“Two-Point” Assembling of Zn(II) and Co(II) Metalloporphyrins Derivatized with a Crown Ether Substituent in Langmuir and Langmuir−Blodgett Films

Abstract

The effect of "two-point" interactions of Zn(II) and Co(II) metalloporphyrins, bearing 15-crown-5 ether peripheral substituents, on their assembling in Langmuir and Langmuir-Blodgett (LB) films was investigated. That is, simultaneously, the central metal ion of the porphyrin was axially ligated by a nitrogen-containing ligand in the emerged part of the Langmuir film on one hand, and a suitably selected cation pertaining in the subphase solution was supramolecularly complexed by the crown ether moiety in the submerged part of the film on the other. The compression and polarity properties of the Langmuir films of the derivatized free-base 5,10,15-triphenyl-20-(benzo-15-crown-5)porphyrin, H2(TPMCP), and the corresponding cobalt(II) and zinc(II) metalloporphyrins, denoted as Co(TPMCP) and Zn(TPCMP), respectively, as well as inclusion complexes of the metalloporphyrins with selected cations were investigated. For the axial ligation of Zn(II) and Co(II), pyrazine (pyz) and 4,4'-bipyridnine (bpy) aromatic as well as piperazine (ppz) and 1,4-diazabicyclo[2.2.2]octane (DABCO) cyclic heteroaliphatic ligands were selected. The films were formed on the water subphase solution in the absence and presence of LiCl, NaCl, or NH4Cl. The Langmuir films were built of monolayer J-type aggregates of tilted porphyrin macrocycles. The porphyrins formed rather labile complexes with the cations in the subphase. Nevertheless, the XPS analysis revealed that these cations were LB transferred together with the porphyrins onto solid substrates. In the Co(TPMCP) Langmuir films formed on the water subphases, Co(II) was complexed by aromatic but not cyclic heteroaliphatic ligands, while, in these films formed on the NaCl subphase solutions, the metalloporphyrin was also complexed by DABCO. In Langmuir films spread on alkaline subphase solutions, both aromatic and heteroaliphatic ligands formed complexes with Co(TPMCP) of different stoichiometries. The X-ray reflectivity and GIXD measurements performed on selected LB films revealed some structure-building effects of the axial ligation.