âOffâ Photochromism of a Polyoxometalate

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Visible-Light-Driven “on” / “off” Photochromism of a Polyoxometalate Diarylethene Coordination Complex Jingjing Xu, Henrieta Volfova, Roger J. Mulder, Lars Goerigk, Gary Bryant, Eberhard Riedle, and Chris Ritchie J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04900 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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Journal of the American Chemical Society

Visible-Light-Driven “on” / “off” Photochromism of a Polyoxometalate Diarylethene Coordination Complex Jingjing Xu,[a] Henrieta Volfova,[b] Roger J. Mulder,[c] Lars Goerigk,[a] Gary Bryant,[d] Eberhard Riedle[b] and Chris Ritchie[a]* [a] School of Chemistry, The University of Melbourne, Melbourne, 3010, Australia [b] Lehrstuhl für BioMolekulare Optik, Ludwig-Maximilians-Universität, Oettingenstrasse 67, München, Germany [c] CSIRO Manufacturing, Clayton, 3168, Victoria, Australia [d] Centre for Molecular and Nanoscale Physics & School of Science, RMIT University, Melbourne, Victoria 3001, Australia Polyoxometalates, Self-assembly, Responsive complexes, Photochromism, Diarylethenes.

ABSTRACT: Herein we report the first photochromic polyoxometalate based diarylethene coordination complex, prepared by ligation of two cobalt (III) incorporated borotungstates [BIIIWVI11O39CoIII]6- with the ditopic pyridyl containing diarylethene (C25H16N2F6S2). The solution state composition, structure and stability of the assembly was probed using 1H and 19F Nuclear Magnetic Resonance Spectroscopy (NMR), Electrospray Ionization Quadrupolar Time of Flight Mass Spectrometry (ESI-QTOF-MS), Ultraviolet-Visible Spectroscopy (UV-Vis) and Small Angle X-ray Scattering (SAXS), revealing that the complex self-organizes to adopt a molecular dumbbell structure due to electrostatic and steric considerations. This conformation is a pre-requisite for the photocyclization reaction. The assembly was found to be switchable between two states using visible light due to the perturbation of the diarylethene electronic structure on coordination to the polyoxometalate. We present photophysical data, including the reaction quantum efficiency of the molecular switch in both directions measured using a custom-built quantum yield determination setup in addition to fatigue resistance on prolonged irradiation.

Introduction Photochromic molecules, and their incorporation into functional materials continue to be the focus of intense research, stimulated by their potential operation as molecular optical switches with tunable electronic and physical structures.1,2 Switching should be achieved readily with minimal fatigue when toggling between “on” and “off” states,3 while being resistant to thermal relaxation. Furthermore, it is desirable that the switch be operated exclusively using visible light.4,5 The spectral changes that are observed in most photo switchable systems on irradiation are commonly due to one of the following: E/Z isomerization around a double bond in molecules such as the azobenzenes, charge separation as found in spiropyrans, or electrocyclization of a 6 π triene system for fulgides and diarylethenes (DAE), when the molecule adopts the photoactive conformation.6,7 Each of these photochrome classes addresses some of the strict criteria to function as a truly reliable molecular switch, although the need for improvement in areas of inadequate performance remains.6–8 Specifically, the diarylethenes although often thermally stable, need to adopt the anti-parallel conformation to undergo electrocyclization, while occasionally suffering poor fatigue resistance in solution due to an irreversible photoreaction on excitation of the ring-closed isomer.9–11 Current approaches to overcome these challenges frequently require elaborate multi-step syntheses to incorporate bulky and/or electron-withdrawing substituents to increase the

population of the photochromic conformer, and minimize formation of photo-generated by-products.6 Meanwhile, strategies to induce a bathochromic shift of the open-form’s absorption spectra include elongation of the conjugated π-system and grafting of sensitizing chromophores. An attachment of triplet sensitizers through conjugated linkers have recently proved to be a very effective new strategy leading to improvement in the overall performance of these photochromes.12,13 Finally, several groups have investigated the coordination of DAE ligands to transition metals, yielding photochromic and electrochromic molecules and solid state structures where alteration of the electronic structure impacts the compounds magnetic and redox behaviours.14,15 Polyoxometalates (POMs) are a class of molecular metal oxides with noteworthy diversity in terms of structure, composition and redox properties.16–19 Synthesized using acid driven condensation, POMs are often assembled with precise control regarding the placement of heteroatoms within their molecular frameworks.20–23 Inspired by recent design strategies towards improving the performance of diarylethene optical switches, we embarked upon a study to provide insight into the influence of POMs on the spectral properties of the widely investigated pyridyl containing diarylethene (C25H16N2F6S2) - 1a (Figure 1). Central to this work was the installation of a single coordinatively unsaturated Co(III) ion within a polyoxotungstate molecular skeleton with low oxidizing power.20 These properties are essential to minimize photo-oxidation of the DAE on coordination to the POM, while the diamagnetic polyanion and slow ligand exchange dynamics of Co(III) assisted characterisation by NMR. To the best of our knowledge this study

