Investigation of tightly coupled porphyrin arrays comprised of identical monomers for multibit information storage

Our prior designs for molecular-based information storage devices have empl oyed multiple redox-active units organized in weakly coupled, covalently li nked arrays. To explore a simpler design, we report here the synthesis of p orphyrin arrays where porphyrins with identical oxidation potentials are di rectly linked to one another instead of joined via a molecular linker. Oxid ative coupling with AgPF6 of zinc(II)-5,15-bis(4-tert-butylphenyl)-10-pheny lporphyrin obtained by a rational synthesis, afforded the expected dimer jo ined by a meso;meso linkage and an unexpected trimer joined by meso-meso li nkages. For attachment to an electroactive surface we synthesized a meso-li nked porphyrin dimer with a thiol-linker in one of the meso positions. The S-acetyl protecting group was used to avoid handling free thiol groups. Cou pling of zinc(II)-5, 10,15-tris(3,5-di-tert-butylphenyl)porphyrin ("upper h alf") and zinc(II)-5[4-(S-acetylthio)phenyl]-10,20 20-bis(3,5-di-tert-butyl phenyl)porphyrin ("lower half") afforded three different meso-linked dimers with the desired dimer as the main product. Electrochemical examination of the meso-linked dimer in solution shows that the first two oxidation poten tials of the array differ by similar to0.15 V and straddle the value exhibi ted by the monomeric constituents. The third and fourth oxidation potential s of the array are also split although to a lesser extent (similar to0.08 V ) than the first and second. For the meso-linked trimer, the first three ox idation waves are also split; however, these waves are severely overlapped. The electrochemical behavior of the dimers and trimer is indicative of str ong electronic interactions among the porphyrins. The thiol-derivatized mes o-linked dimers form self-assembled monolayers (SAMs) on gold via in situ c leavage of the S-acetylthio protecting group. The porphyrin SAM exhibits fo ur well-resolved oxidation waves. Regardless, the meso-meso linkage is rela tively unstable upon formation of the pi -cation radical(s). This character istic indicates that the structural motif is of limited utility for molecul ar information storage elements.