A picosecond time-resolved study on prototropic reactions of electronically excited 1,5- and 1,8-diaminonaphthalenes in aqueous solution

The proton transfer to solvent in the excited state of protonated diaminonaphthalenes, 1,5-diaminonaphthalene (1,5-DAN) and 1,8-diaminonaphthalene (1,8-DAN), in aqueous solution, has been investigated by picosecond time-resolved fluorescence measurements. The deprotonation rate constants of the dications of 1,8-DAN and 1,5-DAN in the excited state to produce the corresponding monocations are determined to be 1.3 × 1010 and 5.6 × 109 s−1, respectively, from dynamic analyses of their fluorescence time profiles. The much larger proton-dissociation rates compared with that of 1-aminonaphthalene (0.6 × 109 s−1) can be attributed to an electron-withdrawing effect due to the ammonium group at the 5- or 8-position in the naphthalene ring. The remarkably large proton-dissociation rate in 1,8-DAN can be ascribed to its larger reaction exergonicity which results from the electrostatic repulsion between the two ammonium groups in the reactant (the dication state) and the stabilization of the monocation state due to hydrogen bonding interactions between the NH3+ and NH2 moieties. The difference in their acidities in the excited state is discussed in terms of the reaction free energy and the proton affinities are evaluated from ab initio MO calculations.