

The exciton dynamics of violanthrone-79 are investigated in solution and in the solid state. In solution, the photoprepared singlet is found to exhibit a strong ground-state bleach and stimulated emission feature, but when sensitized in its triplet state, it exhibits only a narrow and weak ground-state bleach. As supported by density functional theory calculations, this is explained by the triplet state having absorptions in the same region, with a similar oscillator strength, as the ground-state molecule. In solid films, the excited singlet is found to survive for up to 200 ps, giving way to a long-lived transient absorption spectrum with characteristics reminiscent of the triplet in solution. This is interpreted in terms of singlet fission in the solid film.

Photochemical upconversion is a strategy for converting infrared light into more energetic, visible light, with potential applications ranging from biological imaging and drug delivery to photovoltaics and photocatalysis. Although systems have been developed for upconverting light from photon energies in the near-infrared, upconversion from below the silicon bandgap has been out of reach. Here, we demonstrate an upconversion composition using PbS semiconductor nanocrystal sensitizers that absorb photons below the bandgap of silicon and populate violanthrone triplet states below the singlet oxygen energy. The triplet-state violanthrone chromophores luminesce in the visible spectrum following energy delivery from two singlet oxygen molecules. By incorporating organic chromophores as ligands onto the PbS nanocrystals to improve energy transfer, we demonstrate that violanthrone upconverts in the absence of oxygen by the triplet–triplet annihilation mechanism. The change in mechanism is shown by exploiting the magnetic field effect on triplet–triplet interactions.

The electronic structure of benzene is a battleground for competing viewpoints of electronic structure, with valence bond theory localising electrons within superimposed resonance structures, and molecular orbital theory describing delocalised electrons. But, the interpretation of electronic structure in terms of orbitals ignores that the wavefunction is anti-symmetric upon interchange of like-spins. Furthermore, molecular orbitals do not provide an intuitive description of electron correlation. Here we show that the 126-dimensional electronic wavefunction of benzene can be partitioned into tiles related by permutation of like-spins. Employing correlated wavefunctions, these tiles are projected onto the three dimensions of each electron to reveal the superposition of Kekulé structures. But, opposing spins favour the occupancy of alternate Kekulé structures. This result succinctly describes the principal effect of electron correlation in benzene and underlines that electrons will not be spatially paired when it is energetically advantageous to avoid one another.

A dye immersed in a polymer matrix represents a promising material class for organic polariton lasers. While polariton condensation has been established for boron-dipyrromethene blends, it has yet to be extended to other small dye blends. Here, we demonstrate polariton lasing in a molecularly isolated perylene diimide immersed in polystyrene. The emission exhibits threshold behavior, strong directionality, and slight blueshifting of organic polariton systems. We expect that continuous exploration of new dyes will allow improved performance in organic polariton lasing.

We review the pre-quantum theories of electronic structure of Lewis and Langmuir, and how this relates to the post-quantum double-quartet theory of Linnett. Linnett's ideas are put on a firm theoretical footing through the emergence of the wavefunction tile: The 3N-dimensional repeating structure of the N-electron wavefunction. Wavefunction tiles calculated by the dynamic Voronoi Metropolis sampling method are reviewed, and new results are presented for bent bonds of cyclopropane, and electron correlation in Be-O-Be.

Singlet fission is a process whereby a bichromophoric system crosses from an excitonically coupled singlet state to a singlet-coupled triplet pair state. If the electronic structure is described locally, then the process may be described by a formal exchange of electrons. As such, it lends itself to a treatment rooted in the Marcus-Hush description of electron transfer. Here, we use ab initio and density functional electronic structure theories to reveal a Marcus-Hush perspective on singlet fission and propose experiments to probe singlet fission in the spirit of photo-induced electron transfer.

Photon upconversion is a process that creates high-energy photons under low photon energy excitation. The effect of molecular geometry on the triplet fusion upconversion process has been investigated in this work through the design and synthesis of four new 9,10-diphenylanthracene (DPA) derivatives by employing platinum octaethylporphyrin as the triplet sensitizer. These new emitter molecules containing multiple DPA subunits linked together via a central benzene core exhibit high fluorescence quantum yields. Interestingly, large differences in the triplet fusion upconversion performance were observed between the derivatives with the meta-substituted dimer showing the closest performance to the DPA reference. The differences are discussed in terms of the statistical probability for obtaining a high-energy singlet excited state from triplet fusion, f, for both inter- and intramolecular processes and the effect of magnetic field on the upconversion efficiency. These results demonstrate the challenges to be overcome in improving triplet fusion upconversion efficiency based on multichromophoric emitter systems.

