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Coordination-enhanced photochromism in dysprosium dinuclear complexes with photomodulated single-molecule magnet behavior

We report the synthesis of a pyrazine-based bis-tridentate diarylethene (DAE) ligand and the corresponding dinuclear yttrium and dysprosium complexes that all show T-type photochromism. Large changes in the photochromic behaviour are induced by metal coordination, including a tuning of the bleaching rate by several orders of magnitude, characterized through VT-NMR and supported by theoretical calculations. The dysprosium dinuclear complex shows a single-molecule magnet (SMM) behaviour, under an external applied magnetic field, significantly modulated upon photoisomerization. This feature is further analyzed through ab initio calculations.


Introduction
Photochromism, characterized by a reversible change of a substance color under light exposure [1], can be used for photoswitching of a fascinating range of properties [2][3][4], thanks to structural or electronic modifications with implications in catalysis [5], material sciences [6][7][8], medicine [9][10], information technologies [11], and many other fields. The dizzying list of potential and realized applications of photochromic units implies that the tailoring of the characteristics of a photochromic unit to adapt to a specific need should be mastered. For organic photochromism of T type, for which the photoinduced state is metastable and thermally returns to the initial state, such characteristics include excitation wavelengths, quantum yields, fatigability, colorability and rate of thermal bleaching. Systems with fast bleaching are required for photochromic lenses or real-time holography [12][13], while for many biological applications, adaptation of the bleaching rate to meet specific process is key [10,[14][15]]. An extensive literature explores structure-properties relationships in hexaarylbiimidazole [16], spiropyrane and spirooxazine [17] or azobenzene [18][19][20] derivatives as well as less common molecules [21][22]. With some of these Tphotochromes, the impact of metal coordination on photoswitching has been explored and recognized as a valuable strategy to effectively adapt the system properties as well as to design sensors [23]. Metal coordination can, in other cases, block the photochromism, an effect that could be leveraged to enable gated photochromism [24][25]. In most cases, the ubiquitous diarylethene (DAE) photoswitches are considered as P-photochromes since the photoinduced closed isomer generally shows very long half-lives and can be transformed back to the initial open isomer with adequate light irradiation only [2]. However, this stability depends significantly on the nature of the central ethene bridge and, to a lesser extent, on the nature of the aryl groups and of their substitution patterns [26]. Typically, DAE with phenanthrene bridges [27][28] show low photoconversions and fast thermal bleaching of their closed isomer due to the loss of aromaticity upon photocyclisation. For example, a central fluorinated benzene ring can provide fast T-type photochromic DAE with a half-life of 130 ms at room temperature [29]. Recently, Diederich et al. demonstrated that DAE with a central pyrazine bridge like 2,3-bis(2,5-dimethylthiophen-3-yl)pyrazine are also T-photochromes with a thermal half-life of ca. one minute in acetonitrile [30]. Therefore, the pyrazine core that allows for coordination-induced tuning of photochromism, deserves to be further explored to extend this class of T-type DAEs. This work also reports a further significant step towards the design of photoswitchable single-molecule magnets (SMMs) by combining the magnetic bistability of SMMs [31][32][33][34] with the light-switching ability of organic photochromes to provide a new family of photomagnetic systems [35][36][37][38]. We specifically target lanthanide-based complexes since their magnetic properties clearly stand out from those of transitionmetal complexes [39]. We previously obtained hysteresis photomodulation in DAE-dysprosium 1D assemblies [40] and decided to further investigate dinuclear complexes bridged by a DAE. Hence, we report here i) a newly synthesized pyrazine-based bis-tridentate DAE ligand and the corresponding dinuclear yttrium and dysprosium complexes that all show T-type photochromism, ii) large changes in the photochromic behaviour induced by metal coordination, with a tuning of the bleaching rate by several orders of magnitude characterized through VT-NMR and supported by theoretical calculations, and iii) the photomodulation of the resulting SMM properties upon photoisomerization, that is further analyzed through ab initio calculations.

