Exploring the Effect of Electronic Symmetry on the Hyperpolarizabilities of Co(II) and Ni(II) Coordination Complexes with Cytidine Nucleotide

The most significant factor in the design of high-performance nonlinear optical (NLO) materials is electronic symmetry, which directly influences hyperpolarizability and second harmonic generation (SHG) response. This work presents two isostructural one-dimensional coordination complexes, {[Co₂(CMP)₂(BIPY)₂(H₂O)₆]·11H₂O}_n (<b>I</b>) and {[Ni₂(CMP)₂(BIPY)₂(H₂O)₆]·11H₂O}_n (<b>II</b>), (CMP = Cytidine Monophosphate, BIPY = 4,4'-bipyridine), crystallized in the noncentrosymmetric (NCS) P2₁ space group. The cobalt-based complex (<b>I</b>) establishes an NCS environment due to its pronounced octahedral distortion and lower electronic symmetry, coupled with intrachain hydrogen bonding and π-π stacking, resulting in enhanced hyperpolarizability and a robust second-harmonic generation response. Conversely, the nickel-based complex (<b>II</b>) demonstrates comparatively weaker NLO characteristics attributable to its higher symmetry. Experimental and theoretical findings have established that the superior NLO performance of complex (<b>I</b>) is intrinsically linked to its low symmetry, narrow band gap, and significant intermolecular interactions. This research demonstrates that disrupting electronic symmetry can significantly amplify the nonlinear optical response through supramolecular architecture in coordination polymers.