Bright oxygen- and vacancy-derived spin-singlet diamond color centers with metastable spin triplets: OV²⁺ and VOV²⁺

The ST1 diamond color center was experimentally demonstrated to involve a substitutional oxygen atom O$_C$ and carbon vacancy V$_C$, has a spin singlet ground-state, and a metastable electron spin ancilla: a triplet. ST1's structure was left unsolved for more than a decade. With embedded multiconfigurational quantum mechanical theory, we investigate O_C-V_C-derived diamond defects, specifically both 0 and +2-charged coupled O_CV_C, and O_C surrounded by V_Cs along the [110] axis V$_C$O$_C$V$_C$. We found both O_CV_C²⁺ C$_3v$ and V_CO_CV_C²⁺ C$_2v$ to have a spin-singlet ground state 1$¹$A$₁$ and metastable spin triplets. We demonstrate ST1 to be V_CO_CV_C²⁺. The calculated vertical excitation energies of V_CO_CV_C²⁺'s first 1$¹$B$₂$ and second 2$¹$A$₁$ bright spin-singlet excited states closely match ST1's experimental zero phonon line (2.2-2.3 eV). O_CV_C²⁺ $¹$E absorbs much higher (2.8 eV). The two O lone pairs favor V_CO_CV_C over O_CV_C, in a similar manner as the single N lone pair favors formation of N_CV_C centers.