II. Results and Discussion
Synthesis. Pyrazine-based photoswitches have been previously synthesized through the condensation of a bisthiophene dione and diamines to form the central pyrazine cycle [30]. Here, ligand BTP1 was conveniently synthesized by Suzuki coupling from 5-methyl-2-(pyridin-2-yl)-4-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)thiazole [41] and 2,3-dichloropyrazine in 78 % yield. The optimized conditions for this coupling consist in the use of cesium fluoride together with Pd 2 (dba) 3 , tricyclohexylphosphonium tetrafluoroborate and potassium carbonate (Figure 1). Similarly, coupling between 2,5-dimethyl-4-(4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl)thiophene and 2,3-dichloropyrazine afforded the previously published model compound BTP2 [30]. Then, BTP1 was put to react with 2 equivalents of [M(hfac) 3 (Table S1). The compound crystallizes in the space group P2c and the asymmetric unit consists of half a molecule, the other half being generated by a C 2 axis. The most striking feature of this structure is that the compound crystallizes in a non-photoreactive conformation with a distance between the two reactive carbons (C-Me) of 5.3820(3) Å and a parallel orientation of the two pyridylthiazolyl groups (Figure 2). Accordingly, no photocoloration of these crystals could be observed under UV light. Concerning the metal complexes, single crystals of 1Y o and 1Dy o are isostructural (space group C2/c). The dinuclear complexes also possess C 2 symmetry and the asymmetric unit is composed of half a complex. However, the participation of all three nitrogen atoms in a tridentate coordination pocket induces a very different geometry of the ligand in these complexes as compared with the ligand itself (Figure 2). The antiparallel conformation of the two thiazole rings as well as the short distance between the reactive carbons (3.375(5) Å for 1Y o , 3.369(12) Å for 1Dy o ) clearly indicates that a photocyclization reaction is not prevented by conformational issues for these two complexes [2]. However, upon UV irradiation, the crystals show no coloration, probably as a result of the low thermal stability of the closed form (see below) leading to a much faster bleaching rate than the photoisomerization one under these conditions (single crystal, RT). With the prospect of elucidating the magnetic behavior of the dysprosium compound, it is also important to pay attention to the metal ion environment in 1Dy o . First, the coordination sphere around the Dy(III) ion is composed of three hexafluoroacetylacetonate ligands and three nitrogen atoms leading to a coordination number of 9. The dysprosium-oxygen bonds distances are relatively homogeneous in the range 2.342(6)-2.421(7) Å, while the thiazole-nitrogen distance of 2.488(7) Å is clearly much shorter than the two other dysprosium-nitrogen distances (2.604(7) Å and 2.639(6) Å with the pyridine and pyrazine rings, respectively). The continuous shape measurement indicates a geometry close to a spherical-capped square antiprism (CShM parameter of 0.876, see ESI for details) [42]. Finally, the pyrazine core brings the two metal ions of the dinuclear compound in close vicinity (Dy-Dy distance of 7.8710(12) Å). This is in the range for which substantial dipolar coupling can occur.  solid residue turned green upon UV irradiation with quick subsequent fading of the color when irradiation was stopped. We therefore reasoned that the lack of photochromism in solution was due to even quicker thermal fading, indicating that the closed system BTP1 c is less stable than other classical DAE compounds. In dichloromethane and similar conditions, solutions of complexes 1Y o and 1Dy o do turn green under UV light and then progressive bleaching occurs after irradiation. The corresponding electronic absorption spectra are shown in Figure 3 (and S4 for 1Y o .). If BTP1 c could not be observed at RT, the colored species 1Dy c displaying the characteristic visible absorption of DAE could be detected by using a fast scan rate and the fast thermal reopening of the closed isomer was confirmed by the monoexponential decay (t 1/2 = 101 s) of this visible absorption with time. The absorption maximum at 718 nm is clearly red-shifted as compared with previously published pyrazine-based DAEs (643 nm for BTP2 c ). For 1Y c , the behavior was essentially identical to the one of 1Dy c (see Figures S4 and S5). Overall, these experiments indicate that both BTP1 o and 1M o undergo photoisomerization reactions to lead to a closed isomer which is thermally unstable and undergo quick retrocyclization reaction at room temperature. However, the absence of photocoloration in solution in the case of BTP1 o , clearly contrasting with the behavior of the metal complexes, suggests that the coordination significantly affects the thermal stability of the closed isomer. In order to quantitatively examine this effect, variable (low) temperature 1 H NMR spectroscopy was employed.

Study of photochromism by 1 H NMR spectroscopy at variable temperature.
