Dynamics simulations of excited-state triple proton transfer in 7-azaindole complexes with water, water–methanol and methanol

Excited-state triple proton transfer (ESTPT) reactions in 7-azaindole (7AI) complexed with two water, with one water and one methanol, and with two methanol molecules were investigated by dynamics simulations in the first excited state computed with the second order algebraic-diagrammatic construction (ADC (2)) method. The results show that photoexcitation may trigger ultrafast an asynchronous concerted proton transfer via two solvent molecules along an intermolecular hydrogen-bonded network. The probability of occurrence of ESTPT ranges from 32% for 7AI(H2O–MeOH) to 64% for 7AI(MeOH)2. The average time for completing the ESTPT varies between 58 and 85 fs depending on the complex. The proton transfer (rather than hydrogen transfer) nature of the reaction was suggested by the nonexistence of crossings between the ππ* and πσ* states

Theoretical study on excited-state intermolecular proton transfer reactions of 1H-pyrrolo[3,2-h]quinoline with water and methanol

The dynamics of the ultrafast excited-state multiple intermolecular proton transfer (PT) reactions in gas-phase complexes of 1H-pyrrolo[3,2-h]quinoline with water and methanol (PQ(H2O)n and PQ(MeOH)n , where n = 1, 2) is modeled using quantum-chemical simulations. The minimum energy ground-state structures of the complexes are determined. Molecular dynamics simulations in the first excited state are employed to determine reaction mechanisms and the time evolution of the PT processes. Excited-state dynamics results for all complexes reveal synchronous excited-state multiple proton transfer via solvent-assisted mechanisms along an intermolecular hydrogen-bonded network. In particular, excited-state double proton transfer is the most effective, occurring with the highest probability in the PQ(MeOH) cluster. The PT character of the reactions is suggested by nonexistence of crossings between ππ* and πσ* states

Effects of the second hydration shell on excited-state multiple proton transfer: dynamics simulations of 7-azaindole:(H₂O)_1-5 clusters in the gas phase

Dynamics of the multiple excited-state proton transfer (ESPT) in clusters of 7-azaindole with up to five water molecules was investigated with quantum chemical methods. The ultrafast excited-state dynamics triggered by photoexcitation was simulated with the algebraic diagrammatic construction to the second-order scheme. Multiple ESPT through a hydrogen-bonded network is observed in the 100-fs scale. The probability of tautomerization is anti-correlated with the maximum free energy barrier in the excited state. An increasing number of water molecules tends to reduce the barrier by strengthening the hydrogen-bonded network. Barrierless reactions are found already for clusters with four waters. In structures presenting double hydrogen bond circuits, proton transfer happens mostly through the internal circuit by triple proton transfer. The overall role of the second hydration shell is of stabilizing the network, facilitating the proton transfer in the internal circuit. Proton transfers involving the second hydration shell were observed, but with small probability of occurrence. The proton-transfer processes tend to be synchronous, with two of them occurring within 10–15 fs apart