Xingyu Wu
email: xingyu.wu@uha.fr

Bryan Gross

Benjamin Leuschel

Karine Mougin

Sébastien Dominici

Simon Gree

Mehdi Belqat

Vitalii Tkachenko

Benjamin Cabannes-Boué

Abraham Chemtob

Julien Poly

Arnaud Spangenberg
email: arnaud.spangenberg@uha.fr

On-Demand Editing of Surface Properties of Microstructures Made by 3D Direct Laser Writing via Photo-Mediated RAFT Polymerization

Keywords: photo-controlled RAFT polymerization, 3D and 4D printing, micro-and nanostructures, surface modification, multi-material printing

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

4D printing, originated from Skylar Tibbits, describes the capability of objects to transform over time directly off the print bed. [1] This fourth dimension requires printed structures to be programmably active and to have the ability to transform independently. This process greatly facilitates the development of multi-material or customized objects for a large number of applications. [2] However, 3D printing of most current photocurable materials is generally based on conventional radical polymerization. As a result, a polymeric network of "dead" chains is formed due to irreversible termination reactions. Consequently, these chains cannot be used to introduce new monomer units, functionalities or properties in a "living" manner. [1e,3] The emergence of photo-controlled living radical polymerization, [4] especially RAFT polymerization, [5] opened up new perspectives for 4D printing. In 2017, Johnson et al. developed the first-generation living additive manufacturing process. [3] In their approach, a 3D "parent" gel was synthesized via click chemistry, and then photo-redox catalyzed RAFT polymerization was carried out to generate "daughter" gels, which could be chemically and mechanically different or similar to "parent" gel. Later, Boyer's group focused on the direct application of photo-catalyzed RAFT polymerization to the 3D printing process. Their formulations contained an organic dye (photocatalyst), a tertiary amine (co-catalyst), a RAFT agent and acrylates (monomer). A key element for 4D characteristics is the retention of the RAFT chain transfer agent functionality during 3D printing, allowing a facile photochemical post-modification by visible irradiation after the original printing process to tailor surface properties. [6] Very recently, they proposed to use a Norrish type I photoinitiator instead of photocatalyst in a RAFT polymerization system leading to a faster 3D printing process. [7] Surprisingly, to the best of our knowledge, photo-controlled RAFT polymerization has never been described in the context of elaborating 3D micro-and nanostructures by DLW, despite the great potential of this approach for facile surface modifications of such tiny structures. While it is known that micro-and nanostructures with special features have been widely used in photonics, [8] microelectronics, [9] biomedicine, [10] microfluidics [11] and so on, [12] a better, more versatile control of surface properties and modifications could lead to new perspectives in these fields. [13] In our present work, five macro-photoiniferters (Macro-DTCs, Scheme 1a and Table 1 ) were synthesized by photo-controlled RAFT polymerization and one of them (Macro-DTC-01) was selected to perform 3D DLW in presence of air. The initiation system only contains one macrophotoiniferter, no additional compounds. After introducing Macro-DTC-01 into resins, we investigated its performance in 3D micro-printing. Remarkably, 3D structures with sub-500 nm features could be achieved. Following the fabrication of micro-objects, the dithiocarbamate groups incorporated in the networks were reactivated for reconfiguration processes. Two resins were chosen to customize and modify our microstructures' surfaces: a conventional acrylate resin and a functional, stimuli-responsive material. Detailed experimental procedures can be found in the Supporting Information.

Scheme 1. a) The synthesis process of Macro-DTCs. b) The proposed RAFT polymerization mechanism under irradiation. c) Chemical structures of monomers.

Results and Discussion

Macro-photoiniferters Macro-DTCs

As shown in Scheme 1a, Macro-DTCs are derived from methyl 2-((9H-carbazole-9carbonothioyl)thio)-2-methylpropanoate (DTC). Through RAFT polymerization of butyl acrylate (BA), initiated by DTC, five Macro-DTCs with different number-average molecular weights (Mn) (Table 1) were obtained under blue LED irradiation for 24 hours except in case of Macro-DTC-03. The synthesis processes of DTC and Macro-DTCs are detailed in the Supporting Information and Table 1. [14] Moreover, Table 1 lists conversion of BA, Mn, and molecular weight dispersity (Mw/Mn) for each of the five Macro-DTCs. The conversions of BA were calculated from 1 H NMR spectra (Figure S1), Mn and Mw/Mn were obtained from size exclusion chromatography (SEC, Figure S2). The narrow molecular weight distributions, ranging from 1.06 to 1.17 (Table 1), proved that these polymerization processes are well controlled. Based on RAFT polymerization, the mechanism of the polymerization process induced by these macro-photoiniferters is shown in Scheme 1b. Owing to the absorbance of Macro-DTC at blue light (see UV-Vis spectra in Figure S3), [14] photo-excitation of Macro-DTC causes homolytic cleavage of the weak C-S bond, generating a thiocarbonylthio radical and a radical propagating species. The propagating radical species is capable to react with monomer vinyl groups and to interact with other thiocarbonylthio radical species as well as with macromolecules containing dithiocarbamate groups. The recombination of the propagating radical (Pn•) with a thiocarbonylthio radical or its reaction with other macrodithiocarbamates allows a large fraction of the growing polymer chains to become dormant.

The retention of the thiocarbonylthio group as a chain-end throughout the polymerization, repeated activations of dormant thiocarbonylthio-capped polymer chains via transfer reactions, and subsequent monomer additions are key elements of RAFT polymerization. [4g] The ability of a Macro-DTC homologue to initiate polymerization via irradiation at 480 nm or 525 nm has already been investigated in our previous work. [14] Next, we will explore the capabilities of Macro-DTC in 3D printing. b) the reaction time is 29 hours, instead of 24 hours.

3D printing

3D DLW is a very useful tool to generate arbitrary microstructures. In this work, all 2D and 3D

printing were performed on a high resolution 3D-printing system (µFAB-3D from Microlight3D) based on DLW at 532 nm. The fabrication procedure is illustrated in Figure 1a and the printing parameters are detailed in the Supporting Information.

First, we have investigated the performance of Macro-DTC-01/PETA in the printing of 2D microstructures. Pentaerythiritol triacrylate (PETA, chemical structure is shown in Scheme 1c) is a trifunctional monomer, which was added to the formulation as crosslinker to improve the mechanical stability of the resulting microstructures. In order to preserve the ability of the polymer to undergo repolymerization, we first used a limited amount of PETA (20 wt%).

However, as highlighted in Figure S4 (right), the degree of crosslinking within the polymer microstructure was not sufficient to maintain structural integrity after development. By increasing the amount of PETA up to 50 wt%, mechanically stable 3D microstructures were successfully fabricated (Figure S4, left). Therefore, formulations based on Macro-DTC/PETA (50/50, wt%) were systematically employed in all of the following experiments. Furthermore, we studied the impact of the Mn on fabrication conditions, features' size and structuration. For each formulation listed in Table 2, we have first determined the intensity threshold Ith and the damage intensity Idamage defined respectively as the minimum laser intensity required to initiate the polymerization and the laser intensity leading to micro-explosion. To that aim, lines were fabricated at increasing laser intensity with a printing speed of 14.3 µm/s. The results are reported for each formulation in Table 2 and a representative series of lines is given for the formulation Macro-DTC-01/PETA (Figure S5). One can notice that the intensity threshold Ith is higher for higher number average molecular weight Mn, underlining a lower reactivity for formulation based on Macro-DTC with high Mn. This can be explained by a lower concentration of DTC groups for high Mn Macro-DTC. No obvious changes in the damage intensity Idamage were observed. In a second step, we have investigated the impact of laser intensity on the feature size of microstructures. S7). Moreover, as shown by Luo and coworkers, [15] increasing the number of DTC groups in the starting formulation will favor the number of DTC groups in the final polymer. Therefore in order to optimize structuration capability and the ability of the microstructure to be post-functionalized, the formulation Macro-DTC-01/PETA has been selected for the next sections. To demonstrate Macro-DTC-01's performance in DLW, two intricate 3D structures were achieved with a laser intensity of 0.38 mW/cm² and a printing speed of 43.4 µm/s. SEM images (Figure 1b) show that the structures have well defined shapes and sub-µm resolutions (~ 500 nm). Here, it is worth noting that these structures were made from a formulation which contained 50 wt% PETA and 50 wt% Macro-DTC, no other additives. These complex microstructures prove Macro-DTC-01 is an excellent photoiniferter for photopolymerization processes during 3D printing.

