The dual fluorescence on 1-(n,n-dimethylamino)-4-benzonitrile adsorbed on silica and on modified silica surfaces

The effect of adsorption on the dual fluorescence of the title compound (DMABN) is studied on silica gel and on modified silica surfaces (co-adsorption of cyclohexane and of 1-octanol, octadecyl derivatization and removal of silanols by heating to 800°C). Both positive and relative intensity of the long-wavelength emission originating in the twisted charge transfer excited state (CT^- are effected by changes in the interfacial environment. It is concluded that on silica surfaces the CT^- state (originating in DMABN hydrogen-bonded to surface silanols) undergoes effective solvation by orientational relaxation of neighbouring silanols. The apparent surface polarity, estimated from the CT^- emission frequency, is comparable to that of ethanol. Formation of the CT^- state is also indicative of a relatively uninhibited CN rotation in the excited state of the adsorbed molecule which again is similar to the behaviour in ethanol.