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represents the first experimental investigation of a POM-DAE coordination complex, research that aligns with the recent efforts of Mialane, Izzet and co-workers in their study of thermally unstable POM-Spiropyran molecular switches.24 Upon coordination of both the pyridyl groups in 1a to two independent nanoscopic, electron deficient cobalt (III) substituted Keggin anions [BIIIWVI11O39CoIII]6- - 1b, profoundly modified properties were observed for the resulting molecular dumbbell [(C25H16N2F6S2)(BIIIWVI11O39CoIII)2]12- - 2o where the DAE ligand is in its ring-open form. The ring-closed isomer will be referred to as 2c throughout this paper. The composition, structure and elec-

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hexane solutions of 1a and 1b in the ratio 1 : 3 affords the molecular dumbbell 2o (Figure 1). (Detailed synthetic conditions are presented in the Methods and Electronic Supporting Information). Single crystal X-ray diffraction is an important technique for the

Figure 2. ESI-QTOF-MS conducted on a solution of 2o in cyclohexane reveals the presence of two molecular tri-anions (N(C8H17)4)3H6[(C25H16N2F6S2)(BW11O39CoIII)2]3- - A; and (N(C8H17)4)4H5[(C25H16N2F6S2)(BW11O39CoIII)2]3- - B.

Figure 1. Chem-draw and polyhedral representations of the pyridyl containing diarylethene ligand (C25H16N2F6S2) - 1a (top-left), and the polyanion [BIIIWVI11O39CoIII]6- - 1b (bottom-left) that constitute the photochromic coordination complex [(C25H16N2F6S2)(BW11O39CoIII)2]12- - 2o that is shown at the topright as a space-filling diagram. The coordinatively unsaturated Co(III) ion of 1b is shown as a green sphere. Pair distance distribution function analysis (PDDF) of experimental SAXS data (bottomright) indicates that the dominant structure of 2o in solution is that of monodispersed molecular dumb-bells in the photoactive conformation. (Carbon, black; Hydrogen, white; Nitrogen, blue; Boron, pink; Cobalt, dark green; Fluorine, lime green; Sulphur, yellow; Oxygen, red; Tungsten, orange). tronic properties of this compound have been determined and investigated by 1H, 19F NMR, ESI-QTOF-MS, SAXS and UV-VIS. Quantum yields of the photochemical reactions were also determined for the 2o to 2c and 2c to 2o reactions by irradiation with either 400 nm or 625 nm light. Light emitting diodes (LEDs) as part of a custom-built Quantum Yield Determination Setup (QYDS) were used for illumination. The performance of the device was carefully checked using related compounds of known quantum yield, with the results scrutinized on photometric grounds (See ESI). The preparation of 2o was accomplished using an adapted version of the phase-transfer approaches developed independently by Corigliano and Pope.25-26 The Keggin polyanion used in this study was synthesized by inserting Co(II) ion into the borotungstate polyanion [BW11O39]9- followed by oxidation of the Co(II) to yield [BIIIWVI11O39CoIII(H2O)]6-.27–29 Precipitation on the addition of BaCl2 and repeated re-crystallization from hot water was required to obtain a bulk crystalline sample of 1b that was then used in all subsequent synthetic steps. Transfer of 1b into a cyclohexane soluble form was achieved using tetraoctylammonium (TOA) as the phase transfer agent.20,30 Mixing of dry room temperature cyclo-

