Synthesis and photochemistry of 2,6-DI(1-Naphthyl)phenol

When excited, phenols experience enhanced acidity, allowing them to undergo reactions they would not do in the ground state. One of these reactions is excited state intramolecular proton transfer (ESIPT). This reaction involves the transfer of an acidic hydrogen to a basic site within the molecule. Previously, the photochemistry of 2-phenylphenol, 6, and 2,6-diphenylphenol, 8, have been investigated as they undergo ESIPT, and it was found that the addition of the second phenyl group increased the quantum yield. This research aims to see if that trend continues using a different aromatic substituent: naphthalene. 2,6-di(1-Naphthyl)phenol, 14, was prepared via a four-step synthesis involving bromination, methylation, Suzuki coupling, and demethylation with an overall yield of 21%. The photochemistry of 14 was investigated using UV-Vis spectroscopy, NMR spectroscopy, and fluorescence spectroscopy. UV-Vis spectra indicated that ESIPT and electrocyclic ring closing were successful and water mediated, with 10% H2O in acetonitrile being the optimal concentration. UV-Vis spectra completed at 254 nm for 2-(1-naphthyl)phenol, 10, and 14 both showed that the photocyclization was more effective for 10. The quantum yield of the photocyclization of 14 was found to be 0.15. These results show that the addition of the second naphthalene group did not make photocyclization more efficient. However, the same cannot be said about ESIPT to the 2' position since its reaction efficiency could not be determined. Fluorescence spectroscopy was performed to investigate the photophysics of 14 and indicated that excited state proton transfer (ESPT) to solvent was also taking place. More studies need to be performed to determine if ESIPT to the 2' position of 14 is more efficient than 10. The orientation of the OH group will have a greater effect on the ESIPT of the 2' position rather than photocyclization, so the overall quantum yield of 14 could end up higher than that of 10.