Resolution of ultrafast excited state kinetics of bilirubin in chloroform and bound to human serum albumin

The Z–E-isomerisation of bilirubin upon excitation with visible light is a fundamental step in phototherapy of newborns with neonatal jaundice. Here we report results of an ultrafast optical spectroscopy study of bilirubin in CHCl3 as well as bound to human serum albumin. The data show that the initially excited singlet state has sub-ps decay times with major amplitude. Transient absorption measurements reveal that the ultrafast decay of the emission is accompanied by the formation of a transient intermediate which decays on the 15–20 ps timescale. The initial photoprocesses are thus considerably faster than the previously reported fastest lifetimes for bilirubin and this is, to our knowledge, the first time that the earliest processes in excited bilirubin have been resolved.