Photoinduced electron transfer (ET) is one of the most fundamental processes in physics, chemistry, and biology. In the reaction center of natural photosynthesis, photoinduced ET generates a long-lived charge separation (CS) state with ~100% efficiency, leading to light-to-chemical energy conversion. In contrast, photoinduced CS at the interfaces of organic photovoltaic cells and dye-sensitized solar cells generates electron-hole pair, eventually achieving light-to-electricity conversion. However, the interfaces of semiconductor/dye and donor/acceptor (D/A) in artificial photosynthesis and organic photovoltaic cells often suffer from the partial or even large loss of the CS state at the early stage, which has still been controversial and not unveiled owing to inevitable inhomogeneous spatial distribution of D-A components.

In this context D-A linked systems have generated remarkable interest for the last few decades. The D-A covalent linkage can eliminate complex factors arising from diffusion in solutions and assess the photoinduced ET properties precisely with the help of homogenous spatial distribution of the D-A components.  So far these systems have provided fundamental information on photoinduced ET. Impact of various ET parameters, i.e., driving force, electronic coupling, reorganization energy, and temperature, on ET rate has been evaluated by using elaborated D-A linked molecules. In particular, a well-defined D-A linked molecule with a rigid bridge has allowed us to shed light on photoinduced ET more accurately. Here we show unprecedented dependence of the final CS efficiency on D-A interaction (i.e., electronic coupling) that can be changed systematically in the D-A linked models with a one-dimensional (1D) nonconjugated bridge. We have thoroughly examined the photoinduced ET properties by using femtosecond to microsecond time-resolved transient absorption (TRTA) and electron paramagnetic resonance (TREPR) spectroscopies.

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