Complex of copper(ii) hexafluoroacetylacetonate with photochromic spiropyran in the merocyanine form: field-induced slow magnetic relaxation and quantum coherence for CuII (S = 1/2) and photoswitching of spiropyran ligand

Abstract

The interaction of photochromic 8-methoxy-1′,3′,3′-trimethyl-6-nitro-spiro[chromene-2,2′-indole] (MNSP) spiropyran with copper(II) hexafluoroacetylacetonate yields a deep red–violet solution due to a coordination-induced transition of MNSP from its closed form to the colored, open merocyanine (MC) form. The crystal structure of {Cu^II(hfac)₂·MNSP} (1) shows coordination of two oxygen atoms in MNSP to Cu^II, forming a distorted octahedral geometry with four short (1.94–1.97 Å) and two longer (2.22–2.44 Å) Cu–O bonds. The χ_MT value of 0.43 emu K mol⁻¹ at 300 K corresponds to a g-factor of 2.16 for Cu^II (S = 1/2). The electron paramagnetic resonance (EPR) signal fits well with g_∥ = 2.071, g_⊥ = 2.347 (g_iso = 2.167), and A_⊥ = 420 MHz. Slow magnetic relaxation is observed under a static magnetic field of 2000 Oe with χ′′(ν) curves showing well-defined maxima in the 2.0–6.0 K range. The dependence of ln(τ) vs. 1/T is well described using a linear combination of direct and Raman relaxation mechanisms. A magnetic hysteresis loop is observed at 0.5 K, classifying complex 1 as a single-ion magnet. This loop closes at zero field due to quantum tunneling of the magnetization but reopens at fields exceeding ±100 Oe. Pulsed EPR spectroscopy reveals quantum coherence with T_m ∼ 0.3 μs at 10 K. The spin–lattice relaxation time, T₁ = 0.7 ms, correlates well with values obtained from AC susceptibility data. A blue shift of 40–60 nm in the main absorption band of the open MC form of MNSP is observed upon complex formation. The complex dissociates in dilute solutions but reforms reversibly under ultraviolet (UV) and green light excitation. It remains stable in concentrated solutions or films. In solution, green and UV light reversibly switch the ligand between the open and closed forms within the complex, whereas only partial switching is observed in films.