Reversible Photoswitching of a Spin‐Crossover Molecular Complex in the Solid State at Room Temperature

Abstract

Spin-crossover metal complexes are highly promising magnetic molecular switches for prospective moleculebased devices.T he spin-crossover molecular photoswitches developed so far operate either at very low temperatures or in the liquid phase,which hinders practical applications.Herein, we present am olecular spin-crossover iron(II) complex that can be switched between paramagnetic high-spin and diamagnetic low-spin states with light at room temperature in the solid state.T he reversible photoswitching is induced by alternating irradiation with ultraviolet and visible light and proceeds at the molecular level.Spin-crossover (SCO) metal complexes are aw ell-known class of molecular switches whose magnetic,c onductive, color,and other physicochemical properties can be reversibly switched by external triggers,s uch as temperature,p ressure, or light.[1] Six-coordinate iron(II) species,which are the most common SCO complexes,o ffer the opportunity to shift reversibly between diamagnetic low-spin (LS, S = 0) and paramagnetic high-spin (HS, S = 2) electronic states.S uch SCO metal complexes are considered as promising candidates for future applications in molecular electronics and spintronics,c ommunication networks,u ltra-high-density memory systems,displays,and sensors. [2] To be integrated into devices,amolecular switch should ideally operate under ambient conditions:a tr oom temperature and atmospheric pressure.M uch effort has therefore been devoted to the design of SCO complexes that are switchable at room temperature.[3] Systems featuring at hermal hysteresis loop near room temperature and thus showing athermally induced memory effect under ambient conditions have been highly desired. [2a,4] However,a ddressing the spin state with light is more attractive because of short response times,l ow power dissipation, and high selectivity.T hus, bistable coordination polymers that are switchable by means of ap hotoinduced phase transition at room temperature were developed.[5] This type of switching requires strong interaction between monomeric units,asachieved by covalent bonding within the crystal lattice of the polymer.Importantly, switching at the single-molecule level is fundamentally impossible in such systems,t hus diminishing the potential advantage of SCO switches over established bistable bulkphase materials and nanomaterials used in modern technology.P hotoswitching genuinely at the single-molecule level has been realized in the light-induced excited spin state trapping (LIESST) effect.

Light-Responsive Pyrazine-Based Systems: Probing Aromatic Diarylethene Photocyclization

“…DAE photoswitches based on π-linkers that are part of an aromatic ring (such as 4, Scheme 1b) are highly desirable for the functionalization of complex molecular materials, [15][16][17][18] but remain scarcely investigated. [19][20][21] Aromatic DAEs containing phenanthrene 13,22 and phenanthroline 14,15,23 moieties, which were developed by Walko and Feringa (5) 13 or Yam et al (6), 14 were shown to operate under similar conditions to conventional DAE switches (Scheme 1c), while effectively mimicking olefinic systems in accordance with Clar's rule of π-sextets, leaving the central core of these systems with lower aromatic delocalization of the electron density (Scheme 1c). [24][25][26] In contrast, the phenyl-DAE systems are not reported to photocyclize (Scheme 1b), 12 possibly due to the fact that this system is more aromatic and therefore 'resistant' to photocyclization.…”

Switching Magnetic Properties by a Mechanical Motion

“…Recently Khusniyarov et al have used a phenanthroline ligand functionalized by a photochromic diarylethene moiety in a bis(pyrazolyl)borate Fe(II) complex ( Figure 6). Under UV-irradiation, the open HS form is reversibly converted (about 40%) to a LS closed form [49,50]. In parallel, Oshio et al studied similar compounds and showed bidirectional spin-state switching by LIESST and reverse-LIESST in the solid state and in butyronitrile solution [51].…”

Probing the unpaired Fe spins across the spin crossover of a coordination polymer

“…This system exhibits a wide thermal hysteresis in the spin transition, above room temperature. 18 While X-ray absorption spectroscopy (XAS) is frequently used to probe changes in electronic structure, 17,[25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] X-ray core level photoemission at the Fe 2p core has also been used to probe the molecular spin crossover, 27,[41][42][43][44][45] but only in a limited manner. So far, data from X-ray photoemission spectroscopy (XPS) has not been adequate to make a detailed comparison with magnetometry or XAS, nor has the XPS data been sufficient to abstract a critical spin crossover transition temperature.…”

“…One is the light induced activation of the low spin to high spin state switch, i.e. light-induced excited spin-state trapping (LIESST), 4,26,[40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] and soft X-ray-induced excited spin state trapping (SOXIESST). 26 These light and X-ray induced spin state changes occur at temperatures well below the spin crossover temperature (T 1/2 ) where the low spin state should be dominant.…”