Multi-material microstructures

Several experiments were conducted in order to study the possibility to perform localized modifications of 3D microstructures, thanks to the presence of dithiocarbamate groups in the formed polymer. As shown in Figure 2a (top), we used pure trimethylolpropane triacrylate (TMPTA, chemical structure is shown in Scheme 1c) without Macro-DTC to write the letters "µRAFT" onto an assembly of microstructures, created via successive functionalizations of a variable number of layers on top of a PETA base microstructure. Figure 2b shows the "µRAFT" fabricated onto the top of three different, stacked squares. Amongst those squares, exclusively the bottom-layer square (50×50 µm²) was made from 50 wt% PETA and 50 wt% Macro-DTC-01 using a laser intensity of 0.44 mW/cm² and a printing speed of 14.3 µm/s, every subsequent square (repolymerized layer) was made from pure TMPTA without Macro-DTC. The intensities used for fabricating the second and third squares were 0.85 and 0.99 mW/cm², while the printing speed was kept at 14.3 µm/s and resulted in layer thicknesses of 240 and 277 nm, respectively (Figure S8). As shown in the AFM and SEM images (Figure 2b, Figure S9), the height of letters on the top layer (1.92 mW/cm²) is 217 nm and their width 200 nm. Here, the uniform height and exceptionally high resolution of the microstructures on the final layer clearly proves the successful repolymerization. Moreover, reinitiation, reconfigurations and modifications can be achieved, remarkably, across multiple layers using Macro-DTC-01. One major advantage of living polymerization is that it guarantees covalent linking of the microstructures to the supporting polymer surface, which greatly eliminates the adhesion problems between layers. In order to confirm the presence of DTC groups into the repolymerized layers (with a total thickness of 1.4 μm, Figure S10), micro-FTIR experiments were conducted. In Figure S11, we can clearly see peaks at 1666 cm -1 (aromatic C-H), 1451 cm -1 (aromatic C=C), 1326 cm -1 (C=S) and 755 cm -1 (aromatic C-H and C-S) in the spectrum of the repolymerized layers, which are characteristic peaks of dithiocarbamate. [16] The experimental details are described in the Supporting Information. To further demonstrate the versatility of this RAFT polymerization, a spatially controlled polymerization process using N-isopropylacrylamide (NIPAM, Scheme 1c) monomers was performed on the surface of polymerized Macro-DTC-01/PETA squares. Poly (Nisopropylacrylamide) (PNIPAM) is well-known as a temperature-responsive polymer hydrogel, which has been used in the fields of biosensors, actuators and drug delivery. [17] Raising the temperature above the lower critical solution temperature (LCST) of PNIPAM causes it to become hydrophobic and dehydrate, i.e. PNIPAM expels water molecules from the spaces in its polymer network, which results in shrinkage and stiffening of the structure. Upon decreasing the temperature, this process reverses and PNIPAM returns to its swollen, hydrated state. Even though several attractive 3D-printed PNIPAM structures were presented, it remains a challenge to obtain PNIPAM structures exhibiting this temperature stimulus response. [18] This is due to the ratio of monomer to crosslinker used for fabrication: with low quantities of crosslinker, it can hardly form structures, even if the resulting material responds to the stimulus; on the other hand, with high amounts of crosslinker, it can easily produce arbitrary structures, which, however, hardly respond to temperature. In our experiment, Macro-DTC-01 allows us to fabricate responsive patterns via reconfigurations of the surface of a base microstructure. The formulation used for post-modification contains only NIPAM and ethylene glycol. First, as before, a square was made using 50 wt% PETA and 50 wt% Macro-DTC-01; then a drop of the NIPAM solution was applied to cover the square (Figure 2a bottom). A well-organized honeycomb structure was patterned onto the square (Figure 2c) using DLW (1.50 mW/cm²) followed by development using ethanol. The stimuli-responsive property of PNIPAM has been investigated using AFM in an aqueous environment. When the water temperature is 45 °C, the heights are 246 nm (± 18 nm), measured from the top of the PETA square to the top of the bars and the widths are 721 nm (± 46 nm). After decreasing the temperature to 14 °C, both heights (300 ± 14 nm) and widths (987 ± 60 nm) increase, compared to their dimensions at 45 °C.

When the water is heated again to 45 °C, the sizes of structures decrease again as well (heights 258 ± 14 nm, widths 793 ± 55 nm). Figure 2c1 illustrates the evolution of PNIPAM microstructures in height. The AFM images (Figure 2c2) let us easily perceive the changes in height via differing color-map values, and changes in width through variations of the sizes of holes. To quantify these changes, the heights and widths at each temperature are plotted in Figure 2c3. Clearly, Figure 2c shows PNIPAM's shrunk and swollen states under water. Here, it is worth to mention that upon cooling from 45 °C to 14 °C, the structures' heights increase less than their widths. This may be attributed to the organization of polymer chains, which deserves to be further studied.

To highlight the potential of our approach, the functionalization of a 3D microstructure were carried out. For this purpose, one "bridge" microstructure, consisting of four pillars connected by four horizontal hanging bars, was fabricated (0.38 mW/cm², 43.4 µm/s) from Macro-DTC-01/PETA (50/50, wt%) (Figure 3a). Then, functionalization was performed on one of the hanging bars using TMPTA with fluorescein (1.78 mW/cm 2 , 14.3 µm/s) (Figure 3b).

Intensity-mediated surface modification

To further explore the conditions for optimal surface modification, several groups of lines (Figure 4a) made from pure TMPTA without Macro-DTC were fabricated on the assembled squares. The laser intensities used for the first to fourth square were 0.44, 0.85, 0.99 and 1.15 mW/cm², respectively. The first square corresponds to the first polymerized layer, while the second and further squares correspond to repolymerized layers. The distance between two adjacent lines is 3 µm. The laser intensity was increased gradually between lines, while keeping printing speed constant at 14.3 µm/s. In Figure 4b, lines' heights, measured by AFM (Figure 4c), are plotted against laser intensity. The heights increase linearly with laser intensity, except for the lines on the first layer at intensities above 2.1 mW/cm². From Figure 4b, one can also notice that the heights of repolymerized lines clearly decrease with an increasing number of consecutive layers. For instance, while for the lines on the top of the first layer the height is ranging from 100 nm to 1000 nm in function of laser intensity, for the lines on the top of the fourth layer, the height is only tunable between 15 to 220 nm for the same range of laser intensities. Similar trends are observed for lines' widths against laser intensity (Figure S14).

These observations can probably be explained by the formation of dead polymer chains during the whole process. While this indicates that the number of possible successive repolymerizations is certainly limited, this set of data nicely demonstrates that several consecutive post-functionalizations or modifications can be realized in a controlled manner by varying laser intensity. Indeed, the heights of these structures can be tuned accurately from nano-up to micro-scale which is very important in surface functionalization and surface patterning applications. [19] For all fabrications, the printing speed was fixed to 14.3 µm/s.

Conclusion

In summary, we have applied a macro-photoiniferter synthesized by photo-controlled RAFT polymerization for the creation and reconfiguration of 3D microstructures by DLW in presence of air. The simple formula system (Macro-DTC plus one resin) makes sample preparation easy and greatly reduces migration problems during fabrication and in structures. Remarkably, this Macro-DTC is suitable for the fabrication of arbitrary 3D structures with sub-500 nm resolution by DLW, while its retention of RAFT functionalities throughout successive functionalization steps allows the versatile reconfiguration of surfaces across multiple layers and using different materials, e.g. temperature-sensitive polymers. By adapting light intensity and materials, welldefined structures with controlled dimensions and tailored properties can be patterned. The combination of this photo-controlled RAFT polymerization process with 3D DLW provides a strategy to fabricate and reconfigure 3D microstructures by affording access to new functional and stimuli-responsive materials for 4D and multi-material micro-printing.

Figure 1 .

 1 Figure 1. a) The process to fabricate 3D micro-objects using DLW. b) SEM images of 3D microstructures made from PETA and Macro-DTC-01 (50:50, wt%) using a printing speed of 43.4 µm/s and a laser intensity of 0.38 mW/cm². The red circle is used to point to the zoom-in image to see the resolution.

  Finally, it has to be mentioned that no polymerization or repolymerization was observed in pure TPMTA or in absence of Macro-DTC in the conditions of excitation for fabrication and functionalization (FigureS12and S13). These control experiments confirm the crucial role played by Macro-DTC in allowing fabrication and functionalization of microstructures. Based on these observations, one can assume that the multi-layer growth process is induced by photo-mediated RAFT polymerization.

Figure 2 .

 2 Figure 2. a) The process to modify the surface of microstructures using DLW. b) AFM and SEM images of µRAFT at the top layer made from TMPTA without photoinitiator. The first layer is a square made from PETA and Macro-DTC-01 (50:50, wt%), other squares (repolymerized layers) are made from TMPTA alone. c) The evolution of PNIPAM honeycomb microstructures on the square at different temperatures. 1) The schema to illustrate the change

Fluorescein

  has been added to the TMPTA to allow visualization of the functionalized part via fluorescence lifetime imaging (FLIM). It has to be noted that similar structures were also btained without fluorescein. As shown in the inset of the SEM image after functionalization (Figure3a), the repolymerized structure is localized on the top of one hanging bar, demonstrating the exquisite control in the spatial modification of the polymerized microstructure. The height of the functionalized area was estimated to 720 nm from the inset SEM image.

Figure 3 .

 3 Figure 3. a) SEM images (tilt view) of bridges made from Macro-DTC-01/PETA (50:50, wt%) without (left) and with (right) functionalization from TMPTA with fluorescein. Inset: zoom on the functionalized area. b) FLIM image (top view) of the functionalized bridge.

Figure 4 .

 4 Figure 4. a) Layout of assembled square layers with lines. b) The height of lines at the first to fourth layer made from TMPTA alone as a function of laser intensity. The first layer is a square made from PETA and Macro-DTC-01 (50:50, wt%). c) AFM images of these lines and profiles.

Table 1 .

