Triplet state energy of the carotenoid bixin determined by photoacoustic calorimetry

The triplet state energy level of the natural cis-carotenoid bixin (methyl hydrogen 9′Z-6,6′-diapocarotene-6,6′-dioate) was determined using laser-induced photoacoustic calorimetry (PAC) in acetonitrile/methanol (1:1) solution. An energy-transfer process with anthracene as sensitizer in O2-saturated solution was used to populate the carotenoid triplet state. The energy balance of the system allowed the calculation of the triplet energy content of bixin as ET = 18 ± 2 kcal/mol. The result is discussed in relation with both its singlet molecular oxygen quenching and cis → trans isomerization abilities. The quantum yield, Φ = 0.69, for the formation of either the triplet state or the singlet molecular oxygen by the sensitizer anthracene was also determined.