Excited state intramolecular proton transfer (ESIPT) to aromatic carbon

Excited state intramolecular proton transfer (ESIPT) reactions occur when the acidity and basicity of groups on the same molecule are sufficiently enhanced on excitation to permit a proton transfer between these sites. Most studies have been carried out for proton transfers between heteroatoms, though ESIPTs to carbon atoms (alkenyl or alkynyl) have been reported, and these give rise to new and predictable products. This Thesis outlines the discovery of a new class of ESIPT reactions in which aromatic carbon atoms act as the proton accepting group. The reaction either leads to deuterium exchange of the starting material, or gives new products not readily accessible by thermal means.; The photochemical deuterium incorporation at the 2' and 4'-positions of 2-phenylphenol (18) and equivalent positions of related compounds has been studied in D2 O-CH3CN solutions with varying D2O content. Predominant exchange was observed at the 2'-position with an efficiency that is independent of D2O content. The data are consistent with an exchange mechanism that involves ESIPT from the phenol to the 2 '-carbon position of the benzene ring not containing the phenol, to generate the corresponding keto tautomer (an o-quinone methide (QM)). This is the first explicit example of a new class of ESIPT in which an acidic phenolic proton is transferred to an sp2-hybridized carbon of an aromatic ring. Examination of related derivatives led to postulation of a mechanism of ESIPT that requires an initial hydrogen bonding interaction between the phenol proton and the benzene pi-system.; The photochemistry of derivatives containing 1-naphthyl proton accepting rings (59 and 60) was studied. Irradiation of these derivatives in 1:9 H2O-CH3CN gave rise to dihydrobenzoxanthene products. Deuterium exchange was observed in both these products and recovered starting material when D2O was the co-solvent. The reactivity is rationalized by a mechanism involving solvent-mediated ESIPT processes to the 2'- and 7'-positions of the naphthyl ring for both 59 and 60. Evidence for the intermediacy of QM intermediates was obtained by laser flash photolysis (LFP) experiments. These reactions represent the first examples of net product formation (other than simple deuterium exchange) resulting from ESIPT to an aromatic carbon atom.; The photochemistry of naphthols 10, 62, and 63 has been studied in aqueous solution with the primary aim of exploring the viability of such compounds for naphthoquinone methide (NQM) generation. Our results show these derivatives give rise to three types of NQMs ( 61, 88, and 64) on irradiation. Photolysis of the parent 1-naphthol (10) in neutral aqueous solution gave 1,5-NQM 61 as well as the non-Kekule 1,8-NQM 88, both via the process of water-mediated ESIPT, based on observation of deuterium exchange at the 5- and 8-positions, respectively, on photolysis in D 2O-CH3CN. A transient assignable to the 1,5-NQM 61 was observed by LFP experiments. Evidence is presented that suggests a related ESIPT reaction operates in 1-pyrenol (89). The more conjugated 1,5-NQM 64 was formed efficiently via photodehydroxylation of 62; isomeric 63 was unreactive. The efficient photosolvolytic reaction observed for 62 opens the way to design related naphthol systems for application as photoreleasable protecting groups by virtue of the long-wavelength absorption of the naphthalene chromophore.