Optically addressable high-spin states (S ≥ 1) of defects in semiconductors are the basis for the development of solid-state quantum technologies. Recently, one such defect has been found in hexagonal boron nitride (hBN) and identified as a negatively charged boron vacancy (${\mathrm{V}}_{\mathrm{B}}^{-}$). To explore and utilize the properties of this defect, one needs to design a robust way for its creation in an hBN crystal. We investigate the possibility of creating ${\mathrm{V}}_{\mathrm{B}}^{-}$ centers in an hBN single crystal by means of irradiation with a high-energy (E = 2 MeV) electron flux. Optical excitation of the irradiated sample induces fluorescence in the near-infrared range together with the electron spin resonance (ESR) spectrum of the triplet centers with a zero-field splitting value of D = 3.6 GHz, manifesting an optically induced population inversion of the ground state spin sublevels. These observations are the signatures of the ${\mathrm{V}}_{\mathrm{B}}^{-}$ centers and demonstrate that electron irradiation can be reliably used to create these centers in hBN. Exploration of the ${\mathrm{V}}_{\mathrm{B}}^{-}$ spin resonance line shape allowed us to establish the source of the line broadening, which occurs due to the slight deviation in orientation of the two-dimensional B-N atomic plains being exactly parallel relative to each other. The results of the analysis of the broadening mechanism can be used for the crystalline quality control of the 2D materials, using the ${\mathrm{V}}_{\mathrm{B}}^{-}$ spin embedded in the hBN as a probe.

Structural features and dynamical behaviour of the copper(II) bis-complexes with glycine, D-alanine, D-valine, L-serine, L-aspartic acid, L-glutamic acid, L-lysine, L-proline, and sarcosine were studied by combined EPR and NMR relaxation methods. The cis and trans isomers were unambiguously assigned and characterized by EPR data. It was found that addition of a salt background has an influence on the cis–trans isomer equilibrium in favour of the formation of the cis isomer. By comparison of NMRD, DFT computations, and structural data it was shown that only one water molecule is coordinated in the axial position of these complexes. The increased exchange rates of this molecule found for Cu(L-Asp)₂ ²⁻, Cu(L-Glu)₂ ²⁻, Cu(L-LysH)₂ ²⁺, and Cu(L-Pro)₂ were attributed to its pushing out by side chain groups of the ligands. By simulation of NMRD profiles an increase of lifetimes of the copper(II) 2nd coordination sphere water molecules was revealed in the presence of additional carboxylic, alcoholic, or ammonium groups of the ligands, as well as the pyrrolidine ring of proline. The very short lifetimes of the 2nd coordination sphere water molecules (4–13 ps at 298 K) were explained in terms of the Frank–Wen structural model by the existence of cavities which draw in quickly enough water molecules from the 2nd coordination sphere.