Evidence of conformational changes and self-aggregation of 1,2-Bis(4-pyridyl)ethylene in solutions with ethanol

In this work, we report on a concentration and temperature dependent study of the 1,2-Bis(4-pyridyl)ethylene (BPE) – ethanol solutions by means of ultrasonic relaxation spectroscopy. The results revealed two distinct relaxation processes that follow Debye-type frequency dependence. Despite the presence of both processes in the full concentration range studied, the low-frequency relaxation process, related to conformational change between the trans- and gauche-BPE conformers, dominates the acoustic spectra in the low-concentration range, while diminishes at higher concentrations with a parallel enhancement of the high-frequency relaxation process, which is attributed to the self-association of BPE molecules. Quantum mechanical calculations were performed to investigate the energetics of the trans- and gauche-conformers. The trans-species was found more thermodynamically stable than the gauche-conformer. By applying the Synchronous Transit-Guided Quasi-Newton (STQN) methodology, we confirmed the presence of a single transition structure. From the temperature dependence of the acoustic properties, we estimated the activation enthalpy and the enthalpy difference between the two conformers. Density Functional Theory (DFT) calculations have been applied to attain the corresponding enthalpies that were found close to the experimental values. Molecular docking calculations established the self-aggregation mechanism between BPE monomers forming three types of dimeric units, namely the trans-trans, the gauche-gauche and the trans-gauche dimer species with the latter found to be the most thermodynamically favorable. The concentration dependence of the sound velocity, mass density and shear viscosity evidenced the formation of BPE aggregates. From the temperature dependence of the acoustic spectra, the thermodynamic properties of the self-aggregation mechanism of BPE were also determined. Based on the vibrational spectroscopic data, the population of the gauche conformers was found to increase with concentration at the expense of the population of the trans conformers. From the study of the vibrational properties of the system in the short-rage order, we cannot exclude the presence of a specific dimer type in the overall structure. Nevertheless, regarding to the binding score of the trans-gauche dimer (-1.25 kcal/mol), this species is the most thermodynamically stable and most likely its population is higher in dense solutions relative to the other two dimer species.