Excitonics: A Set of Gates for Molecular Exciton Processing and Signaling Nicolas P. D. Sawaya,†,‡ Dmitrij Rappoport,† Daniel P. Tabor,† and Alán Aspuru-Guzik*,†,§ †
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States Intel Laboratories, Santa Clara, California 95054, United States § Senior Fellow, Canadian Institute for Advanced Research, Bioinspired Solar Energy Program, Toronto, ON M5G 1Z8, Canada Downloaded via MOUNT ROYAL UNIV on August 9, 2018 at 11:40:42 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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S Supporting Information *
ABSTRACT: Regulating energy transfer pathways through materials is a central goal of nanotechnology, as a greater degree of control is crucial for developing sensing, spectroscopy, microscopy, and computing applications. Such control necessitates a toolbox of actuation methods that can direct energy transfer based on user input. Here we introduce a proposal for a molecular exciton gate, analogous to a traditional transistor, for regulating exciton flow in chromophoric systems. The gate may be activated with an input of light or an input flow of excitons. Our proposal relies on excitation migration via the second excited singlet (S2) state of the gate molecule. It exhibits the following features, only a subset of which are present in previous exciton switching schemes: picosecond time scale actuation, amplification/gain behavior, and a lack of molecular rearrangement. We demonstrate that the device can be used to produce universal binary logic or amplification of an exciton current, providing an excitonic platform with several potential uses, including signal processing for microscopy and spectroscopy methods that implement tunable exciton flux. KEYWORDS: excitonics, excited states, organic dyes, microscopy, transistor, circuit, binary logic
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input provided by light or by the input of auxiliary excitons. The gate itself is a molecule considered to be of f in its ground singlet state (S0) and on in its first excited singlet state (S1). As described below, when the gate is turned on, excitons migrate through the second excited singlet state (S2), while no excitons (or “excitonic current”) flow when the gate is off. A successful experimental implementation of this S2 exciton gate involves overcoming competing energy transfer and decay processes. Several previously proposed schemes use nanoscale excitonic energy transfer to perform logical operations or to regulate exciton flow. The state of the art can be placed into four broad categories. First, the earliest examples are logical devices based on DNA nanotechnology, in which single-stranded DNA (ssDNA) labeled with chromophores is added to a solution as a logical input.12−16 After the ssDNA binds to complementary base pairs on an existing DNA-based structure, the resulting changes are usually probed optically. Because the movement of ssDNA is diffusion-limited, a single logical operation within this scheme takes seconds to minutes. The second category for exciton gating uses incident light to induce molecular
evising ways to reliably control energy transfer on the nanoscale allows for the further development of nanoscopic devices for spectroscopy, microscopy, photocatalysis, and sensing.1−8 Here we propose a design for an elementary excitonic gate (or excitonic transistor) that uses multiply excited states to control the flow of excitons in organic molecules, resulting in a scheme that provides advantages over previous methods of controlling exciton movement. The concept of a gate is essential in many areas of engineering and science: it allows one to regulate the movement of energy, mass, or charge, leading to useful applications. Ubiquitous gate-like devices include field effect transistors for computing, relays for larger electronics, and valves for modulating fluid flow in civil engineering. By controlling the direction and magnitude of exciton migration (equivalently, of excitonic energy migration), the device proposed in this work aims for such gate-enabled applications on the nanoscale, offering improvements relative to other excitonic gating designs. Besides potential microscopy applications,3−8 it has also been suggested that exciton-based logic might eventually become a low-power alternative to energy-intensive modern computation, perhaps even allowing us to continue following Moore’s law longer than expected.9−11 In this theoretical study, we introduce an exciton gate for controlling the rate of single-exciton transfer, with the actuation © 2018 American Chemical Society
Received: January 23, 2018 Accepted: June 19, 2018 Published: June 19, 2018 6410
DOI: 10.1021/acsnano.8b00584 ACS Nano 2018, 12, 6410−6420
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Cite This: ACS Nano 2018, 12, 6410−6420
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processes in solution.4,15 In addition to usage in biocompatible environments, the S2 exciton gate could provide an alternate route for integrating single-exciton logic into a low-energy highgate-density solid platform, as has been proposed for related excitonic devices.9−11 Regarding advanced tools for spectroscopy, we anticipate that tunable gain behavior may be useful for delivering arbitrary rates of excitonic flux to specific chromophores. For example, to aid in the spectroscopic study of photosynthetic complexes,31,32 one could use S2 exciton gates to direct different amounts of flux to different chromophores. One might be able to probe specific energy transfer pathways through a photosynthetic complex, using multiple S2 exciton gates to simultaneously vary the relative quantity of excitonic flux flowing to different molecules. This would be especially useful if two chromophores are energetically similar, since simply varying light intensity would not allow you to tune relative energy flux rates to the two chromophores. A similar strategy might eventually be useful in photocatalysis, if different photoactivated species require different amounts of energy flux. Additionally, tunable rates of local heating could be achievable, which may be useful in probing some temperature-sensitive processes. In this paper, we begin by describing the S2 exciton gate’s functionality and its design constraints, before summarizing preliminary virtual screening results indicating that it is straightforward to meet these constraints. We then discuss details regarding gate actuation and gain/amplification behavior and show how all binary operations can be performed. Next, we discuss gate fidelity and error rates with the aid of simulation. Finally, we outline fabrication strategies and discuss experimental viability.
rearrangement, especially via light-induced isomerization reactions.9,17−19 When one configuration’s properties allow Förster resonance energy transfer (FRET) to occur while the other does not, this system can be used as an on/off switch for excitonic flow. In the third design, specific chromophores are excited into an optically inaccessible (“dark”) state, which changes the path that excitons take through a chromophoric logic circuit.10,20 These circuits take seconds to minutes to reset because of the long-lived dark states. Finally, a fourth theoretical proposal, explicitly motivated by the need for low-power computation, uses one-dimensional semiconductor channels and quantum dots to perform logical operations,11 intended for integration into traditional silicon chips. The Supporting Information (SI) Section S1 includes comprehensive analyses of these methods’ strengths and limitations in comparison with our proposal. Also notable are the developments in other DNA-based components that might be used to build nanoscopic excitonic circuits, including memory storage21 and excitonic wires.22−25 Our S2 exciton gate proposal exhibits the following functionalities, while only a subset of these features are present in each of the excitonic switching devices summarized above: 1. Gate actuation time scales are