 1 The conversions of BA, Mn and Mw/Mn of Macro-DTCs and their conditions of synthesis under blue LED irradiation.

	Macro-DTC	λLED (nm)

ILED (mW/cm²) [BA]/[DTC] Conversion of BA from 1 H NMR (%) a) determined by SEC in THF eluent.

  Thus, single-voxel structures were prepared from formulations based on Macro-DTC/PETA (50/50, wt%, Table2) at different laser intensities, while keeping a constant exposure time (10 ms). Then, their heights were measured subsequently from SEM images. Similar experiments were conducted with PETA/Macro-DTC-01 to 05, and the voxel heights were plotted as a function of laser intensities (FigureS6). As expected, higher intensity leads to increased voxel dimensions and consequently to lower resolution. Interestingly, Macro-

DTC-01/PETA exhibits the broader window processability by allowing creation of voxel from 0.25 mW/cm² to 2.3 mW/cm², leading to heights from 2.5 to 9 µm. To confirm the better printability provided by Macro-DTC-01/PETA, woodpile microstructures were made from all formulations from Table

2

with a laser intensity close to the laser intensity threshold Ith of each formulation and a printing speed of 14.3 µm/s. The formulation Macro-DTC-01/PETA clearly leads to the best performance in terms of structuration (Figure

Table 2 .

 2 Compositions and reactivities of formulations based on Macro-DTC/PETA.

	Macro-DTC/PETA	Macro-DTC	n of Macro-DTC	I th	I damage
	(50/50 wt%)	(g)	(µmol)	(mW/cm²)	(mW/cm²)
	Macro-DTC-01/PETA	0.262	38.0	0.248	2.3
	Macro-DTC-02/PETA	0.252	21.2	0.446	2.3
	Macro-DTC-03/PETA	0.245	16.8	0.550	2.3
	Macro DTC-04/PETA	0.246	13.1	0.99	2.3
	Macro-DTC-05/PETA	0.283	10.6	1.58	2.3

Acknowledgements

The authors thank the Agence Nationale de la recherche (Project ANR 2PhotonInsight: ANR-16-CE08-347 0020) and Institut Carnot MICA for financial support. B.G, Dr. J.P. and Dr. A.S. thank the IS2M for an emergent grant. This work was also partially supported by an Institutional Research Grant (MIPPI4D) from the Région Grand Est. The authors thank Tatiana Petithory and Ludovic Josien for their help in fluorescence microscopy and SEM, respectively.

The authors declare no competing financial interests.

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Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.


Narayan Ch Jana

Xing-Hui Qi

Paula Brandão

Corine Mathonière
email: corine.mathoniere@u-bordeaux.fr

Anangamohan Panja
email: ampanja@yahoo.co.in

Impact of Positional Isomers on the Selective Isolation of cis-trans Isomers in Cobaltdioxolene Chemistry and Solvation Effects on the Valence Tautomerism in the Solid State

(3-NH 2 py) 2 ]•DMF (2) and trans- [Co(3,5dbcat)(3,5-dbsq)(4-NH 2 py) 2 ]•2DMF (3) (3,5-dbcat 2-and 3,5-dbsq •-stand for 3,5-di-tertbutyl-catacholate and 3,5-di-tert-butyl-semiquinonate, respectively), derived from a redoxactive o-dioxolene ligand in presence of 4-ethylpyridine (4-Etpy), 3-aminopyridine (3-NH 2 py), and 4-aminopyridine (4-NH 2 py), respectively, have been synthesized and investigated with a view to examine possible influence of pyridine derivatives and solvation on the valence tautomeric (VT) process. Single crystal X-ray diffraction data for all compounds at room temperature suggest Co(III)(3,5-dbcat)(3,5-dbsq) charge distribution in these complexes. Further insight into the crystal structures discloses the diverse noncovalent interactions offered by the isomers of aminopyridine in 2 and 3, leading to the first example of selective isolation of cis and trans isomers in cobalt-dioxolene chemistry induced by the positional isomers of the ancillary ligands. Variable temperature magnetic susceptibility data for all compounds between 2 and 300 K are consistent with the structural studies. At elevated temperatures, complex 1 exhibits a VT interconversion from low spin Co(III)-(3,5-dbcat)(3,5dbsq) to high spin Co(II)-(3,5-dbsq)(3,5-dbsq), triggered by the loss of lattice solvent molecules, while a partial interconversion is observed for complex 3 even heating up to 430 K. The present report overall highlights the impact of positional isomers on the selective isolation of cis-trans isomers in cobalt-dioxolene chemistry and solvation effects on the valence tautomerism in the solid state.

INTRODUCTION

The molecular materials that can be reversibly switched between distinguishable states, are of great interest in the scientific community. The molecular switchable compounds are particularly attractive because of the feasibility of fine-tuning in the chemical architecture which allows tailor-made applications, such as sensors, information storage devices, or in the field of molecular electronics. [START_REF] Kumar | Emerging trends in spin crossover (SCO) based functional materials and devices[END_REF][START_REF] Sato | Dynamic molecular crystals with switchable physical properties[END_REF][START_REF] Calzolari | Complex Materials for Molecular Spintronics Applications: Cobalt Bis(dioxolene) Valence Tautomers, from Molecules to Polymers[END_REF][START_REF] Dei | Cobalt-Dioxolene Redox Isomers: Potential Spintronic Devices[END_REF][START_REF] Sedo | Catechol-Based Biomimetic Functional Materials[END_REF][START_REF] Wang | Spin crossover and valence tautomerism conductors[END_REF][START_REF] Rajput | Valence tautomerism and delocalization in transition metal complexes of o-amidophenolates and other redoxactive ligands. Some recent results[END_REF][START_REF] Ferrando-Soria | Molecular magnetism, quo vadis? A historical perspective from a coordination chemist viewpoint[END_REF][START_REF] Guionneau | Towards Spin Crossover Applications[END_REF][START_REF] Sato | Control of Magnetic Properties through External Stimuli[END_REF] Molecule-based systems that exhibit an intramolecular electron transfer (IET) phenomenon can be switched between isomers with different electronic states. [START_REF] Hendrickson | Valence Tautomeric Transition Metal Complexes[END_REF][START_REF] Dei | Quinonoid Metal Complexes: Toward Molecular Switches[END_REF][START_REF] Evangelio | Valence Tautomerism: New Challenges for Electroactive Ligands[END_REF][START_REF] Nadurata | Switching metal complexes via intramolecular electron transfer: connections with solvatochromism[END_REF][START_REF] Li | Magnetic and Optical Bistability Driven by Thermally and Photoinduced Intramolecular Electron Transfer in a Molecular Cobalt-Iron Prussian Blue Analogue[END_REF][START_REF] Aguilà | Switchable Fe/Co Prussian blue networks and molecular analogues[END_REF][START_REF] Mathonière | Metal-to-Metal Electron Transfer: A Powerful Tool for the Design of Switchable Coordination Compounds[END_REF] Valence tautomerism is also a specific phenomenon that involves a stimulated and reversible intramolecular electron transfer between a transition metal and a redox-active ligand. [START_REF] Tezgerevska | Valence tautomerism in metal complexes: Stimulated and reversible intramolecular electron transfer between metal centers and organic ligands[END_REF][START_REF] Fleming | Valence tautomerism in a cobalt-verdazyl coordination compound[END_REF][START_REF] Li | Sophisticated Construction of Electronically Labile Materials: A Neutral, Radical-Rich, Cobalt Valence Tautomeric Triangle[END_REF] Octahedral cobalt complexes with redox non-innocent dioxolene ligand are so far the most popular family of valence tautomeric (VT) complexes particularly those involves 3,5-di-tert-butyl-dioxolene (3,5-dbdiox) and nitrogen-donor ancillary ligands. [START_REF] Boskovic | Valence Tautomeric Transitions in Cobalt-dioxolene Complexes[END_REF][START_REF] Gransbury | DFT Prediction and Experimental Investigation of Valence Tautomerism in Cobalt-Dioxolene Complexes[END_REF][START_REF] Tezgerevska | Valence tautomerism and spin crossover in pyridinophane-cobaltdioxolene complexes: an experimental and computational study[END_REF][START_REF] Drath | One Dimensional Chain and Ribbon Cobalt-Dioxolene Coordination Polymers: A New Valence Tautomeric Compound[END_REF][START_REF] Drath | Valence Tautomerism in One-Dimensional Coordination Polymers[END_REF][START_REF] Beni | Tuning the Charge Distribution and Photoswitchable Properties of Cobalt-Dioxolene Complexes by Using Molecular Techniques[END_REF][START_REF] Pour | Valence tautomerism in catecholato cobalt Bis(phenolate) diamine complexes as models for Enzyme-substrate adducts of catechol dioxygenases[END_REF][START_REF] Wu | Macroscopic Polarization Change via Electron Transfer in a Valence Tautomeric Cobalt Complex[END_REF][START_REF] Alley | Redox Activity and Two-Step Valence Tautomerism in a Family of Dinuclear Cobalt Complexes with a Spiroconjugated Bis(dioxolene) Ligand[END_REF][START_REF] Bubnov | Lattice-Modulated Phase Transition Coupled with Redox-Isomeric Interconversion of o-Semiquinone-Catecholato into Bis(o-semiquinonato) Cobalt Complexes[END_REF][START_REF] Kanegawa | Preparation and Valence Tautomeric Behavior of a Cobalt-Dioxolene Complex with a New TTF-functionalized Phenanthroline Ligand[END_REF][START_REF] Dai | A Switchable Complex Ligand Exhibiting Photoinduced Valence Tautomerism[END_REF][START_REF] Ribeiro | Cobalt lawsone complexes: searching for new valence tautomers[END_REF] In these systems, an intramolecular electron transfer occurs between two redox isomers that exhibit low-spin (ls) Co(III)-catecholate (cat 2-) at low temperature, while the high-spin (hs) Co(II)-semiquinonate (sq•-) state is favored at higher temperatures. [START_REF] Hendrickson | Valence Tautomeric Transition Metal Complexes[END_REF][START_REF] Gransbury | Understanding the Origin of One-or Two-Step Valence Tautomeric Transitions in Bis(dioxolene)-Bridged Dinuclear Cobalt Complexes[END_REF] Thermally and optically induced VT transitions are the most common, while influence of pressure, [START_REF] Roux | Pressure-Induced Valence Tautomerism in Cobalt o-Quinone Complexes: An X-ray Absorption Study of the Low-Spin [Co III (3,5-DTBSQ)(3,5-DTBCat)(phen)] to High-Spin [Co II (3,5-DTBSQ) 2 (phen)] Interconversion[END_REF] soft X-ray [START_REF] Poneti | Soft-X-ray-Induced Redox Isomerism in a Cobalt Dioxolene Complex[END_REF][START_REF] Liang | Charge and Spin-State Characterization of Cobalt Bis(o-dioxolene) Valence Tautomers Using Co Kβ X-ray Emission and L-Edge X-ray Absorption Spectroscopies[END_REF][START_REF] Francisco | Hard X-ray-Induced Valence Tautomeric Interconversion in Cobalto-Dioxolene Complexes[END_REF] and magnetic field [START_REF] Markevtsev | Field-induced spin phase transition in a Co complex[END_REF] on VT transitions have also been reported but in less extent.

