Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 5351−5357
pubs.acs.org/JPCL
Ultrafast Spintronics: Dynamics of the Photoisomerization-Induced Spin−Charge Excited-State (PISCES) Mechanism in SpirooxazineBased Photomagnetic Materials Adam J. Jenkins,† Ziliang Mao,† Aiko Kurimoto,‡ L. Tyler Mix,† Natia Frank,‡ and Delmar S. Larsen*,† †
Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, United States Department of Chemistry, University of Victoria, PO Box 1700 STN CSC1.5, Victoria, British Columbia V8P 5C2up, Canada
‡
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ABSTRACT: The optical control of spin state is of interest in the development of spintronic materials for data processing and storage technologies. Photomagnetic effects at the single-molecule level have recently been observed in the thin film state at 300 K in photochromic cobalt dioxolenes. Visible light excitation leads to ringclosure of a photochromic spirooxazine bound to a cobalt dioxolene, which leads to generation of a high magnetization state. Formation of the photomagnetic state occurs through a photoisomerization-induced spin-charge excited-state process and is dictated by the spirooxazine ligand dynamics. Here, we report a mechanistic investigation by ultrafast spectroscopy in the UV-vis region of the photochemical ring-closing process in the parent spirooxazine, azahomoadamantylphenanthroline spirooxazine, and the photomagnetic spirooxazine cobalt−dioxolene complex. The cobalt appears to stabilize a photomerocycanine transient intermediate, presumably the TCC isomer, formed along the ground-state potential energy surface (PES). Structural changes associated with the TCC isomer induces formation of the high-spin Co(II) form, suggesting that magnetization dymanics can occur along the excited-state PES, leading to ultrafast switching on the ps time scale. We demonstrate the full ring closure of the spiro-oxazine ligand is not required to switch magnetization states which can be induced with a higher yielding isomerization reaction. The ability of this system to undergo optically induced spin state switching on the ps time scale in the solid state makes it a promising canididate for resistive nonvolatile memory technologies.
T
lifetime (10 s) of the photoinduced charge−spin state at 300 K, the optically induced changes in oxidation and spin states can be directly observed for the first time by magnetometry and electronic absorption spectroscopy at ambient temperatures in the solid state.15 The key to initiating the PISCES mechanism is the photoswitching (ring closing) reaction of a modified SO ligand between the open photomerocycanine (PMC) and ringclosed SO conformations (Scheme S1). The PISCES process involves four primary metastable states (Scheme 1). Photoexcitation of the ground-state PMC-ls-Co(III) semiquinone form (S = 1/2) of the complex (1a) leads to ring closure (1b). The SO form is a stronger π acceptor (by 0.3 eV) than the open PMC form and is predicted to stabilize the Co(II) state.16 The resulting change in ligand field is proposed to trigger a ligand-to-metal CT/spin transition from a bound ditert-butyl catecholate (DTBQ) ligand to the ls-Co (III) center to generate a SO-hs-Co(II) bis-semiquinone sextet state (1c) with a resultant increase in magnetization. The lifetime of the photoinduced magnetic state (τ ≈ 10 s) appears to be dictated
he ever growing desire to improved methods for storing and processing data has led to intense research into quantum information processing technologies.1,2 Whereas conventional electronics utilize binary digits, quantum information processing relies on two-state quantum systems, such as photon polarization or electron spin, to form “qubits”. Entanglement and superposition of qubits lead to the generation of an infinite number of states, with an exponential increase in information storage and processing capabilities. Coherent population of spin states by optical excitation in organic materials provides an enticing strategy toward optical spin state manipulation.3−14 We recently reported a novel strategy of generating photoinduced spin states through the coupling of ligand field switching and spin-coupled charge transfer (CT) in a class of electronically bistable cobalt complexes that incorporate a photochromic spirooxazine (SO) ligand.15 A photoisomerization-induced spin−charge excited-state (PISCES) process is observed that involves photoinduced switching of the SO ligand through steady-state irradiation with visible light (550 nm). The photoisomerization initiates a CT spin transition process at the cobalt center, leading to a reversible increase in magnetization in both solution and organic thin films. The PISCES process enables optically induced and reversible changes in charge and spin states. Due to the unusually long © 2018 American Chemical Society
Received: July 11, 2018 Accepted: August 29, 2018 Published: August 29, 2018 5351
DOI: 10.1021/acs.jpclett.8b02166 J. Phys. Chem. Lett. 2018, 9, 5351−5357
Letter
The Journal of Physical Chemistry Letters Scheme 1. Generation of Photomagnetic States of the CoAPSO Complex via a PISCES Processa
a
The APSO ligand is highlighted in red, and the DTBQ ligands are in black. The cobalt metal is yellow and blue for the low-spin Co (III) and highspin Co (II) oxidation states, respectively.
