Photoexcitation of iodide ion-pyrimidine clusters above the electron detachment threshold: Intracluster electron transfer versus nucleobase-centred excitations

Abstract

Laser photodissociation spectroscopy of the I·thymine (I·T) and I·cytosine (I·C) nucleobase clusters has been conducted for the first time across the regions above the electron detachment thresholds to explore the excited states and photodissociation channels. Although photodepletion is strong, only weak ionic photofragment signals are observed, indicating that the clusters decay predominantly by electron detachment. The photodepletion spectra of the I·T and I·C clusters display a prominent dipole-bound excited state (I) in the vicinity of the vertical detachment energy (∼4.0 eV). Like the previously studied I·uracil (I·U) cluster [W. L. Li et al., J. Chem. Phys. 145, 044319 (2016)], the I·T cluster also displays a second excited state (II) centred at 4.8 eV, which we similarly assign to a π-π* nucleobase-localized transition. However, no distinct higher-energy absorption bands are evident in the spectra of the I·C. Time-dependent density functional theory (TDDFT) calculations are presented, showing that while each of the I·T and I·U clusters displays a single dominant π-π* nucleobase-localized transition, the corresponding π-π* nucleobase transitions for I·C are split across three separate weaker electronic excitations. I and deprotonated nucleobase anion photofragments are observed upon photoexcitation of both I·U and I·T, with the action spectra showing bands (at 4.0 and 4.8 eV) for both the I and deprotonated nucleobase anion production. The photofragmentation behaviour of the I·C cluster is distinctive as its I photofragment displays a relatively flat profile above the expected vertical detachment energy. We discuss the observed photofragmentation profiles of the I·pyrimidine clusters, in the context of the previous time-resolved measurements, and conclude that the observed photoexcitations are primarily consistent with intracluster electron transfer dominating in the near-threshold region, while nucleobase-centred excitations dominate close to 4.8 eV. TDDFT calculations suggest that charge-transfer transitions [Iodide n (5p) → Uracil σ*] may contribute to the cluster absorption profile across the scanned spectral region, and the possible role of these states is also discussed.

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

“…Band I in the photodepletion spectrum of I -•A (Figure 4a) being accessed in this region. We have previously shown that dipole-bound excited states can be observed in our instrument following near-threshold anion excitation [27,38,39]. The identity of the shoulder peak at 4.17 eV resembles similar structures present in the nearthreshold photodepletion spectra of anionic alkali halide clusters (e.g.…”

“…Comparing the photophysics of the I -•A cluster with our recent results on I -•pyrimidine clusters [27], two main differences are apparent. Firstly, the relative abundances of the two characteristic photofragments produced by these clusters (i.e.…”

“…PD* is the normalized photodepletion ion count, i.e. PD*= [(IntOFF-IntON)/IntOFF)]/( P), which provides the most straightforward comparison to the electron detachment yield [27].…”

“…TD-DFT spectra of the lowest-energy X -•A7 clusters have also been calculated and are shown in Figure 3. As we have discussed previously [27], such calculations are not expected to accurately predict dipole-bound excited states, since tailored, diffuse functionals are necessary to accurately mimic dipole-bound orbitals [33][34][35]. The TD-DFT calculations predict a strong transition around 4.8 eV for both X -•A7 clusters, which is associated with a nucleobaselocalized  * excitation.…”

Observation of Near-Threshold Resonances in the Flavin Chromophore Anions Alloxazine and Lumichrome

“…37,38 Thus it is now well acknowledged that the formation of a dipole-bound state can act as a doorway to valenceanion formation, and hence to molecular dissociation. [37][38][39][40] This leads us to interpret that our spectroscopic observations as being consistent with It is useful to reflect that the only structural difference between alloxazine and lumichrome, is the presence of lumichrome's two methyl groups. Brauman and co-workers investigated the effects of molecular rotation on the observation of dipole-bound states of anions via photodetachment studies of enolate anions in an ion cyclotron spectrometer over two decades ago.…”