After the discovery of the first VT cobalt-dioxolene complex, [START_REF] Buchanan | Tautomeric catecholate-semiquinone interconversion via metal-ligand electron transfer. Structural, spectral, and magnetic properties of (3,5di-tert-butylcatecholato)(3,5-di-tert-butylsemiquinone)(bipyridyl)cobalt(III), a complex containing mixed-valence organic ligands[END_REF] [Co(III)(3,5dbcat)(3,5-dbsq)(2,2-bpy)] (where 2,2-bpy is 2,2-bipyridine), most of the other VT bis(odioxolene)cobalt complexes have been reported by replacing 2,2-bpy ligand with other bidentate N,N-donor ancillary ligands. Such complexes always adopt the cis orientation of the N-donor atoms due to the structural restraints. By employing monodentate N-donor ligands in place of bidentate N-donor ligands, the trans arrangement of the ancillary ligands can be achieved and several such reports are available in the literature. [START_REF] Schmidt | Goldilocks Effect in Magnetic Bistability: Remote Substituent Modulation and Lattice Control of Photoinduced Valence Tautomerism and Light-Induced Thermal Hysteresis[END_REF][START_REF] Schmidt | Magnetic Bistability in a Cobalt Bis(dioxolene) Complex: Long-Lived Photoinduced Valence Tautomerism[END_REF][START_REF] Mulyana | Solvation effects on the valence tautomeric transition of a cobalt complex in the solid state[END_REF] Most of these complexes exhibit VT transitions in the solid state where intermolecular interactions between the chemical moieties, packing effect and solvent effect influence the characteristics of the electronic transition, inducing abruptness and bistability/hysteresis that will enable application in devices. In this regard, Shultz et al. used different derivatives of monodentate pyridyl N-donor ligand to coordinate [Co (3,5-dbdiox) 2 ] units, which ultimately resulted trans isomer in all the cases. [START_REF] Schmidt | Goldilocks Effect in Magnetic Bistability: Remote Substituent Modulation and Lattice Control of Photoinduced Valence Tautomerism and Light-Induced Thermal Hysteresis[END_REF][START_REF] Schmidt | Magnetic Bistability in a Cobalt Bis(dioxolene) Complex: Long-Lived Photoinduced Valence Tautomerism[END_REF] More recently, Boskovic et al. reported trans form of different solvated VT complexes using 3,5-dbdiox and pyridine ligand. [START_REF] Mulyana | Solvation effects on the valence tautomeric transition of a cobalt complex in the solid state[END_REF] Recently, we have explored how the crystal packing governed by different kinds of noncovalent forces affects the valence tautomeric equilibrium in cobalt-dioxolene system. [START_REF] Panja | Solvent-Triggered Cis/Trans Isomerism in Cobalt Dioxolene Chemistry: Distinguishing Effects of Packing on Valence Tautomerism[END_REF] Moreover, we successfully isolated both cis and trans isomers for the first time in cobalt-dioxolene chemistry and found the crucial role of the solvent polarity in the isolation of these isomers. We have also realized that although the energetics of cis and trans isomers are comparable, the non-covalent interaction in the solid state played an important role for isolation of the isomeric compounds. Apart from that these geometrical differences, different orientations of the ligand-based magnetic orbitals in these isomers may bring distinct electronic properties and hence influences the VT phenomenon. Therefore, it still remains a challenge to synthesize such isomeric compounds in the cobalt-dioxolene chemistry.

As a part of our ongoing research on the transition metal dioxolene chemistry, [START_REF] Panja | Solvent-Triggered Cis/Trans Isomerism in Cobalt Dioxolene Chemistry: Distinguishing Effects of Packing on Valence Tautomerism[END_REF][START_REF] Panja | Unusual structural features in tetrabromocatechol-chelated dinuclear manganese(III) complex: Synthesis, electrochemistry and thermally induced valence tautomerism[END_REF][START_REF] Panja | A series of tetrabromocatecholate chelated cobalt(III) complexes with various N-donor ancillary ligands: syntheses, crystal structures, co-crystallization, thermally induced valence tautomerism and electrochemical studies[END_REF][START_REF] Panja | Valence tautomerism induced nucleophilic ipso substitution in a coordinated tetrabromocatecholate ligand and diverse catalytic activity mimicking the function of phenoxazinone synthase[END_REF][START_REF] Jana | Tuning the geometry and biomimetic catalytic activity of manganese(III)-tetrabromocatecholate based robust platforms by introducing substitution at pyridine[END_REF][START_REF] Jana | Influence of ancillary ligands on preferential geometry and biomimetic catalytic activity in manganese(III)catecholate systems: A combined experimental and theoretical study[END_REF][START_REF] Panja | Introducing Supramolecular Interactions into Robust Bis(tetrabromocatecholate) Chelated Manganese(III) Systems and Biomimetic Catalytic Activity[END_REF][START_REF] Panja | Insight into the origin of catechol oxidase activity in a rare Mn II /Mn III mixed valence ion pair complex: An account of comparative biomimetic catalytic study[END_REF][START_REF] Panja | Valence -Tautomerism -Driven Aromatic Nucleophilic Substitution in Cobalt-Bound Tetrabromocatecholate Ligands -Influence of Positive Charge at the Ligand Backbone on Phenoxazinone Synthase Activity[END_REF] here we aim to develop dioxolene-based complexes with varied geometrical isomers around the metal center by suitable choice of solvent and/or different N-donor ligands. As it has been realized from our previous report, role of the noncovalent interactions in isolation of the geometrical isomers in cobalt-dioxolene chemistry, introduction of the positional isomers of the pyridine derivative may bring different noncovalent interactions, leading to isolation of different products. [START_REF] Panja | Solvent-Triggered Cis/Trans Isomerism in Cobalt Dioxolene Chemistry: Distinguishing Effects of Packing on Valence Tautomerism[END_REF] Therefore, in the present work, we employed two positional isomers of aminopyridine, where the pendent amine group has a potential to establish at least hydrogen bonding interaction in different directions in the solid state. Accordingly, herein, we report the synthesis and structural characterization of three new mononuclear cobalt compounds, trans-[Co (3,5-dbcat) (3,5-dbsq)

(4-Etpy) 2 ]•CH 3 CN (1), cis-[Co(3,5-dbcat)(3,5-dbsq)(3-

EXPERIMENTAL SECTION

Materials and physical methods

Cobalt(II) nitrate hexahydrate was purchased from commercial sources and was used as received. 3,5-di-tert-butyl-catechol (3,5-H 2 dbcat) and 4-Ethylpyridine were purchased from Sigma-Aldrich. 3-aminopyridne and 4-aminopyridine were purchased from SRL Pvt. Ltd., India. Solvents were of analytical grade and used without further purification.

Elemental analyses (C, H, N) were performed by Perkin-Elmer 240C elemental analyzer.

