Rotational Energy Barrier of 2-(2′,6′-Dihydroxyphenyl)benzoxazole: A Case Study by NMR

2-(2′-Hydroxyphenyl)­benzoxazole (HBO) derivatives
represent
an important class of luminescent materials, as they can undergo excited
state intramolecular proton transfer (ESIPT). The material’s
ESIPT properties are dependent on the ratio of two different rotamers,
whose interconversion is poorly understood. By using HBO derivative 4, the rotational energy barrier of 2- (2′,6′-hydroxyphenyl)­benzoxazole
is determined to be 10.5 kcal/mol by variable-temperature NMR. Although
a HBO derivative typically exhibits two rotamers with O···H–O
(e.g., 1a) and N···H–O bonding
(e.g., 1b), correlation of NMR with fluorescence data
reveals
that the rotamer with N···H–O bonding is predominant
in the solution.