Photoreactions are ubiquitous and the foundation of life. The outcome of a photoreaction is mostly determined by its first steps, which happen within few picoseconds. Therefore the study of the initial part of a photoreaction is indispensable for its full understanding. This thesis presents the results of femtosecond pump-probe spectroscopy, the tool of choice for observing photoreactions on an ultrashort timescale, on four different systems: Corrole, Channelrhodopsin 1, Cph1-Phytochrome and a bimolecular ground-state reaction. Corroles are cyclic tetra-pyrroles similar to porphyrins. They show promising characteristics as photosensitizers in photodynamic therapy for cancer. Here, a high triplet yield is crucial. My results show that the addition of bromine to the macrocycle induces efficient intersystem crossing on a 100 ps timescale with a yield near unity. Observation of the ISC in the mid-infrared reveals a distinct spectrum of the triplet state, usable as a marker for the spin-state. Phytochromes are an omnipresent class of photoreceptor proteins initially found in plants. Phytochromes have two semi-stable states, the red and the far-red absorbing state. Recent studies show that these states itself are heterogeneous, but whether this heterogeneity ifluences the photoreaction was still unknown. Using Vis-pump IR/Vis-probe spectroscopy, we show that this is the case. The transient spectra expose two diffferent bleaching bands of the ring-D carboxyl stretching vibration, which we assign to sub-states with diffferent ring-D orientations. Since these two bands show different transient changes, we conclude that the heterogeneity indeed influences the photoreaction. Channelrhodopsin1 is a transmembrane protein which functions as a light-activated cation channel. Channelrhodopsins variants are the primary tools in the new field of Optogenetics. Visible pump visible probe spectroscopy reveals an unusually fast 100 fs all-trans to 13-cis photo-isomerization of the retinal photoreceptor and excitation wavelength dependent dynamics. In the mid-IR fingerprint regi- on,we find evidence that the isomerization outcome depends on the conformation of the ground-state. Using polarisation resolved Vis-pump IR-probe spectroscopy, we assign several priorly unassigned bands in the amide region. Comparison of our results with the C1C2-chimaera- and a homology-structure suggests that ultrafast appearing amide I band, which is strong in CaChR1,has its origin in one of the tryptophans belonging to the retinal cage. Moreover, my data excludes the existence of protonated carboxylic groups coupled to the retinal in ground-state, since no corresponding bands are observed. Bimolecular ground-state reactions are the most common reactions in chemistry. They can be described by transition state theory, in which the reaction is described by a potential energy surface. The initial and the product state are minima separated by a barrier and the shortest path over the barri- er only depends on a subset of coordinates, called reaction coordinates. Since they can be represented by normal modes, it is generally believed that IR-excitation of the correct modes can initiate the reaction. However, this idea was not yet experimentally proven. To find the missing proof and make selective laser-chemistry possible, we applied fs IR-pump IR-probe spectroscopy to a mixture of cyclohexanol and isocyanate, which react to Cyclohexyl-carbanilate at room temperature. We observe the rise of vibrational bands belonging to the product and the rise of bleaching bands belonging to the reagents on a 10 ps scale, which shows that the reaction is initiated by the IR-excitation. We support this finding by comparing the reaction rates with and without IR-illumination, showing a 24 % increase. This finding offers entirely new ways to optimize low yield reactions.
