Ligand-Induced Interconversion of a Coordination-Organized Porphyrin Dimer: a Potential Fluorescence-Based Molecular Memory Monitor

The Solvent Effect on Weak Interactions in Supramolecular Polymers: Differences between Small Molecular Probes and Supramolecular Polymers

In this minireview, weak interactions that occur in supramolecular polymers are discussed. Combination of weak and strong interactions plays an important role in the construction of supramolecular polymers. It is beneficial to separate the contributions of the weak interactions, as well as each solvent effect on the weak interactions. However, it is generally difficult to observe each solvent effect separately at work in each interaction. Small molecular probes are useful to estimate the contributions of the weak interaction. But, the results should be treated with caution when applied to supramolecular polymer systems. To overcome the problems, a new solvent parameter, solvation ability (SA), is introduced, which was determined on the balance point of extended and stacked forms of porphyrin-based interconvertible supramolecular polymers.

Mechanistic Study of the Solvent-Dependent Formation of Extended and Stacked Supramolecular Polymers Composed of Bis(imidazolylporphyrinatozinc) Molecules

Bis(imidazolylporphyrinatozinc) molecules linked through a 1,3-butadiynylene moiety respond to the solvents they are dissolved in to afford exclusively extended (E) or stacked (S) supramolecular polymers. This system is expected to be a solvation/desolvation indicator. However, the principles underlying the solvent-dependent formation of the two types of polymers and the mechanism of the transformation between them are unclear. The formation of the polymers is considered to depend on the two types of complementary coordination bonds that can be formed and the π-π interactions between the porphyrins. In this study, the contributions and solvent dependence of both the coordination bonds and the π-π interactions have been investigated. The results clearly indicate that the coordination bonds are weakly or little solvent-dependent, and that the π-π interactions function effectively only in the inner porphyrins of the S-polymer and are strongly solvent-dependent. Thermodynamic analysis revealed that the formation of the E- or S-polymer in solution is determined by the total energies and the type of solvent used. The transformation of the E- to S-polymer was investigated by gel permeation chromatography. The kinetics of the transformation were also determined. The role of the terminal imidazolylporphyrinatozinc moieties was also investigated: The results indicate that the transformation from the E- to S-polymer occurs by an exchange mechanism between the polymers, induced by attack of terminal free imidazolyl groups on a polymer to zinc porphyrins on other polymers.

A Solvation/Desolvation Indicator Based on van der Waals Interactions between Solvents and Porphyrins

Solvation is a ubiquitous phenomenon associated with molecules in solutions. It often determines the equilibria of molecular systems and the rates of chemical reactions. Van der Waals interactions (a general term) includes weak interactions among noncharged compounds and it contributes significantly to solvation. The distinct observation of van der Waals interaction between solvent and porphyrin derivatives is reported herein. Bis(imidazolylporphyrinatozinc) structures connected through a 1,3-butadiyne moiety give two types of coordination polymers, E (extended) and S (stacked) polymers, exclusively. E polymers have larger solvent-accessible surface areas than the corresponding S polymers. Therefore, E polymers are better solvated than S polymers, providing an indicator of solvation and desolvation for the solvents used. A simple method (like a litmus test) was developed to evaluate the solvation ability of various solvents. Sixty-seven solvents and liquid compounds were tested, under the same conditions, using a conventional UV/Vis spectrometer. The results revealed a new liquid group with high solvation ability towards the porphyrins, and clarified van der Waals interaction assisted by secondary interaction on the substituents. The indicator system should contribute to the solution chemistry of molecules and materials, and to supramolecular chemistry interactions among hetero components.

Coordination-induced sliding motion of a complementary porphyrin-phthalocyanine dimer: fluorescence-based molecular switch

The reversible control of dual (or multiple) states of molecules and supramolecules has attracted much attention for molecular switch, sensor, memory, logic gate, and molecular machine applications. In this paper, a ligand-induced sliding system using a complementary porphyrin-phthalocyanine dimer accompanied by a large fluorescence change is described. An imidazole-appended porphyrinatozinc-phthalocyaninatomagnesium complex 5 was synthesized by the palladium-catalyzed coupling reaction of the corresponding porphyrin and phthalocyanine moieties. The imidazole group of 5 coordinated to the magnesium ion of another 5 was formed spontaneously and dominantly to give a stacked dimer 5ST in non-coordinating solvents, such as chloroform, even at submicromolar concentration (< 10-6 M). When an appropriate amount of dimethyl sulfoxide (DMSO) was added to the 5ST solution, the stacked dimer was transformed almost quantitatively into the extended dimer 5Ex. This transformation was monitored by UV-vis and fluorescence spectroscopies. In the UV-vis spectra, the porphyrin's Soret band is split characteristically because of excitonic coupling between the two closely located porphyrins in 5Ex. At the same time, the fluorescence intensity of 5Ex at 700 nm (ex. 414 nm) increased by a factor of 28 compared with that of 5ST. Removal of the DMSO molecules by extraction with water from the system regenerated 5ST almost completely from 5Ex, and the fluorescence was quenched again. The reversible system can be repeated by cycles of addition and removal of DMSO. The present system can be classified as a coordination-induced sliding system accompanied by a large fluorescence change, and may be applied as a fluorescence-based molecular switch.