Luminescent Mannich bases: Synthesis, crystal structures, and photophysical properties

Three Mannich bases have been obtained starting from 5-bromo-salicylaldehyde: 5-bromo-3-(((2-(dimethylamino)ethyl)(methyl)amino)methyl)-2-hydroxybenzaldehyde (TMEN), 5-bromo-3-((benzyl(methyl)amino) methyl)-2-hydroxybenzaldehyde (MBA), 5-bromo-3-(ethyl(methyl)amino)methyl)-2-hydroxybenzaldehyde (EMA). The crystal structure of MBA has been solved. The UV-Vis absorption and emission spectra were investigated in different solvents, revealing the presence of several species, the neutral forms, the zwitterionic, the anionic and the cationic (the protonated nitrogen) ones. In order to characterize the last two species, further experiments were performed in the presence of hydrogen extracting and hydrogen donating compounds, sodium fluoride (NaF) and tetrabutylammonium hydroxide (TBOH) to evidence the anion, and trichloroacetic acid (TCA) for the protonated cation. The experimental results were rationalized by DFT calculations of the potential energy surfaces (PES) of the S0 and S1 states and of the absorption and emission maxima.