Femtosecond study on the isomerization dynamics of NK88: I. Ground state dynamics after photoexcitation

Recently, optimal control of a photoisomerization reaction in the liquid phase was demonstrated for the first time on the system 3,3'-diethyl-2,2'-thiacyanine (NK88). Additionally, the class of cyanines to which the molecule NK88 belongs draws a lot of attention in different recent theoretical publications. Therefore, a better understanding of the molecular dynamics of this molecular system is of special interest. Experiments using the femtosecond pump-supercontinuum probe technique (PSCP) with an excitation wavelength of 400~nm and a spectral range of 370 to 620~nm for the probe beam have been performed. In order to analyze the dynamics properly the time window has been chosen to comprise the characteristic times of the contributing processes, additionally we have employed two solvents, methanol and ethylene glycol, and have conducted anisotropy measurements. The spectroscopic data have been assigned to different molecular states with the help of DFT (density functional theory) and MP2 (second-order Moller-Plesset perturbation theory) calculations. The analysis of the data has revealed in the most likely model that three different isomers exist with different lifetimes. On the basis of experimental and theoretical data, a conclusive scheme of the isomerization reaction is presented.