Lack of the ESIPT band of aromatic ortho-aminoaldehyde derivatives triggered by N–H vibration

Abstract

The excited state intramolecular proton transfer (ESIPT) reactions and the fluorescence emission spectra of o-aminoaldehyde and o-aminoketone derivatives were systematically studied with density functional theory (DFT) and time-dependent density functional theory (TDDFT). The results suggest that the ESIPT process can be characterized as an ultra-fast process and that N–H vibration plays an important role in fluorescence emissions. The minimum energy pathways (MEP) on the excited states along the proton transfer coordinates (N–H vibration) were constructed for both o-aminoaldehyde and o-aminoketone derivatives, respectively, which showed a small barrier between the normal and tautomer (ESIPT) structures. By comparing the proton transfer barriers (E_b) and the N–H reorganization energies (λ^ele_NH), we find that λ^ele_NH is less than E_b in o-aminoketone derivatives, while λ^ele_NH is greater than E_b in o-aminoaldehyde derivatives. It is clear that protons could move freely in o-aminoaldehyde derivatives, and thus only one normal emission band could be observed. Subsequently, the fluorescence emission spectra upon introduction of the N–H vibration effect can further confirm this mechanism, and the simulated spectra are in good agreement with the experimental observations, in which the o-aminoaldehyde derivatives have only one normal emission band while the o-aminoketone derivatives have two emission bands corresponding to the normal and tautomer structures. Consequently, this work can elucidate the lack of the ESIPT band in o-aminoaldehyde derivatives and also offer new insight into ESIPT by considering the vibronic effect.