Light-induced charge-accumulation in molecular systems with multi-electron donors and acceptors

Abstract

To date, the world’s population mainly relies on fossil fuels for covering its daily energy need. However, not only is the mining of oil, natural gas and coal demanding, but the combustion of these fossil fuels also leads to the release of gases that are harmful to human health, or contribute to global warming. A global shift from fossil fuels to ‘solar fuels’ is therefore highly desirable. The conversion of solar energy into usable chemical fuels, however, is complicated - a central challenge being the accumulation of several redox equivalents to catalyze multi-electron reactions. This thesis therefore addresses the aspect of light-driven charge-accumulation in molecular systems. In the first project (Chapter 3), accumulation of two electrons on a triad consisting of a central naphthalene diimide (NDI) moiety flanked by two [Ru(bpy)3]2+ photosensitizers was investigated. Under continuous irradiation with visible light, two electrons were successfully accumulated on NDI in the presence of a sacrificial electron donor. When the sacrificial donor was replaced with two covalently connected triarylamine electron donors, however, only a singly charge-separated state could be observed in the pentad. The pathways leading to charge-accumulation in case of the triad and the processes preventing the accumulation of charges in the pentad were studied in detail. In the second project (Chapter 4), the impact of charge-accumulation on the catalytic process for BNA+ (an analog of NAD(P)+) reduction was studied. Experiments with a multi-component system as well as with covalently connected molecular systems were performed. Analysis of these measurements revealed that prior charge-accumulation does not lead to an accelerated BNAH formation rate and that the predominant reaction path most likely relies on a disproportionation of the intermediate Rh(II) species. In the third project (Chapter 5), the problems preventing charge-accumulation on the NDI pentad in the first project were addressed. A new concept was developed based on electron donor and acceptor moieties with potential inversion to increase the driving force for the transfer of the second electron. In addition, intermediate electron donors and acceptors were incorporated to establish a redox gradient in analogy to the electron transfer paths in natural photosynthesis. This redox gradient is expected to promote productive electron transfer while the increased spatial separation of terminal donor and acceptor moiety is expected to significantly decrease back-electron transfer. While the successful synthesis of this pentad is still pending, the individual donor and acceptor moieties were examined. A donor triad was synthesized, which upon excitation, showed rapid excited-state quenching by the intermediate electron donor and rapid subsequent hole transfer from the intermediate to the terminal electron donor. The lifetime of this charge-separated state is rather long and therefore shows that back-electron transfer in the triad is significantly retarded by the intermediate electron donor. The overall concept of this pentad is therefore highly promising and could provide the basis for a new generation of charge-accumulative systems.