The hydrogen-atom adduct with anthracene, 9-dihydroanthracenyl radical (C14H11), and its deuterated analogue have been identified by laser spectroscopy coupled to time-of-flight mass spectrometry, supported by time-dependent density functional theory calculations. The electronic spectrum of 9-dihydroanthracenyl radical exhibits an origin band at 19115 cm-1 and its ionization energy was determined to be 6.346(1) eV. The spectra reveal a low-frequency vibrational progression corresponding to a mode described by a butterfly inversion. In the deuterated analogue, a zero-point-energy imbalance along this coordinate is found to lead to a doubling of the observed spectral lines in the progression. This is attributed to quantum-induced symmetry breaking as previously observed in isotopologues of CH5+.

The creation of artificial light-harvesting complexes involves the ordered arrangement of chromophores in space. To guarantee efficient energy-transfer processes, organic dyes must be brought into close proximity, often leading to aggregation and the formation of excimer states. In recent years, the attachment of ligand-based chromophores to nanoparticles has also generated interest in relation to improved solar harvesting and spin-dependent electronic interactions such as singlet fission and upconversion. We explore the covalent attachment of two novel perylene-diimide (PDI) carboxylic acid ligands to silicon dioxide nanoparticles. This allows us to study electronic interactions between the ligands when attached to nanoparticles because these cannot couple to the wide band gap silicon dioxide. One of the synthesized PDI ligands has sterically hindering phenols in the bay position and undergoes minimal optical changes upon attachment, but the other forms an excimer state with a red-shifted and long-lived florescence. As such, molecular structure changes offer a method to tune weak and strong interactions between ligand layers on nanocrystal surfaces.

The anthropogenic rise of carbon emissions has led to increased interest in the innovative utilisation of CO2. CO2 reduction via the Sabatier reaction presents a thermodynamically favourable pathway to reduce CO2; however, to date, there has been a lack of understanding of how plasmonic enhancement may be utilised for this reaction particularly with respect to the role of bimetallic catalysts. In this study, the effect of laser light illumination on Ni-Au/SiO2, at 520 nm and 660 nm, was studied for CO2 reduction over a temperature range of 150 °C to 450 °C, with the catalytic performance being assessed by CO2 conversion along with CH4 and CO selectivity. Under laser light illumination, at 520 nm and 450 °C, CO2 conversion for Ni–Au/SiO2 increased by 79% whilst simultaneously lowering CH4 selectivity, which was attributed to localised surface plasmon resonance effects and promotion of the reverse water-gas shift reaction by the Au. Laser light illumination at 660 nm also introduced a mild conversion increase, however not to the same extent as for 520 nm. XPS analysis of Ni-Au/SiO2 implied that the observed photo-enhancement could be due to plasmonic mediated electron charge transfer from the Au deposits to the Ni deposits. Monometallic Ni and Au samples were examined and compared with the bimetallic system to elucidate the origin of the photo enhancement and to understand the plasmonic and catalytic interactions. The Au/SiO2 and Ni/SiO2 samples demonstrated that the photo-enhancement arose from the presence of the Au, with the increase in CO2 conversion at 520 nm increasing with increasing Au content; Au (∼110%) > Ni-Au (∼80%) > Ni (∼21%). However, the overall better performance of the Ni/SiO2 sample, facilitating high CO2 conversion and CH4 selectivity emphasises that, while plasmonic metal inclusion can induce synergistic effects, the benefits can be muted if it is unable to promote the target reaction.

A perylene (donor-dimer)-acceptor-(donor-dimer) pentamer array is synthesized to investigate the competition between excimer formation and Förster resonance energy transfer. Using time-resolved fluorescence, we show that, upon excitation, the isolated perylene dimer forms an excimer with a time constant of 4.3 ns. However, in the pentamer array, when either of two constituent dimers donate their energy to the acceptor fluorophore, the excimer energy trap is eliminated. The pentamer macromolecule shows broad absorption and reduced self-absorption, at some cost to fluorescence quantum yield.

