Excited state intramolecular proton transfer (ESIPT) occurs in molecules wherein functional groups that exhibit enhanced acidity and/or basicity upon electronic excitation are in close proximity. The ESIPT has been the topic of intensive research because of its fundamental value, as well as many applications.[1] The most common acidic group in the ESIPT reactions is the phenolic OH, whereas the basic site is usually a heteroatom such as pyridine or imidazole nitrogen. Proton transfer to carbon atoms is usually considered intrinsically slow, and therefore, inefficient. Nevertheless, ESIPT from the phenol OH to the carbon atom of an adjacent phenyl, naphthyl, anthryl or pyrenyl ring have been reported.[2] However, in all reported examples involving ESIPT to carbon, the efficiency of the process is low. Herein we report on an investigation of ESIPT and solvent-assisted PT in a series of phenylnaphthols and naphthylphenols 1-6. The process was investigated by preparative irradiations in the presence of D2O wherein regiospecific incorporation of deuterium at C-atoms highlights the basic sites. In addition, photophysical properties of 1-6 were investigated by steady state and time-resolved fluorescence. The experimental observations were rationalized by ab initio calculations.

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