Self-assembly of a porphyrin array via the molecular recognition approach:Synthesis and properties of a cyclic zinc(II) porphyrin trimer based on coordination and hydrogen bonding

Self-assembly of [5-(pyrazol-4-yl)-10,20-bis(p-tolyl)-15- (2-ethoxycarbonyl phenyl)porphyrinato]-zinc(II) (1), designed to have both a coordination sit e and a hydrogen bonding site, leads to a stable cyclic trimer array where coordination of the pyrazole nitrogen to the zinc(ll) ion as well as hydrog en bonding between carbonyl oxygen and pyrazole NH holds each zinc(II) porp hyrin. The recognition event for pyrazole has been confirmed preliminarily in the model studies using [5-(2-ethoxycarbonylphenyl)tris(p-tolyl)porphyri nato]-zinc(II) (3). The zinc(II) porphyrin 3 has large affinity for pyrazol e due to the hydrogen bond between pyrazole and the 2-ethoxycarbonyl group in addition to the coordination bonding accompanied by the conformational c hange of the ethoxycarbonyl group in the coordination process. The H-1 NMR, IR, and UV-vis spectra of 1 and its ESI-MS and VPO measurements have revea led the cyclic trimer structure with an overall association constant of 6.0 x 10(13) M-2 at 22 degreesC. The contribution of the hydrogen bond to the total free energy change in trimer formation is estimated to be 7.5 kcal/mo l based on a reference trimer system without a hydrogen bonding site. The t rimer geometry causes characteristic exitonic interaction between porphyrin units to yield a broad Soret band which is deconvoluted into four componen ts by UV-vis and MCD spectral analyses. Electrochemical measurements have s hown that only the first ring-oxidation process proceeds stepwise in the tr imer.