Excited-state dynamics of the PYP chromophore in solution: Environment and structure effects

This chapter demonstrates that the photo-induced behavior of five analogues of the photoactive yellow protein (PYP) chromophore is strongly dependent on their electron donor-acceptor structure with the help of ultrafast spectroscopy experiments. Compounds for which the carbonyl group is substituted with a strong electron acceptor, in particular the thioester derivative that models the native chromophore of PYP, do not isomerize in aqueous and alcoholic solutions. Their relaxation pathway involves a new transient, the formation of which is sensitive to the polarity and viscosity of the solvent. Hypothetical relaxation mechanisms are discussed in the chapter. The preferred one involves formation of a spectroscopically detectable ground state with a 90. o twisted conformation of the ethylenic bond. The effect of the solvent on the excited-state relaxation is analyzed to stress further the existence of different relaxation pathways in these compounds.