Solutions of BTP1 o and 1Y o in CD 2 Cl 2 were used for the NMR study. At 223 K, the spectrum of BTP1 o is essentially the same as that obtained at room temperature and is consistent with the presence of one conformation in which the two halves of the molecule are magnetically equivalent. Upon irradiation with 313 nm light, the solution turned dark green and the 1 H spectrum consisted in a new set of resonances characterizing the formation of the expected cyclized BTP1 c (80 % at PSS). For instance, in BTP1 c , the singlet signal at 2.30 ppm (2.38 ppm in BTP1 o ) was associated to the methyl groups and the pyrazine protons were observed at 7.71 ppm (8.75 ppm in BTP1 o ), while the overall spectrum showed the preserved symmetry between the two halves of the molecule ( Figure S6). The pyrazine proton signal is more shielded in the closed form due to the aromaticity loss of this pyrazine unit upon cyclization [30]. At 223 K, no thermal evolution of the system was observed in the absence of light for 40 min. Similarly, at 238 K, the closed form of complex 1Y c could be generated by 365 nm irradiation (95% at PSS) and its spectral features were similar, but clearly shifted, compared to those of BTP1 c owing to metal coordination. For instance, the methyl groups showed a single resonance at 2.38 ppm and the pyrazine proton was observed at 7.33 ppm (Figure 4). In addition to the unambiguous characterization of the closed isomers as the photogenerated products, NMR spectroscopy is quantitative and provides valuable information on the photoconversion and on the kinetic of thermal fading. So first, the photoconversion of BTP1 o ( irr = 313 and 365 nm, for a purpose of comparison) and 1Y o ( irr = 365 nm) at 238 K, where no significant thermal back reaction occurred, was followed by recording 1 H NMR spectra between successive periods of illumination. In each spectrum, the integration of the singlet signals for the methyl groups in the open and cyclized forms were measured, allowing the time-evolution of concentrations to be determined (Figure 5). With the same irradiation wavelength of 365 nm, a striking difference was observed between BTP1 and 1Y, with a PSS composition of 31% of BTP1 c and 69% of BTP1 o for the former and of 94 % of 1Y c and 6% of 1Y o for the latter. The photoconversion is also much faster for the complex. A more efficient conversion of BTP1 o was reached upon irradiation at 313 nm, as expected from the absorption profile (Figure 3). The latter was obtained after irradiation at 365 nm for 10 min and showed 84 % conversion. The thermal relaxation was followed in the same way to determine the kinetic parameters of thermal fading for both BTP1 c and 1Y c at different temperatures. For BTP1 c , temperatures of 223 to 247 K were used and the decrease in BTP1 c concentrations always followed a first-order law with the rate constants reported in Table 1. For 1Y c , a similar behavior was observed (T = 260 to 278 K). Fitting of the first order kinetic constant temperature dependence with an Arrhenius law provides the frequency factor and activation energy for the cycloreversion reactions gathered in Table 1. The obtained activation energies (74 kJ.mol -1 for BTP1 c and 67 kJ.mol -1 for 1Y c ) are clearly lower than those observed for P-type classical DTEs (based on the perfluorocyclopentene core) that show typical values as large as 140 kJ.mol -1 [2,26,43]. They are nevertheless comparable to other fast bleaching systems like dithienyl thiazole derivatives [44] (80 to 90 kJ.mol -1 ), phenanthrene-based compounds [45] (50-60 kJ.mol -1 ) as well as perfluorobenzene-based compounds [29] (66 kJ.mol -1 ) and clearly reveal that the origin of this fast bleaching is the low stability of the closed form that correlates with low activation energies for the cycloreversion reaction. This is further confirmed by theoretical investigations (see below). However, the differences in kinetic stability between the ligand and the complex are clearly more governed by the 3000-fold decrease of the frequency factor when going from BTP1 c to 1Y c . While the frequency factor observed for BTP1 c compares quite well with other DTE systems (1.0 10 13 s -1 for 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene with a stable closed form [26], 1.6 10 10 s -1 for 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)thiazolyl structure [44], 2.1 10 12 s -1 for 1,2-bis(2-methyl-5-phenyl-3-thienyl)tetrafluorobenzene), the frequency factor of 1Y c is much lower. This is clearly the main parameter explaining the increased kinetic stability of the closed form upon metal coordination. Kinetic stabilization by lanthanide coordination has been previously observed with terarylene switches [46]. The origin of this stabilization was also a change in the frequency factor. However, the effect observed in our work is a thousand times larger and could provide a new strategy for optimizing the photochromic performances of DAE