HYDROGEN-BONDING-BASED THERMOCHROMIC PHENOL-AMINE COMPLEXES

Variable-temperature UV-vis, C-13 NMR and IR studies showed that proto n-transferred complexes were formed between phenols and amines in apol ar solvents at low temperature. Upon cooling a solution of p-nitrophen ol and diisopropylamine in toluene, the colour of the solution changed from colourless to yellow. This thermochromism was ascribed to the pr oton transfer in the hydrogen-bonding complex. Under UV-vis conditions , butylamine and imidazole also caused similar thermochromism upon com plexation with p-nitrophenol, while triethylamine, quinuclidine and py ridine did not. The thermochromic behaviour was particularly dependent on the stoichiometry of the amine and the phenol: a solution of 3,3'- dibromo-5,5'-dinitro-2,2'-biphenyldiol and diisopropylamine with a mol ar ratio of 1:1 showed no thermochromism, while solutions with 1:2 or higher ratios showed thermochromism, indicating that excess amine is r equired to obtain the proton-transferred species. These results reveal ed that the proton-transferred species forms in apolar solvents at low temperature if an appropriate hydrogen-bonding network between the ph enol and the amine can stabilize it. (C) 1998 John Wiley & Sons, Ltd.