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

ABSTRACT

Laser dissociation spectroscopy of I⁻·adenine (I⁻·A) and ${\text{H}}_2{\text{PO}}_3^{-}$·adenine (${\text{H}}_2{\text{PO}}_3^{-}$·A) has been utilised for the first time to explore how the anion identity impacts on the excited states. Despite strong photodepletion, ionic photofragmentation is weak for both clusters, revealing that they decay predominantly by electron detachment. The spectra of I⁻·A display a prominent dipole-bound excited state in the region of the detachment energy which relaxes to produce deprotonated adenine. In contrast, near-threshold photoexcitation of ${\text{H}}_2{\text{PO}}_3^{-}$·A does not access a dipole-bound state, but instead displays photofragmentation properties associated with ultrafast decay of an adenine-localised π→π* transition. Notably, the experimental electron detachment onset of ${\text{H}}_2{\text{PO}}_3^{-}$·A is around 4.7 eV, which is substantially lower than the expected detachment energy of an ion-dipole complex. The low value for ${\text{H}}_2{\text{PO}}_3^{-}$·A can be traced to initial ionisation of the adenine followed by significant geometric rearrangement on the neutral surface. We conclude that these dynamics quench access to a dipole-bound excited state for ${\text{H}}_2{\text{PO}}_3^{-}$·A and subsequent electron transfer. ${\text{H}}_2{\text{PO}}_3^{-}$·A represents an important new example of an ionic cluster where ionisation occurs from the neutral cluster component and where photodetachment initiates intra-molecular hydrogen atom transfer.

GRAPHICAL ABSTRACT

KEYWORDS:

• Nucleobase
• electron attachment
• photodissociation
• electronic excitation
• gas phase

Acknowledgements

We thank the University of York and the Department of Chemistry for the Horizon OPO laser system and the York Advanced Research Computing Cluster (YARCC) for access to computational resources. R.C. thanks the Department of Chemistry at the University of York for the departmental studentship. We also acknowledge the facilities of the York Centre of Excellence in Mass Spectrometry, which was created thanks to a major capital investment through Science City York, supported by Yorkshire Forward with funds from the Northern Way Initiative, and subsequently received additional support from the EPSRC.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Rosaria Cercola http://orcid.org/0000-0002-2219-6353

Edward Matthews http://orcid.org/0000-0001-5095-621X

Caroline E. H. Dessent http://orcid.org/0000-0003-4944-0413