Langmuir, Vol.12, No.8, 2019-2027, 1996
Langmuir-Blodgett-Films of Stearic-Acid Containing Octakis((Methoxycarbonyl)Methyl)-Meso-Tetrakis(((Eicosanyloxy)Carbonyl)Phenyl)Porphyrin
Molecular mechanics calculations are coupled with a knowledge of properties of porphyrins in solution and available synthetic strategies to computer-designed a lipoporphyrin, [2,3,7,8,12,13,17,18-octakis-((methoxycarbonyl)methyl)-5,10,15,20-tetrakis(((eicosanyloxy)carbonyl)phenyl)porphrin] (LipoP), for future electron-transfer studies and biosensor applications. To prevent separation of the porphyrin and stearic acid phases in the films, the head group was chosen on the basis of earlier studies that showed that its nickel(II) derivative does not pi-pi aggregate in aqueous solution. The lack of aggregation of the porphyrin unit results from the high degree of nonplanarity of the macrocycle and the steric constraints of the twelve bulky peripheral acid substituents. Nonpolar tails consisting of 20-carbon long linear alkanes were attached to the head group to anchor the LipoP head group into the stearic acid films. Metal derivatives of LipoP have been synthesized and characterized by NMR, resonance Raman, and UV-visible spectroscopic methods. Also, the solubility and solution aggregation properties of LipoP were investigated. Finally, Langmuir films of the Ni derivative of LipoP in stearic acid (Sh) (and for comparison also films of nickel protoporphyrin dimethyl ester (PPDME) in SA) were fabricated and characterized by pressure-area isotherms and then transferred as Langmuir-Blodgett (LB) films onto glass or silicon substrates. These NiLipoP-SA and NiPPDME-SALB films were characterized by spectroscopic methods. Resonance Raman spectra show that NiLipoP does not pi-pi aggregate in the films, unlike NiPPDME. Molecular mechanics calculations of the interactions between NiLipoP, a stearic acid monolayer, and surrounding water molecules support a structural interpretation of the spectral and physical properties of the NiLipoP-SA films that locates the porphyrin head group outside of the hydrophobic region of the films while remaining anchored into the film by the four attached hydrocarbon chains.