Better Covalent Connection in a Molecular Triad Enables More Efficient Photochemical Energy Storage

Abstract

Numerous studies have explored the kinetics of light-induced charge separation and thermal charge recombination in donor–acceptor compounds, but quantum efficiencies have rarely been investigated. Here, we report on two essentially isomeric molecular triads, both comprising a π-extended tetrathiafulvalene (ExTTF) donor, a ruthenium­(II)-based photosensitizer, and a naphthalene diimide (NDI) acceptor. The key difference between the two triads is how the NDI acceptor is connected. Linkage at the NDI core provides stronger electronic coupling to the other molecular components than connection via the nitrogen atoms of NDI. This change in molecular connectivity is expected to accelerate both energy-storing charge separation and energy-wasting charge recombination processes, but it is not a priori clear how this will affect the triad’s ability to store photochemical energy; any gain resulting from faster charge separation could potentially be (over)­compensated by losses through accelerated charge recombination. The new key insight emerging from our study is that the quantum yield for the formation of a long-lived charge-separated state increases by a factor of 5 when going from nitrogen- to core-connected NDI, providing the important proof of concept that better molecular connectivity indeed enables more efficient photochemical energy storage. The physical origin of this behavior seems to root in different orbital connectivity pathways for charge separation and charge recombination, as well as in differences in the relevant orbital interactions depending on NDI connection. Our work provides guidelines for how to discriminate between energy-storing and energy-wasting electron transfer reactions in order to improve the quantum yields for photochemical energy storage and solar energy conversion.

Exploiting Orthogonal C–C Cross-Coupling Reactions for Chemistry-on-the-Complex: Modular Assembly of 2,6-Di(quinolin-8-yl)pyridine Ruthenium(II) Photosensitizer Triads

“…In the meantime, Wenger and co-workers reported a detailed study on the oxidative quenching of a similar Ru(dqp) 2 -based assembly with a NDI acceptor, which showed only 6% of charge separation. 24 Their findings corroborate our result and general interpretation, and thus, a more detailed study is in due course but beyond the scope of this study. Nonetheless, the much higher quenching extent of the triad with respect to the dyads indicates the formation of a charge-separated state.…”

“…Notably, previous studies on polymer-bound NDI acceptors showed efficient charge separation, but the assemblies differ markedly in terms of the linkage motif, the mutual spatial orientations, the number of potential acceptor sites and minimum distance between the closest acceptor site and the Ru core due to conformational freedom of the polymer backbone. In the meantime, Wenger and co-workers reported a detailed study on the oxidative quenching of a similar Ru­(dqp) 2 -based assembly with a NDI acceptor, which showed only 6% of charge separation . Their findings corroborate our result and general interpretation, and thus, a more detailed study is in due course but beyond the scope of this study.…”