Influence of solvent basicity on DMABN photophysics

1H-NMR resonance measurements of 4-(dimethylamino)benzonitrile (DMABN), dissolved in dichloromethane at temperatures over the range 300–195 K, confirm that the electronic charge transfer from its dimethylamino group to its cyano group increases with increasing dipolarity of the solvent and hence that the twisted internal charge transfer form of DMABN can be directly created by excitation of its electronic ground state. However, in tetrahydrofuran and 1,4-dioxane, the DMABN molecules form complexes with the solvent, hindering charge transfer and reducing the electron-releasing capability of the dimethylamino group. Irradiating a dilute solution of DMABN with an intensity as low as 7.8 × 1013 photons/s causes the formation of 4-(methylamino)benzonitrile as photoproduct. The emission spectra of DMABN exhibit an isoemission point at a wavelength that is shifted with an increase in solvent dipolarity. The high efficiency with which the photoproduct is obtained seems to depend on the basicity of the solvent and challenges some former photophysical conclusions not considering this fact and drawn by using stronger irradiation sources, as well as the potential use of DMABN-like compounds as solvent viscosity probes.