MONOLAYER ASSEMBLIES MADE OF OCTOPUSPORPHYRINS WITH PYRIDINIUM HEADGROUPS - ELECTRON-TRANSFER REACTIONS IN NONCOVALENT PORPHYRIN-QUINONE PLATELETS IN AQUEOUS-MEDIA

Water-soluble tetraresorcinolporphyrins with eight omega-pyridinium al kyl chains (octopusporphyrins) formed fluorescent and nonfluorescent m onolayer assemblies by anion exchange of the headgroups. Electron micr oscopy of the evaporated solution of the octopusporphyrin having 11-py ridinium-undecanoyl side chains (1) with sodium perchlorate showed pla nar sheets. The uniform thickness of the layer was 4.0 +/- 0.5 nm, cor responding to monomolecular platelets. An exciton calculation on the b asis of the red shift of the Soret band (6 nm) is consistent with a tw o-dimensional arrangement with porphyrin separations of 25.6 and 17.4 Angstrom in the x and y directions, respectively. Organic dianions suc h as anthraquinone-2,6-disulfonate (AQDS(2-)) were more effective than perchlorates or iodides for aggregate formation. Arrays of a 1:3 comp lex of 1/AQDS(2-) produced a curvature to yield nonfluorescent vesicle s. The introduction of dimethyl groups at the bottom of the alkyl chai ns (octopusporphyrin 2) did not lead to enhanced aggregation, while th e octopusporphyrin with long 2,2-dimethyl-C-20-pyridinium chains (3) f ormed fluorescent fibers without assistance of special anions. The ele ctron-transfer reactions of the 1 platelets with perchlorates, in whic h the relative fluorescence intensity was 30% of the monomer, were inv estigated. External addition of negatively charged electron accepters, 1,2-naphthoquinone-4-sulfonate (NQS(-)) and anthraquinone-2-sulfonate (AQS(-)), led to partial quenching of the fluorescence of the central porphyrin layer. In both cases, the corresponding Stern-Volmer plots showed plateaus at sufficiently high concentration of the quinones. Th e results have been evaluated using equations derived for this special case of dynamic quenching by an electrostatically bound quencher. Bin ding constants of 3.4 x 10(4) and 1.7 x 10(5) M-1 and electron-transfe r constants of 5 x 10(9) and 1.3 x 10(9) s(-1) have been calculated fo r NQS(-) and AQS(-), respectively.