Initial photochemistry of bilirubin probed by femtosecond spectroscopy.

Bilirubin is a breakdown product from heme catabolism, and reduced excretion of bilirubin can lead to jaundice. Phototherapy is the most common treatment for neonatal jaundice, a condition frequently encountered in newborn infants. Knowledge of the photochemistry of bilirubin, which is dominated by (ultra)fast components, is necessary for the profound understanding of the processes in phototherapy. Here, we report results from femtosecond fluorescence upconversion measurements on bilirubin and half-bilirubin model compounds, as well as pump-probe absorption measurements on bilirubin. A fast component of ca. 120 fs in the multiexponential fluorescence decay, being only visible in the bilirubin molecule, is interpreted as exciton localization within the molecular halves. The slower components of several hundreds of femtoseconds and a few picoseconds, occurring in bilirubin and the half-bilirubin model, are interpreted as relaxation to a (twisted) intermediate, which decays further with ca. 15 ps to the ground state.