Phototoxicity of cyclometallated Ir(III) complexes bearing a thio-bis-benzimidazole ligand, and its monodentate analogue, as potential PDT photosensitisers in cancer cell killing.

Two novel cyclometallated iridium(III) complexes have been prepared with one bidentate or two monodentate imidazole-based ligands, 1 and 2, respectively. The complexes showed intense emission with long lifetimes of the excited state. Femtosecond transient absorption experiments established the nature of the lowest excited state as 3IL state. Singlet oxygen generation with good yields (40% for 1 and 82% for 2) was established by detecting 1O2 directly, through its emission at 1270 nm. Photostability studies were also performed to assess the viability of the complexes as photosensitizers (PS) for photodynamic therapy (PDT). Complex 1 was selected as a good candidate to investigate light-activated killing of cells, whilst complex 2 was found to be toxic in the dark and unstable under light. Complex 1 demonstrated high phototoxicity indexes (PI) in the visible region, PI > 250 after irradiation at 405 nm and PI > 150 at 455 nm, in EJ bladder cancer cells.

A Near-Infrared Luminescent Cr(III) N-Heterocyclic Carbene Complex.

Photoluminescent coordination complexes of Cr(III) are of interest as near-infrared spin-flip emitters. Here, we explore the preparation, electrochemistry, and photophysical properties of the first two examples of homoleptic N-heterocyclic carbene complexes of Cr(III), featuring 2,6-bis(imidazolyl)pyridine (ImPyIm) and 2-imidazolylpyridine (ImPy) ligands. The complex [Cr(ImPy)3]3+ displays luminescence at 803 nm on the microsecond time scale (13.7 µs) from a spin-flip doublet excited state, which transient absorption spectroscopy reveals to be populated within several picoseconds following photoexcitation. Conversely, [Cr(ImPyIm)2]3+ is nonemissive and has a ca. 500 ps excited-state lifetime.

Direct Determination of the Rate of Intersystem Crossing in a Near-IR Luminescent Cr(III) Triazolyl Complex.

A detailed understanding of the dynamics of photoinduced processes occurring in the electronic excited state is essential in informing the rational design of photoactive transition-metal complexes. Here, the rate of intersystem crossing in a Cr(III)-centered spin-flip emitter is directly determined through the use of ultrafast broadband fluorescence upconversion spectroscopy (FLUPS). In this contribution, we combine 1,2,3-triazole-based ligands with a Cr(III) center and report the solution-stable complex [Cr(btmp)2]3+ (btmp = 2,6-bis(4-phenyl-1,2,3-triazol-1-yl-methyl)pyridine) (13+), which displays near-infrared (NIR) luminescence at 760 nm (τ = 13.7 µs, ϕ = 0.1%) in fluid solution. The excited-state properties of 13+ are probed in detail through a combination of ultrafast transient absorption (TA) and femtosecond-to-picosecond FLUPS. Although TA spectroscopy allows us to observe the evolution of phosphorescent excited states within the doublet manifold, more significantly and for the first time for a complex of Cr(III), we utilize FLUPS to capture the short-lived fluorescence from initially populated quartet excited states immediately prior to the intersystem crossing process. The decay of fluorescence from the low-lying 4MC state therefore allows us to assign a value of (823 fs)-1 to the rate of intersystem crossing. Importantly, the sensitivity of FLUPS to only luminescent states allows us to disentangle the rate of intersystem crossing from other closely associated excited-state events, something which has not been possible in the spectroscopic studies previously reported for luminescent Cr(III) systems.

Active control of mid-wavelength infrared non-linearity in silicon photonic crystal slab.

Natural materials' inherently weak nonlinear response demands the design of artificial substitutes to avoid optically large samples and complex phase-matching techniques. Silicon photonic crystals are promising artificial materials for this quest. Their nonlinear properties can be modulated optically, paving the way for applications ranging from ultrafast information processing to quantum technologies. A two-dimensional 15-µm-thick silicon photonic structure, comprising a hexagonal array of air holes traversing the slab's thickness, has been designed to support a guided resonance for the light with a wavelength of 4-µm. At the resonance conditions, a transverse mode of the light is strongly confined between the holes in the "veins" of the silicon component. Owing to the confinement, the structure exhibits a ratio of nonlinear to linear absorption coefficients threefold higher than the uniform silicon slab of the same thickness. A customised time-resolved Z-scan method with provisions to accommodate ultrafast pump-probe measurements was used to investigate and quantify the non-linear response. We show that optically pumping free charge carriers into the structure decouples the incoming light from the resonance and reduces the non-linear response. The time-resolved measurements suggest that the decoupling is a relatively long-lived effect on the scale comparable to the non-radiative recombination in the bulk material. Moreover, we demonstrate that the excited free carriers are not the source of the nonlinearity, as this property is determined by the structure design.

