Pump‐Pump‐Probe Spectroscopy of a Molecular Triad Monitoring Detrimental Processes for Photoinduced Charge Accumulation

Abstract

Controlling light-induced accumulation of electrons or holes is desirable in view of multi-electron redox chemistry, for example for the formation of solar fuels or for photoredox catalysis in general. Excitation with multiple photons is usually required for electron or hole accumulation, and consequently pump-pump-probe spectroscopy becomes a valuable spectroscopic tool. In this work, we excited a triarylamine-Ru(bpy)3 2+ -anthraquinone triad (bpy = 2,2'-bipyridine) with two temporally delayed laser pulses of different color and monitored the resulting photoproducts. Absorption of the first photon by the Ru(bpy)3 2+ photosensitizer generated a triarylamine radical cation and an anthraquinone radical anion by intramolecular electron transfer. Subsequent selective excitation of either one of these two radical ion species then induced rapid reverse electron transfer to yield the triad in its initial (ground) state. This shows in direct manner that after absorption of a first photon and formation of the primary photoproducts, the absorption of a second photon can lead to unproductive electron transfer events that counteract further charge accumulation. In principle, this problem is avoidable by careful excitation wavelength selection in combination with good molecular design.

Quantitative insights into charge-separated states from one- and two-pulse laser experiments relevant for artificial photosynthesis

“…Given that CSSs typically exhibit intense absorptions over a broad wavelength range, selective photosensitizer excitation is even more difficult under conditions of solar (broadband) irradiation than it is under well-defined lab (monochromatic) conditions. However, this is currently a heavily underexplored area given its importance for accumulative multi-electron transfer 29,33,35,78,81. The molecular triads investigated in this study do not permit charge accumulation, but they are well-suited for studying light-induced charge recombination processes and for quantum yield determinations of this undesired side reaction, which lay the grounds for structural optimizations to avoid that detrimental process.…”

“…In direct consequence, the factors governing the charge-separated state (CSS) formation efficiencies are still poorly understood. Even less explored are the processes that occur upon further excitation of CSSs with a second photon, such as detrimental light-induced charge recombination,33,34 although their deep understanding is of pivotal importance for the efficient accumulation of redox equivalents 29,30,35…”

Proton-coupled multi-electron transfer and its relevance for artificial photosynthesis and photoredox catalysis

“…Pump-pumpprobe experiments can provide valuable mechanistic insight into light-induced PCMET, but such experiments tend to be significantly more challenging than ordinary transient absorption studies. 55,57,64,108 Proton-coupling of multi-electron transfer reactions not only helps to avoid the build-up of charges when accumulating multiple redox equivalents, but in some cases it can even provide a distinct thermodynamic advantage by making a secondary redox process more energetically favorable than a primary redox reaction. For instance, under sufficiently acidic conditions, semiquinone is more readily reduced than p-benzoquinone, and consequently the two-electron reduction of p-benzoquinone to hydroquinone is facilitated.…”

“…This has been shown in recent two-pulse (pump-pump-probe) studies. 55,56 Productive secondary electron transfers, either further oxidation of D + to D 2+ by the excited sensitizer ( Fig. 3c) or further reduction of A À to A 2À ( Fig.…”

Exceptionally Long‐Lived Photodriven Multi‐Electron Storage without Sacrificial Reagents

“…By contrast, as noted above, triarylamines are reversible donors that do not decompose, and hence undesired reverse electron transfers can readily occur after initial charge‐separation. In fact, the oxidized triarylamine is a strong electron acceptor that can easily impede electron accumulation at the central acceptor site due to large driving force and due to covalent attachment . With sacrificial quenchers these issues are far less problematic but their use contradicts the goal of sustainable solar energy conversion.…”