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 alkyl chains (octopusporphyrins) formed fluorescent and nonfluorescent monolayer assemblies by anion exchange of the headgroups. Electron microscopy of the evaporated solution of the octopusporphyrin having 11-pyridinium-undecanoyl side chains (1) with sodium perchlorate showed planar sheets. The uniform thickness of the layer was 4.0 +/- 0.5 nm, corresponding to monomolecular platelets. An exciton calculation on the basis of the red shift of the Soret band (6 nm) is consistent with a two-dimensional arrangement with porphyrin separations of 25.6 and 17.4 Angstrom in the x and y directions, respectively. Organic dianions such as anthraquinone-2,6-disulfonate (AQDS(2-)) were more effective than perchlorates or iodides for aggregate formation. Arrays of a 1:3 complex of 1/AQDS(2-) produced a curvature to yield nonfluorescent vesicles. The introduction of dimethyl groups at the bottom of the alkyl chains (octopusporphyrin 2) did not lead to enhanced aggregation, while the octopusporphyrin with long 2,2-dimethyl-C-20-pyridinium chains (3) formed fluorescent fibers without assistance of special anions. The electron-transfer reactions of the 1 platelets with perchlorates, in which the relative fluorescence intensity was 30% of the monomer, were investigated. 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. The results have been evaluated using equations derived for this special case of dynamic quenching by an electrostatically bound quencher. Binding constants of 3.4 x 10(4) and 1.7 x 10(5) M-1 and electron-transfer constants of 5 x 10(9) and 1.3 x 10(9) s(-1) have been calculated for NQS(-) and AQS(-), respectively.