Ultrafast electronic, infrared, and X-ray absorption spectroscopy study of Cu(I) phosphine diimine complexes.

The study aims to understand the role of the transient bonding in the interplay between the structural and electronic changes in heteroleptic Cu(I) diimine diphosphine complexes. This is an emerging class of photosensitisers which absorb in the red region of the spectrum, whilst retaining a sufficiently long excited state lifetime. Here, the dynamics of these complexes are explored by transient absorption (TA) and time-resolved infrared (TRIR) spectroscopy, which reveal ultrafast intersystem crossing and structural distortion occurring. Two potential mechanisms affecting excited state decay in these complexes involve a transient formation of a solvent adduct, made possible by the opening up of the Cu coordination centre in the excited state due to structural distortion, and by a transient coordination of the O-atom of the phosphine ligand to the copper center. X-ray absorption studies of the ground electronic state have been conducted as a prerequisite for the upcoming X-ray spectroscopy studies which will directly determine structural dynamics. The potential for these complexes to be used in bimolecular applications is confirmed by a significant yield of singlet oxygen production.

Conjugation-length dependence of regioregular oligo 3-alkyl(thienylene-vinylene)s demonstrates polyene-like behaviour with weak electron-electron correlations.

Poly(3-alkyl(thienylene-vinylene)) (P3TV) and its longer oligomers have negligibly low photoluminescence quantum yields, however, the reason for their low yields is currently debated. Here, we prepare a series of regioregular (3-dodecyl)thienylene-vinylene oligomers with n = 2-8 repeat units by iterative Horner-Wadsworth-Emmons reactions, and report their steady-state, transient absorption, and emission spectroscopy. The results presented here demonstrate that 3-alkyl(thienylene-vinylene) oligomers form part of the polyene family. The shortest (n = 2) oligomer emits from the bright 1Bu state, while fluorescence in oligomers with n = 3, 4 is from the formally dark 2Ag state, allowed via Herzberg-Teller vibronic coupling to the nearby bright 1Bu state as described for diphenyl-polyenes. Longer oligomers and the polymer are essentially non-emissive as the 2Ag state can no longer intensity-borrow from the 1Bu state. We demonstrate that the spectral shapes, photoluminescence quantum yield, and transient spectral behaviour can all be explained using a polyene model with weak electronic correlations.

Photophysical Exploration of Two Isomers of Octaethyltrioxopyrrocorphin.

The introduction of three ß-oxosubstituents to octaethylporphyrin by means of an oxidation/rearrangement reaction generates the trioxopyrrocorphin chromophore. Pyrrocorphins (hexahydroporphyrins) are generally nonaromatic, but we recently demonstrated trioxopyrrocorphins to possess considerable aromatic character. This contribution explores the photophysical characteristics of these unusual chromophores. In agreement with density functional theory modeling, the UV-vis and magnetic circular dichroism spectra of the two─out of the four possible─triketone regioisomers investigated conform to the Gouterman model of porphyrinoid optical spectra, in alignment with their aromaticity. Their excited-state dynamics shed further light on the degree to which ß-oxo substitutions tune the photophysical properties of porphyrinoids. Introduction of ß-oxo functionalities increases the rate and yield of intersystem crossing and shortens the triplet state lifetime. Unexpectedly, the singlet oxygen generation yield of both pyrrocorphins remains relatively high, with modes of distortion from planarity likely enhancing triplet energy transfer. This work thus expands our understanding of a rare class of porphyrinoids and further characterizes them as sustaining aromatic porphyrinic π-systems. Our findings suggest triple ß-oxo substitution as a viable route toward the development of novel, high-singlet oxygen yield porphyrinic photosensitizers.

A Single-Crystal Monomer to Single-Crystal Polymer Reaction Activated by a Triplet Excimer in a Zipper Mechanism.

