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Science 3 December 2004:
Vol. 306. no. 5702, pp. 1765 - 1768
DOI: 10.1126/science.1104038
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Reports

Efficient Deactivation of a Model Base Pair via Excited-State Hydrogen Transfer

Thomas Schultz,1* Elena Samoylova,1 Wolfgang Radloff,1 Ingolf V. Hertel,1 Andrzej L. Sobolewski,2 Wolfgang Domcke3

We present experimental and theoretical evidence for an excited-state deactivation mechanism specific to hydrogen-bonded aromatic dimers, which may account, in part, for the photostability of the Watson-Crick base pairs in DNA. Femtosecond time-resolved mass spectroscopy of 2-aminopyridine clusters reveals an excited-state lifetime of 65 ± 10 picoseconds for the near-planar hydrogen-bonded dimer, which is significantly shorter than the lifetime of either the monomer or the 3- and 4-membered nonplanar clusters. Ab initio calculations of reaction pathways and potential-energy profiles identify the mechanism of the enhanced excited-state decay of the dimer: Conical intersections connect the locally excited 1{pi}{pi}* state and the electronic ground state with a 1{pi}{pi}* charge-transfer state that is strongly stabilized by the transfer of a proton.

1 Max Born Institute Berlin, Max-Born-Straße 2a, D-10247 Berlin, Germany.
2 Institute of Physics, Polish Academy of Sciences, Aleja Lotnikow 32/46, PL-02668 Warsaw, Poland.
3 Department of Chemistry, Technical University of Munich, Lichtenbergstraße 4, D-85747 Garching, Germany.

* To whom correspondence should be addressed. E-mail: schultz{at}mbi-berlin.de

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