Thiol-derivatized porphyrins for attachment to electroactive surfaces

The attachment of porphyrin monomers and multiporphyrin arrays in controlled architectures on electroactive surfaces opens many opportunities for electrochemical studies. Toward this goal, we have developed routes for the preparation of thiol-derivatized porphyrin monomers and porphyrin building blocks that require minimal or no handling of free thiols. Routes to S-protected p-thiobenzaldehydes and m-(thiomethyl)benzaldehydes have been developed. TRIO sets of mesosubstituted porphyrins with variation in electrochemical potentials have been prepared for vertical or horizontal orientation with respect to the electroactive surface. In one set, each porphyrin bears one S-protected p-thiophenyl unit and substituents at the three remaining meso-positions. In the other set each porphyrin possesses four S-protected m-(thiomethyl)phenyl units. Tuning the electrochemical potential in the former set has been achieved by variation of the meso substituents (mesityl, 2,4,6-trimethoxyphenyl, n-pentyl, pentafluorophenyl) and in the latter set by variation of the central metal (Zn, Cu, Co, Ag). Six thiol protecting groups ES-cyano, S-(N-ethylcarbamoyl), S-acetyl, S-(9-anthrylmethyl), S-(2,4-dinitrophenyl), S-pivaloyl] have been found to be compatible with porphyrin formation and metalation with zinc. The S-cyano, S-(N-ethylcarbamoyl), and S-acetyl groups undergo in situ cleavage and/or binding on a gold surface. Of these three, only the S-acetyl protecting group is compatible with the Pd-mediated iodo-ethyne coupling conditions for the preparation of multiporphyrin arrays. Three trans-substituted porphyrin building blocks have been prepared for the synthesis of multiporphyrin arrays that can be attached to an electroactive surface. One porphyrin has two mesityl, one p-iodophenyl, and one p-(S-acetylthio)phenyl substituent for vertical positioning, and two porphyrins each have two m-iodophenyl and two S-protected m-(thiomethyl)phenyl substituents for horizontal positioning. Altogether, 16 free base and 16 metalloporphyrins have been prepared. This work establishes the foundation for preparing diverse thiol-derivatized porphyrin monomers and building blocks.