Dynamics of Charge Injection and Charge Recombination in DNA Mini-Hairpins

Steady state spectroscopy, femtosecond transient absorption spectroscopy (fsTA), and femtosecond stimulated Raman spectroscopy (FSRS) of DNA mini-hairpins possessing a diphenylacetylenedi-carboxamide (DPA) linker and 1-3 adenine thymine (A-T) or guanine cytosine (G-C) base pairs have been investigated. Ultraviolet and circular dichroism (UV and CD) spectra are consistent with ground state conformations that are predominantly base-paired and it-stacked for conjugates possessing two or three base pairs; however, they offer no information concerning the conformation of conjugates possessing a single base pair. fsTA spectra are indicative of pi-stacked structures excepted in the case of the conjugate possessing a single G-C base pair. All of the conjugates display transient absorption bands characteristic of the DPA anion radical. Conjugates possessing two or three G-C base pairs display a transient absorption band characteristic of the short-lived G(+center dot) cation radical. The mini-hairpins with 1-3 A-T base pairs do not display the transient absorption band characteristic of the (A(n)(+center dot)) polaron. This implies that an A-tract with three base pairs is too short to support polaron formation.