Directing the path of light-induced electron transfer at a molecular fork using vibrational excitation

Abstract

Ultrafast electron transfer in condensed-phase molecular systems is often strongly coupled to intramolecular vibrations that can promote, suppress and direct electronic processes. Recent experiments exploring this phenomenon proved that light-induced electron transfer can be strongly modulated by vibrational excitation, suggesting a new avenue for active control over molecular function. Here, we achieve the first example of such explicit vibrational control through judicious design of a Pt(II)-acetylide charge-transfer Donor-Bridge-Acceptor-Bridge-Donor "fork" system: asymmetric 13 C isotopic labelling of one of the two -C≡C-bridges makes the two parallel and otherwise identical Donor→Acceptor electron-transfer pathways structurally distinct, enabling independent vibrational perturbation of either. Applying an ultrafast UV pump (excitation)-IR pump (perturbation)-IR probe (monitoring) pulse sequence, we show that the pathway that is vibrationally perturbed during UV-induced electron-transfer is dramatically slowed down compared to its unperturbed counterpart. One can thus choose the dominant electron transfer pathway. The findings deliver a new opportunity for precise perturbative control of electronic energy propagation in molecular devices. Main TextControlling the function of future advanced materials demands a profound understanding of energymatter interactions in molecular systems at the quantum level. 1 There is a growing consensus that energy-and electron-transfer processes occurring far away from equilibrium in condensed-phase systems on femto-to-picosecond timescales do not conform to the Born-Oppenheimer approximation. Indeed, nuclear and electronic wave functions often strongly interact with each other, leading to exotic phenomena whereby vibrational motion can mediate and dictate the rate and efficiency of electronic transitions. [2][3][4][5] With overwhelming evidence that nuclear-electronic (vibronic) interactions play a crucial role in a broad range of systems, including light harvesting and conversion in photosynthetic organisms, 6-11 this phenomenon represents a tremendous opportunity to acquire deeper knowledge and control over the structural traits that govern molecular function and reactivity.Recent efforts led by Rubtsov et al.,12,13 Bakulin et al., 14 and our group 15,16 have shown that one way to experimentally exploit vibronic coupling to modulate molecular function is to introduce targeted vibrational excitation along specific reaction co-ordinates using ultrafast mid-infrared (IR) pulses. This approach enables promoting or suppressing electronic transitions, notably photo-induced electron transfer (ET), an elementary process that pervades the natural world. Although the demonstrated excited state modulations have been remarkably efficient, up to 100% suppression in one case, 15 predictive and directive control of electron transfer in the condensed phase has not been achieved yet.

Solvent tuning of photochemistry upon excited-state symmetry breaking

“…It also explains the lack of success in the elaboration of symmetric molecular systems that could exhibit double excitedstate proton transfer [69][70][71] . Solvent-driven SB leads to a concentration of the excitation on a particular area of a symmetric molecule and results in a strong enhancement of its photochemical reactivity providing effective means of guiding the latter in addition to mode-selective IR excitation [72][73][74][75] . Without symmetry breaking, photochemistry is significantly less efficient, if operative at all, because the excitation density, being evenly distributed over the molecule, is not sufficient to drive a chemical reaction.…”

Coupled harmonic oscillators and their quantum entanglement

“…The behavior of systems that contain coupled harmonic oscillators is currently an area of very active research; this interest is primarily due to the fact that models of such systems are encountered in many applications of quantum and nonlinear physics [1]- [10], molecular chemistry [11]- [13] and biophysics [14]- [16]. In quantum physics specifically, this interest is because of the quantum entanglement for such a system.…”

Shortcuts for Electron‐Transfer through the Secondary Structure of Helical Oligo‐1,2‐Naphthylenes

“…[6] Recently,s ome of us discovered as tereoselective aldol condensation leadingt oc onfigurationally stable, atropisomeric oligo-1,2-naphthylenes, whichd on ot suffer from the problem of rapid interconversion between different wire conformers on the electron transfer timescale, because they are composed of biaryls with defined configuration of stereogenic axes. [7] Because the understanding of one-dimensional electron transfer (Scheme 1a)g ets increasingly complete, there is now growingi nterest in multi-dimensional electron transfer (Scheme 1b-d), for example in foldamers, [8] p-stacked, [9] forked, [10] or circulars tructures. [11] The motivations for such research are diverse and include, for example, the ambition to construct light-harvesting and charge-separatings ystemst hat emulaten atural photosynthesis, to enhance the efficiencyo f organic light emitting diodes (OLEDs), or the desire to control electront ransfer pathways in future molecular electronics applications.D onor-bridge-acceptor compounds with well-defined moleculars tructures are ideally suited to explore the fundamentals of multi-dimensional electron transfer,a nd in our oligo-1,2-naphthylenes the type of unfolding illustrated in Scheme 1c is impossible.…”