Figure 1. Normalized ground-state absorption spectrum for APSO (A) and CoAPSO (B) in toluene. (B) Steady-state illumination of CoAPSO with 450−550 nm light, resulting in conversion of the PMC to the SO given the decrease of the PMC π−π* band at 550 nm and increase of the hsCo(II) MLCT band at 800 nm (inset).
excited states cross and whether there is electronic mixing between states. Additional unresolved questions include the role of vibrational excited (“hot”) ground-state intermediates (GSIs), excited-state evolution, and the nature of conical intersections (CIs) in the photodynamics of the SOs. Here, we report a mechanistic investigation by ultrafast transient absorption (TA) spectroscopy of the primary photochemical ring-closing process in the parent SO, azahomoadamantylphenanthroline SO (APSO), and the SO cobalt-dioxolene complex (CoAPSO). Figure 1A contrasts the ground-state absorption spectra of APSO in the dark-adapted PMC (black curve) and lightadapted SO (red curve) conformations. As with other photochromic SO switches, the PMC state exhibits a strong π−π* transition near 550 nm, while the SO configuration absorbs in the ultraviolet at around 330 nm.15,34 CoAPSO exhibits similar photoactivity with visible light illumination inducing the formation of the SO (and loss of the PMC) as indicated by decreasing 555 nm absorption (Figure 1B).
not by metal-centered excited states, which decay rapidly at room temperature (1−2 ns),17,18 but by the lifetime of the SO metastable state of the photochromic ligand (1c → 1d).15 The principal advantages of the PISCES approach are that (i) the optical modulation of the spin state originates at the single-molecule level and can be controlled by optical gating of the photochromic ligand, and (ii) because the photochrome can photoswitch in the solid state, the system can be incorporated into thin film device architectures. Due to the greater stability of the closed SO form over the open PMC form of most SOs, previous ultrafast dynamics studies focused on the light-induced ring-opening (SO → PMC) reaction,19−32 with the exception of that of Kumar et al.33 who explored the ultrafast ring closing (PMC → SO) photodynamics. Hence, the primary events (timescales, mechanisms, intermediates, etc.) underlying the PISCES mechanism of the modified SOs species in Scheme 1 are still unknown. Questions arise as to whether the potential energy surfaces of the photochrome excited states and metal-centered 5352
DOI: 10.1021/acs.jpclett.8b02166 J. Phys. Chem. Lett. 2018, 9, 5351−5357
Letter
The Journal of Physical Chemistry Letters
Figure 2. TA [(A) APSO; (B) CoAPSO] and kinetics [(C−F) black curves: APSO; red curves: CoAPSO] for APSO and CoAPSO in the visible region. TA data is represented by open circles and the fit by solid lines obtained from the global analysis model in Figure S5. The transient spectra including the initial spectra are shown in more detail in Figure S1. The CoAPSO signals were scaled two-fold to aid in comparison.
However, the ring-closing dynamics of CoAPSO also results in an increase in the weak 700−800 nm transition (Figure 1B inset) that corresponds to a high-spin Co(II) MLCT band.15 The primary dynamics of ring-closure reactions have been extensively studied in the less complex class of spiropyrans.35−38 For spiropyran ring closure, both a singlet (py-BIPS, 6,8-dinitro-BIPS, 6,8-dinitro-BIPS/1′-(2-carboxyethyl)-substituted derivative)35−37 and a triplet38 pathway have been proposed. The singlet pathway involves an initially vibrationally excited S1 population that either directly evolves into the closed SO form35 or relaxes to a hot GSI on the S0 state of the open form before evolving into the closed form.37 Kumar et al. reported the only ultrafast study of the ringclosure excited-state dynamics of a SO chromophore, specifically 1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,3′[3H]phenanthr-[9,10b](1,4)oxazine] (IPSO).33 In contrast to APSO and CoAPSO, IPSO is like most SOs with a closed SO configuration as the dark-adapted ground-state. Independently, Reguro and co-workers and Robb and co-workers computationally studied the thermal and photoisomerization of the ring-closure reaction of 2H-benzo[b][1,4]oxazine39 and 3,4-dihydroquinoline chromophores,40 respectively. In both systems, the photoreaction involved a trans-to-cis isomerization followed by a single-bond rotation prior to ring closure. However, it has also been proposed that thermal ring-closure reactions proceed either through a single-step rotation41 or a two-step inversion pathway42 (Scheme S2). Figure 2 compares the visible TA signals for APSO and CoAPSO. The signals for both resemble the dynamics previously reported for IPSO with several key differences.33
