Effect of electrolytes and temperature on dications and radical cations ofcarotenoids: Electrochemical, optical absorption, and high-performance liquid chromatography studies

The effect of supporting electrolytes and temperature on the behavior of di cations and radical cations of carotenoids is studied. Cyclic voltammograms (CVs) of canthaxanthin (I) at 23 and -25 degrees C show that Car(.+) of I has similar stability during the time of the CV scan, when using tetrabutyl ammonium perchlorate (TBP,PC), tetrabutylammonium tetrafluoroborate (TBATFB ), or tetrabutylammonium hexafluorophosphate (TBAHFP) as supporting electro lyte. However, the stability of Car(2+) decreases when using TBAPC or TBATF B; beta-carotene (II) shows similar behavior. The CV of I at -25 degrees C shows a strong cathodic wave (wave 6) near -0.15 V (vs Ag) with an intensit y about half that of the neutral oxidation wave when TBAPC or TBATFB is the supporting electrolyte. When TBAHFP is used, wave 6 (ca. -0.05 V vs Ag) is ca. 8 times weaker than when TBAPC or TBATFB is used. This wave results fr om the reduction of a species that may be a decay product of Car(2+) of I. Our results show that these electrolytes commonly used in electrochemical s tudies may affect the studied systems to different extents. In simultaneous bulk electrolysis (BE) and optical absorption spectroscopic measurements, the absorption band of Car(2+) of I in the presence of 0.1 M TBAHFP can be observed by lowering the BE temperature to -20 C-degrees. In the presence o f 0.1 M TBAPC or TBATFB, this band is not observed, even at -50 degrees C. Isomerization of neutral I (as shown by HPLC and its blue absorption band s hift) is observed only when the Car(2+) absorption band is absent during BE . This observation, along with an increase of the neutral absorption band a fter stopping BE, suggests that the equilibrium Car + Car(2+) reversible ar row 2Car(.+) is shifted to the left because Car(2+) decays more quickly tha n Car(.+) in the presence of electrolyte and this is a major path for forma tion of cis neutral species from cis Car(.+). The optical absorption of Car (.+) of I at 18 degrees C in the presence of 0.1 M TBAHFP is obtained, whic h suggests that Car(.+) of I is not as unstable at room temperature in the presence of TBAHFP as was thought before.