The CN stretching vibration is a versatile infrared
(IR)
reporter that is useful for a wide range of applications. Aiming to
further expand its spectroscopic utility, herein, we show that, using
4-cyanoindole and 4-cyano-7-azaindole as examples, photoexcitation
can significantly shift the frequency (νCN) and enhance
the molar extinction coefficient (εCN) of this vibrational
mode of aromatic nitriles and that, for these indole derivatives,
the enhancement factor can reach 13. Moreover, we find that while
solvent relaxation at the excited electronic state(s) always leads
to an increase in εCN, its effect on νCN depends on the solute and the solvent. Taken together, these
results demonstrate 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 and electron
transfer processes in which an aromatic nitrile is involved.