Tunable Cr⁴⁺ Molecular Color Centers

Abstract

The inherent atomistic precision of synthetic chemistry enables bottom-up structural control over quantum bits, or qubits, for quantum technologies. Tuning paramagnetic molecular qubits that feature optical-spin initialization and readout is a crucial step toward designing bespoke qubits for applications in quantum sensing, networking, and computing. Here, we demonstrate that the electronic structure that enables optical-spin initialization and readout for S = 1, Cr(aryl) 4 , where aryl = 2,4dimethylphenyl (1), o-tolyl (2), and 2,3-dimethylphenyl (3), is readily translated into Cr(alkyl) 4 compounds, where alkyl = 2,2,2triphenylethyl (4), (trimethylsilyl)methyl ( 5), and cyclohexyl (6). The small ground state zero field splitting values (<5 GHz) for 1− 6 allowed for coherent spin manipulation at X-band microwave frequency, enabling temperature-, concentration-, and orientation-dependent investigations of the spin dynamics. Electronic absorption and emission spectroscopy confirmed the desired electronic structures for 4−6, which exhibit photoluminescence from 897 to 923 nm, while theoretical calculations elucidated the varied bonding interactions of the aryl and alkyl Cr 4+ compounds. The combined experimental and theoretical comparison of Cr(aryl) 4 and Cr(alkyl) 4 systems illustrates the impact of the ligand field on both the ground state spin structure and excited state manifold, laying the groundwork for the design of structurally precise optically addressable molecular qubits.

Spin-flip luminescence

“…Trajectory surface hopping simulations within a linear vibronic coupling model arrived at a similar value of 1.7 ± 0.3 ps [145]. Interestingly, under hydrostatic pressure V3 showed a hypsochromic shift by + 10 cm -1 kbar -1 Scheme 2 Structures of luminescent V III and Cr IV complexes [62,128,130,131,142,144] in contrast to the bathochromic shifts found for Cr4 3+ . This positive shift was rationalized by the combined effect of changes in singlet energy and ground state splitting under pressure [144].…”

“…Intraconfigurational spin-flip transitions can potentially be exploited in optically addressable qubits. Complexes of Cr III , Cr IV , V III and Ni II like Cr4 3+ , Cr17-Cr22, V4, Ni1 2+ and Ni2 2+ (Tables 1 and 2, Schemes 1, 2 and 5) were proposed as suitable candidates [127][128][129][130][131][132]. Their properties are discussed in more detail below.…”

“…Hence spinflip emission from a 1 E → 3 A 2 transition could be achieved with a high ligand field splitting in tetrahedral d 2 complexes [147]. In fact, tetrahedral Cr IV complexes with anionic alkyl or aryl ligands Cr17-Cr22 emit between 897 and 1025 nm at 4 K and were proposed as optically addressable qubit candidates (Scheme 2, Table 2) [130,131]. For the investigation, the complexes Cr18-Cr22 were diluted in isostructural Sn IV host lattices while a Sn IV (2,4-dimethylphenyl) 4 lattice was used for Cr IV (CH 2 CPh 3 ) 4 Cr17 (CH 2 CPh 3 -= 2,2,2-triphenyleth-1-yl).…”

“…A resulting incompatibility with this host lattice in Cr17 served to explain its broad emission compared to the extremely narrow bandwidths obtained for Cr18-Cr22. Because of the stronger nephelauxetic effect in the aryl complexes Cr20-Cr22, their emission is of lower energy than in the alkyl derivatives Cr17-Cr19 [131]. The phosphorescence lifetimes of Cr20-Cr22 in the host lattices at 4 K were determined as 3.3, 5.7 and 6.9 µs, respectively [130].…”

Multiconfiguration Pair-Density Functional Theory for Chromium(IV) Molecular Qubits

“…The coordination environment of the metal center has been shown to impact the spin properties significantly. 16,17,70 Thus, the geometries used for computations are especially crucial.…”