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

Rachel Crespo-Otero,Nawee Kungwan,Mario Barbatti

doi:10.1039/c5sc01902h

Abstract

The nature of the excited-state double proton transfer in 7-azaindole (7AI) dimer-whether it is stepwise or concerted-has been under a fierce debate for two decades. Based on high-level computational simulations of static and dynamic properties, we show that much of the earlier discussions was induced by inappropriate theoretical modelling, which led to biased conclusions towards one or other mechanism. A proper topographical description of the excited-state potential energy surface of 7AI dimer in the gas phase clearly reveals that the stepwise mechanism is not accessible due to kinetic and thermodynamic reasons. Single proton transfer can occur, but when it does, an energy barrier blocks the transfer of the second proton and the dimer relaxes through internal conversion. Double proton transfer takes place exclusively by an asynchronous concerted mechanism. This case-study illustrates how computational simulations may lead to unphysical interpretation of experimental results.

Highlights

A central problem in physical chemistry is to understand how photoinduced multiple proton transfers take place in dimers
Experimental results show that the 7AI dimer in the gas phase excited near the band origin has an ultrafast dynamics with short (0.2–0.6 ps) and long (1–3 ps) time components[2,8,9,10]
Part of the problem is that, so far, all theoretical models guiding the experimental analysis failed to provide a balanced description of the several different diabatic regions of the rst excited state. This imbalance led to prediction of spurious minima, missing conical intersections, wrong descriptions of charge-transfer structures; all of that contributing to biased discussions of the mechanisms

Summary

Introduction

A central problem in physical chemistry is to understand how photoinduced multiple proton transfers take place in dimers. For decades,1–4 7-azaindole (7AI) dimer has been adopted by experimentalists and theorists as a prototype for investigating such processes. A er Zewail and co-workers,[2] based on timeresolved spectroscopy and computational modelling, proposed that photoexcitation near the band origin induces a stepwise double proton transfer in 7AI dimer in the gas phase (Fig. 1), a heated debate took place between advocates of concerted[5] and stepwise mechanisms.[6] This debate, has been shi ed to the excited-state proton transfer (ESPT) of the 7AI dimer in condensed phases,[4,6,7] even though a consensus has never been settled as to the nature of the proton transfer in the gas phase

Methods

Results

Conclusion