Direct Observation of Triplet-Triplet Energy Transfer Between C-Nucleotides of Thioxanthone and Naphthalene in DNA

Investigating the migration of excited-state energy in DNA is crucial for a deep understanding of protection mechanisms and light-induced DNA damage. While numerous reports focused on single electron transfer and Förster-type energy transfer in DNA, studies on the Dexter-type triplet-triplet energy transfer are scarce, in particular those with direct detection of photoexcited triplet states. Herein, we present direct measurements of the distance-dependent triplet-triplet energy transfer rates through DNA by using transient absorption spectroscopy. This was achieved through the synthetic incorporation of thioxanthone as energy donor and naphthalene as energy acceptor into a DNA double strand at defined positions. The energy transfer rates strongly depend on the number of A-T base pairs (up to four) separating the energy donor from the energy acceptor. We observed a fast energy transfer rate with a time constant of 17 ns for the DNA sample in which the donor and acceptor are directly adjacent in the DNA. By analyzing two additional donor-acceptor distances, a steep exponential distance dependence with an attenuation factor of 1.15 Å^−1 could be obtained. Our results demonstrate that DNA acts as a poor conductor of triplet energy when energy donors with triplet energies below 2.7 eV are used, complementing more indirect studies on sensitized DNA damage.