Specific binding of iodide ion to N-confused tetraphenylporphyrin (NC-TPP) at the air–water interface

Abstract

Halide recognition by a porphyrin aggregate at the air-water interface has been studied. π-A Isotherms and UV reflection spectra of mixed monolayers of 5,10,15,20-tetraphenyl-2-aza-21-carbaporphyrin, trivial name ‘N-confused tetraphenylporphyrin’(NC-TPP), with methyl octadecanoate were measured in 1 mmol dm^–3 NaX aqueous solution (X = F, Cl, Br, I). π-A Isotherms of NC-TPP alone were extracted from those of the mixed monolayers at various mixing ratios. Among the anions investigated, only the I^– containing subphase gave a more condensed π-A isotherm and a shifted UV reflection spectrum. The Soret band showed a red shift (465 nm) in NaI solution and blue shifts (425 nm) in pure water and in the other salt solutions, and these shifts were attributed to the occurrence of two different aggregation modes, J-like aggregate and H aggregates, respectively. X-Ray photoelectron spectroscopy (XPS) also demonstrated the incorporation of iodine but no other halogen atoms in the LB films. Loss of counterions (F^–, Cl^–, Br^–) in concurrence with deprotonation during LB transfer apparently induced formation of the J-like aggregate of free-base NC-TPP in the transferred films. In contrast, iodide was kept in the film upon LB transfer, because the aggregation mode of NC-TPP remained unchanged. The important role of the modes of porphyrin aggregation in the anion recognition is discussed.