in DMF hampered the measurement of its transient spectra, we examined the dynamics of the photoexcited states of 2 and 4 in DMF. The energy levels of the ion pairs were calculated to be 1.41 eV for Dp+-Hp--Im and 1.19 eV for Dp'-Hp-Im-. Analysis of the transient absorption spectra of 2 and 4 reveals the formation of Dp+-Hp--Im within 30 ps, and the values of k , and k , were determined to be 1.0 x lo1' and 1.1 x lo9 s-', respectively. Here again the increase of k , is large in DMF, while k , is slightly smaller. As a consequence, the overall quantum yield decreases to roughly 0.1. The polar environment is of course favorable in lowering the energy level of the charge-separated states for the ET process. But it often accelerates the energy-wasteful CR reaction, which is accompanied by a concomitant decrease of the overall quantum yield of the charge separation, as observed in our system. This kinetic aspect may be quite common and should be taken into account in the future design of more elaborate photosynthetic model systems.
In summary, a sequential ET relay resembling that in the RC and ultimately leading to the formation of long-lived charge separated states has been realized in the Dp-Hp-Im triads 1 and 2, which have a molecular composition quite similar to that of the RC. The polarity of solvent is crucial in determining the overall CS quantum yield. The polarity of the solvent is crucial for the overall CS quantum yield.