Assemblies of "hinged" iron-porphyrins as potential oxygen sensors

Sequential self-assembly of a two-component system on a solid support is de scribed with respect to structure and function. Two ligands, which bind to the semiconductor surface through one end and axially ligate a heme analogu e at the other end, are described. Monolayer assemblies of complexes formed by these ligands and iron-porphyrin perform reversible binding of molecula r oxygen. In the manolayer, a metalloporphyrin rin (the sensing unit) is he ld by the intervening ligand that serves as a "hinge'", away from the solid surface. Sensing events based on porphyrin chemistry are communicated via the ligand to the: Solid support. The transduction manifests itself as a ch ange in the solid's surface electronic properties. Synthesis of the ligands and analysis of its complex formation with Fe-III-porphyrin are described. The anisotropic orientation of the porphyrin ring within the ligand cavity , due to restricted rotation around the Fe-III-N imidazole bonds, was probe d by H-I NMR measurements in solution. We show that the porphyrin substitue nts stand as barriers for the free rotation even at room temperature. Molec ular modeling supports the NMR evidence and reveals the stable conformation s for the porphyrin's orientation relative to the solid support, The comple xes wen assembled as films on the (0001) surface of etched n-CdSe single cr ystals, and the Films were characterized using transmission Fourier transfo rm infrared (FTIR) and X-ray photoelectron (XPS) spectroscopies. Contact po tential difference (CPD) and steady-state photoluminescence (PL) measuremen ts of the derivatized CdSe show that the intervening ligands yield better c onjugation and; stronger binding of the sensing unit to the semiconductor s urface, relative to direct adsorption of metalloporphyrins. Furthermore, th e PL changes in the Cdse: can be used to follow the interaction of the surf ace-bound Fe-III-porphyrin-ligand complexes with molecular oxygen, A model is proposed to explain the electronic changes resulting from binding of O-2 to the monolayer.