Molecules Designed to Contain Two Weakly Coupled Spins with a Photoswitchable Spacer

Controlling the charges and spins of molecules lies at the heart of spintronics. A photoswitchable molecule consisting of two independent spins separated by a photo- switchable moiety was designed in the form of new ligand H4L, which features a dithienylethene photochromic unit and two lateral coordinating moieties, and yields molecules with [MM···MM] topology. Compounds [M4L2(py)6] (M = Cu, 1; Co, 2; Ni, 3; Zn, 4) were prepared and studied by single-crys- tal X-ray diffraction (SCXRD). Different metal centers can be selectively distributed among the two chemically distinct sites of the ligand, and this enables the preparation of many double-spin systems. Heterometallic [MM'···M'M] analogues with formulas [Cu2Ni2L2(py)6] (5), [Co2Ni2L2(py)6] (6), [Co2Cu2L2(py)6] (7), [Cu2Zn2L2(py)6] (8), and [Ni2Zn2L2(py)6] (9) were prepared and analyzed by SCXRD. Their composition was established unambiguously. All complexes exhibit two weakly interacting [MM'] moieties, some of which embody two-level quantum systems. Compounds 5 and 8 each ex- hibit a pair of weakly coupled S = 1=2 spins that show quan- tum coherence in pulsed Q-band EPR spectroscopy, as re- quired for quantum computing, with good phase memory times (TM = 3.59 and 6.03 ms at 7 K). Reversible photoswitch- ing of all the molecules was confirmed in solution. DFT cal- culations on 5 indicate that the interaction between the two spins of the molecule can be switched on and off on photo- cyclization.