The six-membered intramolecular hydrogen bond position as a switch for inducing an excited state intramolecular proton transfer (ESIPT) in esters of o-hydroxynaphthoic acids

The substituted naphthalene compounds investigated in this paper, i.e., met hyl 2-hydroxy-3-naphthoate (MHN23), methyl 1-hydroxy-2-naphthoate (MHN12), and methyl 2-hydroxy-1-naphthoate (MHN21), show a strong intramolecular hyd rogen bond (IMHB) in their ground electronic state. The relative position o f the IMHB in the naphthalene skeleton acts as a switch and controls the yi eld of an excited state intramolecular proton transfer (ESIPT) process. As a matter of fact, only MHN23 exhibits a proton transfer (PT) emission and p ossesses a theoretically proved ESIPT mechanism. The role that the ESIPT me chanism plays on the photostability of the molecule MHN23 is unravelled by comparison with the model compounds methyl salicylate (MS), MHN12, and MHN2 1. On one hand, the low photoreaction quantum yield, Phi(r) = 0.00015, and therefore the high photostability of MS, under direct ultraviolet (UV) irra diation, has been explained due to the photophysics of its proton transfer tautomer. On the other hand, (a) the two benzene-fused ring derivatives of methyl salicylate, MHN12 and MHN21, also possess a great photostability to UV radiation, and they do not support an ESIPT mechanism; and (b) although MHN23 exhibits an excited state proton transfer, its efficiency is only of 1.8%, and the photostability is 5 times larger than that of MS. As a result , the photostability of MHN23, MHN12, and MHN21 does not rely on the photop hysics of their proton transfer tautomers but on the nonradiative dynamics of their respective normal tautomers. We present experimental evidence whic h supports the above-mentioned statement on the existence of distinctive no nradiative channels for the molecules MHN23, MHN12, and MHN21.