g‐Engineering in Hybrid Rotaxanes To Create AB and AB₂ Electron Spin Systems: EPR Spectroscopic Studies of Weak Interactions between Dissimilar Electron Spin Qubits

Abstract

Abstract. Hybrid [2]rotaxanes and pseudo-rotaxanes are reported where the magnetic interaction between dissimilar spins is controlled to create AB and AB 2 electron spin systems, allowing independent control of weakly interacting S = 1 / 2 centers Several groups have proposed the idea that molecules could be used as electron spin qubits. [1][2][3][4][5][6] Molecular qubits can be designed such that phase memory times (T m ) approach other solid state electron spin technologies, [7] and have the advantage of being functionalizable by synthetic chemistry methods. Proposed molecular spin qubits include organic radicals, [2] simple coordination complexes, [7] high-spin [1] and low-spin clusters, [3][4][5] and 4f ions. [6] One challenge is to bring together ensembles of such qubits, and particularly to bring together qubits with different electronic g-factors, so called g-engineering, [2,6] which would allow the qubits to be addressed individually and controllably within a complex molecule. Takui and co-workers have done this using elegant organic chemistry, and have reported the operation of the CNOT gate using such an approach. [2] Here we show how similar g-engineering can be achieved using coordination chemistry of 3d-metal cages.We have been studying heterometallic {Cr 7 M} rings for their fascinating spin physics. [8] Where M is Ni, antiferromagnetic exchange coupling gives a well-isolated S = 1 / 2 ground state; pulsed EPR spectroscopy shows these have sufficiently long phase memory times to use as qubits. [9] The chemical robustness and versatility of these units allow their incorporation in hybrid [2]rotaxanes. [10] Here we use a thread, 16 ]} 2; X-ray crystallography [11] confirms that [Cu(hfac) 2 ] has bound to the pyridyl of the rotaxane-thread ( Figure 1a,b and Figure S1 in the Supporting Information). The heterometallic ring is unchanged from the rotaxane 1, containing eight metals in an octagon, bridged by fluoride and pivalates ( Figure 1b). The ammonium group of the thread H-bonds to the fluorides within the metal octagon.The pyridyl group from the thread binds to copper, occupying the apical site of a square- At low temperature (5 K) only the S = 1 / 2 ground state of {Cr 7 Ni} is populated substantially; this state is approximately axially symmetric with ⊥ 7 H 1.78, || 7 H 1.74 where the "unique" axis is perpendicular to the {Cr 7 Ni} plane. [12] The EPR spectrum of 2 at Q-band (34 GHz) and 5 K is very simple with two sets of features, centered at the g-values expected for {Cr 7 Ni} and for Cu ( ⊥ H 2.05, || H 2.30) with resolution of copper hyperfine (A Cu ) on || . Each of these g-features is split into a spectroscopic doublet; i.e. the spectrum has the form of a simple AB spin pattern, where the spin-spin coupling is much smaller than the difference in Zeeman energy (the latter is 0.17 cm -1 for 4 a static field of 1.21 T and g = 2.1 and 1.8). This is a picture familiar from NMR spectroscopy, but examples of AB electron spin patterns resolved by cw EPR are rare. [2a,13] The resoluti...

Heterometallic Rings: Their Physics and use as Supramolecular Building Blocks

“…Ther esult is ac ompound [Cr 6 Zn 2 (m-F) 8 (O 2 C t Bu) 15 (Py-C 6 H 4 -CH 2 -NH 2 -C 2 H 4 -Ph] 2 (35); one of the two Zn sites is five-coordinate, and bound to the pyridyl-donor from the thread. [77] 6. [20] [2]rotaxanes and [3]rotaxanes with threads terminated by ap yridine can also act as ligands for mononuclear copper sites,giving discrete molecules rather than 1D-polymers.…”

“…Ther esult are spectra where the fine structure is resolved in amanner analogous to NMR spectroscopy. [77]…”

“…Q-band EPR spectra of 36 (left) and 37 (right);e xperimental spectra top, simulations below and energy levels shown at the bottom.M odified from Ref [77]. with permission.…”

Molecular spin qudits for quantum algorithms

“…Molecular electron spin qubits , while it remains very robust when linked to one 29,31,70 or more 71 units, leading to their proposal for logic gates.…”

A room temperature phosphorescence encoding [2]rotaxane molecular shuttle

“…Rotaxanes, 1 as one of the challenging topics of molecular machines, have attracted continued interest and attention. A variety of appealing rotaxanes have been fabricated over the recent years, with various functions or applications in such areas as switches, 2 logic gates, 3 artificial muscles, 4 molecular elevators, 5 molecular motors, 6 and so forth.…”