Spin Selectivity in Photoinduced Charge Transfer Mediated by Chiral

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Spin Selectivity in Photoinduced Charge Transfer Mediated by Chiral Molecules John M. Abendroth, Dominik M. Stemer, Brian P. Bloom, Partha Roy, Ron Naaman, David H. Waldeck, Paul S. Weiss, and Prakash Chandra Mondal ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b01876 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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ACS Nano

Spin Selectivity in Photoinduced Charge Transfer Mediated by Chiral Molecules John M. Abendroth,1,2 Dominik M. Stemer,1,3 Brian P. Bloom,4 Partha Roy,5 Ron Naaman,6* David H. Waldeck,4* Paul S. Weiss,1,2,3* and Prakash Chandra Mondal7*

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California NanoSystems Institute, University of California, Los Angeles, Los Angeles, California 90095, United States 2 Department of Chemistry & Biochemistry, University of California, Los Angeles, Los Angeles, California 90095, United States 3 Department of Materials Science & Engineering, University of California, Los Angeles, Los Angeles, California 90095, United States 4 Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States 5 Department of Chemistry, Central University of Rajasthan, Kishangarh 305817, Ajmer, India 6 Department of Chemical and Biological Physics, Weizmann Institute of Science, Rehovot 76100, Israel 7 Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India

*Corresponding authors: [email protected] (RN), [email protected] (DHW) [email protected] (PSW), [email protected] (PCM)

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ABSTRACT Optical control and readout of electron spin and spin currents in thin films and nanostructures have remained attractive yet challenging goals for emerging technologies designed for applications in information processing and storage. Recent advances in room-temperature spin polarization using nanometric chiral molecular assemblies suggest that chemically modified surfaces or interfaces can be used for optical spin conversion by exploiting photoinduced charge separation and injection from well-coupled organic chromophores or quantum dots. Using light to drive photoexcited charge-transfer processes mediated by molecules with central or helical chirality enables indirect measurements of spin polarization attributed to the chiral-induced spin selectivity effect, and of the efficiency of spin-dependent electron transfer relative to competitive radiative relaxation pathways. Herein, we highlight recent approaches used to detect and to analyze spin selectivity in photoinduced charge transfer including spin-transfer torque for local magnetization, nanoscale charge separation and polarization, and soft ferromagnetic substrate magnetization- and chiralitydependent photoluminescence. Building on these methods through systematic investigation of molecular and environmental parameters that influence spin filtering should elucidate means to manipulate electron spins and photoexcited states for room-temperature optoelectronic and photospintronic applications.

KEYWORDS chiral-induced spin selectivity (CISS), spin filter, photoluminescence, fluorescence quenching, optoelectronics, photospintronics, molecular spintronics


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GLOSSARY Chiral-induced spin selectivity effect: enantioselective and spin-dependent interactions between chiral molecules and electrons in charge transmission and transfer Electron helicity: the projection of the spin angular momentum along its linear momentum direction; left-(right-) handed helicity corresponds to antiparallel (parallel) alignment Photospintronics: controlling magnetic states with light, or inversely, recording information dictated by electron spin states by photoluminescence Electron spin polarization: the difference in number of spin-up and spin-down electrons near the Fermi level (EF) divided by the total number of electrons Chiral imprinting: influence of chiral molecules on achiral matter, affording otherwise nonexistent enantioselectivity with chiral environments and circularly polarized light


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Manipulating electron spin orientation and dynamics with light – or conversely, transducing information encoded within electron spins into photons – are attractive routes toward realizing smaller, faster, and more energy-efficient technologies for memory storage,1-5 quantum computation for information processing,6,7 and electronic-, magnetic-, or temperature-based sensors.8-10 On one hand, light can be used to control spin within materials, exemplified by ultrafast laser-induced demagnetization and reorientation in ferri- and ferromagnets,11-15 and generation of surface-spin photocurrents in topological insulators.16,17 On the other hand, monitoring photoluminescence from materials in which the energy or polarization of emitted photons contain electron spin signatures, such as from nitrogen vacancy centers in diamond18-21 or via spin-valley coupling in two-dimensional semiconducting materials,22-27 provides an indirect readout to monitor and to convert spin information into light signals. While diverse, these systems share similar challenges (and opportunities) associated with maintaining spin coherence at surfaces and interfaces, and in controlling magnetic states via doping and proximity effects following photoexcitation.28-34 An emerging possibility to address these challenges is to develop hybrid materials in which surfaces are functionalized with organic molecules.35-41 Chemical approaches using self-assembled monolayers or molecular films to manipulate the chemical,42 electronic,43-45 and magnetic46,47 properties of thin films and nanostructures have been successfully applied to tune material surfaces and interfaces. Moreover, fascinating studies over the past two decades have demonstrated that chiral molecular assemblies can act as electron spin filters,48-55 a phenomenon known as the chiralinduced spin selectivity (CISS) effect (Figure 1).56


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Figure 1. Schematic of the chiral-induced spin selectivity effect in transport of electrons with opposite spin (red and blue) through helical potentials (turquoise).


