Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n'arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. Questions? Contact the NRC Publications Archive team atPublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information. NRC Publications Archive Archives des publications du CNRCThis publication could be one of several versions: author's original, accepted manuscript or the publisher's version. / La version de cette publication peut être l'une des suivantes : la version prépublication de l'auteur, la version acceptée du manuscrit ou la version de l'éditeur. NRC Publications Record / Notice d'Archives des publications de CNRC:http://nparc.cisti-icist.nrc-cnrc.gc.ca/eng/view/object/?id=20900e4b-251a-45fe-bf49-a95e352fe29e http://nparc.cisti-icist.nrc-cnrc.gc.ca/fra/voir/objet/?id=20900e4b-251a-45fe-bf49-a95e352fe29e Sciences, National Research Council of Canada, 100 Sussex DriVe, Ottawa, Ontario K1A 0R6, Canada ReceiVed: March 26, 2007; In Final Form: May 25, 2007 The reaction dynamics of excited electronic states in nucleic acid bases is a key process in DNA photodamage. Recent ultrafast spectroscopy experiments have shown multicomponent decays of excited uracil and thymine, tentatively assigned to nonadiabatic transitions involving multiple electronic states. Using both quantum chemistry and first principles quantum molecular dynamics methods we show that a true minimum on the bright S 2 electronic state is responsible for the first step that occurs on a femtosecond time scale. Thus the observed femtosecond decay does not correspond to surface crossing as previously thought. We suggest that subsequent barrier crossing to the minimal energy S 2 /S 1 conical intersection is responsible for the picosecond decay.

A combination of theory and experiment is used to identify a novel variable excitonic coupling in a series of building blocks for small phenylacetylene dendrons. Systematic changes in the experimental emission spectra, radiative lifetimes, and polarization anisotropies as the number of meta-conjugated branches increases provide evidence for a qualitative change in the electronic structure in the relaxed excited state. The excited state electronic structure is investigated theoretically using ab initio CASSCF and CASPT2 calculations, which indicate the presence of large electronic coupling in the emitting geometry that is not seen for the absorbing geometry of the same molecules. The changes in electronic structure that occur upon excited-state relaxation can be understood in terms of a variable excitonic coupling between the phenylactylene branches, which takes these molecules from the weak coupling to the strong coupling regime as they relax on the excited state. The origin of this geometry-dependent coupling is investigated through the interpretation of ab initio calculations in terms of Fo ¨rster, Dexter, and through-bond charge-transfer interactions. We find that the change in the coupling arises primarily from an increase in the through-bond or charge-transfer component of the coupling, despite the absence of large changes in charge distribution. A theoretical comparison of metaversus para-substituted phenylacetylenes clarifies why this effect is so pronounced in the meta-substituted molecules.

A pair of donor-bridge-acceptor electron-transfer complexes, with a carbazole donor and a naphthalimide acceptor connected by either a para- or meta-conjugated phenylacetylene bridge, are synthesized and studied using time-resolved and steady-state spectroscopy. These experiments show that the charge separation times, which depend on the coupling of the donor and acceptor through the excited bridge moiety, are similar for the two molecules (Meta and Para). The charge recombination time, however, is a factor of 10 slower for Meta than for Para. These results are related to changes in the electronic coupling of the bridge depending on its electronic state, and show that meta-conjugated bridges provide a possible motif for the design of asymmetric molecular wires.