structural characterization of POMs due to their complexity and variable stability in aqueous media.31–33 On transfer of POMs into non-polar organic solvents, crystallization becomes improbable due to the bulky hydrophobic cations required.34,35 It is therefore essential in the absence of crystallographic data that a variety of techniques are utilized to verify the integrity of the polyanion and derivatives thereof.36,37 To this end, the composition of 2o has been confirmed by ESI-QTOF-MS, with two molecular ions observed as salts the mixed TOA and H+ and (N(C8H17)4)3H6[(BIIIWVI11O39CoIII)2(DAE)]3(N(C8H17)4)4H5[(BIIIWVI11O39CoIII)2(DAE)]3- centered at 2453.667 and 2608.865 m/z (See Figure 2). The distinctive isotopic patterns are in excellent agreement with the theoretical spectra. (See ESI – S4-5). To support the compositional information gained from ESI-QTOFMS, we conducted SAXS experiments to probe the solution state structure of 2o. The molecular shape and significant variation of electron density across the molecule due to the metal-oxide and organic constituents, make it particularly amenable to this type of analysis.38–41 X-ray scattering data collected on a dilute (1.2 mM) solution of 2o in cyclohexane was analyzed using two approaches: (i) the PDDF approach (See Figure 1) with 2o appearing to be monodisperse in solution with minimal background x-ray scattering from solvent and tetra-octylammonium bromide; and (ii) by fitting the data to a dumbbell molecular shape model.42,43 This fit (See ESI – S2) supports the following structural conclusions; 2o has a maximum cross-section of 31.9 Å; Intramolecular Keggin separations of 11.7 Å and a dumbbell radius of 5.3 Å, consistent with the geometry optimized conformer of 2o obtained using the reliable, dispersion-corrected density functional theory (DFT) method PBEh3c that is suitable for large transition metal containing systems.44 This structure revealed reactive carbon separations (C1, C6) of 3.542 Å, well under the threshold of 4.200 Å limit determined as the limit for the photocyclization reaction to proceed in the solid state.45 Alteration of this optimized structure by a 180 ° rotation around the C4-C5 bond followed by subsequent optimization identified a second stable rotamer (See ESI - S13-14). This second isomer is 5.9 kcal/mol higher in energy and therefore should be un-

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Journal of the American Chemical Society populated based on thermodynamic considerations at room temperature. It is proposed that self-organization of the structure to yield exclusively the photo-active conformer is driven by both intramolecular steric and electrostatic repulsion between the Keggin anions coordinated to either end of the ditopic ligand 1a. This is an essential structural requirement that enables the photochemical reaction of the coordination complex to proceed.

Irradiation of the 2o NMR sample using a 365 nm LED results in clean conversion to a racemic mixture of (R,R)-2c and (S,S)-2c as evidenced by 1H and 19F NMR (Figures 4 and ESI – S10). 1H resonances for the pyridyl protons HA shift downfield (Δδ ~ + 0.167 ppm), while HB is shifted upfield (Δδ ~ - 0.06 ppm). As the thienyl proton HC experiences the biggest change in proximal electron density due to the electrocyclization reaction, a significant upfield shift

Figure 3. 1H NMR spectra of 1a (bottom) and on addition of one (middle) and three (top) equivalents of [BIIIWVI11CoIIIO39]6- showing the diminishment of ligand-based signals and evolution of resonances corresponding to the formation of 2o. The formation of 2o and its solution state stability was also investigated by 1H and 19F NMR, with experiments conducted in d12cyclohexane to completely negate complications of ligand displacement by coordinating solvents. Titration of 0.5, 1, 1.5 and 2 equivalents of 1b into a solution of 1a resulted in the immediate disappearance of 1a resonances, with the appearance of several illdefined broad resonances that are attributed to the formation of 1 : 1 and 2 : 1 coordination complexes (See Figure 3 (middle)).46 On addition of three equivalents of 1b, three broad aromatic resonances remained indicating complete coordination of 1a to yield exclusively 2o. The aromatic proton resonances of 2o are all shifted downfield with respect to those of 1a, with HA being the most deshielded (Δδ ~ + 0.31 ppm) due to proximity of the Co(III) ion, with HB and HC only subtly influenced (See Figure 3 and ESI – S7). Integration of the aromatic peak areas are in accordance with that expected for 2o. Unfortunately, ligand-based methyl resonances of 2o are completely obscured by signals originating from TOA cations and residual cyclohexane. 19F NMR spectra collected on the same titration samples as for the 1H study also indicate complete complexation (See ESI - S10). Finally, 1H Diffusion-Ordered Spectroscopy (DOSY) experiments were conducted to determine the self-diffusion coefficient of 2o that was found to be D = 1.80 e-10 m2/s. The (spherical) hydrodynamic radius of the assembly was calculated as 12.8 Å, that when considering the differences in longitudinal and lateral diffusion of the molecular dumbbell 2o supports conclusions drawn from the SAXS analysis. On conversion of 2o to 2c (Figure 4), a comparable hydrodynamic radius of 14.0 Å was calculated (See ESI – Table S1). Collectively these experiments validate our hypothesis that 2o is a self-organized assembly, where the organic component is forced to adopt the appropriate conformation to undergo photocyclization. To the best of our knowledge, 2o represents the first diarylethene polyoxometalate coordination complex to be reported in the literature. We will now discuss our findings regarding the photochemical conversion of 2o to 2c.

