Photochemistry of the amine-sensor dye 4-N,N-dioctylamino-4′-trifluoroacetylazobenzene

The chromoreactand 4-N,N-dioctylamino-4′-trifluoroacetylazobenzene (ETHT 4001) has been developed for use in optical sensors to detect aliphatic and aromatic amines. In the present work, the photochemical behaviour of ETHT 4001 and its effects on the sensor characteristics have been investigated. The azo dye exhibits a photochemical reactivity typically observed for donor–acceptor azo dyes with fully reversible photoisomerism. The Z-form exhibits a decrease in extinction and a shift to shorter wavelengths which is caused by the reduced planarity of the chromophore. The quantum yield of the E–Z isomerization was determined to be 0.16 in toluene at −60 °C. At room temperature, however, and if exposed to an irradiance as encountered in a spectrophotometer, the stationary amount of the Z-form is negligibly small. Flash photolysis experiments of ETHT 4001 in toluene result in photochemically induced E–Z isomerization and show that the thermal isomerization is faster for ETHT 4001 (lifetime 5.3 s at room temperature) than for unsubstituted azobenzene. While the kinetics of ETHT 4001 dissolved in plasticised PVC were no longer single exponential, the lifetime of the Z-form was not enhanced in comparison to the toluene solution. The kinetics of the Z–E isomerization of ETHT 4001 in toluene is significantly dependent on the presence of organic acids which is attributed to acid-assisted tautomerization of the Z-ammonium ion into a Z-azonium ion.