Molecules Designed to Contain Two Weakly Coupled Spins with a Photo-switchable Spacer

Controlling the charges and spins of molecules lies at the heart of spintronics. We report the design of a photoswitchable molecule consisting of two independent spins separated by a photo-switchable moiety contained within a new ligand, H4L, that features a dithienylethene photochromic unit and two lateral coordinating moieties, yielding molecules with the [MM···MM] topology. Compounds [M4L2(py)6] (M = Cu, 1; Co, 2; Ni, 3; Zn, 4) have been prepared and described by single crystal X-ray diffraction (SCXRD). The structure allows different metals to be selectively distributed amongst the two chemically distinct sites of the ligand, enabling the preparation of many double-spin systems. Heterometallic [MM’···M’M] analogues with formulae [Cu2Ni2L2(py)6] (5), [Co2Ni2L2(py)6] (6), [Co2Cu2L2(py)6] (7), [Cu2Zn2L2(py)6] (8) and [Ni2Zn2L2(py)6] (9) have been prepared and analyzed by SCXRD. Their composition has been established unambiguously. All complexes exhibit two weakly interacting [MM’] moieties, where some embody two level quantum systems. Compounds 5 and 8 each exhibit a pair of weakly coupled S = ½ spins that show quantum coherence in pulsed Q-band EPR, as required for quantum computing, with good phase memory times (TM of 3.59 and 6.03 μs at 7 K). The reversible photo-switching of all the molecules is confirmed in solution. DFT calculations on 5 indicate that the interaction between both spins of the molecule can be switched ON and OFF upon photocyclization.