Theoretical investigation of the photophysical properties of donor-acceptor dyes containing coumarin and naphthoquinone moieties linked by an aminomethylene bridge

Mannich bases HL1 and HL2 derived from lawsone, heptylamine and 3-formylcoumarins have been synthesized for the investigation of their photophysical properties. The cyclic voltammetry data showed a charge transfer (CT) process from the coumaryl to the naphthoquinonoid group, in spite of the absence of conjugation between these two fragments, with the nitrogen atom playing an important role. Photophysical studies by UV–Visible and fluorescence spectroscopy in acetonitrile revealed low emission quantum yield (Φ), thus confirming the quenching of the coumarin fluorophore by the naphthoquinone moiety. The role of the nitrogen atom in this spacer was investigated by perturbing the electronic density at this site by protonation. The emission spectra of the hydrochlorides HL1·HCl and HL2·HCl revealed increase in the quantum yields in both cases. The geometries of all compounds were optimized at B3LYP/6-31 + G (d,p) level. TD-DFT calculations were carried out with different functionals. The hybrids B3LYP and PBE0 provided the best results with respect to the lowest energy transition and M06-HF failed in this prediction, whereas the long range corrected functionals - CAM-B3LYP and ωB97XD - did not improve the description of this transition. For HL1 and HL2, the lowest energy absorption band around 440 nm (λ3) is assigned to an aminomethyl coumarin (HOMO-1) to naphthoquinone (LUMO) CT. However, for HL1·HCl and HL2·HCl, the contribution of the nitrogen atom was suppressed by protonation and the lowest energy transition is assigned only to the coumarin (HOMO) to naphthoquinone (LUMO) CT.