On the excited-state multi-dimensionality in cyanines

Vibrational coherences in a photoexcited cyanine dye are preserved for the time-scale of diffusive torsional motion to the bottom of the excited-state potential. The coherently excited modes are virtually unaffected by solvent friction and thus distinct from the bond-twisting motion, which is strongly coupled to the surrounding solvent. We correlate the modes apparent in the resonance Raman and the four-wave mixing signal of 1,1′-diethyl-2,2′-cyanine with the understanding of optimal control of isomerization. In turn, the experimental results illustrate that optimal control might be used to obtain vibrational information complementary to conventional spectroscopic data.