Photoinduced electron transfer in donor–bridge–acceptor assemblies: The case of Os(II)-bis(terpyridine)-(bi)pyridinium dyads

Photoinduced electron transfer continues to be a key process for the design of artificial systems capable to perform an efficient solar energy conversion. In particular, linearly-arranged donor–bridge–acceptor dyads have greatly contributed to shine light on the various factors that must be taken into account when designing systems for obtaining long-lived charge separation, a useful property on the route to artificial photosynthesis. Here we summarized the results we recently obtained on the photoinduced electron transfer processes occurring in Os(II)-bis(terpyridine)-(bi)pyridinium dyads. In particular, we will focus on the role of the bridge in forward and backward electron transfer processes, and on the possibility of obtaining efficient photoinduced charge separation even when the driving force for the electron transfer process approaches zero. This latter point can be of considerable interest when several electron transfer steps are considered to ultimately yield long-range charge-separated state, with minimal energy losses from the initial, light-prepared localized excited state.