Synthesis and film-forming properties of ethynylporphyrins

Thermal treatment of ethynyl porphyrin monomers on a surface has been found to yield robust porphyrin films. The scope of this in situ polymerization has been surveyed by the synthesis and characterization of a collection of 20 zinc porphyrins bearing diverse patterns of 1-4 ethyne (or protected ethyne) groups and a variety of nonlinking substituents. Films have been prepared on Si(100), SiO2, Au(111), and glass. The films prepared on Si(100) have been examined by electrochemical methods, which indicate that surface coverages 50-fold greater than those of saturation-coverage monolayers are achievable, although the coverage varies appreciably (10-fold) among the survey group of molecules under a controlled set of film-forming conditions. Variation in these conditions affords control over the number of layers in the film (from a few to tens or more). The electrochemical characteristics of the multilayer films further indicate that the redox thermodynamics are of comparable homogeneity to those of monolayers. Examination of the films by FTIR and resonance Raman spectroscopy indicates the absence of an ethynyl or ethenyl linkage between the porphyrins. SEM analysis indicates the porphyrin polymer films range in thickness from tens to hundreds of nanometers. The in situ polymerization technique not only enables control over the thickness of the porphyrin polymer film but also sidesteps solubility problems that typically result upon solution polymerization. The ability to access the porphyrins in the films electrochemically to form porphyrin cation radicals affords increased surface charge density versus that with a monolayer, suggesting the application of the porphyrin films as materials for electrically addressable, molecular-based information storage.