A combined experimental and theoretical study focused on the elucidation of the polymerization mechanism of the crystal monomer to crystal polymer reaction of a bisindenedione compound in the solid state. The experimental description and characterization of the polymer product have been reported elsewhere and, in this article, we address the first detailed description of the polymerization process. This reaction pathway consists of the initial formation of a triplet excimer state that relaxes to an intermolecularly bonded triplet state that is the starting point of the propagation step of the polymerization. The overall process can be visualized in the monomer starting state as an open zipper in which a cursor or slider is formed by light absorption and the whole zipper is then closed by propagation of the cursor. To this end, variable-temperature electron spin resonance (ESR), femtosecond transient absorption spectroscopy, and vibrational Raman spectroscopic data have been implemented in combination with quantum chemical calculations. The presented mechanistic insight is of great value to understand the intricacies of such an important reaction and to envisage and diversify the products produced thereof.

Photocatalytic Reduction of CO2 to CO in Aqueous Solution under Red-Light Irradiation by a Zn-Porphyrin-Sensitized Mn(I) Catalyst.

This work demonstrates photocatalytic CO2 reduction by a noble-metal-free photosensitizer-catalyst system in aqueous solution under red-light irradiation. A water-soluble Mn(I) tricarbonyl diimine complex, [MnBr(4,4'-{Et2O3PCH2}2-2,2'-bipyridyl)(CO)3] (1), has been fully characterized, including single-crystal X-ray crystallography, and shown to reduce CO2 to CO following photosensitization by tetra(N-methyl-4-pyridyl)porphyrin Zn(II) tetrachloride [Zn(TMPyP)]Cl4 (2) under 625 nm irradiation. This is the first example of 2 employed as a photosensitizer for CO2 reduction. The incorporation of -P(O)(OEt)2 groups, decoupled from the core of the catalyst by a -CH2- spacer, afforded water solubility without compromising the electronic properties of the catalyst. The photostability of the active Mn(I) catalyst over prolonged periods of irradiation with red light was confirmed by 1H and 13C{1H} NMR spectroscopy. This first report on Mn(I) species as a homogeneous photocatalyst, working in water and under red light, illustrates further future prospects of intrinsically photounstable Mn(I) complexes as solar-driven catalysts in an aqueous environment.

Tailoring the Intersystem Crossing and Triplet Dynamics of Free-Base Octaalkyl-ß-oxo-Substituted Porphyrins: Competing Effects of Spin-Vibronic and NH Tautomerism Relaxation Channels.

We demonstrate that ß-oxo-substitution provides effective fine-tuning of both steady-state and transient electronic properties of octaalkyl-ß-mono-oxochlorin and all isomers of the ß,ß'-dioxo-substituted chromophores. The addition of a carbonyl group increases the Qy oscillator strength and red-shifts the absorption spectra. Each oxo-substitution results in a 2-fold increase in the singlet to triplet state intersystem crossing (ISC) rates, resulting in a 4-fold ISC rate increase for the dioxo-substituted chromophores. The effects of oxo-substitution on the ISC rate are thus additive. The progressive increase in the ISC rates correlates directly with the spin-vibronic channels provided by the C═O out-of-plane distortion modes, as evidenced by density functional theory (DFT) modeling. The triplet states, however, were not evenly affected by ß-oxo-substitution, and reduction in the triplet lifetime seems to be influenced instead by the presence of NH tautomers in the dioxoisobacteriochlorins.

Making the Right Link to Theranostics: The Photophysical and Biological Properties of Dinuclear RuII-ReI dppz Complexes Depend on Their Tether.

The synthesis of new dinuclear complexes containing linked RuII(dppz) and ReI(dppz) moieties is reported. The photophysical and biological properties of the new complex, which incorporates a N,N'-bis(4-pyridylmethyl)-1,6-hexanediamine tether ligand, are compared to a previously reported RuII/ReI complex linked by a simple dipyridyl alkane ligand. Although both complexes bind to DNA with similar affinities, steady-state and time-resolved photophysical studies reveal that the nature of the linker affects the excited state dynamics of the complexes and their DNA photocleavage properties. Quantum-based DFT calculations on these systems offer insights into these effects. While both complexes are live cells permeant, their intracellular localizations are significantly affected by the nature of the linker. Notably, one of the complexes displayed concentration-dependent localization and possesses photophysical properties that are compatible with SIM and STED nanoscopy. This allowed the dynamics of its intracellular localization to be tracked at super resolutions.

Solvent-Mediated Activation/Deactivation of Photoinduced Electron-Transfer in a Molecular Dyad.

