Mechanism and Dynamics of Electron Injection and Charge Recombination in DNA. Dependence on Neighboring Pyrimidines

The mechanism and dynamics of photoinduced electron injection and charge recombination have been investigated for several series of DNA hairpins. The hairpins possess a stilbenediether linker, which serves as an electron donor and base pair stems that possess different pyrimidine bases adjacent to the linker. Hairpins with adjacent thymine-adenine (T-A) base pairs undergo fast electron injection and relatively slow charge recombination with rate constants that are not strongly dependent upon the following base pair. Hairpins with adjacent cytosine-guanine (C-G) base pairs undergo reversible electron injection and much faster charge recombination than those with adjacent T-A base pairs. Hairpins with 5-fluorouracil or other halogenated pyrimidines in their first and second base pair undergo fast electron injection and multiexponential charge recombination. The difference in kinetic behavior for the different series of hairpins and its implications for the formation of long-lived charge-separated states are discussed and compared to results reported previously for other electron-donor chromophores

Different Mechanisms for Hole and Electron Transfer along Identical Molecular Bridges: The Importance of the Initial State Delocalization

We report measurements of hole and electron transfer along identical oligo-<i>p</i>-phenylene molecular bridges of increasing length. Although the injection barriers for hole and electron transfer are similar, we observed striking differences in the distance dependence and absolute magnitude of the rates of these two processes. Electron transfer is characterized by an almost distance-independent, fast charge-transfer rate. Hole transfer presents a much slower rate that decreases significantly with the length of the bridge. Time-dependent density functional calculations show that the observed differences can be explained by the delocalization of the respective initial excitation. The evaluation of the initial state is therefore essential when comparing charge-transfer rates between different donor–bridge–acceptor systems

Synthesis and Photophysical Properties of Conjugated and Nonconjugated Phthalocyanine–Perylenediimide Systems

The synthesis and characterization of different conjugated phthalocyanine–perylenemonoimidebenzimidazole [ZnPc-PBIm­(OR)₄] and nonconjugated phthalocyanine–perylenediimide [ZnPc-PDI­(OR)₄] dyads are carried out. UV–vis, ¹H NMR, and electrochemistry measurements reveal the interaction between perylene and phthalocyanine moieties in the ground state in the conjugated hybrid and the lack of interaction in the nonconjugated one. Ultrafast transient absorption measurements show that a state with substantial charge-transfer character is formed in both compounds, but the rates for the formation and recombination from this state are much faster for the conjugated compound