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 diphenylacetylenedicarboxamide (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 π-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 π-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^+• cation radical. The mini-hairpins with 1–3 A-T base pairs do not display the transient absorption band characteristic of the (A_n^+•) polaron. This implies that an A-tract with three base pairs is too short to support polaron formation

Efficient Charge Transport via DNA G‑Quadruplexes

The dynamics and efficiency of photoinduced charge transport has been investigated in DNA capped hairpins possessing a stilbenedicarboxamide (Sa) hole donor and stilbenediether (Sd) hole acceptor separated by DNA G-quadruplex structures possessing 2-to-4 tetrads by means of femtosecond and nanosecond transient absorption spectroscopy with global analysis. The results for the quadruplex structures are compared with those for the corresponding duplex structures having G-C base pairs in place of the G-tetrads. Following photoinduced charge separation to form a contact radical ion pair, hole transport to form the Sa^–•/Sd^+• charge-separated state is slower but more efficient for the quadruplex vs duplex structures. Thus, the G-quadruplex serves as an effective conduit for positive charge rather than as a hole trap when inserted into a duplex, as previously postulated

Charge Separation Mechanisms in Ordered Films of Self-Assembled Donor–Acceptor Dyad Ribbons

Orthogonal attachment of polar and nonpolar side-chains to a zinc porphyrin-perylenediimide dyad (ZnP-PDI, <b>1a</b>) is shown to result in self-assembly of ordered supramolecular ribbons in which the ZnP and PDI molecules form segregated π-stacked columns. Following photoexcitation of the ordered ribbons, ZnP^+•-PDI^–• radical ion pairs form in <200 fs and subsequently produce a 30 ± 3% yield of free charge carriers that live for about 100 μs. Elongating the side chains on ZnP and PDI in <b>1b</b> enhances the order of the films, but does not result in an increase in free charge carrier yield. In addition, this yield is independent of temperature, free energy of reaction, and the ZnP-PDI distance in the covalent dyad. These results suggest that the free charge carrier yield in this system is not limited by a bound charge transfer (CT) state or promoted by a vibronically hot CT state. Instead, it is likely that π-stacking of the segregated donors and acceptors within the ribbons results in delocalization of the charges following photoexcitation, allowing them to overcome Coulombic attraction and generate free charge carriers

Influence of Anion Delocalization on Electron Transfer in a Covalent Porphyrin Donor–Perylenediimide Dimer Acceptor System

Photodriven electron transfer from a donor excited state to an assembly of electronically coupled acceptors has been proposed to enhance charge transfer efficiency in functional organic electronic materials. However, the circumstances under which this may occur are difficult to investigate in a controlled manner in disordered donor–acceptor materials. Here we investigate the effects of anion delocalization on electron transfer using zinc <i>meso</i>-tetraphenylporphyrin (ZnTPP) as a donor and a perylene-3,4:9,10-bis­(dicarboximide) dimer as the acceptor (PDI₂). The PDI units of the dimer are positioned in a cofacial orientation relative to one another by attachment of the imide group of each PDI to the 4- and 5-positions of a xanthene spacer. Furthermore, the distal imide group of one PDI is linked to the <i>para</i>-position of one ZnTPP phenyl group to yield ZnTPP-PDI₂. The data for the dimer are compared to two different ZnTPP-PDI monomer reference systems designed to probe electron transfer to each of the individual PDI molecules comprising PDI₂. The electron transfer rate from the ZnTPP lowest excited singlet state to PDI₂ is increased by 50% relative to that in ZnTPP-PDI, when the data are corrected for the statistics of having two electron acceptors. Femtosecond transient IR absorption spectroscopy provides evidence that the observed enhancement in charge separation results from electron transfer producing a delocalized PDI₂ anion