IR spectra of all the complexes at room temperature and 1 after heating at 380 K followed by cooling the sample back to room temperature were measured using a Thermo Scientific Nicolet iS5 FTIR spectrometer with a universal ATR sampling accessory from 4000 to 400 cm 1 . An Agilent Diode array spectrophotometer (Agilent Carry 60) with a 1-cm-path-length quartz-cell was used to record absorption spectra of the samples. The thermogravimetric analyses (TGA) were performed on a Perkin-15 Elmer Pyris Diamond TGA/DTA analyzer in a dynamic nitrogen atmosphere with a heating rate of 5°C min -1 . The X-band EPR spectra of as synthesized complex 1-3 and after desolvation (for 1) were measured in the solid state using a JEOL JES-FA 200 instrument at room temperature. Room temperature powder X-ray diffraction (PXRD) measurements were carried out using a Rigaku Smart Lab X-ray diffractometer with a Cu-Kα source. 53 The structures were solved using direct methods within SHELXS and the refinement of the structures was carried out using a least squares method based on F 2 with SHELXL-2013. 54 Anisotropic thermal parameters were applied to refine all the non-hydrogen atoms. Hydrogen atoms attached to carbon were placed at geometrically idealized positions with individual isotropic thermal factors equal to 1.2 and 1.5 times of those to which they are attached. Hydrogen atoms attached to nitrogen atoms (N39 and N46 from complex 2 and N23 from complex 3) were located on the difference Fourier map and isotropically treated. The detailed crystallographic data together with refinement details are presented in Table 1.

Synthesis of trans-[Co(3,5-dbcat)(3,5-dbsq)(4-Etpy)

Magnetometry

Magnetic susceptibility measurements were performed on a Quantum Design MPMS-XL SQUID magnetometer working at temperatures between 1.8 K and 380 K and dc magnetic field ranging from -7 and +7 T and on a Microsense VSM (Vibrating Sample Magnetometer) EZ7 working at temperatures between 100 K and 1000 K and dc magnetic field ranging from Goodness-of-fit on

F 2 1.026 1.025 1.041 Final R indices [I≥2(I)] R 1 = 0.0435 wR 2 = 0.1136 R 1 = 0.0430 wR 2 = 0.1050 R 1 = 0.0413 wR 2 = 0.1068 R indices (all data) R 1 = 0.0589 wR 2 = 0.1255 R 1 = 0.0700 wR 2 = 0.1203 R 1 = 0.0557 wR 2 = 0.1178
Largest diff. peak/hole (e Å -3 ) 0.589/ -0.382 0.374/ -0.314 0.529/ -0.238

Optical reflectivity measurements

The surface reflectivity measurements have been performed with a home-built system, operating between 10 and 300 K and in a spectrometric range from 400-1000 nm. A halogentungsten light source (Leica CLS 150 XD tungsten halogen source adjustable from 0.05 mW cm -2 to 1 W cm -2 ) was used as the spectroscopic light. The measurements were calibrated by barium sulphate as reference sample. With this reflectivity technique, we plot the absolute reflectivity (AR) at a specific wavelength  as

AR() = (R sample () -R dark ()) / (R ref ()- R dark ()
). The corresponding spectra AR = f() can be viewed as a mirror image of the absorbance spectra, meaning when the sample absorbs efficiently (or weakly) the light, a low (or high, respectively) value of AR is measured. As the samples are potentially photosensitive, the light exposure time was minimized during the experiments keeping the samples in the dark except during the spectra measurements when white light is shined on the sample surface (P = 0.08 mW cm -2 ). For all the excitation/de-excitation experiments performed at 10 K, the sample was initially placed at this temperature keeping the sample in the dark to avoid any excitation. Heating and cooling measurements were carried out at 4 K min -1 . For white light irradiation, the source described above was used, but in a continuous manner with a power of 0.08 mW cm

UV-vis spectral study

Electronic absorption spectrum of complex 1 was recorded in acetonitrile whereas the spectra of 2 and 3 were measured in DMF solvent to check the nature of the compounds in the solvents from which they synthesized (Figure S1a). UV-vis spectra of 2 and 3 were also recorded in acetonitrile with no noticeable change in electronic transitions (Figure 1, left).

UV-vis spectral study proves stability of cis-isomer (2) over a period of 1 day even at measurable temperature range (Figure S2). UV-vis spectra in different solvents further suggest that both 2 and 3 are thermodynamically controlled products and their formation is not dependent of the solvent polarity (Figure S1b). Here, involvement of the pendant amine groups of aminopyridines in different non-covalent interactions could play important roles behind the selective isolation of cis and trans isomeric complexes of 2 and 3, respectively (vide infra). In all the complexes, shoulders/bands at around 580 nm band is associated with the ls-Co(III)-(cat)(sq) tautomer, and the ∼770 nm band is characteristic of the hs-Co(II)-(sq) 2 tautomer in agreement with the previous studies on similar systems as well as recently reported TD-DFT studies on closely related systems. [START_REF] Gransbury | DFT Prediction and Experimental Investigation of Valence Tautomerism in Cobalt-Dioxolene Complexes[END_REF][START_REF] Tezgerevska | Valence tautomerism and spin crossover in pyridinophane-cobaltdioxolene complexes: an experimental and computational study[END_REF][START_REF] Drath | One Dimensional Chain and Ribbon Cobalt-Dioxolene Coordination Polymers: A New Valence Tautomeric Compound[END_REF][START_REF] Janetzki | A Convenient DFT-Based Strategy for Predicting Transition Temperatures of Valence Tautomeric Molecular Switches[END_REF] The bands at around 400 nm can tentatively be assigned as a ligand-based n-* transition. The band at around 580 nm is due to the localized semiquinonate -* transition in ls-Co(III)-(cat)(sq) tautomer, [START_REF] Janetzki | A Convenient DFT-Based Strategy for Predicting Transition Temperatures of Valence Tautomeric Molecular Switches[END_REF] whereas the bands at around 750 nm may be assigned as the metal-to-ligand charge transfer (MLCT) band from the e g orbital of Co(II) center to * orbital of 3,5-dbsq •-in the hs-Co(II)-(sq) 2 species.

The spectral features nicely agree with the presence of both ls-Co(III)-catecholate and hs-Co(II)-semiquinonate tautomers in solution at room temperature for these compounds. ] isomer as reported earlier. [START_REF] Schmidt | Goldilocks Effect in Magnetic Bistability: Remote Substituent Modulation and Lattice Control of Photoinduced Valence Tautomerism and Light-Induced Thermal Hysteresis[END_REF][START_REF] Adams | Bistability in the [CoII(semiquinonate)2] to [CoIII(catecholate)(semiquinonate)] valence-tautomeric conversion[END_REF] The increase of intensity of the band at 750 nm is associated with the MLCT transition from the e g orbital of Co(II) center to 3,5dbsq •-in the high spin Co(II) isomer. Finally, the appearance of isosbestic points at 457 and 650 nm is consistent with the presence of at least two different species, originating from the VT process. Thus complex 1 was found to undergo in solution a thermally induced VT process through an electron transfer from the 3,5-dbcat ligand to Co(III) ion to generate high spin Co(II) isomer. [START_REF] Schmidt | Goldilocks Effect in Magnetic Bistability: Remote Substituent Modulation and Lattice Control of Photoinduced Valence Tautomerism and Light-Induced Thermal Hysteresis[END_REF][START_REF] Adams | Bistability in the [CoII(semiquinonate)2] to [CoIII(catecholate)(semiquinonate)] valence-tautomeric conversion[END_REF] Complexes 2 and 3 also exbibit minor changes of the spectral bands in the range 400-800 nm at different temperatures, suggesting that VT process is less active at around room temperature (Figure S3).

IR spectroscopy

The oxidation states of the redox active dioxolene ligands are usually characterized by IR spectroscopy. In the solid state, the C-O stretching modes, such as the intense absorption bands at ca. 1600-1675 cm -1 for quinone, ca. 1400-1550 cm -1 for semiquinone, and ca. 1250 cm -1 for catecholate, are especially useful to distinguish the oxidation states of such ligands.

In the IR spectra of these complexes (Figures S4-S6), the bands at ca. 1480 cm -1 and 1250-1300 cm -1 can be assigned to the C-C ring and C-O stretching of the catecholate ligands, respectively, while the bands at around 1440 and 1580 cm -1 are characteristic of semiquinonate ligands, consistent with the reported results on similar complexes. [START_REF] Gransbury | DFT Prediction and Experimental Investigation of Valence Tautomerism in Cobalt-Dioxolene Complexes[END_REF][START_REF] Tezgerevska | Valence tautomerism and spin crossover in pyridinophane-cobaltdioxolene complexes: an experimental and computational study[END_REF][START_REF] Drath | One Dimensional Chain and Ribbon Cobalt-Dioxolene Coordination Polymers: A New Valence Tautomeric Compound[END_REF][START_REF] Drath | Valence Tautomerism in One-Dimensional Coordination Polymers[END_REF][START_REF] Mulyana | Solvation effects on the valence tautomeric transition of a cobalt complex in the solid state[END_REF][START_REF] Lynch | Mixed-valence semiquinonecatecholate-iron complexes[END_REF] A characteristic stretching band associated with the coordinated pyridine molecules are observed in between 450 and 650 cm -1 . [START_REF] Mulyana | Solvation effects on the valence tautomeric transition of a cobalt complex in the solid state[END_REF] The IR spectra of all the complexes at room temperature also display bands over the range 2850 to 2960 cm -1 , typical for the stretching frequencies of the C-H bond of the tert-butyl groups. [START_REF] Mulyana | Solvation effects on the valence tautomeric transition of a cobalt complex in the solid state[END_REF] Based on these IR data, we deduce that each complex contains two o-dioxolene forms (i. e. semiquinonate and catecholate) of the ligands at room temperature. These conclusions are confirmed by the EPR spectroscopic studies at room temperature of these samples that reveal the signal of a radical at g = 2.00 (Figure S7), further indicating the existence of a radical species, 3,5-dbsq •-, in the solid state.

