Quantum Control Spectroscopy of Competing Reaction Pathways in a Molecular Switch

Excitation with shaped femtosecond laser pulses is a logical extension of coherent two-dimensional (2D) spectroscopy. Here we combine quantum control and information from 2D spectroscopy to analyze the initial steps in three competing reaction pathways of an isomerizing merocyanine dye. Besides the achievement of control objectives, we show how excitation with tailored pulses can be used to retrieve photochemical information that is inaccessible or experimentally demanding to obtain with other approaches

Measuring Charge-Separation Dynamics via Oligomer Length Variation

We study the optically induced charge-transfer dynamics in donor–acceptor oligomers of different chain lengths. The combination of systematic synthesis, electrochemical measurements, and ultrafast transient absorption spectroscopy allows us to determine the charge-transfer properties and dynamics in donor–acceptor systems of confined lengths. Calculations within Marcus and Jortner electron-transfer theory explain the different charge-recombination times. For compounds in which complete charge separation can occur we deduce fast equilibration between different charge-transfer configurations prior to charge recombination. Thus, monoexponential charge-recombination kinetics are observed, as only the smallest-barrier configuration leads to relaxation to the ground state. The systematic oligomer length variation along with time-resolved spectroscopy allows us to determine how far apart charges can be separated in multichromophore donor–acceptor systems. Such information is relevant for understanding on a microscopic level the charge carrier mobility in materials for organic electronics and photovoltaics