Cavity-Forming Cofacial Porphyrin Dimers via Cooperative Zn2+ Binding: Structure, Formation Pathways, and Host Properties

Porphyrins bearing four 2,2'-bipyridine ligands linked through acetylene or diacetylene spacers, ZnPor(C2bpy)4 and ZnPor(C4bpy)4 were designed to self-assemble into cavity-containing cofacial dimers through Zn2+ coordination. Electrospray ionization mass spectrometry confirmed the formation of [(ZnPor(Cnbpy)4)2Zn4]8+ (n = 2, 4). For ZnPor(C2bpy)4, Bayesian Markov chain Monte Carlo (MCMC) analysis of through-space 1H NMR chemical shift changes identified a cofacial dimer as the probable solution structure. Titration of the porphyrins with Zn(OTf)2 produced highly sigmoidal changes in their UV-Vis absorption spectra. Spectral deconvolution by multivariate curve resolution-alternating least-squares, followed by Bayesian MCMC equilibrium analysis, identified the most probable intermediate distributions and supported a stepwise pathway involving side-by-side (J-type) porphyrin dimers before formation of the final cofacial (H-type) dimer. The cavities exhibited spacer-dependent guest selectivity: ZnPor(C2bpy)4 encapsulated electron-deficient aromatic guests through π-π stacking, whereas ZnPor(C4bpy)4 accommodated 4,4'-bipyridine and 4,4'-dipyridylacetylene through ditopic axial coordination. r2SCAN-3c geometry optimizations were consistent with the proposed binding modes. These results establish Bayesian MCMC analysis as a powerful approach for resolving multiparameter systems, including multicomponent supramolecular assemblies, while explicitly accounting for uncertainty.