Nonstatistical Photoinduced Processes in Gaseous Organic Molecules

Abstract

Processes that proceed in femtoseconds are usually referred to as being ultrafast, and they are investigated in experiments that involve laser pulses with femtosecond duration in so-called pump probe schemes, where a light pulse triggers a molecular process and a second light pulse interrogates the temporal evolution of the molecular population. The focus of this review is on the reactivity patterns that arise when energy is not equally distributed on all the available degrees of freedom as a consequence of the very short time scale in play and on how the localization of internal energy in a specific mode can be thought of as directing a process toward (or away from) a certain outcome. The nonstatistical aspects are illustrated with examples from photophysics and photochemistry for a range of organic molecules. The processes are initiated by a variety of nuclear motions that are all governed by the energy gradients in the Franck–Condon region. Essentially, the molecules will start to adapt to the new electronic environment on the excited state to eventually reach the equilibrium structure. It is this structural change that is enabling an ultrafast electronic transition in cases where the nuclear motion leads to a transition point with significant coupling between to electronic states and to ultrafast reaction if there is a coupling to a reactive mode at the transition point between the involved states. With the knowledge of the relation between electronic excitation and equilibrium structure, it is possible to predict how the nuclei move after excitation and often whether an ultrafast (and inherently nonstatistical) electronic transition or even a bond breakage will take place. In addition to the understanding of how nonstatistical photoinduced processes proceed from a given excited state, it has been found that randomization of the energy does not even always take place when the molecule takes part in processes that are normally considered statistical, such as for example nonradiative transitions between excited states. This means that energy can be localized in a specific degree of freedom on a state other than the one that is initially prepared. This is a finding that could kickoff the ultimate dream in applied photochemistry; namely light excitation that leads to the rupture of a specific bond.

Non‐statistical dynamics for the allene oxide to cyclopropanone conversion

“…Current pursuits in organic reaction mechanisms almost require some treatment for or discussion of dynamics as so many investigations of reactions keep turning up new details with a deeper analysis. A number of reviews help to frame the current concerns on the topic, [ 1–21 ] while one of Carpenter's more recent reviews [ 22 ] might be one of the more accessible reviews on the topic for the organic chemistry community. The work presented here focuses on an organic reaction mechanism, and of the many different reasons for non‐statistical reaction dynamics, this paper focuses on intramolecular vibrational energy redistribution (IVR).…”

Vibrational Funnels for Energy Transfer in Organic Chromophores

“…The efficiency of energy transfer processes in multichromomoric molecular systems has nonstatistical character due to its ultrafast aspect, quantum-mechanical delocalization and localization, and nontrivial participation of specific nuclear motions. 17,18 The presence of one or a few main vibrational funnels avoids the energy dissipation among numerous vibrational motions or the solvent/bath degrees of freedom. 19,20 The so-called active vibrational modes are defined as modes that actively participate in the energy transfer by coupling the electronic states, and different strategies are proposed to identify them 19,21−26 in the context of conical intersections.…”

“…The efficiency of energy transfer processes in multichromomoric molecular systems has nonstatistical character due to its ultrafast aspect, quantum-mechanical delocalization and localization, and nontrivial participation of specific nuclear motions. , The presence of one or a few main vibrational funnels avoids the energy dissipation among numerous vibrational motions or the solvent/bath degrees of freedom. , The so-called active vibrational modes are defined as modes that actively participate in the energy transfer by coupling the electronic states, and different strategies are proposed to identify them ,− in the context of conical intersections. − Atomistic nonadiabatic dynamics simulations are a useful pragmatic methodology to analyze these relevant vibrations. − Herein, we make use of the NEXMD software package, , particularly developed to describe photoinduced dynamics in relatively large conjugated multichromophoric molecules. NEXMD has proved to be suitable for the analysis of exciton redistributions and vibronic dynamics during electronic relaxation in this class of molecular systems. , …”