Photo-activation of Single Molecule Magnet Behavior in a Manganese-based Complex

A major roadblock to fully realizing molecular electronic devices is the ability to control the properties of each molecule in the device. Herein we report the control of the magnetic properties of single-molecule magnets (SMMs), which can be used in memory devices, by using a photo-isomerizable diarthylenthene ligand. Photo-isomerization of the diarylethene ligand bridging two manganese salen complexes with visible light caused a significant change in the SMM behavior due to opening of the six-membered ring of diarylethene ligand, accompanied by reorganization of the entire molecule. The ring-opening activated the frequency-dependent magnetization of the complex. Our results are a major step towards the realization of molecular memory devices composed of SMMs because the SMM behaviour can be turned on and off simply by irradiating the molecule.

Scientific RepoRts | 6:23785 | DOI: 10.1038/srep23785 repeatable. From static and dynamic magnetic measurements, only the complex with dae-o exhibited SMM behavior. This is unlike our previous complexes, where the magnetic behaviour is only modified due to opening/ closing of the dae ligand. In other words, for the first time, we show that photo-isomermization of the dae ligand can be used to turn on and off the SMM behaviour.

Results and Discussion
Structural Description. Crystallographic data for 1c and 1c-Vis are summarized in Table S1. 1c crystallized in the orthorhombic space group P2 1 /cn (Fig. 1). The Mn III ions are hexacoordinated with an N 2 O 2 atom set from 2,2′-ethylenebis(nitrilomethylidene)phenol (salen 2− ), one oxygen atom from the carboxylato group of dae-c 2− , and one oxygen atom from a coordinated methanol molecule. The two Mn III ions exhibited Jahn-Teller distortion with elongation of the oxygen-metal distance perpendicular to the N 2 O 2 atom set of salen 2− (O9-O7 and O5-O10 axes for Mn1 and Mn2, respectively) ( Table S2).
The dae-c 2− ligand acts as a monodentate ligand bridging Mn(salen) monomers via the carboxylato groups. In the packing diagrams of 1c ( Figure S2), Mn(salen) and dae-c 2− units are not organized in organic and inorganic sub networks. Two non-coordinated methanol solvent molecules are present in the asymmetric unit. No π -π interactions were observed in the crystal. However, hydrogen bonds between the non-coordinated and coordinated oxygen atoms of the dae-c 2− ligand and non-coordinated methanol molecules were found. In addition, there are hydrogen bonds between the fluorine atoms of dae-c 2and the hydrogen atoms of the solvent molecules. The intramolecular Mn III ···Mn III distance across the dae-c 2− ligand was determined to be 13.943 Å, whereas the nearest intermolecular distance was found to be 6.533 Å.
Crystals of 1c-Vis were obtained by irradiating those of 1c with sunlight, and it crystallized in the P2 1 /cn space group. The coordination modes of the Mn III ions are similar to those in 1c ( Figure S3) (Table S2). Since photo-isomerization of the complex induced constraint in the crystal, the atoms could not be refined anisotropically. Packing diagrams for 1c-Vis ( Figure S4) are similar to those for 1c. The intramolecular Mn III ···Mn III distance across the dae-o 2− ligand was determined to be 15.147 Å, and the nearest interunit Mn III ···Mn III distance was found to be 8.168 Å. These distances are larger than those in 1c During photo-opening of the dae ligand, the non-coordinated methanol evaporated. The solvent available void by unit cell was determined to be 277.6 Å 3 in 1c-Vis, whereas it was determined to be 479.1 Å 3 in 1c. In the case of the coordinated methanol molecules, only the oxygen atoms could be localized, and we could not determine if the carbon atoms of the methanol molecules were too disordered to be localized or if the methanol molecules were actually water molecules.
The crystal structure of 1c-Vis after UV irradiation could not be obtained due to a loss of crystallinity.