Herein is presented a molecular dyad comprised of a [Ru(bpy)3]2+ photosensitizer and an anthraquinone (AQ) acceptor coupled by an ethynyl linker ([Ru(bpy)2(bpy-cc-AQ)]2+) in which activation/deactivation of photoinduced electron-transfer from the [Ru(bpy)3]2+ photosensitizer to the AQ acceptor is achieved and characterized as a function of the dielectric constant and hydrogen-bond donating ability of the solvent used. It is demonstrated that the rate of photoinduced electron-transfer can be modulated over several orders of magnitude (105-1011 s-1) by choice of solvent. Nanosecond transient absorption spectra are dominated by MLCT signals and exhibit identical decay kinetics to the corresponding emission signals. Ultrafast transient absorption and time-resolved infrared spectroscopies provide direct evidence for the formation of the charge-separated (CS) state and rapid (on the order of a few picoseconds) establishment of an excited-state pseudoequilibrium.

Photophysics of Cage/Guest Assemblies: Photoinduced Electron Transfer between a Coordination Cage Containing Osmium(II) Luminophores, and Electron-Deficient Bound Guests in the Central Cavity.

A heterometallic octanuclear coordination cage [Os4Zn4(Lnap)12]X16 (denoted Os•Zn; X = perchlorate or chloride) has been prepared (Lnap is a bis-bidentate bridging ligand containing two pyrazolyl-pyridine chelating units separated by a 1,5-naphthalenediyl spacer group). The {Os(NN)3}2+ units located at four of the eight vertices of the cube have a long-lived, phosphorescent 3MLCT excited state which is a stronger electron donor than [Ru(bipy)3]2+. The chloride form of Os•Zn is water-soluble and binds in its central cavity the hydrophobic electron-accepting organic guests 1,2,4,5-tetracyanobenzene, 1,4-naphthoquinone and 1-nitronaphthalene, with binding constants in the range 103-104 M-1, resulting in quenching of the phosphorescence arising from the Os(II) units. A crystal structure of an isostructural Co8 cage containing one molecule of 1,2,4,5-tetracyanobenzene as a guest inside the cavity has been determined. Ultrafast transient absorption measurements show formation of a charge-separated Os(III)/guest•- state arising from cage-to-guest photoinduced electron transfer; this state is formed within 13-21 ps, and decays on a time scale of ca. 200 ps. In the presence of a competing guest with a large binding constant (cycloundecanone) which displaces each electron-accepting quencher from the cage cavity, the charge-separated state is no longer observed. Further, a combination of mononuclear {Os(NN)3}2+ model complexes with the same electron-accepting species showed no evidence for formation of charge-separated Os(III)/guest•- states. These two control experiments indicate that the {Os(NN)3}2+ chromophores need to be assembled into the cage structure to bind the electron-accepting guests, and for PET to occur. These results help to pave the way for use of photoactive coordination cages as hosts for photoredox catalysis reactions on bound guests.

Anion-Mediated Photophysical Behavior in a C60 Fullerene [3]Rotaxane Shuttle.

By addressing the challenge of controlling molecular motion, mechanically interlocked molecular machines are primed for a variety of applications in the field of nanotechnology. Specifically, the designed manipulation of communication pathways between electron donor and acceptor moieties that are strategically integrated into dynamic photoactive rotaxanes and catenanes may lead to efficient artificial photosynthetic devices. In this pursuit, a novel [3]rotaxane molecular shuttle consisting of a four-station bis-naphthalene diimide (NDI) and central C60 fullerene bis-triazolium axle component and two mechanically bonded ferrocenyl-functionalized isophthalamide anion binding site-containing macrocycles is constructed using an anion template synthetic methodology. Dynamic coconformational anion recognition-mediated shuttling, which alters the relative positions of the electron donor and acceptor motifs of the [3]rotaxane's macrocycle and axle components, is demonstrated initially by 1H NMR spectroscopy. Detailed steady-state and time-resolved UV-vis-IR absorption and emission spectroscopies as well as electrochemical studies are employed to further probe the anion-dependent positional macrocycle-axle station state of the molecular shuttle, revealing a striking on/off switchable emission response induced by anion binding. Specifically, the [3]rotaxane chloride coconformation, where the ferrocenyl-functionalized macrocycles reside at the center of the axle component, precludes electron transfer to NDI, resulting in the switching-on of emission from the NDI fluorophore and concomitant formation of a C60 fullerene-based charge-separated state. By stark contrast, in the absence of chloride as the hexafluorophosphate salt, the ferrocenyl-functionalized macrocycles shuttle to the peripheral NDI axle stations, quenching the NDI emission via formation of a NDI-containing charge-separated state. Such anion-mediated control of the photophysical behavior of a rotaxane through molecular motion is unprecedented.

