<br/>In the visible spectral range it is often difficult due to spectral overlap to distinguish between multiple molecular species such as different conformers, isomers or molecules involved in different kinds of intermolecular interactions and to selectively excite them. IR spectroscopy is usually more selective, however, IR excitation does not offer the possibility to trigger photochemistry of the selected species. The VIPER 2D-IR experiment, where VIPER stands for VIbrationally Induced Electronic Resonance, is a mixed IR/VIS two-dimensional spectroscopy that combines IR selectivity with electronic excitation and the possibility to investigate subsequent dynamics.<br/>To achieve this goal, the VIPER 2D-IR pulse sequence uses a selective IR pulse to vibrationally tag a molecular species, followed by an off-resonant visible pulse and an IR probe pulse. Only molecules that are vibrationally excited will be transferred to the electronically excited state, because the IR pulse shifts the molecule into resonance with the UV-Vis pulse. In this fashion, one can select even between molecules with identical UV/VIS spectrum by exploiting differences in the IR spectrum. A strong vibronic coupling between the initially excited vibration and the electronic transition is not essential for the generation of a VIPER signal, because a modulation of the visible cross-section can as well be achieved by intramolecular vibrational energy redistribution (IVR) into Frank-Condon-active modes.<br/>We report on several useful applications of the VIPER 2D-IR experiment, such as (1) probing chemical exchange processes which occur on time scales that are slower than the vibrational lifetime, (2) monitoring the photochemistry of a particular molecular species within a mixture of species, even though they are in fast equilibrium, as well as (3) selecting one species for electronic excitation out of a mixture of molecules with virtually identical UV/VIS spectra.

Subensemble-Selective photochemistry by mixed IR/VIS two-dimensional spectroscopy

253rd National Meeting (2017)

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