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.

Singlet fission is an excitation multiplication process in molecular systems that can circumvent energy losses and significantly boost solar cell efficiencies; however, the nature of a critical intermediate that enables singlet fission and details of its evolution into multiple product excitations remain obscure. We resolve the initial sequence of events comprising the fission of a singlet exciton in solids of pentacene derivatives using femtosecond transient absorption spectroscopy. We propose a three-step model of singlet fission that includes two triplet-pair intermediates and show how transient spectroscopy can distinguish initially interacting triplet pairs from those that are spatially separated and noninteracting. We find that the interconversion of these two triplet-pair intermediates is limited by the rate of triplet transfer. These results clearly highlight the classical kinetic model of singlet fission and expose subtle details that promise to aid in resolving problems associated with triplet extraction.

Organic semiconductors with polar sidechains have been identified as a promising class of materials for the field of bioelectronics. These materials, also called organic mixed ionic/electronic conductors (OMIECs), can exchange ions with aqueous electrolytes when electronic charge carriers are injected, transported, and stored in the bulk of the material. [1] Recent developments of OMIECs based on redox-active conjugated polymers [2][3][4][5][6][7][8] and novel device concepts [9,10] have opened up new pathways for bioelectronic devices including integrated circuits for electroencephalogram (EEG) monitoring [9] or low-power voltage amplifiers based on organic electrochemical transistors (OECTs). [11] Specifically, the OECT has drawn significant attention in the field of organic bioelectronics. It operates by electrochemically modulating the conductivity of a redox-active channel material with an electrolyte that is often aqueous, Avoiding faradaic side reactions during the operation of electrochemical devices is important to enhance the device stability, to achieve low power consumption, and to prevent the formation of reactive side-products. This is particularly important for bioelectronic devices, which are designed to operate in biological systems. While redox-active materials based on conducting and semiconducting polymers represent an exciting class of materials for bioelectronic devices, they are susceptible to electrochemical side-reactions with molecular oxygen during device operation. Here, electrochemical side reactions with molecular oxygen are shown to occur during organic electrochemical transistor (OECT) operation using high-performance, state-of-the-art OECT materials. Depending on the choice of the active material, such reactions yield hydrogen peroxide (H 2 O 2 ), a reactive side-product, which may be harmful to the local biological environment and may also accelerate device degradation. A design strategy is reported for the development of redox-active organic semiconductors based on donor-acceptor copolymers that prevents the formation of H 2 O 2 during device operation. This study elucidates the previously overlooked side-reactions between redox-active conjugated polymers and molecular oxygen in electrochemical devices for bioelectronics, which is critical for the operation of electrolyte-gated devices in application-relevant environments.

Novel p-type semiconducting polymers that can facilitate ion penetration, and operate in accumulation mode are much desired in bioelectronics. Glycol side chains have proven to be an efficient method to increase bulk electrochemical doping and optimize aqueous swelling. One early polymer which exemplifies these design approaches was p­(g2T-TT), employing a bithiophene-co-thienothiophene backbone with glycol side chains in the 3,3′ positions of the bithiophene repeat unit. In this paper, the analogous regioisomeric polymer, namely pgBTTT, was synthesized by relocating the glycol side chains position on the bithiophene unit of p­(g2T-TT) from the 3,3′ to the 4,4′ positions and compared with the original p­(g2T-TT). By changing the regio-positioning of the side chains, the planarizing effects of the S–O interactions were redistributed along the backbone, and the influence on the polymer’s microstructure organization was investigated using grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. The newly designed pgBTTT exhibited lower backbone disorder, closer π-stacking, and higher scattering intensity in both the in-plane and out-of-plane GIWAXS measurements. The effect of the improved planarity of pgBTTT manifested as higher hole mobility (μ) of 3.44 ± 0.13 cm2 V–1 s–1. Scanning tunneling microscopy (STM) was in agreement with the GIWAXS measurements and demonstrated, for the first time, that glycol side chains can also facilitate intermolecular interdigitation analogous to that of pBTTT. Electrochemical quartz crystal microbalance with dissipation of energy (eQCM-D) measurements revealed that pgBTTT maintains a more rigid structure than p­(g2T-TT) during doping, minimizing molecular packing disruption and maintaining higher hole mobility in operation mode.

Amphiphilic donor-acceptor meso-ethynyl porphyrins with polar pyridinium electron-acceptor head groups and hydrophobic dialkyl-aniline electron donors have high molecular hyperpolarizabilities (as measured by hyper-Rayleigh scattering) and high affinities for biological membranes. When bound to water droplets in dodecane, or to the plasma membranes of living cells, they can be used for second harmonic generation (SHG) microscopy; an incident light of wavelength 840 nm generates a strong frequency-doubled signal at 420 nm. Copper(II) and nickel(II) porphyrin complexes give similar SHG signals to those of the free-base porphyrins, while exhibiting no detectable two-photon excited fluorescence.

Organic semiconductors can be employed as the active layer in accumulation mode organic electrochemical transistors (OECTs), where redox stability in aqueous electrolytes is important for long‐term recordings of biological events. It is observed that alkoxy‐benzo[1,2‐b:4,5‐b′]dithiophene (BDT) copolymers can be extremely unstable when they are oxidized in aqueous solutions. The redox stability of these copolymers can be improved by molecular design of the copolymer where it is observed that the electron rich comonomer 3,3′‐dimethoxy‐2,2′‐bithiophene (MeOT2) lowers the oxidation potential and also stabilizes positive charges through delocalization and resonance effects. For copolymers where the comonomers do not have the same ability to stabilize positive charges, irreversible redox reactions are observed with the formation of quinone structures, being detrimental to performance of the materials in OECTs. Charge distribution along the copolymer from density functional theory calculations is seen to be an important factor in the stability of the charged copolymer. As a result of the stabilizing effect of the comonomer, a highly stable OECT performance is observed with transconductances in the mS range. The analysis of the decomposition pathway also raises questions about the general stability of the alkoxy‐BDT unit, which is heavily used in donor–acceptor copolymers in the field of photovoltaics.

Donor-acceptor (D-A) polymers are promising materials for organic electrochemical transistors (OECTs), as they minimized etrimental faradaic side-reactions during OECT operation, yet their steady-state OECT performance still lags far behind their all-donor counterparts.W er eport three D-A polymers based on the diketopyrrolopyrrole unit that affordO ECT performances similar to those of all-donor polymers,hence representing asignificant improvement to the previously developed D-A copolymers.I na ddition to improved OECT performance,DFT simulations of the polymers and their respective hole polarons also reveal ap ositive correlation between hole polaron delocalization and steadystate OECT performance,p roviding new insights into the design of OECT materials.I mportantly,w ed emonstrate how polaron delocalization can be tuned directly at the molecular level by selection of the building blocks comprising the polymers conjugated backbone,t hus paving the way for the development of even higher performing OECT polymers.