Near-threshold electron transfer in anion-nucleobase clusters: does the identity of the anion matter?

Abstract

Laser photofragmentation spectroscopy of the I-•adenine (I-•A) and H2PO3-•adenine (H2PO3-•A) clusters has been utilized for the first time across the electron detachment thresholds to explore how the anion identity affects intra-cluster electron transfer. Ionic photofragmentation is weak for both clusters, despite strong photodepletion, revealing that both clusters decay predominantly by electron detachment. The spectra of I-•A display a prominent dipole-bound excited state in the region of the vertical detachment energy, which relaxes to produce deprotonated adenine. In contrast, photoexcitation of H2PO3-•A in the near-threshold region does not access a dipole-bound state, but instead displays photofragmentation properties associated with ultrafast decay of an adenine-localized -* transition. Notably, the experimental electron detachment onset of H2PO3-•A is around 4.7 eV. This value is substantially reduced compared to the VDE expected for detachment of a simple anion-dipole complex. The lower VDE of H2PO3-•A can be traced to initial ionization of the adenine, which is followed by significant rearrangement of a hydrogen atom on the neutral surface. We conclude that these dynamics quench access to a dipole-bound excited state for H2PO3-•A. Despite the fact that the excess negative charge is located on H2PO3in the ground-state cluster, the anion in this cluster does not act as a free electron source to initiate free electron attachment or dissociation in the nucleobase. However, the H2PO3-•A cluster represents an important new example of an anionic cluster where ionization occurs from the initial neutral moiety of the cluster, and where photodetachment initiates intra-molecular hydrogen atom transfer.

Observation of Enhanced Dissociative Photochemistry in the Non-Native Nucleobase 2-Thiouracil

“…The pattern of photofragment action spectra observed here for the m/z 58 and m/z 93 photofragments is reminiscent of behaviour we observed recently in studies of iodide-nucleobase complexes [55,56,64]. These complexes similarly display a dipole-bound excited state in the vicinity of the VDE, which decays with production of the respective valence anion produced upon low-energy electron attachment to the nucleobase.…”

“…However, these absorptions are not evident in our gaseous experimental spectrum presented in Figure 2, probably due to the dominance of electron detachment (Section S2). In previous anionic systems we studied, these excited states have been more clearly visible in the photofragment production spectra [44,55,56]. We therefore turn to inspecting the photofragmentation channels of [2-TU-H] − to further characterise the excited states and photochemistry.…”

Electron detachment dynamics of the iodide-guanine cluster: does ionization occur from the iodide or from guanine?

“…One such series of experiments have involved gas-phase iodide ion-nucleobase clusters. Photoexcitation of these clusters provides a novel route for probing low-energy electron-nucleobase coupling in a highly controlled environment [13][14][15][16]. In these experiments, the iodide ion is photodetached to produce a 'spectator' iodine atom and a low-energy free electron with a well-defined kinetic energy that can be captured by an adjacent molecule [17][18][19].…”

“…In these experiments, the iodide ion is photodetached to produce a 'spectator' iodine atom and a low-energy free electron with a well-defined kinetic energy that can be captured by an adjacent molecule [17][18][19]. The resulting TNI dynamics can then be monitored either via time-resolved photoelectron spectroscopy or photofragment action spectroscopy [13][14][15][16][17][18][19][20][21].…”

“…Further studies have been performed to investigate the electron attachment dynamics in thymine, cytosine and adenine via laser photodissociation studies of the I -•N (N = thymine, adenine, cytosine) clusters [14,15], as well as via TRPEI [20,22,23]. These experiments provide a complex picture of the interplay of dipole-bound and valence-anion states formed following near-threshold excitation of the clusters, with the TNIs participating in cluster dissociation following internal conversion back to the electronic ground states.…”