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Photodehydration mechanisms of quinone methide formation from 2-naphthol derivatives

Photogeneration of quinone methides (QM) from 3- hydroxymethyl-2-naphthol (1) and 3-(2-hydroxy-2- adamantyl)-2-naphthol (2) was investigated using femtosecond and nanosecond transient absorption spectroscopy and supported with theoretical analysis of possible reaction pathways. Our results indicate that adamantylnaphthol 2 after UV excitation undergoes a non-adiabatic excited state intramolecular proton transfer coupled with dehydration via a conical intersection, delivering the corresponding QM (S0) within 1 ns. Surprisingly, in naphthol 1 on femtosecond time scale only formation of radical cation and solvated electrons was observed where the photoionization is a competing process not connected to the photochemical reaction of QM formation. Radical cations decay fast (45 ps) due to the back electron transfer. By quenching with ascorbate we identified phenoxyl radicals as a QM precursor, which are formed in a slower H-transfer reaction taking place on nanosecond time scale. Our combined experimental and theoretical investigation point to a polycyclic substituent effect, which profoundly changes the photochemical reaction pathway.

femtosecond transient absorption ; naphthols ; quinone methides ; conical intersection ; photodehydration

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