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

Tracking Photoinduced Charge Separation in DNA: from Start to Finish

The initial studies of the dynamics of photoinduced charge separation conducted in our laboratories 20 years ago found strongly distance-dependent rate constants over short distances but failed to detect intermediates in the transport of positive charge (holes). These observations were consistent with the single-step superexchange or tunneling mechanism that had been observed for numerous donor-bridge-acceptor systems at that time. Subsequent studies found weak distance dependence for hole transport over longer distances in DNA, characteristic of incoherent hopping of either localized or delocalized holes. The introduction of synthetic DNA capped hairpin constructs possessing hole donor and acceptor chromophores (or purine bases) at opposite ends of a base-pair domain made it possible to determine the time required for transit of charge from one chromophore to the other and, in some cases, to distinguish between the transit time and the much faster initial charge injection time. These studies eliminated conventional tunneling as a viable mechanism for charge transport in DNA except at very short donor-acceptor separations; however, they did not establish the presence or nature of intermediates in the charge separation process. Recent studies in our laboratories have succeeded in identifying key intermediates as well as untangling the dynamics and efficiency of the charge separation process from start to finish. The dynamics of the initial charge injection process is dependent upon both its free energy and the stacking of the hole donor chromophore and adjacent purine base. The transport of positive charge (holes) over multiple base pairs in duplex DNA occurs most efficiently via repeating adenine bases, known as A-tracts. The transit time across an A-tract is strongly dependent upon the free energy for hole injection, whereas the efficiency of charge separation depends on the competition between charge delocalization and charge recombination in the contact radical ion pair. The guanine cation radical has been detected both by femtosecond transient absorption and by stimulated Raman spectroscopies when the guanine is located near the chromophore employed for hole injection into an A-tract. Replacement of guanine by its derivative 8-phenylethynylguanine (EG), permits tracking of hole transport across longer poly(purine) sequences as a consequence of the stronger transient absorption and stimulated Raman scattering for EG^+• vs G^+•. We have recently obtained evidence based on femtosecond transient absorption spectroscopy for the formation of delocalized A-polarons in A-tracts possessing four or more A-T base pairs. Similar methods have been used to track hole transport across less-common DNA structures including diblock and triblock poly(purines), locked nucleic acids, three-way junctions, and G-quadruplexes. Similar methods are have been applied to the study of photoinduced electron transport in DNA.

Ground- and Excited-State Interactions of a Psoralen Derivative with Human Telomeric G-Quadruplex DNA

G-quadruplex DNA has been a recent target for anticancer agents, and its binding interactions with small molecules, often used as anticancer drugs, have become an important area of research. Considering that psoralens have long been studied in the context of duplex DNA but that very little is known about their potential as G-quadruplex binders and their excited-state interaction with the latter has not been explored, we have studied herein the binding of a planar water-soluble psoralen derivative, 4'-aminomethyl-4,5',8-trimethylpsoralen (AMT), with the 22-mer human telomeric G-quadruplex-forming sequence, AGGG(TTAGGG)₃, labeled here as (hTel22), and investigated the consequences of photoexcitation of AMT by calorimetric and spectroscopic techniques. The results show an enthalpy-driven 1:1 binding of AMT with hTel22 via end-stacking mode. Fluorescence quenching experiments on 6-fluorescein amidite-labeled oligomers indicate that the binding site is nearer to the 3' end of hTel22 in the diagonal loop region. Femtosecond time-resolved transient absorption measurements indicate electron transfer from the guanine moiety of hTel22 to photoexcited AMT, leading to the formation of a radical pair species (AMT^•-G^•+), which survives for 30 ps and is favored by a parallel/quasi-parallel orientation between the two. The findings reveal psoralens as a prospective class of compounds for the development of anticancer therapeutics by targeting the G-quadruplex DNA.

X-ray Generated Recombination Exciplexes of Substituted Diphenylacetylenes with Tertiary Amines: A Versatile Experimental Vehicle for Targeted Creation of Deep-Blue Electroluminescent Systems

Customizable and technology-friendly functional materials are one of the mainstays of emerging organic electronics and optoelectronics. We show that recombination exciplexes of simple substituted diphenylacetylenes with tertiary amines can be a convenient source of tunable deep-blue emission with possible applications in organic electroluminescent systems. The optically inaccessible exciplexes were produced via recombination of radiation-generated radical ion pairs in alkane solution, which mimics charge transport and recombination in the active layer of practical organic light-emitting diodes in a simple solution-based experiment. Despite varying and rather poor intrinsic emission properties, diphenylacetylene and its prototypical methoxy (donor) or trifluoromethyl (acceptor) monosubstituted derivatives readily form recombination exciplexes with N,N-dimethylaniline and other tertiary amines that produce emission with maxima ranging from 385 to 435 nm. The position of emission band maximum linearly correlates with readily calculated gas-phase electron affinity of the corresponding diphenylacetylene, which can be used for fast computational prescreening of the candidate molecules, and various substituted diphenylacetylenes can be synthesized via relatively simple and universal cross-coupling reactions of Sonogashira and Castro. Together, the simple solution-based experiment, computationally cheap prescreening method, and universal synthetic strategy may open a very broad and chemically convenient class of compounds to obtain OLEDs and OLED-based multifunctional devices with tunable emission spectrum and high conversion efficiency that has yet not been seriously considered for these purposes.