Photoinduced Double Proton Transfer: Inter‐ and Intramolecular Cases

Photoinduced two‐proton tautomerization has been studied in two types of chromophores: (i) alcohol complexes of azaaromatic molecules possessing both proton donor and acceptor groups; (ii) constitutional isomers of porphyrin. The reaction path for the intermolecular process may involve solvent reorientation around the excited chromophore. In this case, rapid internal conversion is activated, efficiently competing with proton transfer. Another possibility arises if cyclic, doubly‐hydrogen‐bonded complexes exist already in the ground state. Excitation of such species leads to a fast tautomerization, which is not stopped even at low temperatures. Excited‐state double proton transfer also has been observed in cyclic dimers in the crystal phase. In porphyrin isomers, the rate of the process is a function of the strength of the intramolecular hydrogen bond. Dependence of the phototautomerization rate on temperature and on the nature of the surrounding matrix has also been studied.