[Ru(phen)2dppz]2+ (phen = 1,10-phenanthroline, dppz = dipyridophenazine) and closely related complexes have previously been observed to have an undetectably small quantum yield of photoluminescence in water but a moderate emission yield when bound to DNA. This so-called “light-switch” effect is a critical factor in the utility of these complexes as spectroscopic probes for DNA. Here we describe a detailed investigation of the photophysics of [Ru(phen)2dppz]2+ in aqueous solution, and in mixtures of acetonitrile and water, by time-resolved absorption and emission spectroscopies. The emission of the complex in water has been measured for the first time. A prompt initial emission, derived from a metal-to-ligand charge-transfer (MLCT) excited state typical for polypyridyl−ruthenium complexes, is observed along with a delayed emission attributed to a novel MLCT species. The small quantum yield of photoluminescence for [Ru(phen)2dppz]2+ in water, and in water/acetonitrile, depends upon efficient formation of a novel MLCT species, followed by its rapid radiationless decay. The MLCT interconversion is assigned to an intramolecular charge-transfer process that is induced by the polarity and proton donating ability of the solvent.

Ultrafast emission and absorption spectroscopies were used to measure the kinetics of DNA-mediated electron transfer reactions between metal complexes intercalated into DNA. In the presence of rhodium(III) acceptor, a substantial fraction of photoexcited donor exhibits fast oxidative quenching (>3 x 10(10) per second). Transient-absorption experiments indicate that, for a series of donors, the majority of back electron transfer is also very fast (approximately 10(10) per second). This rate is independent of the loading of acceptors on the helix, but is sensitive to sequence and pi stacking. The cooperative binding of donor and acceptor is considered unlikely on the basis of structural models and DNA photocleavage studies of binding. These data show that the DNA double helix differs significantly from proteins as a bridge for electron transfer.

The distance dependence of electron transfer (ET) rates between donors and acceptors in a DNA environment has been an area of intense research and heated discussion, especially with regard to the recent report of ultrafast ET over a separation of >40 Å. This paper is focused on quantitative modeling of the photodynamics and gel electrophoresis/photocleavage experiments of the untethered DNA/metallointercalator system. Simulations are compared to previously published experimental results in order to quantitatively examine the possible involvement of long-range ET and donor/acceptor clustering in the DNA-mediated ET between metallointercalators. Comparison of the simulation results with a broad range of experimental data strongly indicates that a fairly typical distance dependence for ET (β ≈ 1 Å -1 ) coupled to donor/acceptor clustering is able to account for all features in the data and for related photocleavage studies.

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