Excited State, Charge Transfer, and Spin Dynamics in DNA Hairpin Conjugates with Perylenediimide Hairpin Linkers

A series of short DNA hairpins (nG) using perylene-3,4:9, 10-bis(dicarboximide) (PDI) as the hairpin linker was synthesized in which the distance between the PDI and a guanine-cytosine (G-C) base pair is systematically varied by changing the number (n - 1) of adenine-thymine (A-T) base pairs between them. Due to the relatively large hydrophobic surface of PDI, the nG hairpins dimerize in buffer solutions. The photophysics and photochemistry of these hairpins were investigated using femtosecond transient absorption and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Photoexcitation of the self-assembled PDI dimer within each nG hairpin results in subpicosecond formation of its lower exciton state ((1)*PDI2) followed by formation of an excimer-like state ((1)*(PDI2)-P-X) with tau = 10-28 ps. Both of these states are lower in energy than (1)*PDI, so that neither can oxidize A, C, and T. Electron transfer from G to (1)*PDI2 is faster than formation of (1)*(PDI2)-P-X only for 1G. Electron transfer from G to (1)*(PDI2)-P-X for 2G-8G, occurs by the superexchange mechanism and, thus, becomes exponentially less efficient as the G-PDI2 distance increases. Nevertheless, TREPR studies show that photoexcitation of 2G and 4G produce spin-correlated radical ion pairs having electron spin polarization patterns indicating that a low yield of charge separation proceeds from (1)*(PDI2)-P-X by the radical pair intersystem crossing (RP-ISC) mechanism to initially yield a singlet radical ion pair. The strong spin-polarization of the radical ion pairs makes it possible to observe them, even though their concentration is low. As expected, the hairpin lacking G (0G) and that having the longest G-PDI2 distance (8G) display no TREPR radical ion pair signals. Hairpins 0G, 2G, 4G, and 8G all exhibit triplet EPR spectra at 85 K. Simulations of the spectra show that (3)*PDI is produced mainly by a spin-orbit-induced intersystem crossing mechanism, while the spectra of 2G and 4G have 5% and 21% contributions, respectively, from (3)*PDI produced by charge recombination of radical ion pairs that originate from RP-ISC. These low percentages of RP-ISC derived (3)*PDI result mainly from the low yield of radical ion pairs in 2G and 4G.