ON/OFF Photoswitching and Thermoinduced Spin Crossover with Cooperative Luminescence in a 2D Iron(II) Coordination Polymer

Abstract

A 2D coordination polymer, {[Fe(L) 2 (NCSe) 2 ]•6MeOH•14H 2 O} n (1; L = 2,5-dipyridylethynylene-3,4-ethylenedioxythiophene), has been synthesized based on a redox active luminescence ligand. 1 possesses a 2D [4 × 4] square-grid network where the iron(II) center is in a FeN 6 octahedral coordination environment. 1 displays reversible thermoinduced high-spin (HS; S = 2) to diamagnetic low-spin (LS; S = 0) ON/OFF spin-state switching with a T 1/2 value of 150 K. Interestingly, optical reflectivity and photomagnetic studies at 10 K under light irradiation revealed an efficient conversion to a photoinduced metastable HS excited state from a LS ground state. Remarkably, the photoexcited HS state can be reversibly switched ON and OFF by using 625 and 850 nm light-emitting-diode lights. Intriguingly, the thermal dependence of the luminescence intensity of the maximum emission at 524 nm for 1 shows a minimum at around the spin-crossover (SCO) temperature, indicating a cooperative nature between the SCO and luminescence properties. Theoretical calculations confirmed the above findings.

Reversible Photo- and Thermo-Induced Spin-State Switching in a Heterometallic {5d-3d} W2Fe2 Molecular Square Complex

“…The average Fe–N bond distances are 1.948 and 1.947 Å for Fe(1) and Fe(2) centers, respectively, which lie in the typical range of LS Fe­(II) ion in a FeN 6 distorted octahedral coordination surrounding. , In addition, the octahedral distortion parameters (Σ, Θ, and ζ) and CShM factor lie in the typical range for LS iron­(II) complexes (Tables S4 and S5). In the [W 2 Fe 2 ] square motif, the W–CN–Fe edges are almost identical with a value between 5.206(1) and 5.218(1) Å, and angles at the Fe corners are closer to orthogonal in comparison to W corners (N(1)–Fe(1)–N(4) = 92.5(3)°, N(2)–Fe(2)–N(3) = 92.0(3)°, C(1)–W(1)–C(2) = 73.0(3)°, and C(3)–W(2)–C(4) = 73.5(3)°). The cyanide bridges are slightly more bent on Fe sides (Fe–N–C = 172.6(8)–174.6(7)°) than W sides, which remained almost linear (W–C–N = 177.0(7)–179.8(8)°) (Table S3).…”

“…Switchable molecular magnetic materials have been representing an amazing class of systems offering a large range of desired physical properties with enormous potential application in quantum science and technologies, , mainly those exhibiting magnetic, optical, and/or electric bistabilities, which includes spin crossover (SCO), − single molecule magnets (SMMs), − single chain magnets (SCMs), − metal-to-metal electron transfer (MMET), − etc. Switching of the magnetic properties in iron­(II)-based SCO materials originates from their reversible spin-state switching between a low-spin (LS) and high-spin (HS) state upon application of an external stimulus, e.g., temperature, pressure, light irradiation, and magnetic field, with a drastic alteration in electric and optical behaviors. − Importantly, SCO occurring at around or above room temperature is desirable for developing SCO-based electronics and spintronics devices amiable for practical use. ,,− …”

Monitoring spin-crossover phenomena via Re(i) luminescence in hybrid Fe(ii) silica coated nanoparticles

“…The first of them combines the SCO active moiety and the emitter in a single coordination compound. The emitter can be an organic fluorophore or luminescent complex which acts as: (i) ligand towards the Fe(II) centres; [8][9][10][11][12][13][14][15][16][17] (ii) counterion; 18 or (iii) guest molecules inserted within the cavities of three-dimensional SCO frameworks. 19,20 In these cases, the location of both components can be studied through structural elucidation techniques allowing the accurate determination of the nature of the correlation between the SCO and the lumine-scence phenomena.…”

Construction of Magneto‐Fluorescent Bifunctional Spin‐Crossover Fe(II) Complex from Pyrene‐Decorated Pybox Ligand

“…[14][15][16][17][18][19][20] Impressively, some cases in these bifunctional complexes display strong synergy between SCO and fluorescence emission. [21][22][23][24][25][26][27][28][29][30][31][32][33] The research on these bifunctional complexes will create numerous molecule-based functional materials that display potential applications in biosensors, thermometry, optical memory devices, and biological imaging, etc. However, metal ions such as Co II and Fe II can easily quench fluorescence, and it is still a great challenge to maintain fluorescence in an SCO molecule.…”