Review
Importance of covalence, conformational effects and tunneling-barrier heights for long-range electron transfer: Insights from dyads with oligo-p-phenylene, oligo-p-xylene and oligo-p-dimethoxybenzene bridges

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Abstract

This review reports on our recent studies of phototriggered charge transfer in rigid rod-like donor-bridge-acceptor molecules in liquid solution as well as between randomly dispersed electron donors and acceptors in frozen organic glasses. Investigation of the distance dependence of the rates of these reactions provides detailed insight into the various factors that govern long-range charge transfer efficiencies. The importance of covalence can be probed by a comparison of charge tunneling through a frozen toluene matrix to tunneling across an oligo-p-xylene bridge. The distance decay constants for these two processes are β = 1.26 Å−1 and β = 0.52 Å−1, respectively, indicating that charge tunneling across a covalent xylene–xylene contact is ∼2 orders of magnitude more efficient than that across a noncovalent toluene–toluene contact. Conformational effects were investigated by comparing hole tunneling across oligo-p-xylene and oligo-p-phenylene bridges. The latter are significantly more π-conjugated and mediate long-range hole tunneling with β = 0.21 Å−1 between a ruthenium–phenothiazine donor–acceptor couple. Quantitative analysis indicates that in this particular instance, tunneling across a phenylene–phenylene contact is roughly 50 times more efficient than tunneling across a xylene–xylene contact. The use of oligo-p-dimethoxybenzene wires instead of the structurally very similar oligo-p-xylene bridges was found to lead to a strong acceleration of long-range hole transfer rates: The 23.5-Å charge transfer step across four xylene units occurs within 20 μs, but the charge transfer over the same distance across four dimethoxybenzene units takes only 17 ns. This is attributed to a tunneling-barrier effect that is caused by a large difference in oxidation potentials between the two types of bridges.

Introduction

Long-range electron transfer reactions play important roles in chemistry, biology, and physics. Controlled electron flow over long distances is necessary for photosynthesis and respiration [1]. Controlling (long-range) electron transfer rates is also a prerequisite for artificial light-to-chemical energy conversion [2], and efficient charge transport across molecular bridges is needed for a molecular electronics technology [3]. This has stimulated much experimental and theoretical work on biological and artificial donor-bridge-acceptor systems. Biological examples include ruthenium(II)-modified azurins [4], [5] and other proteins and peptides equipped with inorganic photosensitizers [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], work that yielded detailed insight into how protein/peptide backbone mediates long-range charge transfer. Similar research on DNA has led to a relatively clear picture of charge transport in this biomolecule [16], [17], [18], [19], [20]. Among the numerous artificial donor-bridge-acceptor systems investigated, the early work on rigid saturated hydrocarbon [21], [22] and aromatic bridges [23], [24] is particularly noteworthy since much of the later research has built on these landmark studies. This later work includes for example the investigation of long-range charge and energy transfer across oligo-p-phenylene [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], oligo-p-phenylene vinylene (OPV) [39], [40], [41], [42], [43], oligo-p-phenylene ethynylene (OPE) [44], [45], [46], [47], [48], [49], and oligo-fluorene bridges [50], [51], [52]. Some of these molecular entities act as true molecular wires in that they mediate long-range charge transfer with very shallow distance dependences, and this makes them interesting for molecular electronics applications. Other research is more concerned with establishing long-lived charge-separated states which store solar energy that could potentially be used to drive other chemical reactions that are thermodynamically uphill [2], [53], [54], [55], [56]. In this context, porphyrin–fullerene molecules have led to particularly spectacular results [57], [58], [59], but systems based on ruthenium(II) [60], [61], [62], [63], iridium(III) [64], [65], [66], and platinum(II) [67], [68], [69], [70] have also attracted significant attention. Despite all these efforts, the conversion of light energy into useful chemical energy remains a challenge as illustrated for example by photochemical hydrogen production: Many of the newly explored systems still rely on the use of sacrificial electron donors [71], [72], [73], [74].

Our own research in the area of long-range electron transfer has been concerned with more fundamental problems. Over the past three years we have sought to explore the importance of factors such as covalence, molecular conformation, and so-called tunneling-energy effects on the rates of long-range charge transfers [75]. Here, we give a review of this work in which we have focused mostly on rigid rod-like donor-bridge-acceptor molecules with ruthenium(II) and rhenium(I) sensitizers to phototrigger the charge transfers. As molecular bridges, oligo-p-xylenes have become our benchmark as they represent an intermediate case between strongly π-conjugated wires and relatively poor conductors such as alkanes. This allows investigation of charge tunneling processes by the superexchange mechanism over reasonably long distances without interference from incoherent charge hopping mechanisms. Starting from the oligo-p-xylene systems, it is possible to obtain quantitative insight into the importance of covalent linkages between the individual bridging units by comparison to long-range charge tunneling through a frozen toluene matrix (Section 2). A direct comparison of the distance dependence of long-range charge transfer rates across oligo-p-xylene and oligo-p-phenylene bridges addresses the importance of conformational effects (Section 3), and tunneling-energy effects were explored by comparing oligo-p-xylene bridges to structurally similar oligo-p-dimethoxybenzene molecules (Section 4).

Section snippets

Importance of covalence: comparison of long-range charge transfer across covalent xylene and noncovalent toluene bridges

Mechanistic investigations of long-range electron transfer reactions frequently aim at exploring their distance dependences. In order to extract meaningful information from such studies, it is usually necessary to keep electron donors and acceptors at a fixed distance. This is often done by connecting them via rigid spacers [2], but it is also possible to randomly disperse non-connected donors and acceptors in solid or viscous matrices in which diffusion is much slower than the electron

Conformational effects: comparison of oligo-p-xylene and oligo-p-phenylene bridges

Using nearly identical methodologies as those employed for the synthesis of the donor-bridge-acceptor molecules from Fig. 3, analogous oligo-p-phenylene bridged rhenium(I)–phenothiazine as well as xylene- and phenylene-bridged ruthenium(II)–phenothiazine dyads are synthetically accessible (Fig. 4) [97], [105], [106].

Fig. 5 shows the optical absorption spectra obtained from dichloromethane and acetonitrile solutions of these dyads. Inspection of these UV–vis data reveals the presence of the

Tunneling-barrier variation through change of the donor redox potential: ruthenium versus rhenium

Depending on whether rhenium(I) or ruthenium(II) photosensitizers are used, different distance dependences for charge transfer across oligo-p-xylene bridges are observed. For the rhenium–phenothiazine dyads (Fig. 4, upper left), charge tunneling across the xylene spacers proceeds with β = 0.52 Å−1 (Fig. 2) [97], whereas for the ruthenium–phenothiazine dyads (Fig. 4, upper right) β = 0.77 Å−1 for exactly the same bridge (Fig. 6) [105]. A similar observation has been reported previously for long-range

Summary and conclusions

The use of oligo-p-xylene bridges as benchmark systems provides a means for quantitative investigation of the importance of covalence, conformational effects, and tunneling-barrier effects for long-range charge transfer. Distance dependence studies of electron transfer rates play a central role in this research. The comparison of charge transfer through a frozen toluene matrix to that across covalent oligo-p-xylene spacers (Section 2) shows that charge tunneling rates increase by two orders of

Acknowledgments

This work was financed by the Swiss National Science Foundation (grant number PP002-110611) and the University of Geneva.

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