Hole Tunneling and Hopping in a Ru(bpy)32+-Phenothiazine Dyad with a Bridge Derived from oligo-p-Phenylene

Mathieu E Walther,Oliver S Wenger

doi:10.1021/ic201446x

Abstract

A molecular dyad was synthesized in which a Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine) photosensitizer and a phenothiazine redox partner are bridged by a sequence of tetramethoxybenzene, p-dimethoxybenzene, and p-xylene units. Hole transfer from the oxidized metal complex to the phenothiazine was triggered using a flash-quench technique and investigated by transient absorption spectroscopy. Optical spectroscopic and electrochemical experiments performed on a suitable reference molecule in addition to the above-mentioned dyad lead to the conclusion that hole transfer from Ru(bpy)(3)(3+) to phenothiazine proceeds through a sequence of hopping and tunneling steps: Initial hole hopping from Ru(bpy)(3)(3+) to the easily oxidizable tetramethoxybenzene unit is followed by tunneling through the barrier imposed by the p-dimethoxybenzene and p-xylene spacers. The overall charge transfer proceeds with a time constant of 41 ns, which compares favorably to a time constant of 1835 ns associated with equidistant hole tunneling between the same donor-acceptor couple bridged by three identical p-xylene units. The combined hopping/tunneling sequence thus leads to an acceleration of hole transfer by roughly a factor of 50 when compared to a pure tunneling mechanism.

Highlights

Long-range charge transfer in donor-bridge-acceptor systems can proceed through tunneling or hopping mechanisms.[1]
Hole transfer across four p-dimethoxybenzene units was found to be more than 3 orders of magnitude more rapid than equidistant hole transfer across four p-xylene spacers.[10]
A combination of hopping and tunneling steps is responsible for phototriggered hole transfer in the RuÀPTZ dyad

Summary

’ INTRODUCTION

Long-range charge transfer in donor-bridge-acceptor systems can proceed through tunneling or hopping mechanisms.[1] As long as the one-electron reduced or oxidized states of the bridge cannot be populated directly by the transferring charge carriers, superexchange-mediated tunneling is the only viable reaction mechanism. This is typically the case for alkane bridges,[2] weakly π-conjugated molecular rods,[3] and for many proteins.[4] As soon as there are reducible or oxidizable molecular units in the bridge, electron or hole hopping may become important. A dyad composed of the same donorÀacceptor couple bridged by three identical p-xylene spacers, hereafter referred to as RuÀxy3-PTZ, was available from a prior investigation and served as an additional reference molecule.[11]

’ RESULTS AND DISCUSSION

’ SUMMARY AND CONCLUSIONS

’ REFERENCES