derivatives. Clearly, further specific investigations are needed to be able to rationalize this effect. However, since variations of the electronic effects in otherwise similar organic structures can lead to a wide range of frequency factor (from 10 9 s -1 to 10 13 s -1 ) [47], we suggest that the involvement of nitrogen doublets in coordination bonds could play a role. We used a "traditional" DFT approach, PCM-B97X/6-311G(d,p) to investigate the free energies of the different possible conformations (see Figure 6 and the SI for details) that are labeled depending on their symmetry : parallel (P) or antiparallel (AP). Functionals of the B97 family have been shown to be accurate for various properties of DTE switches [48][49], and we computed the free energies using the harmonic vibrational frequencies. The most stable conformation P6 presents the C 2 point group and shows a parallel orientation of the thiazolyl rings (Figure 6) that resembles the X-rays structure of BTP1 o . According to a Boltzmann distribution, this conformation represents 88 % of the conformers mixture at 298 K while the next two most stable isomers are P2 (C 1 parallel conformation, +5.2 kJ.mol -1 as compared to P6) and AP2 (C 2 antiparallel conformation, +10.3 kJ.mol -1 ). This latter photoactive form represents 1.4 % of the conformers' mixture at 298 K according to the calculations. We can therefore deduce that i) the lowtemperature NMR-detected conformer is P6 which presents a parallel conformation; ii) there should be a quick interconversion between the parallel and anti-parallel forms. The closed isomer was also investigated and different conformers corresponding to the rotation along the pyridyl-thiazolyl bond were computed. The most stable of these lies 138.9 kJ.mol -1 higher in energy than the reactive open conformation AP2. The transition state corresponding to the cycloreversion path was also computed, using a broken-symmetry approach with the same DFT level, and the corresponding activation energy (69.0 kJ.mol -1 ) matches very well the experimental one (74.2 kJ.mol -1 ). For the records, a M06-2X calculation yields a very similar value of 73.9 kJ.mol -1 , such a difference of less than 5 kJ.mol -1 being in the BS-DFT error bar. Clearly, the reason for the fast bleaching of the closed form is the instability of BTP1 c and the consequent low cycloreversion activation energy as compared with classical DTE compounds.  Figure S8). Then, the SMM properties of the systems were characterized through dynamic magnetic measurements in which an oscillating magnetic field with variable frequency is used to trigger the system response. Note that at a molecular level, the two dysprosium centers in 1Dy o are equivalent by symmetry and are expected to lead to the same magnetic relaxation. This should be true also for the closed complex since closing only occurs on conformers having C 2 antiparallel symmetry. On all samples clear frequency dependence of the out-of-phase susceptibility (χ M ''), a signature of a dynamic behavior of the magnetization, is observed between 1.8 and 9 K provided that an external magnetic field (H dc ) is applied (Figures 7 and S9-S12). The optimal field is H dc = 1200 Oe for all samples. This is associated with slow relaxation of the magnetization related to the SMM behavior of each sample investigated. This behavior can be further characterized by extracting the systems relaxation times (τ) using a modified Debye law [51] that considers four different types of magnetic relaxation (Figures 8 and S13-S14): In this equation, each term relates to a different mechanism for relaxation, namely direct relaxation, quantum tunneling of magnetization, Raman relaxation and Orbach mechanisms that have different temperature dependence as follow: τ -1 = τ direct -1 + τ tunnel -1 + CT n + τ 0 -1 exp(-U eff /k B T) (2) On our series of samples, the fitting procedure shows that magnetic relaxation is governed by Raman and tunnel relaxations (see experimental section for the details). The following parameters have been obtained in agreement with what is found in the literature (1Dy o : C = 0.110 ± 0.005 s -1 K -1 , n = 6.03 ± 0.06, τ tunnel = 0.190 ± 0.003 s; 1Dy c' : C = 6.71 ± 0.60 s -1 K -1 , n = 3.49 ± 0.07, τ tunnel = 9.3 ± 0.26 10 -3 s; 1Dy o' : C = 0.61± 0.05 s -1 K -1 , n = 4.85 ± 0.08, τ tunnel = 41 ± 0.8 10 -3 s) [33,52]. From this analysis, it is visible that the less crystalline materials (1Dy c' and 1Dy o' ) relax faster than the crystalline one (1Dy o ) and that all the magnetic relaxation properties are different between 1Dy o and 1Dy o' mainly because of the crystallinity difference between the two samples. But the main information from the dynamic magnetic measurements is that when photoconversion is performed on two samples with similar shaping and crystallinity (1Dy c' and 1Dy o' ) a significant acceleration of the magnetic relaxation is observed when going from the closed to the open form (Figure 8).   Table S2 for values).
