Hydrogen bond formations of 1-aminoanthracene in the ground and excited electronic states with protic solvent molecules

The absorption and fluorescence spectra of 1-aminoanthracene (AA) in various solvents have been compared with those of 1-dimethylaminoanthracene (DMAA). This comparison has demonstrated that the amino group of AA in the ground electronic state acts as a proton-acceptor to form the A-type hydrogen bond with alcohol, while that of AA in the excited electronic state acts as a proton-donor to form the B-type one. In the cyclohexane-ethanol mixed solvent, the solute-solvent configuration for the fluorescing excited state of AA changes with the ethanol concentration; it keeps the same type of hydrogen bond with ethanol as that (A-type) in the ground electronic state in the lower ethanol concentrations up to 0.2% and exhibits an intramolecular-like rearrangement of the A-type hydrogen bond into the B-type one in the ethanol concentration range of 0.2-1%. This hydrogen bond rearrangement occurs in cooperation with a bulk reorientation of solvent molecules in the range of 1-100%. AA forms the hydrogen-donor-acceptor-type excimer in the nonpolar fluid matrix of MP (a 1 : 1 mixed solvent of methylcyclohexane and isopentane) at low temperature.