Entanglement of states is one of the most surprising and counter-intuitive consequences of quantum mechanics, with potent applications in cryptography and computing. In organic materials, one particularly significant manifestation is the spin-entangled triplet-pair state, which mediates the spin-conserving fission of one spin-0 singlet exciton into two spin-1 triplet excitons. Despite long theoretical and experimental exploration, the nature of the triplet-pair state and inter-triplet interactions have proved elusive. Here we use a range of organic semiconductors that undergo singlet exciton fission to reveal the photophysical properties of entangled triplet-pair states. We find that the triplet pair is bound with respect to free triplets with an energy that is largely material independent (∼30 meV). During its lifetime, the component triplets behave cooperatively as a singlet and emit light through a Herzberg–Teller-type mechanism, resulting in vibronically structured photoluminescence. In photovoltaic blends, charge transfer can occur from the bound triplet pairs with >100% photon-to-charge conversion efficiency.

From biological complexes to devices based on organic semiconductors, spin interactions play a key role in the function of molecular systems. For instance, triplet-pair reactions impact operation of organic light-emitting diodes as well as photovoltaic devices. Conventional models for triplet pairs assume they interact only weakly. Here, using electron spin resonance, we observe long-lived, strongly-interacting triplet pairs in an organic semiconductor, generated via singlet fission. Using coherent spin-manipulation of these two-triplet states, we identify exchange-coupled (spin-2) quintet complexes co-existing with weakly coupled (spin-1) triplets. We measure strongly coupled pairs with a lifetime approaching 3 µs and a spin coherence time approaching 1 µs, at 10 K. Our results pave the way for the utilization of high-spin systems in organic semiconductors. The dynamics of spin-dependent reactions impact organic systems across scales of complexity. In vivo radicalpair recombination has been implicated in the biological mechanism for avian navigation and in photosynthesis, while in organic semiconducting materials triplet spin-reactions can determine efficiencies in light-emitting diodes and photovoltaics 1-5. One such process, singlet fission, enables efficient production of two triplet excitons from an initially excited singlet state 6-8. This carrier multiplication process has enabled photovoltaic devices with over 100% external quantum efficiencies and holds promise as a means of harnessing the solar spectrum more efficiently 9,10. Fission proceeds from a photogenerated singlet exciton to an overall spin-zero triplet-pair state, conserving spin and enabling efficient triplet-pair formation. This initial pure singlet state can further decohere into the triplet-pair eigenstates, which we study here. Understanding how these triplet-pair states interact, annihilate, and move is critical for harnessing them in optoelectronic or spintronic applications. The fate of triplet pairs depends not only on their electronic degrees of freedom, but also on their spin properties, such as the pair spin coherence time. To date, spin dynamics of triplet pairs have predominantly been explored passively via photoluminescence experiments 11-13 , which do not allow for direct triplet-pair manipulation. Spin resonance techniques allow for active spin control but have previously been limited to continuous-wave (cw) studies of triplet pair-states 14,15 , although transient spin resonance has provided insight into triplet-transfer and triplet-charge interactions 16,17. Here we focus on the early-time behaviour of the non-equilibrium population of tripletpair states formed following singlet fission and before thermalization. We report the observation of exchange-coupled triplet pairs forming pure spin-quintet (total spin S = 2) states. Quintet states have been observed previously, for example in synthetic compounds that utilize directly bonded radical species 18 or in materials with degenerate ground state orbitals 19. Here we observe, i...

Singlet exciton fission (SF), the conversion of one spin-singlet exciton (S) into two spin-triplet excitons (T), could provide a means to overcome the Shockley-Queisser limit in photovoltaics. SF as measured by the decay of S has been shown to occur efficiently and independently of temperature, even when the energy of S is as much as 200 meV less than that of 2T. Here we study films of triisopropylsilyltetracene using transient optical spectroscopy and show that the triplet pair state (TT), which has been proposed to mediate singlet fission, forms on ultrafast timescales (in 300 fs) and that its formation is mediated by the strong coupling of electronic and vibrational degrees of freedom. This is followed by a slower loss of singlet character as the excitation evolves to become only TT. We observe the TT to be thermally dissociated on 10-100 ns timescales to form free triplets. This provides a model for 'temperature-independent' efficient TT formation and thermally activated TT separation.

Spin-bearing molecules are promising building blocks for quantum technologies as they can be chemically tuned, assembled into scalable arrays, and readily incorporated into diverse device architectures. In molecular systems, optically addressing ground-state spins would enable a wide range of applications in quantum information science, as has been demonstrated for solid-state defects. However, this important functionality has remained elusive for molecules. Here, we demonstrate such optical addressability in a series of synthesized organometallic, chromium(IV) molecules. These compounds display a ground-state spin that can be initialized and read out using light, and coherently manipulated with microwaves. In addition, through atomistic modification of the molecular structure, we vary the spin and optical properties of these compounds, indicating promise for designer quantum systems synthesized from the bottom-up.

We demonstrate the successful incorporation of a solution-processable singlet fission material, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), into photovoltaic devices. TIPS-pentacene rapidly converts high-energy singlet excitons into pairs of triplet excitons via singlet fission, potentially doubling the photocurrent from high-energy photons. Low-energy photons are captured by small-bandgap electron-accepting lead chalcogenide nanocrystals. This is the first solution-processable singlet fission system that performs with substantial efficiency with maximum power conversion efficiencies exceeding 4.8%, and external quantum efficiencies of up to 60% in the TIPS-pentacene absorption range. With PbSe nanocrystal of suitable bandgap, its internal quantum efficiency reaches 170 ± 30%.