UV-Visible Spectroscopy.
Since the dae ligand undergoes reversible isomerization between closed and opened ring forms upon visible light (opening process) and UV irradiation (closing process) in solution and the solid state [16][17][18][19][20][21][22] , we studied the photo-isomerization processes in the solid state using KBr pellets of 1c before and after visible irradiation ( Figure S5). In the absorption spectra, there were two bands around 380 and 580 nm, which were assigned to be π -π * transitions of the dae 2− ligand 23 . Irradiation of the KBr pellets of 1c with visible light (λ > 480 nm) caused a pronounced color change from black to pale brown, which is characteristic of dae 2− -c and dae 2--o, respectively. After visible light irradiation, the two absorption bands disappeared due to ring opening. After UV light irradiation (λ > 365 nm) of 1c-Vis, the initial spectrum was recovered, meaning that the photocyclization reaction was reversible. In other words, isomerization between the closed and opened isomers occurs reversibly in the solid state.  The magnetic data obeys the Curie-Weiss law, affording Curie constant C = 5.81 and 5.71 cm 3 K mol −1 and Weiss temperature θ = − 1.16 and − 1.12 K for 1c and 1C-Vis, respectively. The C values obtained from fitting are close to the spin-only value of 6.11 cm 3 K mol −1 corresponding to two high-spin Mn III ions. Moreover, the negative θ values confirm that there are antiferromagnetic interactions between the Mn III ions.

Static Magnetic
The field dependence of the magnetization, M vs. applied field (H), at 1.82 K for 1c and 1c-Vis were similar. M of the complexes increased continually with an increase in H up to 4.55 Nμ B at 5 T with a decrease in the slope at 1.5 T. The M values did not saturate up to 8.00 Nμ B (S = 4), which is the M value expected for two high-spin Mn III ions. The absence of saturation and the observed slowdown were ascribed to the antiferromagnetic interactions and magnetic anisotropy due to the Jahn-Teller distortion of the high-spin Mn III ions. Hysteresis was not observed for either complex ( Figure S6). Dynamic Magnetic Properties. Ac magnetic susceptibilities were obtained for polycrystalline samples of 1c and 1c-Vis as a function of T, H and frequency. No out-of-phase signal was observed for 1c, and weak frequency dependence was observed for 1c-Vis ( Figure S7). In order to suppress potential quantum tunneling relaxation of the M, an H was applied to cause a mismatch in the + m s and − m s magnetization levels 24 . For 1c, an H less than 4000 Oe was not sufficient to observe a peak in the out-of-phase signal, characteristic of slow relaxation of the magnetization ( Figure S8). Detailed H-dependent ac measurements at 1.9 K showed that an H of 5000 Oe was needed to observe slow magnetic relaxation for 1c-Vis ( Figure S9). In-phase (χ′ ) and out-of-phase magnetic susceptibilities (χ″ ) for 1c-Vis were determined in an H of 5000 Oe at various T (Fig. 3a). The relaxation time τ was determined by using a generalized Debye model for a distributed single relaxation process of M, as show in equations (1) and (2), where χ adia and χ iso are the adiabatic and isotherm susceptibilities, respectively, τ is the relaxation time, and α the distribution coefficient of τ 25,26 .
The obtained τ values for 1c-Vis at several T were fitted using the Arrhenius law (equation 3), where τ 0 is the pre-exponential factor and Δ eff is the effective energy barrier for the reversal of M (Fig. 3b). Δ eff /k B and τ 0 were determined to be 13.13 K and 1.008 × 10 −7 s at 5000 Oe, respectively. These values are comparable to those determined for other [Mn 2 ] SMMs 27,28 .
The magnetic properties significantly changed upon photo-isomerization of the ligand. The closed form did not exhibit slow magnetic relaxation, whereas the open form exhibited clear frequency-dependent magnetic relaxation. The change in the magnetic properties is due to two reasons. First, the slight change in inter-and intra-unit Mn III ···Mn III distances upon photo-isomerization of the ligand could affect the magnetic interactions between the magnetic centers, although this is not supported by the results of the dc measurements. Second, the π-conjugation of the two thiophene rings are delocalized over the entire molecule in the closed ring form, whereas it is localized in the open ring form. The change in π-conjugation of the dae moiety upon irradiation can change the super-exchange interactions between the two Mn III ions. Due to the extended conjugation π-system, the quantum tunneling relaxation of M is dominant in the closed ring form. To clearly determine the mechanism and origin of the photo control of the magnetic properties, theoretical calculations, including the aromaticity of the ligand, the intramolecular interactions trough the π-conjugated bond and through dipole-dipole interactions, the change of the coordination geometry around the Mn ions, and the intermolecular interactions, are needed. Due to the scope of this report, the results of the computational studies will be reported later.
To determine the reversibility and repeatability of this activation, the sample was successively irradiated with visible and UV light over one day at room temperature, and magnetic measurements were performed at 1.9 K at each wavelength (Fig. 4). After visible irradiation, a clear peak appeared, whereas no peak appeared after irradiation with UV light, demonstrating that the SMM behavior could be switched on/off reversibly and repeatedly. The peak intensity after the second opening decreased slightly in comparison to the first one due to the fatigue of the complex, where the dae ligand itself or its coordination to Mn ion gradually degraded.

