Structure and Dynamics of Electron Injection and Charge Recombination in i-Motif DNA Conjugates

The dynamics of electron injection have been investigated in intramolecular i-motif conjugates possessing stilbenediether (Sd) and perylenediimide (PDI) chromophores separated by either four or six hemiprotonated cytosine C-C base pairs assembled with synthetic loops. Circular dichroism spectra are consistent with the formation of i-motif structures in the absence or presence of Sd and PDI chromophores. The fluorescence of the Sd chromophore is essentially completely quenched by neighboring C-C base pairs, consistent with their function as an electron donor and electron acceptor, respectively. However, the fluorescence of the PDI chromophore is only partially quenched. The dynamics of electron injection from singlet Sd into the i-motif and subsequent charge recombination has been determined by femtosecond transient absorption (fsTA) spectroscopy and compared with the results for electron injection and charge recombination in Sd-linked hairpins possessing cytosine-guanine (C-G) or 5-fluorouracil-adenine (F-A) base pairs. While charge injection is ultrafast (<0.8 ps) for the i-motifs, charge transport across the i-motif C-C base pairs to the PDI electron trap is not observed. The absence of electron transport is related to the structure of the stacked C-C base pairs in the i-motif.