Electronic coupling between photo-excited stacked bases in DNA and RNA strands with emphasis on the bright states initially populated.

In biology the interplay between multiple light-absorbers gives rise to complex quantum effects such as superposition states that are of extreme importance for life, both for harvesting solar energy and likely protecting nucleic acids from radiation damage. Still the characteristics of these states and their quantum dynamics are a much debated issue. While the electronic properties of single bases are fairly well understood, the situation for strands is complicated by the fact that stacked bases electronically couple when photoexcited. These newly arising states are denoted as exciton states and are simply linear combinations of localised wavefunctions that involve N - 1 ground-state bases and one base in its excited state (cf. the Frenkel exciton model). There is disagreement over the number of bases, N, that coherently couple, i.e., the spatial extent of the exciton, and how electronic deexcitation back to the ground state occurs. The importance of dark charge-transfer states has been inferred both from time-resolved fluorescence and transient absorption experiments. These states were suggested to be responsible for long deexcitation times but it is unclear whether 'long' is tens of picoseconds or nanoseconds. In this review paper, we focus on the bright states initially populated and discuss their nature based on information obtained from systematic absorption and circular dichroism experiments on single strands of different lengths. Our results from the last five years are compared with those from other groups, and are discussed in the context of successive deexcitation schemes. Pieces to the puzzle have come from different experiments and theory but a complete description has yet to emerge. As such the story about DNA/RNA photophysical decay mechanisms resembles the tale about the blind men and the elephant where all see the beast in different, correct but incomplete ways.