Hydrogen molecule, photoinduced complexes

To understand the fundamental photochemical processes in biologically relevant molecular systems, prototype molecules like phenol or indole - the chromophores of the amino acids tyrosine respective trypthophan - embedded in clusters of ammonia or water molecules are an important object of research. Numerous studies have been performed concerning the dynamics of photoinduced processes in phenol-ammonia or phenol-water clusters (see e. g. [1,2]). As a main result a hydrogen transfer reaction has been clearly indicated in phenol(NH3)n clusters [2], whereas for phenol(H20)n complexes no signature for such a reaction has been found. According to a general theoretical model [3] a similar behavior is expected for the indole molecule surrounded by ammonia or water clusters. As the primary step an internal conversion from the initially excited nn state to a dark 7ta state is predicted which may be followed by the H-transfer process on the 7ia potential energy surface. 

Examples of the photoinduced loss of a molecule of hydrogen to give a coordinately unsaturated complex are found in the conversion of IrH2(dppe)2 and IrClH2(CO)(PPh3)2 into Ir(dppe)2 and IrCl(CO)(PPh3)2, respectively ... 

The tautomerization at room temperature is caused by photoinduced proton transfer and due to the large distance between O and N atoms it cannot proceed intramoleculary. The process is solvent assisted as seen from the measured emission spectra in different solvents [53] in apolar or nonprotic polar solvents only the emission coming from 2 IE is detected, while in hydrogen-bonded solvents (alcohols or water for instance) a new strongly Stokes-shifted band appears in addition. This new band has been attributed to emission by a phototautomer 21K, formed by an adiabatic excited state intramolecular proton transfer (ESIPT) process in a complex between the excited enol form and solvent molecules [54, 55] (Figure 12.10). 

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