Triisopropylsilylethynyl-substituted acenes (TIPS-acenes) have received prominent attention in the field of singlet fission, the pentacene derivative being an exothermic singlet fission material, and the tetracene being a prototypical endothermic material. Little attention has been given to TIPS-anthracene, which is expected to exhibit exothermic triplet-triplet annihilation, despite literature reports to the contrary. We show that there is some evidence for singlet fission in TIPS-anthracene solutions, and that it does exhibit triplet-triplet annihilation. We apply anti-Stokes action spectroscopy to determine the upconversion efficiency of a composition of TIPS-anthracene and platinum octaethylporphyrin. At a bias equivalent to 0.86 suns, the composition exhibited an annihilation efficiency of 3.2%, which may be compared to diphenylanthracene, which yielded 9.2% under the same conditions. We attribute the low efficiency to a combination of a shorter triplet lifetime and low lying T2 and T3 states. The results are supported by ab initio quantum chemical calculations.

The limiting efficiency of a solar cell enhanced by both up- and downconversion is calculated to be 44.6% under one sun illumination using a detailed balance formalism. We show that solar cells are enhanced by both spectral conversion and photon recycling. This device architecture has the advantage of being able to enhance existing technologies after relatively few modifications. Importantly, we show that this device architecture's limiting efficiency peaks close to the band gap of silicon, unlike other spectral converters with three absorbing thresholds.

Photochemical upconversion uses sensitized triplet-triplet annihilation in bimolecular compositions to convert lower energy photons to higher energy photons. For high efficiency under low illumination, usually a high sensitizer concentration is desirable. However, here we show that the upconversion sensitizer can reduce the emitter triplet lifetime by dynamic quenching, with rate constants on the order 106 M-1 s-1, leading to diminishing returns beyond a certain concentration. These results serve as a warning to designers of photochemical upconvertors that higher concentrations of sensitizers are not necessarily beneficial to upconversion performance.

Multihydroxy-anthraquinone derivatives [i.e., 1,2,4-trihydroxyanthraquinone (124-THAQ), 1,2,7-trihydroxyanthraquinone (127-THAQ), and 1,2,5,8-tetrahydroxyanthraquinone (1258-THAQ)] can interact with various additives [e.g., iodonium salt, tertiary amine, N-vinylcarbazole, and 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine] under household green LED irradiation to generate active species (cations and radicals). The relevant photochemical mechanism is investigated using quantum chemistry, fluorescence, cyclic voltammetry, laser flash photolysis, steady state photolysis, and electron spin resonance spin-trapping techniques. Furthermore, the multihydroxy-anthraquinone derivative-based photoinitiating systems are capable of initiating cationic photopolymerization of epoxides or divinyl ethers under green LED, and the relevant photoinitiation ability is consistent with the photochemical reactivity (i.e., 124-THAQ-based photoinitiating system exhibits highest reactivity and photoinitiation ability). More interestingly, multihydroxy-anthraquinone derivative-based photoinitiating systems can initiate free radical crosslinking or controlled (i.e., reversible addition−fragmentation chain transfer) photopolymerization of methacrylates under green LED. It reveals that multihydroxy-anthraquinone derivatives can be used as versatile photoinitiators for various types of photopolymerization reactions.

Despite being at the heart of chemical thought, the curly arrow notation of reaction mechanisms has been treated with suspicion - the connection with rigorous molecular quantum mechanics being unclear. The connection requires a view of the wavefunction that goes beyond molecular orbitals and rests on the most fundamental property of electrons. The antisymmetry of electronic wavefunctions requires that an N-electron wavefunction repeat itself in 3N dimensions, thus exhibiting tiles. Inspection of wavefunction tiles permits insight into structure and mechanism. Here, we demonstrate that analysis of the wavefunction tile along a reaction coordinate reveals the electron movements depicted by the curly arrow notation for several reactions. The Diels-Alder reaction is revealed to involve the separation and counter propagation of electron spins. This unprecedented method of extracting the movements of electrons during a chemical reaction is a breakthrough in connecting traditional depictions of chemical mechanism with state-of-the-art quantum chemical calculations.

