Photoenhancement of the C≡N Stretching Vibration Intensity of Aromatic Nitriles

Abstract

The CN 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 CN stretching vibration can serve as a sensitive IR probe of charge and electron transfer processes in which an aromatic nitrile is involved.

Good Vibrations Report on the DNA Quadruplex Binding of an Excited State Amplified Ruthenium Polypyridyl IR Probe

“…Due to the strong absorbance of D 2 O in this region of the spectrum, the TRIR spectra of rac - 1 were recorded in H 2 O and show a transient at 2232 cm –1 whose intensity is significantly greater than the associated bleach band (see Figure a). The notable enhancement of the molar absorptivity of the ν­(CN) vibration in the excited state is in agreement with recent observations for indole substituted nitriles and linked to the electron density in the aromatic system . The transient band is shifted to a lower wavenumber by ca .…”

“…The notable enhancement of the molar absorptivity of the ν(C�N) vibration in the excited state is in agreement with recent observations for indole substituted nitriles and linked to the electron density in the aromatic system. 80 The transient band is shifted to a lower wavenumber by ca. 18 cm −1 from the position of the bleach ground state absorption, indicating the presence of a weaker CN bond in the 3 MLCT* excited state relative to the ground state.…”

C≡N Stretching Frequency as a Convenient Reporter of Charge Separation in Molecular Systems

“…This large frequency shift (Δν CN ≈ 123–130 cm –1 ) clearly indicates that in both cases, the electronic excited single state(s) accessed via UV excitation has a strong CT character, resulting in an increase in the local charge density on the nitrile group. As observed in a previous study, this charge increase on the nitrile group leads to a significant enhancement in its oscillator strength. Moreover, as shown (Figures S5 and S6), the decay kinetics of the nitrile stretching bands of these two indole derivatives match their respective fluorescence decay kinetics, hence indicating that in both cases, the fluorescence state is L a in DMSO.…”

Photophysics of Two Indole-Based Cyan Fluorophores

“…However, unlike in the case of 7AI, we did not observe a detectable “tautomer” emission band for 4CN7AI in alcohols (Figure S5). In addition, a recent time-resolved infrared (IR) study by Liu et al . also indicated that there is no detectable “tautomer” population at the electronically excited state(s) of 4CN7AI.…”

“…However, unlike in the case of 7AI, we did not observe a detectable "tautomer" emission band for 4CN7AI in alcohols (Figure S5). In addition, a recent timeresolved infrared (IR) study by Liu et al 39 there is no detectable "tautomer" population at the electronically excited state(s) of 4CN7AI. Taken together, these findings suggest that if a "tautomer" of 4CN7AI is formed via a double proton transfer process in protic solvents at its electronically excited state(s), this "tautomer" species must decay to the ground electronic state on an ultrafast timescale.…”