1. On the different strength of photoacids
- Author
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Martina Schiazza, Umberto Raucci, Federico Coppola, Maria Gabriella Chiariello, Paola Cimino, Nadia Rega, Greta Donati, Cimino, Paola, Raucci, Umberto, Donati, Greta, Chiariello, MARIA GABRIELLA, Schiazza, Martina, Coppola, Federico, and Rega, Nadia
- Subjects
Hybrid implicit/explicit solvation model ,Intermolecular excited-state proton transfer ,TD-DFT ,Physical and Theoretical Chemistry ,010304 chemical physics ,Chemistry ,Intermolecular force ,010402 general chemistry ,Photochemistry ,01 natural sciences ,Potential energy ,Molecular electronic transition ,0104 chemical sciences ,Molecular dynamics ,Solvation shell ,Chemical physics ,0103 physical sciences ,Molecule ,Density functional theory ,Singlet state - Abstract
In spite of the detailed information provided by advanced time-resolved spectroscopy, the understanding of the excited-state proton transfer (ESPT) reactivity remains difficult to obtain at molecular level. In this work we studied three photoacids showing different strength: the 8-hydroxy-1,3,6-pyrenetrisulfonate weak photoacid, the N-methyl-6-hydroxyquinolinium strong photoacid and the phenol-carboxyether dipicolinium cyanine (QCy9) superphotoacid, focusing on the intermolecular ESPT toward a solvent molecule or a base molecule in aqueous solution. To this aim, the ground and the first singlet excited-state potential energy surfaces of the three systems were characterized by means of the time-dependent density functional theory and a hybrid implicit/explicit model of the solvent. Main structural and photophysical features of the photoacids were assessed and satisfactorily compared with the experimental data. Energy profiles along the PT coordinate were analyzed in both the electronic states. We reproduced many important features of the photoacidity experimentally observed. The results suggest that the relative strength is mainly due to the different extent of charge transfer caused by the electronic transition in proximity of the acid group. Remarkably, we found that even in the case of the strongest photoacid (QCy9), showing a ESPT rate as rapid as to escape the solvent dynamics control, the PT is modulated and supported by the first solvation shell of the proton-accepting molecule. However, a complete understanding of this fascinating field needs the full account for the electronic and the molecular dynamics in play at different timescales.
- Published
- 2016