Ultrafast ring opening reaction of a photochromic indolyl-fulgimide

The photo-induced ring-opening reaction C → E of a photochromic indolyl-fulgimide is investigated by sub-picosecond pump-probe spectroscopy in the visible and infrared spectral range. The thermally stable C-isomer shows a pronounced absorption in the visible spectral range without spectral overlap with the absorption of the E- and Z-isomer. The comparison of infrared and visible transient absorption data allows to assign the observed exponential 2.1 ps dynamics to the ring-opening reaction and the non-exponential 10-40 ps component to the cooling of the hot molecules. The transient absorption spectra taken 80 ps after photo-excitation are identical with the steady state difference spectra.