Nonlinear and emissive {[M$^{III}$(CN$_{6}$]$^{3–}$ ···polyresorcinol} (M = Fe, Co, Cr) cocrystals exhibiting an ultralow frequency Raman response

Optically active functional noncentrosymmetric architectures might be achieved through the combination of molecules with inscribed optical responses and species of dedicated tectonic character. Herein, we present the new series of noncentrosymmetric cocrystal salt solvates (PPh$_{4}$)$_{3}$[M(CN)$_{6}$](L)$_{n}$·msolv (M = Cr(III), Fe(III), Co(III); L = polyresorcinol coformers, multiple hydrogen bond donors: 3,3′,5,5′-tetrahydroxy-1,19-biphenyl, DiR, n = 2, or 5′-(3,5-dihydroxyphenyl)-3,3″,5,5″-tetrahydroxy-1,19:3′,1″-terphenyl, TriRB, n = 1) denoted as MDiR and MTriRB, respectively. The hydrogen-bonded subnetworks {[M(CN)$_{6}$]$^{3¯}$;L$_{n}$}$_{∞}$ of dmp, neb, or dia topology are formed through structural matching between building blocks within supramolecular cis-bis(chelate)-like {[M(CN)$_{6}$]$^{3¯}$; ;(H$_{2}$L)$_{2}$(HL)$_{2}$} or tris(chelate)-like {[M(CN)$_{6}$]$^{3¯}$; (H$_{2}$L)$_{3}$} fragments. The quantum-chemical analysis demonstrates the mixed electrostatic and covalent character of these interactions, with their strength clearly enhanced due to the negative charge of the hydrogen bond acceptor metal complex. The corresponding interaction energy is also dependent on the geometry of the contact and size matching of its components, rotational degree of freedom and extent of the $\pi$-electron system of the coformer, and overall fit to the molecular surroundings. Symmetry of the crystal lattices is correlated with the local symmetry of coformers and {complex;(coformer)$_{n}$} hydrogen-bonded motifs characterized by the absence of the inversion center and mirror plane. All compounds reveal second-harmonic generation activity and photoluminescence diversified by individual UV–vis spectral characteristics of the components, and interesting low-frequency Raman scattering spectra within the subterahertz spectroscopic domain. Vibrational (infrared/Raman), UV–vis electronic absorption (experimental and calculated), and $^{57}$Fe Mössbauer spectra together with electrospray ionization mass spectrometry (ESI-MS) data are provided for the complete description of our systems.