Singlet fission is a process whereby two triplet excitons can be produced from one photon, potentially increasing the efficiency of photovoltaic devices. Endothermic singlet fission is desired for maximum energy conversion efficiency, and such systems have been shown to form an excimer-like state with multi-excitonic character prior to the appearance of triplets. However, the role of the excimer as an intermediate has, until now, been unclear. Here we show, using 5,12-bis((triisopropylsilyl)ethynyl)tetracene in solution as a prototypical example, that, rather than acting as an intermediate, the excimer serves to trap excited states, to the detriment of singlet fission yield. We clearly demonstrate that singlet fission and its conjugate process, triplet-triplet annihilation, occur at a longer intermolecular distance than an excimer intermediate would impute. These results establish that an endothermic singlet fission material must be designed that avoids excimer formation, thus allowing singlet fission to reach its full potential in enhancing photovoltaic energy conversion.

A gellated organic light upconversion layer comprising tetraphenylporphryrin sensitizers and diphenylanthracene emitters is attached to a green-emitting high-brightness tandem-OLED to generate deep-blue light. The upconversion efficiency is enhanced by incorporating a longpass filter between the upconverter and the OLED as well as a shortpass filter on the outside of the upconverter.

Luminescent solar concentrators based on molecular dyes are a promising approach to light collection for photovoltaics owing to their potential low cost and wide light acceptance angles. However, they readily suffer from self-absorption, which rapidly reduces device efficiency. We use a perylene-based sensitizer-emitter system to reduce self-absorption. The sensitizer and emitter are copolymerized to enhance energy transfer to the emitter. The sensitizer is susceptible to yield-reducing H-aggregation. We show that a composite polymer can be used to reduce H-aggregation, while maintaining efficient energy transfer.

Singlet exciton fission is an exciton multiplication process that occurs in certain organic materials, converting the energy of single highly-energetic photons into pairs of triplet excitons. This could be used to boost the conversion efficiency of crystalline silicon solar cells by creating photocurrent from energy that is usually lost to thermalisation. An appealing method of implementing singlet fission with crystalline silicon is to incorporate singlet fission media directly into a crystalline silicon device. To this end, we developed a solar cell that pairs the electron-selective contact of a high-efficiency silicon heterojunction cell with an organic singlet fission material, tetracene, and a PEDOT:PSS hole extraction layer. Tetracene and n-type crystalline silicon meet in a direct organic-inorganic heterojunction. In this concept the tetracene layer selectively absorbs blue-green light, generating triplet pairs that can dissociate or resonantly transfer at the organo-silicon interface, while lower-energy light is transmitted to the silicon absorber. UV photoemission measurements of the organic-inorganic interface showed an energy level alignment conducive to selective hole extraction from silicon by the organic layer. This was borne out by current-voltage measurements of devices subsequently produced. In these devices, the silicon substrate remained well-passivated beneath the tetracene thin film. Light absorption in the tetracene layer created a net reduction in current for the solar cell, but optical modelling of the external quantum efficiency spectrum suggested a small photocurrent contribution from the layer. This is a promising first result for the direct heterojunction approach to singlet fission on crystalline silicon.

We apply a combination of state-of-the-art experimental and quantum-chemical methods to elucidate the electronic and chemical energetics of hydrogen adduction to a model open-shell graphene fragment. The lowest-energy adduct, 1H-phenalene, is determined to have a bond dissociation energy of 258.1 kJ mol−1, while other isomers exhibit reduced or in some cases negative bond dissociation energies, the metastable species being bound by the emergence of a conical intersection along the high-symmetry dissociation coordinate. The gas-phase excitation spectrum of 1H-phenalene and its radical cation are recorded using laser spectroscopy coupled to mass-spectrometry. Several electronically excited states of both species are observed, allowing the determination of the excited-state bond dissociation energy. The ionization energy of 1H-phenalene is determined to be 7.449(17) eV, consistent with high-level W1X-2 calculations.

© 2017 American Chemical Society. Low-energy photons, which are not used by a solar cell, can be converted to higher-energy photons by photon upconversion. The figure-of-merit of upconvertors, J UC , is given in mA cm -2 under 1 sun excitation conditions. A device-relevant J UC should not be less than 0.1 mA cm -2 . However, to date, the highest J UC reported is in the 10 -2 mA cm -2 range. In this Perspective, we analyze the shortcomings of previously reported devices and unfold a roadmap toward device-relevant, high-efficiency upconvertors.