Directly Coupled Versus Spectator Linkers on Diimine PtII Acetylides-Change the Structure, Keep the Function?

Modification of light-harvesting units with anchoring groups for surface attachment often compromises light-harnessing properties. Herein, a series of [donor-acceptor-anchor] platinum(II) diimine (bis-)acetylides was developed in order to systematically compare the effect of conjugated versus electronically decoupled modes of attachment of protected anchoring groups on the photophysical properties of light-harvesting units. The first examples of "decoupled" phosphonate diimine PtII complexes are reported, and their properties are compared and contrasted to those of carboxylate analogues studied by a diversity of methods. Ultrafast time-resolved IR and transient absorption spectroscopy revealed that all complexes have a charge-transfer (CT) lowest excited state with lifetimes between 2 and 14 ns. Vibrational signatures and dynamics of CT states were identified; the assignment of electronic states and their vibrational origin was aided by TDDFT calculations. Ultrafast energy redistribution accompanied by structural changes was directly captured in the CT states. A significant difference between the structures of the electronic ground and CT excited states, as well as differences in the structural reorganisation in the complexes bearing directly attached or electronically decoupled anchoring groups, was discovered. This work demonstrates that decoupling of the anchoring group from the light-harvesting core by a saturated spacer is an easy approach to combine surface attachment with high reduction potential and ten times longer lifetime of the CT excited state of the light-absorbing unit, and retain electron-transfer photoreactivity essential for light-harvesting applications.

Twisted Carotenoids Do Not Support Efficient Intramolecular Singlet Fission in the Orange Carotenoid Protein.

Singlet exciton fission is the spin-allowed generation of two triplet electronic excited states from a singlet state. Intramolecular singlet fission has been suggested to occur on individual carotenoid molecules within protein complexes provided that the conjugated backbone is twisted out of plane. However, this hypothesis has been forwarded only in protein complexes containing multiple carotenoids and bacteriochlorophylls in close contact. To test the hypothesis on twisted carotenoids in a "minimal" one-carotenoid system, we study the orange carotenoid protein (OCP). OCP exists in two forms: in its orange form (OCPo), the single bound carotenoid is twisted, whereas in its red form (OCPr), the carotenoid is planar. To enable room-temperature spectroscopy on canthaxanthin-binding OCPo and OCPr without laser-induced photoconversion, we trap them in a trehalose glass. Using transient absorption spectroscopy, we show that there is no evidence of long-lived triplet generation through intramolecular singlet fission despite the canthaxanthin twist in OCPo.

A stronger acceptor decreases the rates of charge transfer: ultrafast dynamics and on/off switching of charge separation in organometallic donor-bridge-acceptor systems.

To unravel the role of driving force and structural changes in directing the photoinduced pathways in donor-bridge-acceptor (DBA) systems, we compared the ultrafast dynamics in novel DBAs which share a phenothiazine (PTZ) electron donor and a Pt(ii) trans-acetylide bridge (-C[triple bond, length as m-dash]C-Pt-C[triple bond, length as m-dash]C-), but bear different acceptors conjugated into the bridge (naphthalene-diimide, NDI; or naphthalene-monoimide, NAP). The excited state dynamics were elucidated by transient absorption, time-resolved infrared (TRIR, directly following electron density changes on the bridge/acceptor), and broadband fluorescence-upconversion (FLUP, directly following sub-picosecond intersystem crossing) spectroscopies, supported by TDDFT calculations. Direct conjugation of a strong acceptor into the bridge leads to switching of the lowest excited state from the intraligand 3IL state to the desired charge-separated 3CSS state. We observe two surprising effects of an increased strength of the acceptor in NDI vs. NAP: a ca. 70-fold slow-down of the 3CSS formation-(971 ps)-1vs. (14 ps)-1, and a longer lifetime of the 3CSS (5.9 vs. 1 ns); these are attributed to differences in the driving force ΔGet, and to distance dependence. The 100-fold increase in the rate of intersystem crossing-to sub-500 fs-by the stronger acceptor highlights the role of delocalisation across the heavy-atom containing bridge in this process. The close proximity of several excited states allows one to control the yield of 3CSS from ∼100% to 0% by solvent polarity. The new DBAs offer a versatile platform for investigating the role of bridge vibrations as a tool to control excited state dynamics.

