Excited State Intramolecular Proton Transfer (ESIPT) from Phenol to Carbon in Selected Phenylnaphthols and Naphthylphenols

ESIPT and solvent-assisted ESPT in isomeric phenyl naphthols and naphthyl phenols 5–8 were investigated by preparative photolyses in CH3CN–D2O, fluorescence spectroscopy, LFP, and ab initio calculations. ESIPT takes place only in 5 (D-exchange Φ = 0.3), whereas 6–8 undergo solvent-assisted PT with much lower efficiencies. The efficiency of the ESIPT and solvent-assisted PT is mainly determined by different populations of the reactive conformers in the ground state and the NEER principle. The D-exchange experiments and calculations using RI-CC2/cc-pVDZ show that 5 in S1 deactivates by direct ESIPT from the OH to the naphthalene position 1 through a conical intersection with S0, delivering QM 14 that was detected by LFP (τ = 26 ± 3 ns). ESIPT to position 3 in 5 is possible but it proceeds from a less-populated conformer and involves an energy barrier on S1. In solvent-assisted PT to naphthalene position 4 in 5, zwitterion 17 is formed, which cyclizes to stable naphthofuran photoproducts 9–12. The regiochemistry of the deuteration in solvent-assisted PT was correlated with the NBO charges of the corresponding phenolates/naphtholates 5 ‑ –8 ‑ . Combined experimental and theoretical data indicate that solvent-assisted PT takes place via a sequential mechanism involving first deprotonation of the phenol/naphthol, followed by the protonation by H2O in the S1 state of phenolate/naphtholate. The site of protonation by H2O is mostly at the naphthalene α-position.