Structural Study

The neutral complex 1 was isolated from an acetonitrile/water solvent mixture and X-ray structural analysis at 296 K reveals that it crystalized in the triclinic space group Pī in which the asymmetric unit contains one half of each of two crystallographically independent complex molecules and one solvent acetonitrile molecule. A representative complex molecule along with selected atom numbering scheme is depicted in Figure 2, and important bond distances and bond angles are given in Table 2 and Table S1, respectively. The metal center in each crystallographically independent complex molecule sits on an inversion center and the coordination environments around the metal centers in both complex molecules are eventually identical with general formula [Co(III)(3,5-dbcat)(3,5-dbsq)(4-Etpy) 2 ]. The geometry of the metal centers in these complex molecules can be best described as a slightly distorted octahedral structure being connected with two o-dioxolene ligands and two pyridyl groups in trans configuration. [START_REF] Schmidt | Goldilocks Effect in Magnetic Bistability: Remote Substituent Modulation and Lattice Control of Photoinduced Valence Tautomerism and Light-Induced Thermal Hysteresis[END_REF][START_REF] Schmidt | Magnetic Bistability in a Cobalt Bis(dioxolene) Complex: Long-Lived Photoinduced Valence Tautomerism[END_REF][START_REF] Mulyana | Solvation effects on the valence tautomeric transition of a cobalt complex in the solid state[END_REF] 6) Å, respectively. These values lie in between typical distances for semiquinonate and catecholate ligands, consistent with disorder of the 3,5-dbsq •-and 3,5-dbcat 2-redox states of the ligand over the two o-dioxolene positions in both complex molecules. [START_REF] Schmidt | Goldilocks Effect in Magnetic Bistability: Remote Substituent Modulation and Lattice Control of Photoinduced Valence Tautomerism and Light-Induced Thermal Hysteresis[END_REF][START_REF] Schmidt | Magnetic Bistability in a Cobalt Bis(dioxolene) Complex: Long-Lived Photoinduced Valence Tautomerism[END_REF][START_REF] Mulyana | Solvation effects on the valence tautomeric transition of a cobalt complex in the solid state[END_REF][START_REF] Adams | Pulsed Laser Photolysis and Thermodynamics Studies of Intramolecular Electron Transfer in Valence Tautomeric Cobalt o-Quinone Complexes[END_REF][START_REF] Paquette | Opto-Spintronics: Photoisomerization-Induced Spin State Switching at 300 K in Photochrome Cobalt-Dioxolene Thin Films[END_REF][START_REF] Mörtel | A valence tautomeric cobalt-dioxolene complex with an anchoring group for prospective chemical grafting to metal oxides[END_REF] The "metrical oxidation state" (MOS) as proposed by

Brown [START_REF] Brown | Metrical Oxidation States of 2-Amidophenoxide and Catecholate Ligands: Structural Signatures of Metal-Ligand π Bonding in Potentially Noninnocent Ligands[END_REF] was calculated considering all the C-O and C-C bond distances of o-dioxolene ligand and found to be -1.46 and -1.50 for two crystallographically independent dioxolene ligands present in the asymmetric unit of 1, that further supports the intermediate oxidation state of the redox active o-dioxolene ligand in these complex molecules. The Bond Valence Sum (BVS) calculations were further performed to determine the oxidation state of cobalt using the following general expression as proposed by Brown and Altermatt: [START_REF] Brown | Bond-Valence Parameters Obtained from a Systematic Analysis of the Inorganic Crystal Structure Database[END_REF]

V i = Σs ij = Σ exp[(r 0 -r ij )/b],
where r 0 and r ij are empirical and crystallographic bond distances between atoms i and j, respectively; b is the "universal parameter" and is set equal to 0.37; s ij is the valence between atoms i and j; V i is the sum of all valences s ij of a coordination sphere around the metal ion, and its value is the approximation of the formal oxidation state. [START_REF] Brown | Chemical and Steric Constraints in Inorganic Solids[END_REF] The BVS values for two crystallographically independent complex molecules in 1 are calculated to be 3.243.26 and 3.24 considering empirical bond distances either of +II or +III oxidation states of cobalt, respectively, further confirming the +III oxidation state of cobalt at room temperature in 1. From the detailed analysis of the crystal structure, it is found that a T-shape C-H•••π interaction is present in the molecule to establish the solid-state stability of the complex (Figure 3). The π•••π interaction between the pyridyl ring provides further stability in the solid state (Figure 3). sitting on an inversion center, and one DMF molecule are present in the asymmetric unit in 3.

The molecular pictograms along with atom numbering scheme for 2 and 3 are depicted in Figure 4, and the bond parameters can be found in Table 2 and Table S1. The coordination environment around the metal centers is identical with that found in 1 and consists of N ligands. [START_REF] Schmidt | Goldilocks Effect in Magnetic Bistability: Remote Substituent Modulation and Lattice Control of Photoinduced Valence Tautomerism and Light-Induced Thermal Hysteresis[END_REF][START_REF] Schmidt | Magnetic Bistability in a Cobalt Bis(dioxolene) Complex: Long-Lived Photoinduced Valence Tautomerism[END_REF][START_REF] Mulyana | Solvation effects on the valence tautomeric transition of a cobalt complex in the solid state[END_REF][START_REF] Adams | Pulsed Laser Photolysis and Thermodynamics Studies of Intramolecular Electron Transfer in Valence Tautomeric Cobalt o-Quinone Complexes[END_REF][START_REF] Paquette | Opto-Spintronics: Photoisomerization-Induced Spin State Switching at 300 K in Photochrome Cobalt-Dioxolene Thin Films[END_REF][START_REF] Mörtel | A valence tautomeric cobalt-dioxolene complex with an anchoring group for prospective chemical grafting to metal oxides[END_REF] Moreover, the MOS values in 2 of -1.20 and -1.91 are apparent for the isolated semiquinone and catecholate ligands, respectively [START_REF] Panja | Valence tautomerism induced nucleophilic ipso substitution in a coordinated tetrabromocatecholate ligand and diverse catalytic activity mimicking the function of phenoxazinone synthase[END_REF] and the MOS value is found to be -1.50 in 3, suggesting the intermediate oxidation state of the o-dioxolene ligand in this complex. [START_REF] Brown | Metrical Oxidation States of 2-Amidophenoxide and Catecholate Ligands: Structural Signatures of Metal-Ligand π Bonding in Potentially Noninnocent Ligands[END_REF] The BVS values of 2 and 3 considering either the empirical bond parameters of Co(II) and Co(III) are found to be 3.17

Magnetic properties

The magnetic properties of 1-3 have been studied using a SQUID magnetometer working up to 380 K. The first measurements have been performed below 300 K to avoid the solvent loss for the three compounds (Figure 6), and then cycles at higher temperatures (not more than 400 K) have been studied. As shown in Figure 6, the three compounds exhibit similar magnetic properties below 300 K.

They all have a T product around 0.4 cm 3 mol -1 K at 300 K. This value is in agreement with the structural data of all three compounds that show discrete molecules of [Co III (3,5dbcat)(3,5-dbsq)(L) 2 ] where only the radical ligand 3,5-dbsq •-has a spin equal to ½. Based on the TGA results these compounds are stable up to 380 K for 1 and 2 and 430 K for 3 (Figure S8) allowing a magnetic study up to 380 K, the maximum available temperature with our squid magnetometer. Although compounds 2 and 3 do not show significant modifications of their magnetic properties during a temperature cycle (heating 10380 K and then cooling 38010 K, Figure S9), compound 1 shows an intriguing behavior. When 1 is heated above 320 K, a sharp increase of the T product is observed and a value of 2.4 cm 3 mol -1 K is obtained at 380 K suggesting that the compound is now in a high spin state. To understand the important magnetic changes of 1 upon heating, we have investigated further IR spectroscopic studies at room temperature of 1 after heating at 380 K. As shown in Figure 7, the catecholate bands of as synthesized complex 1 at ca. 1250-1300 cm -1 and 1480 cm -1 almost disappeared when the solid sample is heated at 380 K. On the other hand, the relative intensity of semiquinone bands at around 1440 and 1580 cm shown in equation 1. [START_REF] Li | Sophisticated Construction of Electronically Labile Materials: A Neutral, Radical-Rich, Cobalt Valence Tautomeric Triangle[END_REF][START_REF] Mulyana | Solvation effects on the valence tautomeric transition of a cobalt complex in the solid state[END_REF] The disappearance of acetonitrile CN stretching band at ca. 2248 cm -1 after the thermal heating (Figure S4) as well as about 5% mass loss up to 380 K in the thermo gravimetric studies which is equivalent to 92 % mass loss of lattice acetonitrile molecule, further indicating that the desolvation triggers valence tautomeric isomerism. As

shown by the elemental analyses (Table S2) and IR spectroscopic studies, the desolvated species (1 heat380K ) does not include any more solvent even when it was kept under solvent vapour overnight. We have further carried out the PXRD studies, and from Figure S10, it is clear that the sample almost becomes amorphous when heated at 380 K and only a partial fraction of the pristine phase remained intact in this process. This result suggests that the crystal packing of the complex is almost destroyed after desolvation. Thus, this study overall suggests the thermally induced VT conversion of the complex in expense of an irreversible loss of the solvent molecules from the crystal lattice as shown in equation 1.

