TUNING EXCITED STATE INTRAMOLECULAR PROTON TRANSFER TO C- ATOM IN 2-PHENYLPHENOL

MARIA T. BAUMGARTNER; LILIANA B. JIMENEZ; LEANDRO MENA

Dublín

Simposio; International Symposium on Photochemistry PHOTOIUPAC 2018; 2018

Excited state intramolecular proton transfer (ESIPT) reactions are important and ubiquitous photochemical processes. In most cases, proton travels from an acidic donor moiety (usually a OH or NHR group) to a basic site (carbonyl oxygen or heterocyclic nitrogen atom) in the same molecule.Besides the common cases, there are systems in which a carbon atom behaves as the proton acceptor, but these examples are quite rare. In their seminal work1, Wan et al. demonstrated that aromatic carbon atoms in excited state can be basic enough to receive the proton, giving rise to o-quinone methides (o-QMs). Since then, they have widely studied the generation of QMs via ESIPT from phenolic OH to the corresponding C-atoms in different hydroxybiaryl systems.Taking the first study about 2-phenylphenol as a ?model case?, and in order to contribute to the understanding of this unusual type of ESIPT, we decided to analyse the impact that electron-donating and withdrawing substituents have on the ESIPT efficiency of 2-phenylphenol.For this purpose, compounds 1-5 were synthetized and purified following previously published procedures. Then, irradiation of CH3CN-D2O (3:1) solutions of 1-5 under 254 nm light was performed and the deuterium (D) exchange was determined by 1H NMR as a measure of ESIPT extent. It was found a linear correlation between the electronic character of substituents and the efficiency of the process. D-incorporation rises as the donating character of substituent increases, whereas ESIPT is almost negligible with strong electron withdrawing groups. Photoacidity of the studied phenylphenols must be taken into account in order to rationalize these results.Experimental observations together with theoretical calculations suggest that a critical competition exists between ESIPT and adiabatic deprotonation in protic solvents. As a consequence, the efficiency of ESIPT process can be finely tuned by modifying the electronic nature of the groups attached to the phenol moiety.