Effect of Molecular Crowding on Complexation of Metal Ions and 8-Quinolinol-5-Sulfonic Acid

Kinetic study on bimolecular photochemical quenching of tris(2,2'-bipyridyl)ruthenium(II) complex in water-ethylene glycol binary media

The bimolecular photochemical quenching of the tris(2,2'-bipyridyl)ruthenium (II) complex by ferricyanide ions in water-ethylene glycol binary media was investigated using a spectrophotometric approach, as previously reported by our research group. Time-resolved spectroscopy revealed that dynamic photochemical quenching occurred via a diffusion-dominated pathway. The static quenching process was found to occur significantly when the concentration of ethylene glycol was greater than 40 wt%. The analysis of the Stern-Volmer plots revealed that the emitters and quenchers tended to show ionic associations in water-ethylene glycol compared to our previous results with a water-glycerol binary solvent system. This is attributed to hydrophobicity, which is consistent with pioneering works that report that the addition of ethylene glycol to pure water forms hydrophobic regions, leading to dehydration of the complex ions.

Elucidating the Quenching Mechanism of Tris(2,2'-bipyridyl)ruthenium(II) Complex in the Water-Glycerol Binary System Based on the Microscopic Structure of the Media

Kinetic studies on the photochemical quenching reaction of the tris(2,2'-bipyridyl) ruthenium(II) complex ([Ru(bpy)3]2+) in water-glycerol binary media were conducted based on the Einstein-Smoluchowski (E-S) theory. Dynamic and static quenching behaviors were analyzed by comparing results from time-resolved spectroscopy and emission spectroscopy. While the dynamic quenching reaction aligns well with the E-S theory, static quenching was observed, leading to a notable increase in the overall photoquenching reaction rate constant. Employing chromatography and infrared spectroscopy, we correlated the microscopic molecular structure of the binary solvent system and the solvation environment around the emitters with the reaction mechanism. This correlation was found to correspond to ion pair formation and the confinement effect of the emitter, respectively.

Molecular crowding effect in Hantzch pyridine synthesis in polyethylene glycol aqueous solution

In a molecular crowding environment, the kinetics and thermodynamics differ from those in a diluted solution. Although the molecular crowding effect has been extensively investigated, its fundamental kinetics and thermodynamics remain unclear. In this study, we investigated the change in the rate constant (k) of the Hantzch pyridine reaction in a molecular crowding environment using polyethylene glycol (PEG). While the k value increased to a PEG concentration (CPEG) of 10 vol%, a decreasing trend was observed for CPEG > 20 vol%. This intriguing behavior was analyzed based on the increase in reactant activity due to volume exclusion and the decrease in water activity due to osmotic pressure. Volume exclusion and osmotic pressure had opposing effects on the reaction, which were positive for volume exclusion and negative for osmotic pressure. We found that k decreased when the negative effect of the osmotic pressure surpassed the volume exclusion effect.

J-aggregation of 5, 10, 15, 20-tetraphenyl-21H, 23H-porphinetetrasulfonic acid in a molecular crowding environment simulated using dextran

In a molecular crowding environment, different thermodynamics is often observed in a dilute solution. One such example is the promotion of the formation of amyloids, which are causal agents of Alzheimer's disease. Although a considerable number of molecular crowding studies have been reported, its effect remains unclear. In this study, we investigated a J-aggregation of a porphyrin derivative, 5, 10, 15, 20-tetraphenyl-21H,23H-porphinetetrasulfonic acid (TPPS), in a molecular crowding environment simulated by dextran (Dex) in HClO₄, HCl, and NaCl solutions. The changes in the number of monomers in the J-aggregate (n) with the concentration of Dex (C_Dex) depended on the type of solution. No change in n was observed in the NaCl solution, which indicated that the Dex solution did not affect the J-aggregation because of the ionic strength effect. In the HCl solution, the aggregation behavior changed with the pH. Further, at a low pH, the electrostatic interactions promoted J-aggregation by the volume exclusion of Dex, while the aggregation was suppressed at a high pH owing to steric hindrance. A different aggregation mechanism, involving the hydrogen bonding between NH in the center of the TPPS macrocyclic frame and the SO₃H and ClO₄^- functional groups, was responsible for the J-aggregation in the HClO₄ solution. Moreover, the n value increased owing to the volume exclusion effect. We expect that this study will be useful for further elucidation of the molecular crowding effect.