Ultrafast above-threshold dynamics of the radical anion of a prototypical quinone electron-acceptor

Abstract

. (2013) 'Ultrafast above-threshold dynamics of the radical anion of a prototypical quinone electron-acceptor.', Nature chemistry., 5 (8). pp. 711-717. Further information on publisher's website:https://doi.org/10.1038/nchem.1705Publisher's copyright statement:Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Quinones are found throughout nature as key electron acceptor intermediates, 1,2 with examples including plastoquinone which is involved in the electron transfer chain of photosystem II, and ubiquinone (coenzyme Q10) which plays a key role in aerobic cellular respiration. 3 The central moiety responsible for the electron accepting ability in quinones is para-benzoquinone (pBQ), shown in Figure 1c. Electron transfer reactions involving pBQ can be highly exergonic and are therefore often classed as being in the Marcus inverted region. [4][5][6][7][8][9][10] This is shown schematically by the green path in Figure 1a, where a barrier between the Gibbs free energy of the reactants and products lowers the rates of the electron transfer process. However, even in the earliest experimental verifications of the inverted region for intramolecular electron transfer, several electron acceptors based on pBQ showed marked deviations from the expected behaviour, with transfer rates approaching those of a barrierless reaction. 11 It has been proposed that such deviations may involve electronically excited states of the product radical anion of para-benzoquinone (pBQ• -), 12 which could provide reaction pathways that bypass the barrier, as shown in Figure 1a with purple arrows. Figure 1b shows the location of these resonances.From the point of view of an electron approaching pBQ, these anionic resonances in the detachment continuum can capture an electron. Subsequent formation of the anionic ground state through internal conversion would redistribute the excess internal energy amongst all the vibrational modes. In a condensed-phase environment, this energy will be quenched by the surroundings. However, the initially formed excited states of pBQ• -can be unbound with respect to electron loss. For the above picture to be feasible, internal conversion must be able to compete with autodetachment. Electron attachment spectra suggest that it can, 26-28 but how does this occur given that these resonances are in some cases > 1 eV above the detachment threshold?In order to gain a fundamental understanding of the processes involved following electron capture, it is necessary to observe the relaxation dynamics in real t...

Internal conversion outcompetes autodetachment from resonances in the deprotonated tetracene anion continuum

“…The mechanism through which above-threshold population is funnelled to the ground electronic state is internal conversion via conical intersections that result from strong non-adiabatic electronic state couplings. [24][25][26][27][28] …”

“…The mechanism through which above-threshold population is funnelled to the ground electronic state is internal conversion via conical intersections that result from strong non-adiabatic electronic state couplings. [24][25][26][27][28] Complete internal conversion to the ground electronic state can be assisted by intermediate states that are close in energy, or by a ladder of intermediate states. In particular, bound electronic states of the anion have been shown to be important intermediates.…”

“…22 Although spontaneous autodetachment from a resonance is typically very fast (tens to hundreds of femtoseconds), non-adiabatic internal conversion can compete and even dominate. [24][25][26][27][28] Indeed, many electron impact studies on conjugated molecules have observed efficient formation of long-lived anions despite the incident electron energies and molecular configurations incompatible with an electric dipole-mediated capture mechanism. 22,29 As a specific example, mass spectrometry measurements on tetracene and pentacene have shown the formation of long-lived anions with lifetimes of ~10 ms following capture of electrons with up to ~3 eV of kinetic energy.…”

Anion resonances and above-threshold dynamics of coenzyme Q0

“…We have recently used a combination of frequency-, angle-, and time-resolved photoelectron imaging (FAT-PI) to study the simplest para-quinone anion, para-benzoquinone (pBQ), 10,11 and also menadione (vitamin K 3 ), 12 which also is an active electron mediator in biology. Briefly, pBQ and its resonances has been the subject of several studies over the last few Paper PCCP 2 | Phys.…”

“…Chem. Phys., 2015, 00, [1][2][3][4][5][6][7][8][9][10][11] This journal is © The Royal Society of Chemistry 2015…”