Figure 4. A generalized representation of the photoinduced ringclosing and ring-opening process (top), and 1H NMR spectra showing the evolution of 2c from 2o on irradiation with 365 nm light. is observed (Δδ ~ - 0.74 ppm). Evolution and diminishment of resonances in the 19F spectra on irradiation are consistent with conclusions drawn from the 1H spectra. Additionally, the simplicity of the spectra implies population of only the photo-active “anti-parallel” (C2-symmetric) conformers of 2o, in agreement with our SAXS analysis (See ESI – S2).47 No attempt was made to separate the photoproducts. Absorption spectra of 1a and 1b, the molecular components of 2o in cyclohexane are consistent with the literature (See ESI – S1519).20,46 Both species absorb strongly in the UV due to their respective π-π* (ε = 31500 M-1 cm-1 @ 297 nm) and ligand to metal charge transfer (LMCT) O2p → W5d/Co3d electronic transitions (ε = 32900 M-1 cm-1 @ 249 nm).6, 20,48 Neither component displays strong absorption in the visible, λ > 400 nm, with a weak (ε = 124 M-1 cm-1, Co → Co: d-d) absorptive feature for 1b at 683 nm. On mixing, and subsequent complexation to form 2o, a new broad absorptive feature centered at 400 nm and tailing well into the visible is evident (ε = 8702 M-1 cm-1) (Figure 5 and ESI – S20-21). This new transition is tentatively assigned as an 1A1 → 1T2 d-d transition as previously discussed by Weakley, Hill and Malik for similar



CoIII containing POM pyridyl complexes.27,49 The high molar absorptivity is attributed to the low symmetry of the CoIII environment and the electron donating strength of 1a (Figure 5). A shift of the Co → Co d-d transition (Δλ ~ + 67 nm) from 683 nm to 616 nm in line with previous literature is also observed on complexation of 2o.20 The thermal stability of 2o was monitored at room temperature ≈ 20 °C over 4 days, with negligible change in the absorption spectra observed over this time.

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due course. Excitation into the tail of the d-d transition at 450 nm also promotes the conversion of 2o to 2c. To investigate the photochemical conversion of 2o via the 1T2 d-d transition, reaction quantum yield measurements were conducted using an improved version of the Quantum Yield Determination Setup (QYDS) of our own design.51 In comparison to standardized chemical actinometry, this method offers a rapid and facile determination of the quantum yield under standard lab conditions with-

Figure 5. Time dependent UV-Vis spectroscopy showing the photochemical conversion of 2o to 2c and vice versa in response to irradiation with 2.95 mW of 400 nm light and 20.74 mW of 625 nm light (upper and lower right panels). Molar absorptivities for 2o and 2c extracted from 1H NMR data are plotted alongside the emission profiles of the LEDs used in this study (upper left panel), and colour changes observed on conversion from 2o (left) to 2c (right) (lower left panel). Irradiation of a cyclohexane solution of 2o using either 365 or 400 nm LEDs results in the clean conversion to 2c. We propose that this conversion is the result of combined ligand and POM based excitation pathways. Two isosbestic points are observed at 400 and 448 nm accompanied by the growth of a new absorptive feature centered at 602 nm that is hyposochromically shifted by 17 nm when compared to that of 1a(closed)42 (Figure 5 and ESI - S21). Irradiation of a cyclohexane solution of 1a(open) with 400 nm light does not induce electrocyclization, instead, the back reaction 1a(closed) → 1a(open) is induced.50 Molar absorptivity’s for 2o and 2c were determined based on NMR analysis as indicated earlier, with 80 % conversion at the photostationary state on irradiation with 365 nm light. We suggest that the observed photochromism is facilitated by the combination of the short-lived POM based excited states (< 10 ps) and the rapid electrocyclization rates frequently observed for DAEs (< 10 ps). Transient absorption (TA) studies are required to validate this hypothesis. These studies are beyond the scope of the current work and will be conducted and reported in

