Distance-dependent activation energies for hole injection from protonated 9-amino-6-chloro-2-methoxyacridine into duplex DNA

The recent investigation of the apparently anomalous attenuation factor (β > 1.5 Å^-1) for photoinduced hole injection into DNA duplexes modified by protonated 9-amino-6-chloro-2-methoxyacridine (X⁺) led to the conclusion that in addition to the electronic couplings, the activation energy must also be distance-dependent. In this communication we report the verification of this postulate by direct measurements of the activation energies for a series of (X⁺)-modified DNA duplexes which sample an appreciable range of donor-acceptor distances (∼4-11 Å). The resulting changes in thermal activation energy can be explained within the framework of a distance-dependent reorganization energy.