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 (q_CN) and its stretching frequency (ν_CN). However, it is unclear whether the corresponding frequency and charge properties of structurally different nitrile compounds can be described by a single linear ν_CN-q_CN relationship. Herein, we compute the q_CN magnitudes of a large number of nitrile-containing molecules whose ν_CN values cover a spectral range of ca. 200 cm^-1 and are measured under different experimental conditions. Our results reveal that ν_CN indeed exhibits a linear dependence on q_CN, with a slope of 637 ± 30 cm^-1/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_a state.