The TA spectra for both APSO and CoAPSO exhibit strong negative bleaches centered at around 550 nm that are flanked by positive absorptions (Figure 2A,B). The red-flanked absorptions in both systems blue shift on a ∼ps time scale, which is resolved as a growth in the 600 nm kinetics (Figure 2E,F; black curves). Both blue-flanking absorptions persist longer for CoAPSO than those for APSO, while the redflanking absorptions decay more quickly to expose negative bleach signals. The APSO and CoAPSO kinetics are also multiphasic. In the 600 and 650 nm kinetics in Figures 2E,F, there were shortlived decays (600 nm) and is characteristic of the vibrational relaxation of similar rapidly generated hot excitedstate populations.45,46 Concurrent with vibrational relaxation is a partial population loss back to the original trans-PMC-S0 population (point 0) presumably faciliated by a CI between the S1 and S0 surfaces. After relaxation, the resulting relaxedPMC-S1 population evolves through a parasitic CI back to the original trans-PMC-So population on a 13 ps time scale (Figure 3D). A weak portion of the population (Figure 3E) is successful in completing the ring-closing reaction (point 5) with an EADS that strongly resembles the bleach of the PMC form (Figure 1A; black curve). The yield for the ring-closure reaction is estimated at 2% from a simple comparison of the amplitude of initial and terminal bleaches (Figure S2C). However, it is unclear if this reaction occurs via an excited-state mechanism (e.g., point 3 → point 5) or a ground-state mechanism (point 3 → point 4 → point 5) because no clear cis-PMC-So population is observed in the TA signals. Hence, neither productive pathway is included in Figure 4. The EADS are qualitatively similar for APSO (Figure 3A− E) and CoAPSO (Figure 3F−J), suggesting similar evolution dynamics for the two systems (Figure 4; thick yellow and blue curves). However, two important differences can be resolved: (1) the biphasic vibrational relaxation kinetics are distinctly slower in CoAPSO (500 vs 400 fs and 5.5 vs 1.5 ps) and (2) the relaxed-PMC-S1 population decays more slowly (25 vs 13
ps). For APSO, relaxed-PMC-S1 decays directly into transPMC-S0 (mostly), and for CoAPSO, the relaxed-PMC-S1 population coexists with a new population that is not observed in APSO and absorbs at 425 nm (Figure 3I; purple curve). This population is ascribed to the cis-PMC-S0 (Figure 4; point 4) and persists on a 280 ps time scale before decaying back into trans-PMC-S0. A simple comparison of 500 fs and 280 fs EADSs estimates that 17% of the excited-state population evolves into cis-PMC (Figure S2A). As with the terminal APSO EADS (Figure 3E), the terminal spectrum for CoAPSO (Figure 3J) strongly resembles the CoAPSO bleach (Figure 2); the yield for successful ring-closing is estimated at 4% (Figure S2B). As with APSO, it is unclear if the increased ring-closure reaction yield is due to manipulating the S1 or S0 surfaces. The long-living cis-PMC-S0 population would facilitate the groundstate evolution, while the longer-lived PMC-S1 populations would also facilitate the excited-state mechanism. The cobalt-benzoquinone components in CoAPSO stabilize the cis-PMC-S0 population (Figure 4; point 4), and the EADS for this population has an absorption peak at around 425 nm (Figure 3I) with a weak, but resolvable, absorption that extends to the near-IR (NIR). Figure 5 compares the kinetics of APSO and CoAPSO probed at 800 nm. The APSO NIR signals are monophasic with a rapid rise and 400 fs decay attributed to the vibrational relaxation phase resolved in the visible TA data (Figure 3B); however, the subsequent structural dynamics are not reflected in the NIR kinetics. In contrast, the CoAPSO NIR signals are appreciably more complex (Figure 5B) with nonmonotonic evolution and a long-living positive absorption. Because these data exhibit the same 280 ps kinetics resolved for the cis-PMC-S0 population in the visible TA data (Figure 3I), the terminal relaxation phase can be ascribed to decay of cis-PMC-S0. Its absence in the APSO signals (Figure 5A) further reinforces the interpretation of the visible TA data discussed above. Complexation of APSO to the electronically bistable cobalt dioxolene fragment leads to the appearance of a weak band in the 800 nm region assigned to a forbidden Co(II)benzoquinone MLCT band of the CoAPSO complex (Figure 1B, inset). In the trans-PMC-S0 state of CoAPSO, the absorbance of this band is weak due to a small population of the Co(II) form and dominant population of the PMC-lsCo(III) form of the complex (Scheme 1). The ligand-to-metal CT intrinsic to the PISCES process (Scheme 1) involves electron transfer from the catecholate form of the DTBQ ligand to ls-Co-III, resulting in a broad 800 nm absorption assigned to a hs-Co(II)-semiquinone CT state, typically 5355
DOI: 10.1021/acs.jpclett.8b02166 J. Phys. Chem. Lett. 2018, 9, 5351−5357
The Journal of Physical Chemistry Letters
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observed in the Co(II) state of the parent diimine cobalt dioxolenes.17,18 The observation of this CT band upon population of the hs-Co(II) state was initially postulated to occur after full ring closure. It is likely that the broad absorption of the cis-PMC-S0 population of CoAPSO (Figure 3I) extends to the 800 nm probe kinetics (Figure 5B) and can be ascribed to this hs-Co(II) MLCT band. The
