A framework for multiexcitonic logic.

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.

Anisotropic Multiexciton Quintet and Triplet Dynamics in Singlet Fission via Pulsed Electron Spin Resonance.

The quintet triplet-pair state may be generated upon singlet fission and is a critical intermediate that dictates the fate of excitons, which can be exploited for photovoltaics, information technologies, and biomedical imaging. In this report, we demonstrate that continuous-wave and pulsed electron spin resonance techniques such as phase-inverted echo-amplitude detected nutation (PEANUT), which have emerged as the primary tool for identifying the spin pathways in singlet fission, probe fundamentally different triplet-pair species. We directly observe that the generation rate of high-spin triplet pairs is dependent on the molecular orientation with respect to the static magnetic field. Moreover, we demonstrate that this observation can prevent incorrect analysis of continuous-wave electron spin resonance (cw-ESR) measurements and provide insight into the design of materials to target specific pathways that optimize exciton properties for specific applications.

An All-Photonic Molecular Amplifier and Binary Flip-flop.

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.

Comment on "Boosted molecular mobility during common chemical reactions".

The apparent "boosted mobility" observed by Wang et al (Reports, 31 July 2020, p. 537) is the result of a known artifact. When signal intensities are changing during a nuclear magnetic resonance (NMR) diffusion measurement for reasons other than diffusion, the use of monotonically increasing gradient amplitudes produces erroneous diffusion coefficients. We show that no boosted molecular mobility is observed when shuffled gradient amplitudes are applied.

Controlled Diffusion of Photoswitchable Receptors by Binding Anti-electrostatic Hydrogen-Bonded Phosphate Oligomers.

Dihydrogen phosphate anions are found to spontaneously associate into anti-electrostatic oligomers via hydrogen bonding interactions at millimolar concentrations in DMSO. Diffusion NMR measurements supported formation of these oligomers, which can be bound by photoswitchable anion receptors to form large bridged assemblies of approximately three times the volume of the unbound receptor. Photoisomerization of the oligomer-bound receptor causes a decrease in diffusion coefficient of up to 16%, corresponding to a 70% increase in effective volume. This new approach to external control of diffusion opens prospects in controlling molecular transport using light.

Enhanced Diffusion of Molecular Catalysts is Due to Convection.

Intriguing reports of enhanced diffusion in enzymes and molecular catalysts have spurred significant interest in experimental and theoretical investigations, and the mechanism of this phenomenon is the topic of lively debate. Here we use time-resolved diffusion NMR methods to measure the diffusion coefficients (D) of small molecule species involved in chemical reactions with high temporal resolution. We show the enhanced diffusion of small molecules cannot be explained by reaction velocity, and that apparent measurements of enhanced diffusion by small molecules appear to be caused by bulk fluid flow processes such as convection.

Time-Resolved Diffusion NMR Measurements for Transient Processes.

A general procedure for measurement of time-resolved diffusion coefficients of molecular species by NMR is described, including the use of methanol for fast temperature-independent gradient calibration.

Photochromic switching behaviour of donor-acceptor Stenhouse adducts in organic solvents.

We report photochromic donor-acceptor Stenhouse adducts (DASAs) capable of fully reversible photoisomerization with visible light in organic solvents including chloroform, acetonitrile and benzene. The rates of photoisomerization and thermal reversion can be tuned by altering the electronics of the donor adduct. X-Ray crystallography and photo-NMR experiments unambiguously establish molecular structures.