ADSORPTION OF ORIENTED METHYL PYRIDINIUM PORPHYRINS ON SOLID SUBSTRATES

Free base methyl pyridinium porphyrins were adsorbed on microscope gla ss slides, quartz plates or silicon wafers from a berate buffer soluti on at pH 10. Adsorption of the porphyrins is strongly enhanced by elec trostatic interaction of the positively charged methyl pyridinium side groups with the SiO surface groups. Using UV-visible absorption spect roscopy, reflectometry and total internal reflection fluorescence (TIR F), the surface coverage, adsorption kinetics and the orientation of t he porphyrins relative to the substrate surface were found to depend o n the number and arrangement of the pyridinium groups attached to the porphyrin macrocycle, i.e. for the di-cis- (DiMPyP), the tri(TriMPyP) and the tetra- (TMPyP) methyl pyridinium substituted compounds. For al l compounds the initial adsorption follows first-order kinetics. The l imiting value of the absorbance of TriMPyP and TMPyP corresponds appro ximately to a full monolayer coverage. Adsorption of DiMPyP steadily i ncreases beyond monolayer coverage at increasing exposure periods, how ever. TIRF measurements indicate that the TMPyP molecular plane is pre ferentially oriented parallel with respect to the surface on adsorptio n, in contrast to DiMPyP and TriMPyP which have a more random orientat ional distribution.