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Opto-Spintronics: Photoisomerization-Induced Spin State Switching at 300 K in Photochrome-Cobalt Dioxolene Thin Films Michelle M Paquette, Daniel Plaul, Aiko Kurimoto, Brian O Patrick, and Natia L Frank J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09190 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018
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Journal of the American Chemical Society
Opto-Spintronics: Photoisomerization-Induced Spin State Switching at 300 K in Photochrome Cobalt-Dioxolene Thin Films Michelle M. Paquette,1†‡ Daniel Plaul1§‡, Aiko Kurimoto,1 Brian O. Patrick,2 and Natia L. Frank1* 1Department 2Department
of Chemistry, University of Victoria, PO Box 1700 STN CSC, Victoria, British Columbia, Canada, V8W 2Y2. of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada, V6T 1Z1.
ABSTRACT: Controllable quantum systems are under active investigation for quantum computing, secure information processing, and nonvolatile memory. The optical manipulation of spin quantum states provides an important strategy for quantum control with both temporal and spatial resolution. Challenges in increasing the lifetime of photoinduced magnetic states at T > 200 K have hindered progress towards utilizing photomagnetic materials in quantum device architectures. Here we demonstrate reversible light-induced magnetization switching in an organic thin film at device operating temperatures (300–330 K). By utilizing photochromic ligands that undergo structural changes in the solid state, the changes in ligand field associated with photoisomerization modulate the ligand field, and in turn the oxidation and spin state of a bound metal center. Green light irradiation (exc = 550 nm) of a spirooxazine cobaltdioxolene complex induces photoisomerization of the ligand that in turn triggers a reversible intramolecular charge-transfer coupled spin-transition process at the cobalt center. The generation of photomagnetic states through conversion between a low-spin Co(III)semiquinone doublet and high-spin Co(II)-bis-semiquinone sextet state has been demonstrated in both solution and the solid state, and is described as a Photoisomerization-Induced Spin–Charge Excited State (PISCES) process. The high transition temperature (325 K) and long-lived photoinduced state ( = 10 s at 300 K) are dictated by the photochromic ligand. Theory provides effective modelling of the phenomenon and long-term strategies to further modulate the lifetimes of photomagnetic states for quantum information technologies at the single molecule level.
INTRODUCTION The optical manipulation of spin quantum states, optospintronics, provides an important strategy for quantum control with both temporal and spatial resolution.1-5 Controllable quantum systems are under active investigation for quantum computing, secure information processing, and nonvolatile memory.6-12 Since the first observation in 1967 of light-induced changes in magnetization in yttrium-doped iron garnets,13 there has been intense interest in understanding and controlling the photophysics of photomagnetic effects. Ultrafast photomagnetic effects in manganites,14 carbon homologs,4 doped spinels,15 garnets,16 and colloidal quantum dots17 have been observed at cryogenic temperatures. Photoinduced magnetic effects have also been extensively investigated in molecular and polymeric Prussian blue analogs18-29 and spincrossover complexes30-39 at low temperatures. The lifetime of photomagnetic states observed in these systems however, is often too short to measure at higher temperatures (T >100 K). This is because the metal-centered excited states generated by photoinduced charge transfer processes (ps-ns) typically relax rapidly to the ground state even at cryogenic temperatures. In addition, the high thermal absorption coefficients of metalorganics lead to significant contributions from photothermal and photoacoustic effects to the magnetization. Longer-lived photoinduced magnetic states based on ligand-driven spincrossover (LD-LISC) processes have been observed at higher temperatures in solution and dilute media.40-46 In these systems, a subtle ligand-field change induced by cis-trans isomerization of stilbene derivatives leads to changes in the spin state at bound transition metal centers. The large structural changes associated
with cis-trans isomerization, however, typically require the solution-state for conversion which leads to difficulties in incorporating LD-LISC systems into solid-state architectures. The challenge for the last 50 years therefore has been to develop functional materials that exhibit long-lived photomagnetic states in the solid state at T > 200 K. We have developed a strategy in which we couple optically bistable photochromic ligands to electronically bistable metal complexes. The photoresponsive element is a negative photochromic spirooxazine ligand that exhibits photoisomerization in the solid state.47-48 Photochromic spirooxazines are efficient optical switches that exhibit lightinduced structural changes with rapid switching times (ps), high fatigue resistance (>1000 cycles), and solid-state photochromism,49 giving rise to applications in optical coatings, switches, sensors, and optical logic gates.50-56 In most spirooxazines, the ground state is the closed spirooxazine (SO) form and the metastable state is the open photomerocyanine (PMC) form (Figure 1). UV irradiation of the ring-closed SO form initiates C–O bond cleavage and isomerization to give the ring-opened PMC form. Isomerization is thermally and photochemically reversible, and PMC SO conversion can occur either with visible light excitation or thermally.48 An unusual class of spirooxazines that exhibit negative photochromism has been developed in our laboratory, the azahomoadamantyl-spirooxazines, in which the ground state is the PMC form (Figure 1).48 These negative photochromes exhibit visible light-induced ring closure and solid-state photochromism.47 Integration of phenanthroline into the spirooxazine backbone generates photochromic ligands
