Characterization of Porphyrin Surface Orientation in Monolayers on Au(111) and Si(100) Using Spectroscopically Labeled Molecules

Abstract

The synthesis and surface IR characterization are reported for a series of porphyrins bearing vibrational spectroscopic labels that afford distinction of the two in-plane axes of the porphyrin ring. The labeled porphyrins include three different types of tethers for surface attachment, those containing either methylthio (−CH2S−) or benzylthio (−BzS−) for attachment to either Au or Si and a tripodal alkenyl group for attachment to Si. The spectroscopic labels are placed so as to enable distinction between the methyl group of a p-tolyl substituent that lies along the molecular axis distal to the tether versus the p-tolyl substituents that lie along the orthogonal in-plane axis lateral to the tether. The porphyrins include isotopically labeled species containing CD3 and species wherein a CF3 replaces CH3. The spectroscopically labeled molecules allow evaluation of both the tilt angle (θ) with respect to the surface normal and the rotation angle (φ) about the molecular axis. These two angles cannot be uniquely determined for typical porphyrins because the in-plane modes of the porphyrin are (nearly) degenerate, and unique vibrational signatures cannot be identified that define the two orthogonal in-plane axes. The surface IR studies suggest that all of the porphyrins on both Au and Si exhibit a distribution of tilt and rotation angles. The distribution of φ angles is (nearly) random about the molecular axis; the distribution of tilt angles is less broad, owing to steric interactions between the porphyrin substituents and the surface. The surface coverage affects the distribution of both the tilt and rotation angles. At lower surface coverage, the molecules exhibit larger tilt angles and rotation angles, that is, the porphyrin is more coplanar with the surface. The fact that all of the porphyrins, which bear structurally different types of tethers, on both Au and Si exhibit qualitatively similar surface orientation characteristics suggests that the adsorption geometry is primarily controlled by properties intrinsic to the porphyrin macrocycle rather than by properties of the tether and/or the surface.