Morphological selection of the self-assembled nanostructure of zwitterionic porphyrins between nanorod and nanosheet by the small displacement of the peripheral anionic position is demonstrated based on the J-aggregates of zwitterionic porphyrin isomers, meso-tetrakis(5-sulfonatothienyl)porphyrin diacid and meso-tetrakis(4-sulfonatothienyl)porphyrin diacid.
Various porphyrin diacids are known to show controlled self-assembly by inorganic anions. Following the unexpected finding that meso-tetrakis(4-sulfonatothienyl)porphyrin diacid shows Cl(-) specific aggregation in spite of having the anionic substituents, the aggregation behavior of the diacid in aqueous solution with Cl(-) was investigated in detail. We found that Cl(-) induces the H-aggregate, followed by the transformation into the J-aggregate with increasing Cl(-) concentration. For the J-aggregate formation, negatively charged sulfonic acid groups were suggested to be of minor influence. The J-aggregate forms nanoscale macrostructures composed of highly oriented molecules on substrate. To our knowledge, this is the first report of both H- and J-type aggregate formation of a porphyrin diacid in the presence of an aqueous inorganic anion. These results would open the gate for controlling the chromophore packing structure of a porphyrin diacid complexed with an inorganic anion by varying the composition.
To tailor functional nanomaterials, the co-assembly of self-assembling dyes in a homogeneous way would be a promising approach because the electronic properties can be tuned by the mixing ratio. Although porphyrins are important supramolecular building blocks with unique optical properties, a homogeneously mixed J-aggregate system of porphyrins has not been reported yet. Herein, we focused on three kinds of zwitterionic porphyrin diacids, H(4)TSPP(2-), H(4)T(5-STh)P(2-) and H(4)T(4-STh)P(2-), due to their capability to form J-aggregates with distinguished optical properties and well-defined nanostructures. In this study, we investigated the co-assembly behaviours of the zwitterionic porphyrins in aqueous solution by UV-vis and RLS, and investigated the morphology of the resultant homogeneously mixed J-aggregates by AFM. In the case of the combination of H(4)TSPP(2-) and H(4)T(5-STh)P(2-), they readily co-assemble to form homogeneously mixed J-aggregates with different types of binary excitonic bands, whereas the combination of H(4)T(4-STh)P(2-) and other porphyrins results in the dominant formation of the individual pure J-aggregates. Deposited homogeneously mixed J-aggregates of H(4)TSPP(2-) with H(4)T(5-STh)P(2-) consist of rod-shaped nanostructures, whose height changes discontinuously upon varying the mixing ratio. These results would provide new insights into the electronic properties and the nanostructure of self-assembled multicomponent materials.
Controlling the adsorption behavior of bulk-phase self-assembling dye molecules at solid/liquid interfaces is of importance for application to various devices. Herein, we report an unexpected phenomenon on the adsorption behaviors of bulk J-aggregating water-soluble porphyrin diacids. A comparative study on the adsorption amounts of J-aggregated meso-tetrakis(4-sulfonatophenyl)porphyrin diacid from freshly prepared and pre-aged solutions revealed enhanced adsorption through the self-assembly process (EASAP). The aggregate structure formed by EASAP is almost identical to the one from preformed J-aggregate solutions. The generation ratio of J-aggregates at an interface and in bulk strongly depends on the interface-to-volume ratio of the solutions. The surface property of cuvettes and coexisting inorganic ions has no significant effects on EASAP. While EASAP occurs in the J-aggregations of the other water-soluble porphyrin diacids, it is suggested that self-assembly properties play an important role in the adsorption proportion. These results will provide new insight into the adsorption equilibrium of bulk self-assembling molecules at solid/liquid interfaces.
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