Fluorescent ligands for optical sensing of alcohols: Synthesis and characterisation of p-N,N-dialkylamino-trifluoroacetylstilbenes

Novel fluorescent ligands have been synthesised which interact with specific alcohols resulting in a change of absorbance as well as fluorescence. 4-trifluoracetyl-4'-(di-N-butylamino)stilbene (ETHT 4003) exhibits an absorption at around 450 nm in organic solvents, whereas, upon reaction with primary alcohols, the maximum shifts to around 360 nm. The fluorescence emission of the positively solvatochromic dye ranges from 500 to 700 nm and, upon reaction with alcohols, is also blue-shifted to around 450 nm. The significant shift of both absorbance and fluorescence is based on the chemical conversion of the trifluoroacetyl group of the dye into the hemiacetal, thus causing a change in the electron distribution of the dye and a spectral shift of around 100 nm. The reaction is selective for alcohols, which are not sterically hindered to approach the trifluoroacetyl group, i.e. for primary alcohols. An example for the analytical application of the ligands is given. An optical sensor is prepared by embedding 4-trifluoracetyl-4'-(di-N-butylamino)stilbene in plasticised PVC and coating the layer with white PTFE. The microporous PTFE coating provides an optical isolation and prevents interferences of ions. The sensor exhibits a sensitive range from 5-50 vol% ethanol with maximum signal changes to be in the range of 40%. The detection limit is 2 vol% ethanol.