Another useful parameter to characterize the magnetic relaxation of a molecule is the distribution (α) of its relaxation times. This parameter is extracted from Cole-Cole plots (Figure 7, Tables 2 and S3-S8) and is expected to be 0 if all molecules relax simultaneously and 1 on the contrary (spin glass-like behavior). Additionally, from the difference between the measured and the ideal Cole-Cole plot, the relaxing fraction (RF) that is the part of the sample that relaxes slowly, can be extracted (RF = 1-(χ s /χ T ), with χ s being the adiabatic and χ T the isothermal susceptibility). For 1Dy o as expected, almost all the sample relaxes slowly and RF (2K) = 94 % and RF (7K) = 87%. This is associated with quite low α values of α (2K) = 0.21 and α (7K) = 0.02. In the photoisomerized sample (1Dy c' ), the relaxing fraction diminishes, especially in the high temperature region with RF (2K) = 93 % and RF (7K) = 56% and the distribution of the relaxation times is severely increased with α (2K) = 0.45 and α (7K) = 0.25. This is combined with a peak broadening in the χ M " vs. frequency curves (Figure 7) as often observed when non-purely crystalline samples are compared to crystalline material. When 1Dy c' is converted back to 1Dy o' , the complex relaxes fairly well when compared to 1Dy c' (RF (2K) = 91 %, RF (7K) = 83%, α (2K) = 0.24 and α (7K) = 0.15) and peak broadening is diminished. However, all its relaxing properties are not recovered when compared to the crystalline sample 1Dy o . These differences could be ascribed to a distribution of slightly different coordination Dy III geometries in the less crystalline compounds as compared with 1Dy o . Overall, a clear finding of these magnetic measurements is that the closing reaction induces an acceleration of the magnetic relaxation (τ), an increase of the relaxation time distribution (α) and a diminution of the relaxation fraction in the high-temperature regime when 1Dy c' is compared with the open analogue (1Dy o' ). Moreover, as observed in our previous studies, these magnetic measurements confirm that the effect of photoconversion on the SMM behavior of magnetic molecules has to be investigated on comparable sampling, i.e. single crystals when single-crystal to single-crystal photoconversion is observed [40] or evaporated solution when photoconversion occurs at the liquid state [53].   The calculated magnetic susceptibility (T) and magnetization (M) for the open and closed forms of 1Dy are shown in Figure S17. As expected from the similar electronic structures, the calculated static magnetic properties for the investigated compounds are almost identical between each other and reproduce properly the experimental data. At very low temperature, an intramolecular antiferromagnetic interaction between the Dy 3+ ions is found to be responsible for the decrease of the T product. Using the dipolar approximation, isotropic dipolar magnetic coupling constants (J ) of -0.070, -0.059 and -0.081 cm -1 are calculated for 1Dy o -XRD, 1Dy o -DFT and 1Dy c -DFT, respectively. Interestingly, the representation of the magnetic transition moments between the lowest states of 1Dy o -XRD, 1Dy o -DFT and 1Dy c -DFT is found to be very sensitive to the structural changes (see Figure 10), suggesting possibly different dynamic magnetic properties. It is particularly interesting to look at possible Orbach mechanisms since we could not extract such information from the experimental data. In the three structures, an Orbach mechanism for the relaxation of the magnetization through ES1 is favored, suggesting slow relaxation of magnetization barriers of 39, 89 and 66 cm -1 for 1Dy o -XRD, 1Dy o -DFT and 1Dy c -DFT, respectively. In the latter two systems, thermally-assisted quantum tunneling relaxation mechanisms start to be non-negligible in ES1 with calculated transversal magnetic transition moment of 1.53 and 0.65 B, respectively. In 1Dy o -XRD, the transversal Orbach mechanism is the dominant one because of the almost perpendicular orientation of the GS and ES1 easy magnetic axes. However, it has to be mentioned that the GS in 1Dy o -XRD already exhibits strong QTM, suggesting possible fast relaxation, in line with the fact that no slow relaxation is observed in zero applied external field. This stronger QTM mechanism compared to 1Dy o -DFT is attributed to the more contracted coordination sphere in 1Dy o -XRD (see Figure S16). Overall, the opening or closing of the bridging ligand does not drastically modify the dipolar magnetic coupling between the two metallic centers. However, the structural re-organization of this central ligand leads to sizable changes in the coordination sphere around the Dy centers that induces modifications of the crystalfield splitting of the 6 H 15/2 level, and hence of the magnetic relaxation pathways.

III. Conclusions
We have designed a new pyrazine-core DAE ligand with fast T photochromism that exhibits a large change of bleaching time upon metal coordination with slower kinetic allowing its observation and use for properties control. In particular, the dysprosium complex shows a rare example of lanthanide dinuclear complex bridged by a photochromic ligand [38] and allows to study the impact of the isomerization on the slow relaxation of magnetization. We demonstrated both experimentally and theoretically that the main effect explaining the slower bleaching rate in the complexes is a change in the frequency factor of several orders of magnitude. We believe that this could provide a new strategy for optimizing the photochromic performances of DAE derivatives and we are currently implementing changes in the metal ion precursors in order to fully explore this effect.

IV. Additional Information
Supporting information is available online. Correspondence and requests for materials should be addressed to the corresponding author.

V. Materials and Methods
See supporting information for complete details about the synthesis and NMR spectra.