Two‐dimensional exchange spectroscopy (2D EXSY) is a powerful method to study the interconversion (chemical exchange) of molecular species in equilibrium. This method has recently been realized in femtosecond 2D‐IR spectroscopy, dramatically increasing the time resolution. However, current implementations allow the EXSY signal (and therefore the chemical process of interest) only to be tracked during the lifetime (T1) of the observed spectroscopic transition. This is a severe limitation, as typical vibrational T1 are only a few ps. An IR/Vis pulse sequence is presented that overcomes this limit and makes the EXSY signal independent of T1. The same pulse sequence allows to collect time‐resolved IR spectra after electronic excitation of a particular chemical species in a mixture of species with strongly overlapping UV/Vis spectra. Different photoreaction pathways and dynamics of coexisting isomers or of species involved in different intermolecular interactions can thus be revealed, even if the species cannot be isolated because they are in rapid equilibrium.
The VIPER (vibrationally promoted electronic resonance) 2D‐IR experiment selects a species within a mixture according to its IR spectrum and electronically excites it. The relaxation barrier for 2D‐IR exchange spectroscopy can thus be broken and chemical exchange tracked over long timescales. The photochemistry of the IR‐selected species such as distinct conformers can be studied in the presence of other species with virtually identical UV/Vis spectra.