Upon excitation to the singlet excited state, phenols experience increased acidity. This allows them to undergo an excited state intramolecular proton transfer (ESIPT), in which the phenolic proton is transferred to a basic site within the same molecule. To increase the efficiency of ESIPT reactions for 2-phenylphenol (10) derivatives, two previously determined strategies include the addition of a bulky substituent ortho to the hydroxyl group or incorporation of a second hydroxyl group ortho to the phenyl ring. This research aims to combine the two strategies into one molecule, 4,6-ditert-butyl-2-phenylbenzene-1,3-diol (18), to see if both strategies can work together to increase the ESIPT reaction efficiency. The new phenylphenol derivative 18 was prepared via a Suzuki coupling reaction of 2,6-dimethoxyphenylboronic acid and iodobenzene, followed by a demethylation with BBr3, and finally an electrophilic aromatic substitution reaction to add the two tert-butyl groups to the ring, resulting in a yield of 8%. The compound was characterized by 1H and 13C{1H} NMR spectroscopy, mass spectrometry (MS), and product studies.
Compound 18 was found to have undergone ESIPT to the 2’-position, followed by a reverse proton transfer (RPT), however, there was a very low approximate quantum yield. Initial quantum yield measurements suggest the ESIPT quantum yield is around 0.07. This is lower than what has been reported for 10, suggesting that adding two tert-butyl groups and two hydroxyl groups does not increase the efficiency of deuterium exchange, contrary to what was originally hypothesized. The low quantum yield is believed to be a result of the “loose bolt” effect, suggesting internal conversion (IC) is likely the dominant pathway from the singlet excited state back to the ground state.