Dual Time-Scale Proton Transfer and High-Energy, Long-Lived Excitons Unveiled by Broadband Ultrafast Time-Resolved...

In contrast to the immense amount of research on electronically excited DNA, surprisingly little has been done about the excited states of RNA. Herein, we demonstrate an ultrafast broadband time-resolved fluorescence and fluorescence anisotropy study to probe directly the intrinsic fluorescence and overall dynamics of the fluorescence from a homopolymeric adenine·uracil RNA duplex adopting the A-form structure. The results unveiled complex deactivation through distinctive multichannels mediated by states of varied energy, a character of charge transfer, and a lifetime from sub-picosecond to nanoseconds. In particular, we observed an unprecedented kinetic isotopic effect and participation of unusual proton transfer from states in two discrete energies and time domains. We also identified a high-energy nanosecond emission that we attributed to its fluorescence anisotropy to long-lived weakly emissive excitons not reported in DNA. These distinguishing features originate from the stacking, pairing, and local hydration environment specific to the A-form conformation of the adenine·uracil double helix.