Ultrafast Dynamics of Photoisomerization and Subsequent Unfolding of an Oligoazobenzene Foldamer.

Investigating and deciphering the dynamics of photoswitchable foldamers provides a detailed understanding of their photoinduced conformational transitions, resembling similar processes in photoresponsive biomacromolecules. We studied the ultrafast dynamics of the photoisomerization of azobenzene moieties embedded in a foldamer backbone and the resulting conformational helix-coil transition by time-resolved femtosecond/picosecond pump-probe spectroscopy in the visible and infrared region. During E → Z photoisomerization of the azobenzenes, the complexity of the photoinduced conformational transition of the pentameric foldamer 105 is reflected in distinct spectral characteristics and a 2-fold slower decay of the excited-state absorption bands compared to the monomer M (τ4,foldamer = 20 ps, τ4,monomer = 9 ps). Time-resolved IR experiments reveal the vibrational features of the monomer and the foldamer after photoexcitation, with an additional time constant for the foldamer (τ = 150 ps), indicating the initial steps of unfolding of the helical conformation, which are supported by density functional theory calculations. Our results record the overall sequence of photoinduced structural changes in the foldamer, starting from the initial ultrafast isomerization of the azobenzene unit(s) and ending with the complete unfolding on a later time scale. From our experiments, we could gain insight into the coupling of primary photoisomerization events ("cause") and secondary unfolding processes ("effect") in these oligoazobenzene foldamers.