Photoelectron imaging of the isolated adenosine-5´-triphosphate dianion excited to the 1 ππ* states reveals that electron emission is predominantly parallel to the polarization axis of the light and arises from subpicosecond electron tunneling through the repulsive Coulomb barrier (RCB). The computed RCB shows that the most probable electron emission site is on the amino group of adenine. This is consistent with the photoelectron imaging: excitation to the 1 ππ* states leads to an aligned ensemble distributed predominantly parallel to the long axis of adenine, the subsequent electron tunneling site is along this axis, and the negatively charged phosphate groups guide the outgoing electron mostly along this axis at long range. Imaging electron tunneling from polyanions combined with computational chemistry may offer a general route to probing the intrinsic photo-oxidation site and dynamics as well as overall structure of complex isolated species.

A dipole-bound state is found to survive despite its disruption by an alkyl chain.

The excited state dynamics of the doubly deprotonated dianion of adenosine-5′-triphosphate, [ATP–H2]2 –, has been spectroscopically explored by time-resolved photoelectron spectroscopy following excitation at 4.66 eV. Time-resolved photoelectron spectra show that two competing processes occur for the initially populated 1ππ* state. The first is rapid electron emission by tunneling through a repulsive Coulomb barrier as the 1ππ* state is a resonance. The second is nuclear motion on the 1ππ* state surface leading to an intermediate that no longer tunnels and subsequently decays by internal conversion to the ground electronic state. The spectral signatures of the features are similar to those observed for other adenine-derivatives, suggesting that this nucleobase is quite insensitive to the nearby negative charges localized on the phosphates, except of course for the appearance of the additional electron tunneling channel, which is open in the dianion.

A carboxylated adenosine analog (C-Ado−) has been synthesized and probed via time-resolved photoelectron spectroscopy in order to induce intra-molecular charge transfer from the carboxylic acid moiety to the nucleobase. Intra-molecular charge transfer can be exploited as starting point to probe low-energy electron (LEE) damage in DNA and its derivatives. Time-dependent density functional theory (TD-DFT) calculations at the B3LYP-6311G level of theory have been performed to verify that the highest occupied molecular orbital (HOMO) was located on carboxylic acid and that the lowest occupied molecular orbital (LUMO) was on the nucleobase. Hence, the carboxylic acid could work as electron source, whilst the nucleobase could serve the purpose of electron acceptor. The dynamics following excitation at 4.66 eV (266 nm) were probed using time-resolved photoelectron spectroscopy using probes at 1.55 eV (800 nm) and 3.10 eV (400 nm). The data show rapid decay of the excited state population and, based on the similarity of the overall dynamics to deoxy-adenosine monophosphate (dAMP–), it appears that the dominant decay mechanism is internal conversion following 1ππ* excitation of the nucleobase, rather than charge-transfer from the carboxylic acid to the nucleobase.