Adiabatic Deprotonation as an Important Competing Pathway to ESIPT in Photoacidic 2-Phenylphenols.
ESIPT (Excited State Intramolecular Proton Transfer) to C atom in 2-phenylphenol is known to occur as an intrinsically inefficient process; however, to the best of our knowledge, a structure-ESIPT efficiency relationship has not been elucidated yet. Here we show that there exists a competitive interplay between photoacidity and ESIPT efficiency for 2-phenylphenol system. The attachment of electron withdrawing groups to the phenol moiety promotes adiabatic deprotonation in excited state and diminish the charge transfer character of excitations, both factors contributing to decrease the ESIPT reaction yield. On the other hand, unfavorable conformational distribution in ground state also appears as another important aspect responsible for the low ESIPT extent of 2-phenylphenol. A new derivative bearing electron donor, bulky substituents at ortho and para positions of the phenol ring shows an outstanding ESIPT performance, which demonstrates that the efficiency of the process can be significatively enhanced by modifying the substitution pattern. We anticipate our results will help guide molecular design to produce new compounds with high ESIPT efficiency.