Cooperative Self-Assembly of Double-Strand Conjugated Porphyrin Ladders

Abstract

A series of conjugated zinc porphyrin oligomers, from the dimer through to the hexamer, form stable ladder complexes with linear bidentate ligands such as 1,4-diazabicyclo[2.2.2]octane (DABCO) and 4,4‘-bipyridyl (Bipy). The 1H NMR spectra of these ladders confirm their structures and show how they fray and dissociate in the presence of excess ligand. The thermodynamics of these equilibria were elucidated by spectrophotometric titration, using multivariate global factor analysis, in two different solvents (chloroform and toluene). Ladder formation and dissociation exhibit many indications of positive cooperativity, including all-or-nothing two-state assembly, sigmoidal isotherms, large Hill coefficients, and narcissistic self-sorting. Ladder formation increases the planarity and conjugation, resulting in a reduction in the gap between the highest occupied and lowest unoccupied molecular orbitals.