Among the numerous artificial analogues of the natural photosynthetic reaction center, many contain porphyrin and naphthalene diimides (NDI) as building blocks. We are investigating the ultrafast excited-state dynamics of a series of molecular arrays consisting of a zinc porphyrin (ZnP) unit acting as an electron donor substituted by an increasing number of NDI acceptors (n=1 to 4), and synthesized in the group of S. V. Bhosale (RMIT University, Melbourne). The transient electronic absorption spectra are very similar independently of n and do not present any feature that could be assigned to a charge-separated state. This points to a faster charge recombination (CR) than charge separation (CS). However, the band in the 400−450 nm region appearing after a few picoseconds with n=2 is characteristic of the vibrationally hot ground state of ZnP band becomes more pronounced with increasing n. As confirmed by global target analysis, this can be explained by an acceleration of CS as n gets larger. Increasing n speeds up CS but does not affect CR.
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