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

Previously, several studies have shown that, for a set of structurally related nitrile compounds, there could be a linear relationship between the total charge on the nitrile group () and its stretching frequency (ν). However, it is unclear whether the corresponding frequency and charge properties of structurally different nitrile compounds can be described by a single linear ν- relationship. Herein, we compute the magnitudes of a large number of nitrile-containing molecules whose ν values cover a spectral range of ca. 200 cm and are measured under different experimental conditions. Our results reveal that ν indeed exhibits a linear dependence on , with a slope of 637 ± 30 cm/charge. Because the nitrile moiety is a commonly used building block in electronic donor-acceptor (D-A) molecular systems, we believe that this linear relationship will find utility in a wide range of applications where such D-A constructs are used, such as in organic photovoltaic assemblies. In addition, we apply this linear relationship to characterize the degree of charge transfer upon photoexcitation of two indole derivatives, 5-cyanoindole and 6-cyanoindole, and are able to show that in both cases, the fluorescence emission arises from a charge-transfer or L state.