From the above studies, it is now reasonable to consider that 1 heat380K is a mixture of the pristine compound and a desolvated phase. When it is cooled, a small jump is observed around 330 K. This bump may come from a partial reverse thermal-induced valence tautomerism in the desolvated phase. Below K, the T decreases monotonously to show a T product of 2 cm 3 K at 300 K. Finally, a third measurement has been performed after exposure of the heated crystals in a vial containing CH 3 CN solvents to see if initial state of 1 is reachable with this method. The resulted T values (Figure 6, see cycle , consistent with the tendency observed in magnetic properties. In contrast to compound 2 which shows rapid decomposition started from 383 K, compound 3 is stable at least up to 430 K as suggested by TGA data (Figure S8). We have therefore investigated the magnetic properties of 3 up to 430 K using a VSM magnetometer. Figure S11 shows the temperature dependence of the T product of compound 3 during a temperature cycle between 150 K and 430 K (cooling and heating modes). Complex 3 shows an increase of its T product from 0.4 cm 3 mol -1 K at 405 K to reach the value of 0.85 cm 3 mol -1 K at 430 K. Before the cooling, the sample is kept at this temperature and the T product continues to increase to 1.12 cm 3 mol -1 K. This behavior is probably due to a progressive loss of the solvent molecules in the compound. During further cooling, a small decrease is observed and the T product stays above 1 cm 3 mol -1 K, a value far from the theoretical value of 2.57 cm 3

Optical reflectivity

Some complexes based on Co ion, 3,5-dbcat 2-and 3,5-dbsq •-ligands are known to exhibit photo-induced properties [START_REF] Schmidt | Goldilocks Effect in Magnetic Bistability: Remote Substituent Modulation and Lattice Control of Photoinduced Valence Tautomerism and Light-Induced Thermal Hysteresis[END_REF][START_REF] Schmidt | Magnetic Bistability in a Cobalt Bis(dioxolene) Complex: Long-Lived Photoinduced Valence Tautomerism[END_REF][START_REF] Sato | Photo-induced Valence Tautomerism in Co Complexes[END_REF][START_REF] Beni | Unprecedented optically induced long-lived intramolecular electron transfer in cobalt-dioxolene complexes[END_REF] that correspond to a photo-induced valence tautomerism.

These photo-induced can be followed by optical and/or magnetic measurements because the two valence isomers present distinct optical and magnetic properties. To check if our compounds show such properties, we have used optical reflectivity to measure photo-induced optical properties. We have selected compound 1 because it shows the most complete conversion from one state to the other one. The optical reflectivity spectra of 1 and 1 after heating at 380 K (noted here after 1 heat380 ) are shown on Figure S12 and Figure 9. 1 and 1 heat380 exhibit different spectra in agreement with the nature of their valence states. We have then measured the wavelength dependence using a set of 14 LEDs (working between 1100 nm to 365 nm with a power of 2 mW/cm 2 ) on the optical spectra for 1 and 1 heat380 (Figure S13). Where as 1 heat380 does not present significant modifications, 1 shows a small photoactivity in the visible range. An additional measurement has been done for 1 using a 405-nm LED with a power of 10mW/cm 2 . Even in these conditions, the photo-induced modifications remain small (Figure S14) meaning that the measured photoactivity of 1 is probably a surface effect.

NH 2 py) 2 ]Scheme 1 .

 21 Scheme 1. Possible redox isomers of a dioxolene ligand (top) and the schematic representation of the synthetic routes to 1-3 and the crystallographic bond distances (Å) in 1-3.

- 1 .

 1 8 and +1.8 T. The polycrystalline samples for SQUID (m = 23.36 mg for 1, m = 8.94 mg for 2 and m = 7.43 mg for 3) are introduced in a sealed polypropylene bag and for VSM ((m = 9.2 mg for 1) are introduced in quartz capsule. Experimental data were corrected for sample holder and intrinsic diamagnetic contributions.

  -2 . 14 Light Emitting Diodes (LEDs) from Thorlabs operating at 365 -385 -405 -455 -505 -530 -590 -625 -660 -735 -780 -850 -940 -1050 nm were used for excitation experiments.RESULT AND DISCUSSIONSynthesisAn acetonitrile-water (5:1 v/v) mixture of cobalt(II) nitrate hexahydrate, 3,5-di-tertbutylcatechol (3,5-H 2 dbcat) and 4-ethylpyridine in 1:2:4 molar ratio under aerobic condition resulted in an instantaneous olive-green solution, which upon standing at ambient temperature afforded dark-green crystals of 1 after two days. On the other hand, under a similar reaction condition, a mixture of Co(NO 3 ) 2 .6H 2 O, 3,5-H 2 dbcat and aminopyridines (3aminopyridine for 2 and 4-aminopyridine for 3) in a DMF-water (5:1 v/v) mixture led to formation of blue and dark green solutions, which afforded blue block and plate shaped green crystals for 2 and 3, respectively. In all the cases, the precursor cobalt(II) salt is air-oxidized to cobalt(III) in the presence of the redox active ligand and ancillary pyridyl ligands. Excess amount of pyridines were used to deprotonate 3,5-H 2 dbcat ligand and to provide basic environment for the requirement of oxidation of cobalt(II). Isolation of the crystals and the structure determinations of 2 and 3 ultimately reveal a unique result in this cobalt-dioxolene chemistry where cis and trans isomers were selectively isolated in reasonable yields by using positional isomers of the ancillary aminopyridyl ligands. 3-aminopyridine resulted a cis isomer (2) while 4-aminopyridine afforded a trans isomer (3) under an identical reaction condition. All the complexes are soluble in common organic solvents including acetonitrile and DMF.

Figure 1 :

 1 Figure 1: (left) UV-vis spectra of 1-3 in acetonitrile at [complex] = 1x10 -4 M, whereas the inset spectra were recorded at [complex] = 1x10 -3 M and (right) Variable temperature UV-vis spectra of 1 in CH 3 CN in the temperature range of 283-323 K.

Figure 2 .

 2 Figure 2. Ellipsoid view of one crystallographically independent molecule and one solvent acetonitrile molecule present in 1 with partial atom numbering scheme. Ellipsoids are drawn at 30% probability. Hydrogen atoms are omitted for clarity.

Figure 3 :

 3 Figure 3: Molecular packing of 1 showing π•••π and C-H•••π interactions.

2 O 4 donor

 24 sites. The geometry of the metal centers in these complexes are again a distorted octahedral structure; however, the connectivity of the ligands is different in these complexes as the ligands are bonded to the metal center in cis configuration in 2, while they are coordinated to the metal center in trans fashion in 3. The Co-N bond distances in 2 and 3 lie within the range 1.9493(12)-1.963(2) Å, and the Co-O bond distances vary in between 1.8674(17)-1.9073(17) Å, which are again typical for the low spin cobalt(III) complexes.41- 43,58-63 In both complexes, the dioxolene C-O and C-C bond lengths are in the range 1.300(9)-1.3215(17) Å, and 1.408(3)-1.442(3) Å for 2 and 3 respectively, which are inbetween the values reported for catecholate (1.33-1.39 Å) and semiquinonate (1.27-1.31 Å)

  and 3.22-3.23, respectively, further supporting the +III oxidation state of cobalt at room temperature in 2 and 3. In summary, both complexes can be formulated as [ls-Co(III)(3,5-dbcat)(3,5-dbsq)] units with two ancillary aminopyridine ligands at the temperature of the data collection. Details structural analysis reveals that one hydrogen atom of the amine group from one of the coordinated 3-NH 2 py ligands in 2 is engaged in hydrogen bonding interaction with one of the catecholate oxygen atoms from a neighboring molecule (N-H•••O distance 3.149 Å) while the other hydrogen atom forms relatively stronger hydrogen bond with the oxygen atom of solvated DMF molecule with N-H•••O distance 2.996 Å (Figure 5). The second 3-NH 2 py ligand only establishes a N-H•••π interaction with aromatic ring of the one of the dioxolene units as depicted in Figure 5. On the other hand, the symmetry equivalent 4-NH 2 py ligands establish a well directional strong hydrogen bond with dimension N-H•••O of 2.882 Å and the N-H•••π (Figure 5) interaction in the solid state in 3. Thus, different types of noncovalent interaction offered by the aminopyridine isomers could be responsible of the isolation of cis and trans isomers of 2 and 3, respectively, in the solid state.

Figure 4 .

 4 Figure 4. X-ray structures of 2 (left) and 3 (right) with partial atom numbering scheme. Ellipsoids are drawn at 30% probability. Hydrogen atoms are omitted for clarity.

Figure 5 .

 5 Figure 5. Molecular packing in 2 and 3 showing hydrogen bonding and N-H•••π (for 3) interactions.

Figure 6 .

