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

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 eleNH), we find that λ eleNH is less than E b in o -aminoketone derivatives, while λ eleNH 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.