The nitrile (CN) stretching vibration is one of the most utilized infrared (IR) reporters in chemical and biological sciences, due to the fact that (1) it exhibits an exquisite dependence on various molecular and/or environmental properties, (2) it is a localized mode, (3) it is easily distinguishable from other molecular vibrations, and (4) nitrile-containing compounds ubiquitously exist in natural and synthetic products. For example, nitrile-derivatized amino acids have been used to study protein conformational dynamics, protein hydration, protein electric field, protein-ligand interaction, enzymatic reaction, etc.
Aiming to further expand its spectroscopic utility, a research team led by Prof. Feng Gai at College of Chemistry and Molecular Sciences(CCME), Peking University (PKU) and Prof. Wenkai Zhang of Beijing Normal University (BNU) demonstrated that, using 4-cyanoindole and 4-cyano-7-azaindole as examples, photoexcitation to higher electronic state(s) can significantly shift the frequency (CN) and enhance the molar extinction coefficient (CN) of this vibrational mode of aromatic nitriles and that, for these indole derivatives, the enhancement factor is greater than 10-fold. In addition, they found that while solvent relaxation at the excited electronic state(s) always leads to an increase in CN, its effect on CN depends on the solute and the solvent. Taken together, their findings demonstrated that solvent relaxation can differently affect the local environment of the nitrile group and its conjugation with the indole ring and, more importantly, that the CN stretching vibration can serve as a sensitive IR probe of charge/electron transfer processes wherein an arylnitrile is involved.
The research was published in The Journal of Physical Chemistry Letters (ASAP Article; https://doi.org/10.1021/acs.jpclett.2c02418). Prof. Feng Gai at CCME, PKU and Prof. Wenkai Zhang of BNU are the corresponding authors. Other co-authors are Jinsong Liu (BNU), Ranran Feng (PKU), and Liang Zhou (BNU). The research was supported by the National Natural Science Foundation of China, Beijing National Laboratory for Molecular Sciences, and Peking University.
Original link for the paper: https://pubs.acs.org/doi/10.1021/acs.jpclett.2c02418
