Stepwise double excited-state proton transfer is not possible in 7-azaindole dimer

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.MB and NK would like to thank the support from the Deutscher Akademischer Austauschdienst (DAAD) and from the Deutsches Zentrum f¨ur Luft- und Raumfahrt (DLR) through the Thai- German mobility gran

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

Kinetics of the hydrogen abstraction ·C2H5 + alkane → C2H6 + alkyl reaction class: an application of the reaction class transition state theory

This paper presents an application of the reaction class transition state theory (RC-TST) to predict thermal rate constants for hydrogen abstraction reactions at alkane by the C2H5 radical on-the-fly. The linear energy relationship (LER), developed for acyclic alkanes, was also proven to hold for cyclic alkanes. We have derived all RCTST parameters from rate constants of 19 representative reactions, coupling with LER and the barrier height grouping (BHG) approach. Both the RC-TST/LER, where only reaction energy is needed, and the RC-TST/BHG, where no other information is needed, can predict rate constants for any reaction in this reaction class with satisfactory accuracy for combustion modeling. Our analysis indicates that less than 50% systematic errors on the average exist in the predicted rate constants using either the RC-TST/LER or RC-TST/BHG method, while in comparison with explicit rate calculations, the differences are within a factor of 2 on the average. The results also show that the RC-TST method is not sensitive to the choice of density functional theory used

Kinetics of 1,6-hydrogen migration in alkyl radical reaction class

The kinetics of the 1,6-intramolecular hydrogen migration in the alkyl radical reaction class has been studied using the reaction class transition state theory (RC-TST) combined with the linear energy relationship (LER) and the barrier height grouping (BHG) approach. The RC-TST/LER, where only reaction energy is needed, and RC-TST/BHG, where no other information is needed, are found to be promising methods for predicting rate constants for any reaction in the 1,6-intramolecular H migration in alkyl radicals reaction class. Direct comparison with available experimental data indicates that the RC-TST/LER, where only reaction energy is needed, can predict rate constants for any reaction in this reaction class with satisfactory accuracy

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

The effect of Hydrogen Bonding on the Excited-State Proton Transfer in 2,(2'-hydroxyphenyl)benzothiazole: a TDDFT molecular dynamics study

The dynamics of the excited-state proton transfer (ESPT) in a cluster of 2-(2′-hydroxyphenyl)benzothiazole (HBT) and hydrogen-bonded water molecules was investigated by means of quantum chemical simulations. Two different enol ground-state structures of HBT interacting with the water cluster were chosen as initial structures for the excited-state dynamics: (i) an intramolecular hydrogen-bonded structure of HBT and (ii) a cluster where the intramolecular hydrogen bond in HBT is broken by intermolecular interactions with water molecules. On-the-fly dynamics simulations using time-dependent density functional theory show that after photoexcitation to the S1 state the ESPT pathway leading to the keto form strongly depends on the initial ground state structure of the HBT–water cluster. In the intramolecular hydrogen-bonded structures direct excited-state proton transfer is observed within 18 fs, which is a factor two faster than proton transfer in HBT computed for the gas phase. Intermolecular bonded HBT complexes show a complex pattern of excited-state proton transfer involving several distinct mechanisms. In the main process the tautomerization proceeds via a triple proton transfer through the water network with an average proton transfer time of approximately 120 fs. Due to the lack of the stabilizing hydrogen bond, intermolecular hydrogen-bonded structures have a significant degree of interring twisting already in the ground state. During the excited state dynamics, the twist tends to quickly increase indicating that internal conversion to the electronic ground state should take place at the sub-picosecond scale

Exceptional electron conduction in two-dimensional covalent organic frameworks

Most organic/polymeric semiconductors are p-type semiconductors, whereas their n-type versions are limited in both availability and carrier mobility. How to develop high-rate n-type organic/polymeric semiconductors remains challenging. Here, we report an approach to high-rate n-type semiconductors via topology-directed polycondensation of conventional p-type knots with n-type isoindigo linkers to form non-conjugated tetragonal and hexagonal two-dimensional polymeric frameworks. The polymers are planar in conformation and show flattened frontier levels, which enable electrons to move along the non-conjugated polymeric backbones. The eclipsed face-to-face stack reduces reorganization energy and greatly strengthens electronic coupling, thus enabling band-like electron conduction perpendicular to polymer layers. A device recording electron mobility as high as 8.2 cm2 V−1 s−1 was achieved with Hall effect measurements, whereas time- and frequency-resolved terahertz spectroscopy revealed a benchmark mobility of 13.3 cm2 V−1 s−1. These new mechanistic insights with exceptional mobility open the way to high-rate n-type organic/polymeric semiconductors