out the necessity for any additional calibration measurements (i.e. determination of the illumination power via a photo sensitive substance with a known quantum yield). The combination of the welldefined spectral bands of LEDs, the precisely determined absorbed power of the irradiation light at the sample and the constant monitoring of the transmitted power enables determination of the reaction quantum yield with a relative error under 5 %. To increase the sensitivity and precision of the measurement, illumination is continued until the photostationary state is reached. The progress of the isomerisation is then modelled in detail and the model fitted to the observations (see ESI – S22 and S23). We determined the quantum yield for both the forward and back reaction on excitation with 400 nm and 625 nm (see Fig. 5). The excitation wavelength of 400 nm coincides with an isosbestic point where the photostationary state is reached with a composition of 66 % 2c and 34 % 2o. The measurement reveals a relatively low quantum yield of 0.97 %, which is to be expected due to the significant electron withdrawing capacity of the coordinated polyanion. Photocycloreversion required considerably higher illumination power


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Journal of the American Chemical Society as well as longer reaction times to complete the isomerization process. Typically, the quantum yield for the photoinduced ring opening of diarylethenes is one or two orders of magnitude lower than for the forward reaction.52–54 Correspondingly we found that cycloreversion process to proceed with a quantum yield of 0.022 %. Despite the inefficiency of the process, we successfully completed the reaction to yield exclusively 2o.

Jingjing Xu: 0000-0002-3838-624X Henrieta Volfova: 0000-0001-5206-328X Roger Mulder: 0000-0002-8740-802X Lars Goerigk: 0000-0003-3155-675X Gary Bryant: 0000-0001-5483-7592 Eberhard Riedle: 0000-0002-2672-5718

Conclusions

Chris Ritchie: 0000-0002-1640-6406

In conclusion, we have synthesized the first diarylethene-polyoxometalate coordination complex, 2o, and verified its solution state integrity and molecular structure using a suite of techniques, that agree with a stable photoactive geometry optimized DFT structure. The molecular assembly can be switched between two stable forms on irradiation with visible light, with ring closure achieved via a lower energy pathway compared to that of the uncoordinated ligand. This is evidenced by the red-shifted absorption profile of 2o with respect to 1a. Reaction quantum yield analysis of switching in both directions indicate the presence of significant relaxation pathways that don’t promote the photochemical reaction. We can speculate that two main reasons are responsible for the low quantum yield of 2o. Firstly, coordination of the POMs will change the electron density at the two reactive carbons involved in the ring closing process. Secondly, the rigidity of 2o may impact the internal rotation that is required for bond formation. Finally, the precise nature of the electronic transitions occurring on excitation have not yet been determined, where underlying charge transfer processes that do not induce ring closure may be occurring. Whether it is the rate (probability) of the closing channel or rather the increased rate of the effective internal conversion that leads to the observed inefficiency, these processes will be investigated by time-resolved spectroscopy and reported in due course.

Funding Sources All authors gratefully acknowledge financial contributions from their respective funding sources. C.R - (ARC - DE130100615), L.G - National Computational Infrastructure (NCI) National Facility within the National Computational Merit Allocation Scheme (project ID: fk5), E.R - (Deutsche Forschungsgemeinschaft through the SFB 749 (project B5) and the Research Training Group ‘Chemical Photocatalysis’ (GRK 1626). ACKNOWLEDGMENTS Associate Profs. Colette Boskovic and Trevor Smith are thanked for discussions regarding various aspects of the article. Mr Kristian Davies is thanked for preliminary experimental work. REFERENCES (1)

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Nonetheless, the development of a reliable approach for the preparation of coordination complexes such as that presented in this article opens the door to the development of a multitude of unique photochromic compounds. These envisaged compounds will include the coordination of POMs of varying sizes, shapes and charges to DAE ligands such as 1a and the determination of structure-activity relationships. With the above considerations, a route to higher photochromic efficiencies should be achieved through modifications of the molecular architectures and electronic structures.

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ASSOCIATED CONTENT

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Supporting Information Detailed materials and methods of all techniques used in the study (SAXS, Chromatography, ESI-MS, NMR, UV-Vis, QYDS, Fatigue experiments, FT-IR, Computational details.

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Corresponding Author *[email protected] Present Addresses

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School of Chemistry, The University of Melbourne, Melbourne, 3010, Australia

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Author Contributions The manuscript was written with contributions from all authors, and have approved the final version of the manuscript.

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