On the mechanism of vibrational control of light-induced charge transfer in donor–bridge–acceptor assemblies

Abstract

Nuclear-electronic (vibronic) coupling is increasingly recognized as a mechanism of major importance in controlling the light-induced function of molecular systems. It was recently shown that infrared light excitation of intramolecular vibrations can radically change the efficiency of electron transfer, a fundamental chemical process. We now extend and generalize the understanding of this phenomenon by probing and perturbing vibronic coupling in several molecules in solution. In the experiments an ultrafast electronic-vibrational pulse sequence is applied to a range of donor-bridge-acceptor Pt(II) trans-acetylide assemblies, for which infrared excitation of selected bridge vibrations during ultraviolet-initiated charge separation alters the yields of light-induced product states. The experiments, augmented by quantum chemical calculations, reveal a complex combination of vibronic mechanisms responsible for the observed changes in electron transfer rates and pathways. The study raises new fundamental questions about the function of vibrational processes immediately following charge transfer photoexcitation, and highlights the molecular features necessary for external vibronic control of excited-state processes.

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

“…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.…”

“…Figure 2b displays the 2DIR spectrum in the ground electronic state of 1* at an IR pump -IR probe delay of 3 ps. Negative-positive peak pairs along the diagonal correspond to de-population of the ground state and associated population of the v=1 state, giving rise to ground state bleaching, stimulated emission (v=1 0), and excited state absorption (v=1 2) red-shifted from the ground state bleach by the vibrational anharmonicity (typically ~ 20 cm -1 for C C stretches in Pt-acetylides 16 ). Coupling between the ( 12 C) and ( 13 C) vibrations would appear as off-diagonal (cross-) peaks.…”

“…Another important component in our system is the metal centre which acts as a vibrational energy transfer bottleneck that contributes to the localization of injected quanta of energy on either arm of the molecule. [38][39][40][41][42] Furthermore, the presence of the metal offering a two-step charge-separation process starting from a metal-to-acceptor charge transfer followed by reductive quenching from the donor is a particularly effective design strategy to achieve IR-control: 15,16,35,36 it allows the rapid population of a far-fromequilibrium gateway state where an IR pump can be introduced, and in this way perturb an otherwise symmetrical system in a spatially selective manner at a crucial crossroad where small structural and energetic fluctuations dictate the chosen electronic pathway. 43 Complementary to such excited state evolution is the ability to delay the control pulse with respect to actinic excitation, thus influencing reactivity at the decisive moments of light-induced molecular function [44][45][46][47] and circumventing the myriad of processes occurring immediately after electronic excitation, which would rapidly scramble the selectivity of mode-specific excitation.…”

“…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.…”

Electron transfer across a thermal gradient

“…Even in this well-understood limit different considerations apply under different conditions, and different levels of descriptions were applied to account for the molecular nature of the solvent (89), the dimensionality of the process (90)(91)(92)(93)(94)(95)(96)(97)(98)(99), and the definition of the reaction coordinate. Extensions to equilibrium situations have ranged from considerations of deviation from transition state theory (TST) to the description of control by external fields (99)(100)(101).…”