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that, when bound to paramagnetic metal centers, exhibit high photoresponsivities.57 In these complexes, photoisomerization induces ligand field changes that modify the redox potentials and the ground-state and excited-state energies of the metal center. Computation and experiment demonstrate that the closed (SO) form of the spirooxazine ligands is a more powerful -acceptor than the open (PMC) form by 0.3 eV.58
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hs-Co(II) states occurs through an intramolecular ligand-tometal electron transfer that is coupled to a spin transition process at the cobalt center. The process has been termed valence tautomerism or, alternatively, redox isomerism. The thermal ls-Co(III) / hs-Co(II) transition temperature (T1/2) is linearly correlated with the reduction potential of the ancillary diimine (NN) ligand.77 A lower diimine reduction potential leads to greater stabilization of the hs-Co(II) form and a higher T1/2. Ligand control of the ls-Co(III) / hs-Co(II) population of states suggests a strategy of modulating the driving force for charge transfer and spin states by ligand structure. Earlier attempts at incorporating photochromic ligands into electronically bistable metal complexes has led to complexes that exhibit either low transition temperatures (T1/2 ~ 50 K)78 or photoswitching only in the solution-state.41 Scheme 1. Photoisomerization-Induced Spin–Charge Excited State (PISCES) process
Figure 1. Light-induced ring opening and closing processes for azahomoadamantyl-spirooxazines (top) and the parent class of spirooxazines (bottom).
Once in possession of photochromic phenanthroline– spirooxazine ligands that exhibit changes in ligand field upon photoisomerization, we incorporated these ligands into metal complexes for development of useful photoswitchable systems. In a given coordination complex, certain optical properties (e.g., energies of d–d or MLCT transitions), electrochemical properties (e.g., redox potentials), or magnetic properties (e.g., magnitude of spin–orbit coupling) should scale gradually as a function of ligand field strength or symmetry. The modulation of such properties through ligand photoisomerization could indeed provide a path to photoswitchable coordination complexes. However, an optimal way to take advantage of the modest ligand field changes observed upon isomerization of the phenanthroline–spirooxazine ligands is to coordinate them to electronically bistable metal complexes. Classic examples of electronically bistable coordination complexes are those which exhibit spin-crossover,30, 34, 59-60metal-to-metal charge transfer processes, 26, 30, 33, 61-67 or redox isomerism.68-73 In such systems, small changes in the environment of the metal (either within the coordination sphere, or through external perturbations such as variations in temperature or pressure) can potentially lead to abrupt changes in metal-based optical, electrochemical, or magnetic properties that are correlated with changes in oxidation and/or spin state. Cobalt-dioxolenes are an important class of electronically bistable metal complexes that possess two distinct oxidation/spin states. The application of external stimuli (temperature, pressure, magnetic field, soft X-rays, and light), can modulate the population of the low spin and high spin states, leading to a change in magnetization and electrical properties.68, 70-76 At low temperature, the cobalt dioxolene system exists in a low-spin Co(III)(catecholate)(semiquinone– •) doublet state (1a/1b, Scheme 1). With increasing temperature, entropic contributions favor population of the high-spin Co(II)(semiquinone–•)2 sextet state (1c/1d), resulting in an increase in magnetization. The transition between ls-Co(III) and
We report here the generation of photomagnetic states with unusually long lifetimes (s) in the thin film state at T > 300 K. Incorporation of a negative azahomoadamantyl-spirooxazine photochromic ligand that undergoes photoisomerization in the solid state48 into an electronically bistable system (cobaltdioxolenes) with a high magnetic transition temperature (T1/2 ~ 325 K) leads to complex 1 (Scheme 1). The photoinduced structural change initiates a reversible intramolecular charge transfer from a low-spin doublet state to a high spin state that is reversible. As the driving force (Go) for either metal-to-ligand or metal-to-metal charge transfer processes is dependent on redox potential, a change in ligand field modulates the driving force for charge-transfer processes in the metal complex. As the charge-transfer process is coupled to a spin transition, the result is optical modulation of the spin state without direct excitation to metal-centered excited states. The lifetime of the spin state is then dictated by the lifetime of the metastable state of the photochromic ligand. The optical gating of magnetization is described as a Photoisomerization-Induced Spin–Charge Excited State (PISCES) process in which the lifetime of the magnetization state is dictated entirely by the photochrome dynamics in a ligand-controlled process (Scheme 1). The advantages of the PISCES strategy over metal-centered based processes are (i) the generation of long-lived photomagnetic states that can be generated at room-temperature in the solid state, (ii) generation of a ligand-controlled photomagnetic state that can be modified by changes in ligand structure through
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synthesis, (iii) the optical modulation of spin state that originates at the single molecule level, and (iv) a system that can be incorporated into solid-state device architectures through simple solution-based thin-film processing techniques.