 6 Figure 6. (left) Thermal variation of the T products for 1, 2 and 3 under a magnetic field of 1 T. (right) Thermal variation of the T products for 1 under different temperature cycles. (a) 2 K-300 K (b) 150 K-380 K and (c) after immersion of the crystals (see text).

  -1 increases significantly, which suggests the conversion of [Co(III)-(3,5-dbcat)(3,5-dbsq)] to [Co(II)-(3,5-dbsq)(3,5-dbsq)] as

  (c)) are very similar around room temperature to 1 heat380K at the end of cycle (b). The immersed crystals of 1 heat380K are not back in the initial isomer [Co III (3,5-dbcat)(3,5-dbcat)(4-Etpy) 2 ] as stated earlier. Therefore, the observed magnetic behaviour for compound 1 is probably due to an irreversible valence tautomer induced by solvent loss. The EPR studies have also been performed to confirm the VT process in 1. At room temperature, the signal of a radical with g = 2.00 could be detected, indicating the existence of a radical species, 3,5dbsq •-, consistent with the crystal structure of the complex at room temperature. After the heating of the sample at 380 K to remove solvent acetonitrile molecules from the crystals, the EPR spectrum again recorded at room temperature that displays the stronger signal intensity (Figure8), suggesting the increase of the magnetic signal through the desolvation induced valence tautomerism conversion from [Co III(3,5-dbcat)(3,5-dbsq)(4-Etpy) 2 ] to [Co II (3,5dbcat)(3,5-dbcat)(4-Etpy) 2 ]

mol - 1 K

 1 for [Co(II)(3,5-dbsq)(3,5-dbsq)(4-Etpy) 2 ]. Finally compound 3 shows only a partial conversion towards the high spin isomer induced by solvent loss. In conclusion of the magnetic study, along this series of compounds, compound 1 has the most interesting magnetic behavior. The solvated form of 1 has a low spin configuration in agreement with the low-spin valence isomer [Co(III)(3,5-dbcat)(3,5-dbsq)(4-Etpy) 2 ].CH 3 CN. When heated above 340 K, a simultaneous solvent loss and a valence tautomerism occur to stabilize the high-spin valence isomer [Co(II)(3,5-dbsq)(3,5-dbsq)(4-Etpy) 2 ] at 380 K.

Figure 7 .

 7 Figure 7. ATR-IR spectra of as synthesized complex 1 and after heating at 380 K.

Figure 8 .

 8 Figure 8. EPR spectra of 1 as from synthesized (blue) and after heating at 380 K (red) (frequency: 9.44 GHz; modulation: 1.0 mT; power: 0.711 mW). Optical reflectivity

Figure 9 .

 9 Figure 9. Optical reflectivity spectra at 10 K for compound 1 (in the pristine form) and in 1 heat380 (1 after heating at 380 K).

2 ]•CH 3 CN (1). All

	manipulations were
	carried out under aerobic condition for the synthesis of all three complexes. An aqueous
	solution (2 mL) of Co(NO 3 ) 2 •6H 2 O (0.072 g, 0.25 mmol) was added to an acetonitrile
	solution (10 mL) of 3,5-H 2 dbcat (0.111 g, 0.5 mmol) and 4-ethylpyridine (4-Etpy) (0.107 g, 1
	mmol) with stirring and the resulted solution was filtered and kept at ambient temperature for
	crystallization. Dark green crystals of 1 suitable for single-crystal X-ray diffraction were
	obtained in two days. Yield: 0.155 g (82%). Anal. Calcd. for C 44 H 61 CoN 3 O 4 : C, 70.01; H,
	8.14; N, 5.57. Found: C, 69.85; H, 7.89; N, 5.42 IR (cm -1 ): 2954w, 1617s, 1574m, 1478m,
	1457m, 1389s, 1282s, 1247m, 1208m, 1068m, 984m, 900m, 685w, 542m.

Synthesis of cis-[Co(3,5-dbcat)(3,5-dbsq)(3-NH 2 py) 2 ]•DMF (2). Complex 2 was

	synthesized from DMF/water (5:1 v/v) mixture by adopting a very similar method as that
	employed for 1 but 3-aminopyridine was used instead of 4-ethylpyridine. The dark-blue
	block-crystals suitable for X-ray diffraction were obtained from the reaction mixture upon
	standing at ambient temperature within a couple of days. Yield: 0.162 g (85%). Anal Calcd.
	for C 41 H 59 CoN 5 O 5 : C, 64.72; H, 7.82; N, 9.20. Found: C, 64.55; H, 7.59; N, 9.05. IR (cm -1 ):
	3445br, 3333s, 3210w, 2954s, 1660m, 1617m, 1602m, 1580s, 1437s, 1382s, 1280m, 1193m,
	1058m, 1021m, 975m, 909m, 829m, 804m, 698m, 602m, 518m.

Syntheses of trans-[Co(3,5-dbcat)(3,5-dbsq)(4-NH 2 py) 2 ]•2DMF (3). Complex 3 was

	Found: C, 63.25; H, 8.05; N, 10.03. IR (cm -1 , KBr): 3429br, 3321m, 2944w, 1668s, 1633m,
	1517m, 1456m, 1381m, 1278m, 1203s, 1089m, 1058m, 984m, 850m, 680m, 542m, 487s.
	X-ray Crystallography
	Single crystal X-ray diffraction measurements of the complexes were made on a Bruker
	Smart Apex-II CCD diffractometer with monochromated Mo-Kα radiation (λ = 0.71073 Å) at
	room temperature. Several scans in φ and ω directions were made to increase the number of
	redundant reflections and were averaged during refinement cycles. Bruker APEX-II suite
	(v2.0-2) program was used for data collection, data reduction, structure solution and
	refinement. Reflections were then corrected for absorption, inter frame scaling and other
	systematic errors with SADABS programme.
	synthesized from DMF/water (5:1 v/v) mixture following the same methodology as that
	applied for the synthesis of 1 but 4-aminopyridine was used in place of 4-ethylpyridine.
	Green plate shaped crystals suitable for X-ray crystallography were obtained within a couple
	of days. Yield: 0.165 g (79%). Anal Calcd. for C 44 H 67 CoN 6 O 6 : C, 63.29; H, 8.09; N, 10.07.

Table 1 .

 1 Crystal data and structure refinement for complexes 1-3. Empirical formula C 44 H 61 CoN 3 O 4 C 41 H 59 CoN 5 O 5 C 44 H 67 CoN 6 O 6

	1	2	3

Table 2 .

 2 Selected bond lengths Å and oxidation state parameters for 1-3 crystalize in the monoclinic unit cell with space group P2 1 /c and C2/c, respectively. The asymmetric unit of complex 2 contains one neutral molecule of [Co(III)(3,5-dbcat)(3,5-dbsq)(3-NH 2 py) 2 ] and one DMF solvent, while half of the complex molecule of [Co(III)(3,5-dbcat)(3,5-dbsq)(4-NH 2 py) 2 ],

		1 a
		A	B	2	3
				1.9073(17)
	Co-O (dioxolene)	1.8799(12) 1.8849(12)	1.8795(13) 1.8760(13)	1.9042(17) 1.8674(17)	1.8835(10) 1.8815(10)
				1.8806(16)
	Co-N (pyridine)	1.9451(15) 1.9482(19)	1.950(2) 1.963(2)	1.9493(12)
				1.300(3)
	C-O (dbsq •-/dbcat 2-)	1.322(2) 1.319(2)	1.322(2) 1.315(2)	1.302(3) 1.357(3)	1.3215(17) 1.3204(16)
				1.357(3)
	C-C (dbsq•-/dbcat 2-)	1.432(2)	1.435(3)	1.442(3) 1.408(3)	1.4230(19)
	MOS b	-1.46	-1.50	-1.20 -1.91	-1.50

a A and B denote two independent molecules in the asymmetric unit; b Metrical Oxidation State X-ray structural analysis at 296 K shows that both 2 and 3

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi: Supplementary data including bond angles, IR and UV-vis spectral data, cyclic voltammogram, EPR spectral data, magnetic measurements and optical reflectivity studies.

Accession Codes

CCDC 2073857-2073859 for 1-3 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION

Corresponding Author

Anangamohan Panja, Department of Chemistry, Panskura Banamali College, Panskura RS, WB 721152; Department of Chemistry, Gokhale Memorial Girls' College, 1/1 Harish Mukherjee Road, Kolkata-700020, India. E-mail: ampanja@yahoo.co.in Corine Mathonière, Univ. Bordeaux, CNRS, Bordeaux INP, ICMCB, UMR 5026, F-33600 Pessac, France; Univ. Bordeaux, CNRS, Centre de Recherche Paul Pascal, CRPP, UMR 5031, F-33600 Pessac, France. E-mail: corine.mathoniere@u-bordeaux.fr

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

There are no conflicts to declare.

For Table of Contents Use Only

Impact of Positional Isomers on the Selective Isolation of cis-trans Isomers in Cobalt-dioxolene Chemistry and Solvation Effects on the Valence Tautomerism in the Solid State

Narayan Ch. Jana, Xing-Hui Qi, Paula Brandão, Corine Mathonière, and Anangamohan Panja Influence of positional isomers of aminopyridine on the selective isolation of cis-trans isomers in cobalt-dioxolene chemistry and effect of solvation on valence tautomerism have been explored.