CVN Stretching Frequency as a Convenient Reporter of Charge Separation in Molecular Systems

Previously, several studies haveshown that, for a setof structurallyrelated nitrile compounds, there could be a linear relationship betweenthe total charge on the nitrile group (q (CN)) and its stretching frequency (& nu;(CN)). However, itis unclear whether the corresponding frequency and charge propertiesof structurally different nitrile compounds can be described by asingle linear & nu;(CN)-q (CN) relationship. Herein, we compute the q (CN) magnitudes of a large number of nitrile-containing molecules whose & nu;(CN) values cover a spectral range of ca. 200 cm(-1) and are measured under different experimental conditions.Our results reveal that & nu;(CN) indeed exhibits a lineardependence on q (CN), with a slope of 637 & PLUSMN; 30 cm(-1)/charge. Because the nitrile moietyis a commonly used building block in electronic donor-acceptor(D-A) molecular systems, we believe that this linear relationshipwill find utility in a wide range of applications where such D-Aconstructs are used, such as in organic photovoltaic assemblies. Inaddition, we apply this linear relationship to characterize the degreeof charge transfer upon photoexcitation of two indole derivatives,5-cyanoindole and 6-cyanoindole, and are able to show that in bothcases, the fluorescence emission arises from a charge-transfer orL(a) state.