Bilirubin is a neurotoxic product responsible for neonatal jaundice, which is generally treated by phototherapy. The photoreaction involves ultrafast internal conversion via an elusive intermediate and Z-E isomerization with minor yield (less than 3% in solution). The structure of the intermediate remains unclear. Here, the combination of UV-vis and mid-IR ultrafast transient absorption spectroscopy reports a comprehensive picture of the mechanism and provides essential structural information about the intermediate species. Thus, spectral dynamics during the earliest ps unveils a wavepacket travelling from the Franck-Condon region to the crossing point with a dark state. The latter shows a tighter molecular skeleton than the ground state and decays with 15 ps time constant. Remarkably, the relative contribution of a non-decaying component increases linearly with pump energy, suggesting that Z-E isomerization could also be triggered by two-photon excitation. Implications for the photochemistry of protein-bound open tetrapyrroles are discussed.
Carreira-Blanco, C., Singer, P., Diller, R., & Pérez Lustres, J. L. (2016). Ultrafast deactivation of bilirubin: Dark intermediates and two-photon isomerization. Physical Chemistry Chemical Physics, 18(10), 7148–7155. https://doi.org/10.1039/c5cp06971h