Spherical Self-aggregates Formed with an Alanine-functionalized Porphyrin Derivative in Organic Solvent

Abstract

It is well known that porphyrins and phthalocyanines tend to align into one-dimensional aggregates and, therefore, are of much interest in relation to the creation of novel supramolecular architectures, such as nanowires, discotic liquid crystals, and helical ribbon structures, etc. The major driving forces operating in these architectures are considered to be π-π stacking, intermolecular hydrogen bonding interactions, and/or van der Waals interactions. For example, Shinkai and co-workers reported that self-assembled porphyrin derivatives having amide groups efficiently form fiber structures by π-π stacking as well as through intermolecular hydrogen bonding interactions in organic solvents. Particularly, the hydrogen bond forming groups in porphyrin derivatives play an important role in the formation of the final aggregation mode. Therefore, we designed an alanine-functionalized porphyrin derivative in order to form self-assembled superstructures through intermolecular hydrogen bonding interactions. The alanine-functionalized porphyrin is constructed by introducing four alkyl alanine residues onto the macrocyclic skeleton. We report here the preparation and the aggregate formation of 1 in organic solvent, as demonstrated by field-emission scanning electron microscopy (FE-SEM), energy-filtered transmission electron microscopy (EF-TEM), confocal laser scanning microscopy (CLSM), FT-IR, and fluorescence spectroscopy studies. Alanine-functionalized porphyrin 1 was synthesized in six steps as shown in Scheme 1. The alanine ethyl ester was added to a solution of the corresponding tetraphenyl carboxy acid 3. After de-ethylation of 5, treatment with 6 and diaminoacetylene 10 afforded the desired product 1 as a dark brown powder. Also, compound 2 without alanine moieties was prepared as reference. The aggregation behavior of 1 was investigated in organic solvents such as toluene or chloroform by using several microscopic, and spectroscopic methods. In order to obtain visual insights into the aggregation mode, we observed the superstructure of self-assembled 1 and 2 by FE-SEM and EF-TEM. Figure 1 shows FE-SEM and EF-TEM images of the self-assembled 1 formed in toluene. The FE-SEM and EF-TEM images of unstained samples in toluene show a spherical structure with uniform diameter of ≈600 nm, which have no hollow cavity. In contrast, compound 2 which lacked alanine moieties did not reveal any morphology due to high solubility in organic solvents. These results indicate that the alanine moiety plays a critical role in forming the self-assembled spherical structure. Also, the findings suggest that the self-assembled spherical structure of 1 might be induced mainly by strong intermolecular hydrogen bonding interactions between alanine and alanine moieties in organic solvents. Hydrogen bonding interactions of alanine moieties of 1 in toluene were evaluated by FT-IR measurements and compared with those of 2 without the alanine moiety. The absorption frequencies originating from the N-H deformation band and from the C=O stretching vibrations of 1 shifted to higher and lower wavenumbers (N-H: 1534 and C=O: 1623 cm−1), as compared to the comparable absorption frequencies for the reference compound 2 (N-H: 1556 and C=O: 1638 cm−1), indicating the formation of hydrogen bonds for almost all the amide residues of 1 (Figure S1). To have an insight on the role of hydrogen bonding between amide groups in the self-assembly of 1, we simplified the molecule 1 by replacing the alkyl chain after the amide group with methyl. It is clearly shown in Figure S2 that the hydrogen bonding between amide groups play an important role in growing by self-assembly. In particular, all the four chains seem to be involved in the intermolecular hydrogen bonding in the self-assembly. The thermodynamic stability of uuuu, uudd, and udud structures is virtually equivalent (within 0.12 kcal/mol) at the AM1 calculations (see calculation results and Scheme S1 in Supporting Information). In our experiment, the spherical self-assembled structure was