Conclusions
Irradiation of a photochromic dae ligand was used to repeatedly turn on and off the SMM behaviour of Mn(salen) complexes. When the complex with the closed ring form of dae (1c) was converted to 1c-Vis by irradiating with visible light, slow relaxation of the magnetization was observed, whereas it was not for 1c. This change in behaviour is due to slight changes in inter-or intra-unit Mn III ···Mn III distances upon photoisomerization of dae. In addition, the superexchange interactions between the two Mn III ions caused by the π-conjugation of the two thiophene rings affects the magnetic properties of the two forms. To the best of our knowledge, this is the first example of a coordination assembly where the magnetic properties are activated upon irradiation with visible light, and it is a significant advance in the area of molecular memory using SMMs.  and characterization were carried out in the dark to prevent the closed ring isomer from undergoing photo-cycloreversion. Physical Measurements. Solid-state UV/Vis absorption spectra of 1c were measured using a KBr matrix on a Shimadzu UV-3100 spectrophotometer before and after irradiation with visible light of λ = 480 nm (1c and 1c-Vis respectively) and after irradiation at λ = 356 nm (1c-UV). Measurements were carried out at room temperature.

Synthesis of [{Mn(salen)MeOH} 2 (dae-c)]·(MeOH) 2 (1c).
Dc susceptibility measurements were performed on a Quantum Design MPMS-5S superconducting quantum interference device (SQUID) magnetometer with a polycrystalline sample in applied magnetic fields of 100 Oe in the range of 1.8-20 K, 500 Oe in the range of 18-50 K, and 1000 Oe in the T range of 45-300 K. Experimental data were corrected for the sample holder contribution, and the diamagnetism of the sample was calculated from Pascal's constants. The ac susceptibilities was acquired on a Quantum Design PPMS-6000 physical property measurement system with an ac field amplitudes of 3 Oe below 1000 Hz and 1 Oe over 1000 Hz with and without a static dc field. After measuring the magnetic properties of 1c, the samples were alternately irradiated with sunlight and UV light for 1 day outside of the measurement system at room temperature, and the magnetic properties were measured at 1.9 K after each irradiation.
Single crystals were mounted on a glass rod, and crystallographic data were collected on a Rigaku Saturn70 CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 nm) produced by a VariMax microfocus X-Ray rotating anode source at 103 K. Data processing was performed using the Crystal Clear crystallographic software package. The structures were solved by using direct methods via SIR-92 or SIR-2011 31 and refined using the full-matrix least-squares technique included in SHELXL-2013 32 . The final cycles of full-matrix least-squares refinements on F 2 converged with unweighted and weighted agreement factors of R 1 = Σ ||F o |−|F c ||/Σ |F o | (I > 2.00σ(I) for R 1 ), and wR 2 = [Σ w(F o 2 − F c 2 ) 2 ]/Σ w(F o 2 ) 2 ] 1/2 (all reflections), respectively. Anisotropic thermal parameters were assigned to all non-hydrogen atoms for 1c. The hydrogen atoms were set in calculated positions and refined using a riding model with a common fixed isotropic thermal parameter. CCDC-1443151 and 1443152 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.