Femtosecond linear dichroism of DNA-intercalating chromophores: Solvation and charge separation dynamics of [Ru(phen)_2dppz]^(2+) systems
The DNA-intercalating chromophore [Ru(phen)_2dppz]^(2+) has unique photophysical properties, the most striking of which is the "light-switch" characteristic when binding to DNA. As a dimer, it acts as a molecular staple for DNA, exhibiting a remarkable double-intercalating topology. Herein, we report femtosecond dynamics of the monomeric and the covalently linked dimeric chromophores, both free in aqueous solution and complexed with DNA. Transient absorption and linear dichroism show the electronic relaxation to the lowest metal-to-ligand charge-transfer (CT) state, and subpicosecond kinetics have been observed for this chromophore for what is, to our knowledge, the first time. We observe two distinct relaxation processes in aqueous solution with time constants of 700 fs and 4 ps. Interestingly, these two time constants are very similar to those observed for the reorientational modes of bulk water. The 700-fs process involves a major dichroism change. We relate these observations to the change in charge distribution and to the time scales involved in solvation of the CT state. Slower processes, with lifetimes of approx 7 and 37 ps, were observed for both monomer and dimer when bound to DNA. Such a difference can be ascribed to the change of the structural and electronic relaxation experienced in the DNA intercalation pocket. Finally, the recombination lifetime of the final metal-to-ligand CT state to the ground state, which is a key in the light-switch process, is found in aqueous solution to be sensitive to structural modification, ranging from 260 ps for [Ru(phen)_2dppz]^(2+) and 360 ps for the monomer chromophore derivative to 2.0 ns for the dimer. This large change reflects the direct role of solvation in the light-switch process.
Additional Information© 2000 The National Academy of Sciences. Contributed by Ahmed H. Zewail, March 21, 2000. B.Ö. would like to thank The Royal Swedish Academy of Sciences for the Berzelianum Scholarships of 1998 and 1999 that made his visit to the California Institute of Technology possible. The Swedish part of the project was supported by the Natural Science Research Council (NFR). The Laboratory for Molecular Sciences at the California Institute of Technology is supported by the National Science Foundation. The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. Article published online before print: Proc. Natl. Acad. Sci. USA, 10.1073/pnas.100127397. Article and publication date are at www.pnas.org/cgi/doi/10.1073/pnas.100127397
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