Charge Transfer, Symmetry, and Dissipation in Donor-Acceptor Molecules

We study charge transfer between donor-acceptor molecules subject to a mirror symmetry constraint in the presence of a dissipative environment. The symmetry requirement leads to the breakdown of the standard single reaction coordinate description, and to a new charge transfer model, in the limit of low temperature, based on two independent reaction coordinates of equal relevance. We discuss implications of these results to charge transfer between DNA base pairs, whose geometrical configuration is modified by the addition of the migrating charge in conformity with the discussed symmetry constraint.

Figure 1

Two different reaction paths in the $Y_1,Y_2$ plane of the nuclear coordinates. The locus of degeneracy points between the donor and the acceptor state energies are indicated by the lines $Y_1=\pm Y_2$. Charge transfer reactions take place within the Landau-Zener region centered at the intersection of the reaction path with each degeneracy line. The top trajectory, with a large impact parameter $q$, crosses the two degeneracy lines separately, and the corresponding 1D Landau-Zener regions, not shown in the picture, are of order $l_{\mathrm{L}\mathrm{Z}}(q)$. At the crossing points, charge transfer to the acceptor state $|R\downarrow ⟩$ (dashed arrows) is allowed, according to the single reaction coordinate description. The 2D Landau-Zener length $\xi$ defines a region surrounding the origin where single-reaction coordinate theory fails. At the center of the 2D region one encounters strong resonant tunneling within a distance $\alpha$ from the origin. The bottom trajectory, with a low impact parameter $q$, enters both the 2D region of width $\xi$ and the coherent tunneling region of width $\alpha$. Here the charge transfer process must be described within a 2D framework.

Figure 2

Side view of charge induced structural deformation in DNA base pairs. Neutral base pairs, shown in the center, have a mirror symmetry that is broken by the addition of an external charge.