RESULTS AND DISCUSSION Preparation of cobalt bis(dioxolene) spiro[azahomoadamantylphenanthrolinoxazine], Co(diox)2(APSO) was accomplished through the complexation of spiro[azahomoadamantyl-phenanthrolinoxazine] (APSO)48 with a mononuclear cobalt di-tert-butyl benzoquinone (DTBQ) complex [Co(3,5-DTBQ)2(pyridine)2] to give purple needles in 79% yield. The complex is air- and moisture-stable in the solid state with a high decomposition point of 518 K (220 °C) (Figure S1). tBu
N
N
N
O
N
tBu
O
tBu h
O
h or
O
N
O tBu
tBu N
N
O
N
N
O
N
tBu
tBu
tBu
tBu
O
h or
N
N
O
h
O
N
O
1d) PMC-hs-Co(II) state
O
1b) SO-ls-Co(III) state
O
tBu
O O
tBu
1a) PMC-ls-Co(III) state
N
N
tBu
O
open PMC form in a trans–trans–cis geometry about the azomethine backbone, as in the parent ligand (Figure 3, S9).48 The ligands about the cobalt center form an octahedral complex, with only slight deviations from ideal geometry. Redox isomeric Co complexes exhibit large differences in electronic structure between the ls-Co(III) / hs-Co(II) and Cat2–/SQ–• forms, and the oxidation states of the Co center and DTBQ ligands can be determined from an analysis of the bond lengths. The Co–O and Co–N bond lengths are expected to be 0.10–0.20 Å longer in the hs-Co(II) species than in the ls-Co(III) species due to population of the antibonding orbitals in the hs-Co(II) state. For complex 1, the Co(1)–N(1) and Co(1)–N(2) bond lengths are 1.955(8) and 1.945(9) Å, respectively (Table 2), within the range expected for a ls-Co(III) species (1.92–1.95 Å).77, 79 The dioxolene-based Co–O bonds lengths range from 1.895(6) for Co(1)–O(2) to 1.869(6) for Co(1)–O(4) and are consistent with the assignment of Co(III) oxidation state (1.86– 1.92 Å). In addition, the average Co–O and Co–N bond distances of 1.879 Å and 1.950 Å, respectively, lead to a bond valence sum analysis80 of 3.2 (theoretical value 3.0), consistent with the ls-Co(III) state.77
tBu
N
tBu
O O O O tBu
tBu
1c) SO-hs-Co(II) state
Figure 2. Four possible electronic states of complex 1 are formed by a combination of open (PMC) and closed (SO) states of the photochrome, and the low-spin d6 Co(III) and high-spin d7 Co(II) states of the cobalt-dioxolene.
Under a given set of conditions (solvent, temperature) complex 1 can adopt four possible forms (1a–1d, Figure 2), due to both ligand isomerization between the open (PMC, 1a/1d) and closed (SO, 1b/1c) forms of the APSO ligand, and electronic bistability between the ls-Co(III) (1a/1b) and hsCo(II) (1c/1d) states of the cobalt center. In solution at 300 K, 1H NMR spectroscopy (toluene-d8) reveals a mixture of states with a 4:1 ratio of the PMC:SO forms (Figures S2 and S3). The PMC/SO thermal equilibrium constant, KT = [PMC]/[SO], corresponds to a free energy difference between PMC (1a/1d) and SO (1b/1c) forms of Go = 3.5 kcal/mol, with the PMC form lower in energy. Broadened and downfield-shifted phenanthroline-based resonances are associated with complexation to the paramagnetic hs-Co(II) metal center, which exhibit narrowing and upfield shifts with decreasing temperature, consistent with population of the ls-Co(III) form at low temperatures. Distinct resonances for the ls-Co(III) and hs-Co(II) forms are not observed due to rapid interconversion between the hs-Co(II) and ls-Co(III) states at room temperature relative to the NMR timescale (k ≈ 10–8 s–1 in the parent phenanthroline complex).74 To summarize, at room temperature in solution, by 1H NMR spectroscopy, the complex exists as a mixture of four states dominated by the open PMC form, with a mixture of ls-Co(III) and hs-Co(II) states. Crystallographic analysis at 90 K of a single crystal obtained from diethyl ether reveals the PMC-ls-Co(III) form (Figures 3 and S9). Complex 1 crystallizes in the triclinic space group P-1 with two molecules per unit cell (Figure S10). The azahomoadamantyl-phenanthrolinoxazine ligand exists in the
Figure 3. Complex 1 viewed along the b-axis depicting the Co-O bonds and the open PMC form of the spirooxazine. Color code: N, blue; O, red; and Co, yellow. Hydrogen atoms have been omitted for clarity.
The oxidation state of the dioxolene ligands can also be inferred from an analysis of their C–O and C–C bond lengths. The C–O bond length for the DTBCat ligand is ~0.06 Å longer than that for the DTBSQ ligand. In addition, ligands in the SQ–• oxidation state should exhibit C–C bonds of different bond orders alternating between greater single (>1.40 Å) and double (
