Synthesis of strapped porphyrins: toward isolation of the chromophore on semiconductor surfaces.

Strapped porphyrins, designed to bind planar to semiconductor surfaces, were synthesized as model dyes to study the sensitization processes on metal oxide semiconductor nanoparticles surfaces. The strap was added to limit porphyrin-porphyrin stacking and contacts with the semiconductor, while constraining the anchoring groups' orientation. Diester ZnPorOMe(2) (2b) was made of a Zn(II) 5,10,15,20-tetraphenylporphyrin (ZnTPP) with the 5 and 15 meso-phenyl groups tethered by a 1,4-phenylenebis(butyl-4-oxy) unit (the strap) and carrying a methylbenzoic ester (the anchoring group) in the meta position. The synthesis involved a useful intermediate, dibromide ZnPorOBr(2) (2a). The X-ray structure of the corresponding metal-free H(2)PorOBr(2) shows that, in the crystal, the strap is located on one side above the macrocycle and tilted with respect to the porphyrin plane. The characterization included (1)H-(1)H COSY, (1)H-(13)C HMQC, and DEPT. The spectroscopic and electrochemical properties of 2b varied little when compared to ZnTPP, suggesting that the properties are not greatly influenced by the tether or the anchor groups. The HOMO-LUMO gap for 2b, at 2.14 V, was typical for a ZnTPP derivative. Compound 2b was bound to TiO(2) and ZrO(2) nanoparticle films. On TiO(2), the fluorescence emission was fully quenched, while the emission on insulating ZrO(2) was similar to the solution spectra.