“Conformational” Solvatochromism: Spatial Discrimination of Nonpolar Solvents by Using a Supramolecular Box of a π‐Conjugated Zinc Bisporphyrin Rotamer

A Five-layer π-Aromatic Structure Formed through Self-assembly of a Porphyrin Trimer and Two Aromatic Guests

In this study we synthesized two- and four-armed porphyrins - bearing two carboxyl and four 2-aminoquinolino functionalities, respectively, at their meso positions - as a complementary hydrogen bonding pair for the self-assembly of a D₂ -symmetric porphyrin trimer host. Two units of the two-armed porphyrin and one unit of the four-armed porphyrin self-assembled quantitatively into the D₂ -symmetric porphyrin trimer, stabilized through ammidinium-carboxylate salt bridge formation, in CH₂ Cl₂ and CHCl₃ . The porphyrin trimer host gradually bound two units of 1,3,5-trinitrobenzene between the pair of porphyrin units, forming a five-layer aromatic structure. At temperatures below -40 °C, the rates of association and dissociation of the complexes were slow on the NMR spectroscopic time scale, allowing the 1 : 1 and 1 : 2 complexes of the trimer host and trinitrobenzene guest(s) to be detected independently when using less than 2 eq of trinitrobenzene. Vis titration experiments revealed the values of K₁ (2.1±0.4×10⁵  M^-1 ) and K₂ (2.2±0.06×10⁴  M^-1 ) in CHCl₃ at room temperature.

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.

Structural Modulation of Chromic Response: Effects of Binding-Site Blocking in a Conjugated Calix[4]pyrrole Chromophore

Herein, we modulate the chromic response of a highly colored tetrapyrrole macrocycle, namely, tetrakis(3,5-di-tert-butyl-4-oxocyclohexadien-2,5-yl)porphyrinogen (OxP) by structural modification. N-Benzylation at the macrocyclic nitrogen atoms leads to stepwise elimination of the two calix[4]pyrrole-type binding sites of OxP and serial variation of the chromic properties of the products, double N-benzylated Bz₂OxP and tetra N-benzylated Bz₄OxP. The halochromic (response to acidity) and solvatochromic (response to solvent polarity) properties were studied by using UV/Vis spectroscopy and NMR spectroscopy in nonpolar organic solvents. Titration experiments were used to generate binding isotherms to elucidate their binding properties with difluoroacetic acid. Differences in the halochromic properties of the compounds allowed construction of a colorimetric scale of acidity in nonpolar solvents, as the compounds in the series OxP, Bz₂OxP, and Bz₄OxP are increasingly difficult to protonate but maintain their propensity to change color upon protonation. The concurrent effects of binding-site blocking and modulation of acidity sensitivity are important new aspects for the development of colorimetric indicators.

Synthesis, spectroscopic, electrochemical redox, solvatochromism and anion binding properties of β-tetra- and -octaphenylethynyl substituted meso-tetraphenylporphyrins

β-Phenylethynyl (PE) porphyrins exhibited unique photophysical properties and solvatochromic behavior due to ICT from the porphyrin core to PE moieties. Further, Zn(ii) porphyrins were utilized for anion sensing.