Insight into the Origin of Trapping in Polymer/Fullerene Blends with a Systematic Alteration of the Fullerene to Higher Adducts.

The bimolecular recombination characteristics of conjugated polymer poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,5-bis 3-tetradecylthiophen-2-yl thiazolo 5,4-d thiazole)-2,5diyl] (PDTSiTTz) blended with the fullerene series PC60BM, ICMA, ICBA, and ICTA have been investigated using microsecond and femtosecond transient absorption spectroscopy, in conjunction with electroluminescence measurements and ambient photoemission spectroscopy. The non-Langevin polymer PDTSiTTz allows an inspection of intrinsic bimolecular recombination rates uninhibited by diffusion, while the low oscillator strengths of fullerenes allow polymer features to dominate, and we compare our results to those of the well-known polymer Si-PCPDTBT. Using µs-TAS, we have shown that the trap-limited decay dynamics of the PDTSiTTz polaron becomes progressively slower across the fullerene series, while those of Si-PCPDTBT are invariant. Electroluminescence measurements showed an unusual double peak in pristine PDTSiTTz, attributed to a low energy intragap charge transfer state, likely interchain in nature. Furthermore, while the pristine PDTSiTTz showed a broad, low-intensity density of states, the ICBA and ICTA blends presented a virtually identical DOS to Si-PCPDTBT and its blends. This has been attributed to a shift from a delocalized, interchain highest occupied molecular orbital (HOMO) in the pristine material to a dithienosilole-centered HOMO in the blends, likely a result of the bulky fullerenes increasing interchain separation. This HOMO localization had a side effect of progressively shifting the polymer HOMO to shallower energies, which was correlated with the observed decrease in bimolecular recombination rate and increased "trap" depth. However, since the density of tail states remained the same, this suggests that the traditional viewpoint of "trapping" being dominated by tail states may not encompass the full picture and that the breadth of the DOS may also have a strong influence on bimolecular recombination.

Spin Statistics for Triplet-Triplet Annihilation Upconversion: Exchange Coupling, Intermolecular Orientation, and Reverse Intersystem Crossing.

Triplet-triplet annihilation upconversion (TTA-UC) has great potential to significantly improve the light harvesting capabilities of photovoltaic cells and is also sought after for biomedical applications. Many factors combine to influence the overall efficiency of TTA-UC, the most fundamental of which is the spin statistical factor, η, that gives the probability that a bright singlet state is formed from a pair of annihilating triplet states. The value of η is also critical in determining the contribution of TTA to the overall efficiency of organic light-emitting diodes. Using solid rubrene as a model system, we reiterate why experimentally measured magnetic field effects prove that annihilating triplets first form weakly exchange-coupled triplet-pair states. This is contrary to conventional discussions of TTA-UC that implicitly assume strong exchange coupling, and we show that it has profound implications for the spin statistical factor η. For example, variations in intermolecular orientation tune η from to through spin mixing of the triplet-pair wave functions. Because the fate of spin-1 triplet-pair states is particularly crucial in determining η, we investigate it in rubrene using pump-push-probe spectroscopy and find additional evidence for the recently reported high-level reverse intersystem crossing channel. We incorporate all of these factors into an updated model framework with which to understand the spin statistics of TTA-UC and use it to rationalize the differences in reported values of η among different common annihilator systems. We suggest that harnessing high-level reverse intersystem crossing channels in new annihilator molecules may be a highly promising strategy to exceed any spin statistical limit.

Emissive spin-0 triplet-pairs are a direct product of triplet-triplet annihilation in pentacene single crystals and anthradithiophene films.

Singlet fission and triplet-triplet annihilation represent two highly promising ways of increasing the efficiency of photovoltaic devices. Both processes are believed to be mediated by a biexcitonic triplet-pair state, 1(TT). Recently however, there has been debate over the role of 1(TT) in triplet-triplet annihilation. Here we use intensity-dependent, low-temperature photoluminescence measurements, combined with kinetic modelling, to show that distinct 1(TT) emission arises directly from triplet-triplet annihilation in high-quality pentacene single crystals and anthradithiophene (diF-TES-ADT) thin films. This work demonstrates that a real, emissive triplet-pair state acts as an intermediate in both singlet fission and triplet-triplet annihilation and that this is true for both endo- and exothermic singlet fission materials.