Counterion Modulation of Spin Crossover and Single-Ion Magnet Behavior in Photoresponsive Cobalt(II) Terpyridine Complexes

Ligands bearing potentially photoactive groups may enable control over SCO and SIM behavior, an important step in designing strong candidates for implementing quantum technologies. In this work, two isoestructural cobalt(II) compounds of formula [CoII(E-4-TPE)2]X2 [X = ClO4– (1) and PF6– (2); E-4-TPE = (E)-4′-styryl-2,2′/6′,2″-terpyridine], were synthesized, characterized and investigated through magnetic measurements. The pair of compounds exhibit a counterion-dependent modulation of the SCO and SIM properties in the solid state, as 1 undergoes a complete thermally-induced spin transition and has slower relaxation times (τ) whereas 2 exhibits an incomplete spin transition (evidenced by residual high-spin signals in Q-band EPR spectrum) and faster relaxation times. Moreover, 1H NMR and UV-Vis spectra of the E-4-TPE ligand and 1 suggest the occurrence of E→Z photoisomerization upon UV light irradiation (λmax = 355 nm) in solution, but not in solid state.