Ultrafast Photoswitching in a Copper‐Nitroxide‐Based Molecular Magnet

Abstract

Molecular compounds with photoswitchable magnetic properties have been intensively investigated over the last decades due to their prospective applications in nanoelectronics, sensing and magnetic data storage. The family of coppernitroxide-based molecular magnets represents a new promising type of photoswitchable compounds. We report the first study of these appealing systems using femtosecond optical spectroscopy. We unveil the mechanism of ultrafast (< 50 fs) spin state photoswitching and establish its principal differences compared to other photoswitchable magnets. On this basis, we propose potential advantages of copper-nitroxide-based molecular magnets for the future design of ultrafast magnetic materials.Among various photoswitchable materials, special attention has been given to spin-crossover (SCO) compounds and the light-induced excited spin state trapping (LIESST) phenomenon, which allow optical manipulation of the magnetization by switching the molecules between high-and low-spin states. [1, 2] In the solid state it is also possible to reach new phases through light-control of structural order between SCO molecules. [3] LIESST has been extensively investigated in common SCO compounds, with the complete photocycle revealed and dynamics of photoswitching studied on femtosecond timescale in solution [4] and the solid state. [5] More recently, another type of promising spin-crossover-like compounds has emerged. These copper-nitroxide-based molec-

Novosibirsk Free Electron Laser—Facility Description and Recent Experiments

“…The molecular magnet undergoes thermo-and light-induced magnetostructural transitions [39], [40] and at temperatures less than 20 K can be switched from a ground state (G) to a metastable state (M) by near-IR-vis light [ Fig. 13(a)] [41]. The activation energy of the relaxation processes is about cm [42].…”

Light‐Induced Spin Transitions in Copper‐Nitroxide‐Based Switchable Molecular Magnets: Insights from Periodic DFT+U Calculations

“…Recently, a family of switchable compounds with SCO‐like behavior and LIESST‐like phenomena, based on Cu II and nitronyl nitroxide (NIT) ligands have been reported . These compounds of formula Cu(hfac) 2 L R consist of heterospin polymer‐chain complexes of Cu II hexafluoroacetylacetonato (hfac) with stable pyrazole‐substituted nitronyl nitroxides, L R , with R=Me, Et, Pr, i Pr, Bu.…”

“…The relaxation to the ground SS state is nonexponential and has a self‐decelerating character . The process has been also analyzed by femtosecond optical spectroscopy for a Cu(hfac) 2 L Pr complex . Figure shows the UV/Vis absorption spectra of the WS and SS states of Cu(hfac) 2 L Pr , which are dominated by the nitronyl nitroxide bands in the λ =450–700 nm region .…”

“…The process has been also analyzed by femtosecond optical spectroscopy for a Cu(hfac) 2 L Pr complex . Figure shows the UV/Vis absorption spectra of the WS and SS states of Cu(hfac) 2 L Pr , which are dominated by the nitronyl nitroxide bands in the λ =450–700 nm region . Similar UV/Vis spectra have been reported for the related Cu(hfac) 2 L Me and Cu(hfac) 2 L Et‐CP complexes (where L Et‐CP =2‐(1‐ethylpyrazol‐4‐yl)‐4,5‐bis(spirocyclopentane)‐4,5‐dihydro‐1 H ‐imidazole‐3‐oxide‐1‐oxyl) .…”

“…Similar UV/Vis spectra have been reported for the related Cu(hfac) 2 L Me and Cu(hfac) 2 L Et‐CP complexes (where L Et‐CP =2‐(1‐ethylpyrazol‐4‐yl)‐4,5‐bis(spirocyclopentane)‐4,5‐dihydro‐1 H ‐imidazole‐3‐oxide‐1‐oxyl) . The most significant difference between both states is a strong band at λ =500 nm observed only at low temperature (SS state), and tentatively assigned to a charge transfer band (metal‐to‐ligand or ligand‐to‐metal) . A further feature is the displacement of the structure band centered at ∼650 nm, associated with the nitroxide radicals, to a higher energy .…”