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While the mechanisms of spin filtering in chiral molecules are not yet fully understood, several theoretical models attribute the observed phenomena to unconventional Rashba-like (momentum-dependent) spin-orbit coupling,57-63 and conclude that chirality (inversion dissymmetry) is essential to observe spin polarization. Briefly, spin filtering is hypothesized to arise from the coupling between the magnetic moment of an electron and effective magnetic fields generated by electron propagation through a stereogenic center or along a molecular helix. These effective magnetic fields lift the spin degeneracy (a result of the Zeeman effect), thereby aligning the electron spin parallel (right-handed helicity) or antiparallel (left-handed helicity) to the direction of the electron’s linear momentum. When the chirality of a stereogenic center, or the handedness of a molecular helix, is reversed, the sign of the spin-orbit interaction changes, and the preferred longitudinal spin orientation of an electron transmitted through the chiral system should be reversed. The room-temperature spin polarization of electrons transmitted through self-assembled films of chiral molecules demonstrates the exciting opportunities that chiral materials may provide for next-generation organic spintronic devices capable of operation above cryogenic conditions.64 While a variety of creative measurement strategies have been employed to test spin selectivity in electron transport through chiral molecules, oligomers, and polymers (and are the subject of some current reviews),65-67 this Review addresses recent studies that use visible light to excite organic chromophores or quantum dots (QDs) functionalized with chiral tethers and to drive photoinduced charge-transfer reactions. These experiments monitor competitive non-radiative and radiative relaxation processes as a function of external magnetic field, light polarization, and/or molecular and helical handedness to probe the spin selectivity in electron transfer and to establish a foundation for future experiments that optimize spin polarization.


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First, we highlight favorable strategies for controlling photoinduced static and dynamic magnetization and spin pumping, and mediating spin-dependent charge transfer with light by chemical functionalization of material surfaces and interfaces. Next, we review experiments designed to monitor photoluminescence as an indirect probe of spin selectivity in charge transfer, and how the chirality of ground or excited electronic states of nanoparticles may be manipulated by chiral adsorbates. Throughout, we emphasize analytical methods that promise to identify and to quantify the spin polarization arising from the CISS effect, and highlight recent and occasionally counterintuitive findings provided by these complementary measurement techniques. We conclude by summarizing the potential for utilizing the CISS effect in photovoltaics, directing photochemical reactions, and for quantum computing. Progress made in these directions, and opportunities based on advances in related disciplines will move the field beyond the observation of the CISS effect, and to move it toward tunable and useful applications.


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SPIN POLARIZATION IN PHOTOINDUCED CHARGE TRANSFER THROUGH CHIRAL MOLECULES Many experimental approaches have utilized the robust photophysical behavior of QDs functionalized with chiral molecular anchors to monitor spin-polarized charge transfer between QDs and underlying magnetized substrates.68 Upon electrical excitation, tethered QDs may engage in charge transfer with a nearby substrate.69 In the case when a chiral tether is used to anchor the QD to the surface, charge transfer is accompanied by net spin polarization. Building upon a comprehensive body of research outlining the effect of QD size,70-72 composition,72,73 and tether69,72,74-77 these studies have unveiled chirality-induced phenomena within this familiar charge-transfer paradigm. Spin-polarized charge transfer through stereogenic centers in chiral molecules or oligomers with helical chirality may be readily studied either by ensemble techniques, via electrochemical photocurrent measurements,72,78 or by more localized scanning probe methods.79 The robustness of anchored QD films makes them attractive for the study of the CISS effect in the electron tunneling regime (~5 nm). Furthermore, anchored QD films may be produced using diverse molecular attachment strategies, enabling study of spin-selective electron tunneling through a wide range of chiral systems. For spintronic applications, films of chiral molecules and oligomers boast a range of attractive features, foremost among those are their suitability for miniaturization and (sub)nanoscale patterning. Analogous to conventional organic semiconductor-based devices, organic spintronic devices may be easily prepared via traditional solution processing methods including spin coating, drop casting, dip coating, or vapor deposition, and may be patterned with nanoscale resolution using soft lithographies.80,81 These patterning techniques are well suited for productionscale film formation, and are equally adaptable to the continued miniaturization of components


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expected in electrical and spintronic devices in contrast to the permanent magnetic layers required by traditional inorganic spin valves. Further, chiral organic molecular assemblies are energetically efficient, passive spin filters. Next, we describe how the experimental methods and results from earlier work investigating photoinduced charge-transfer reactions mediated by chiral molecular films, assemblies of helical oligopeptides, and proteins may be used to tailor future studies designed to elucidate intriguing mechanistic aspects of the CISS effect, and to develop practical applications.

Nanoscale Charge Separation with Chiral-Imprinted Quantum Dots Selective to Light Polarization. In photovoltaic devices for applications including photoelectrochemical water splitting and artificial photosynthesis,82,83 solar fuel generation,84,85 and photodetection,86 charge separation of electrons and holes upon excitation with light is necessary to generate usable photocurrents. Generation of photoexcited states and the subsequent separation into long-lived positive and negative charge carriers is typically achieved in semiconducting materials, i.e., doped silicon, by formation of p-n junctions capable of separating electron-hole pairs efficiently due to the built-in potential of the depletion region upon application of an applied bias voltage and irradiation with light of energy greater than the optical band gap (Eg). However, because of maximum doping concentrations in inorganic semiconductors (ca. 10−2 dopants per cubic nanometer), over which the materials are considered degenerate, miniaturization of photovoltaic device components remains a significant challenge.87 Peer et al. demonstrated a way to overcome the doping-controlled limitations associated with miniaturization of photovoltaic devices to enable efficient charge separation at the

Spin Selectivity in Photoinduced Charge Transfer Mediated by Chiral

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