Control of the Aggregation Properties of Tris(maltohexaose)‐Linked Porphyrins with an Alkyl Chain

Abstract A series of porphyrins with three maltohexaose units and one alkyl chain (ethyl, butyl, hexyl, decyl, and hexadecyl) has been synthesized. Aggregation properties were investigated by absorbance and circular dichroism (CD) spectra. These trismaltohexaosylated tetraphenylporphyrins exhibited high water solubility. Although these porphyrins exhibited sharp Soret bands in DMSO solution, the Soret band broadened in a concentration‐dependent manner in aqueous solution. This is the result of the aggregation of porphyrin molecules in aqueous solution. Interestingly, trismaltohexaosylated porphyrins with long alkyl chains of a certain length exhibited a broad Soret band in water without concentration dependency due to the stability of such aggregates. The CD spectra were silent in DMSO solution in all porphyrin derivatives, indicating that the maltohexaose moiety does not interact with the porphyrin chromophore in the monomeric state. However, in water, a splitting Cotton effect was observed at the Soret band. Intensity of this CD signal decreased as the length of the alkyl chain was extended. These observations show the formation of a chiral, face‐to‐face aggregate, in which the porphyrin rings are in close proximity to each other for short alkyl chain derivatives. The porphyrin derivatives with long alkyl chains exhibit edge‐to‐edge aggregation, where hydrophobic interaction of the alkyl chains separates the porphyrin chromophores.