Cooperative self-assembly of double-strand conjugated porphyrin ladders

A series of conjugated zinc porphyrin oligomers, from the dimer through to the hexamer, form stable ladder complexes with linear bidentate ligands suc h as 1,4-diazabicyclo[2.2.2]octane (DABCO) and 4,4'-bipyridyl (Bipy). The H -1 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 mu ltivariate global factor analysis, in two different solvents (chloroform an d toluene). Ladder formation and dissociation exhibit many indications of p ositive cooperativity, including all-or-nothing two-state assembly, sigmoid al isotherms, large Hill coefficients, and narcissistic self-sorting. Ladde r formation increases the planarity and conjugation, resulting in a reducti on in the gap between the highest occupied and lowest unoccupied molecular orbitals.