Highly exoergic electron transfer (ET) from the photoexcited donor (pyrene) to a 1→2 branching dendritic network of acceptors (methylviologen moieties) was investigated with the help of pump-probe spectroscopy. The dependence of the survival of the injected electrons on the dendron generation could be satisfactorily explained in terms of the simple site-to-site hopping mechanism for the dendron family in which the pyrene donor is linked to the network via a (-CH2-)4 bridge. In contrast, an analogous family with a shorter, single -CH2- bridge at the apex of the dendron exhibited a much more complex behavior which indicated a close competition between electron transfer and vibrational cooling. Counterintuitively, slowing both the electron injection and charge recombination at the apex increases the yield of electrons captured by the dendritic network. A direct comparison of the dendrons with a strictly analogous linear array of acceptors was also carried out. For an identical bridging unit the observed electron capture and survival scale very well with the branching of the acceptor network. The observation of vibrationally assisted electron hopping in the -CH2- bridged system has prompted us to investigate in more detail the vibrational cooling of the monoreduced MV+● radical ion in a variety of media, including supramolecular host molecules. The experimentally observed cooling rates correlated well with the density of vibrational states states of the medium. This was corroborated by classical MD simulations which had shown the same trends albeit at approximately two times higher absolute rates than the experiment. Overally, it was concluded that highly exoergic photoinduced ET processes occurring at the time scale faster than 10-20 ps are likely to exhibit non-equilibrium effects and deviate substantially from the predictions of the equilibrium-based Marcus theory.

Electron hopping and vibrational cooling in dendritic arrays of acceptors

2019 Fall National Meeting

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