Communication: Photoactivation of nucleobase bound platinumIImetal complexes: Probing the influence of the nucleobase

Investigating the mapping of chromophore excitations onto the electron detachment spectrum: photodissociation spectroscopy of iodide ion-thiouracil clusters

Laser photodissociation spectroscopy (3.1-5.7 eV) has been applied to iodide complexes of the non-native nucleobases, 2-thiouracil (2-TU), 4-thiouracil (4-TU) and 2,4-thiouracil (2,4-TU), to probe the excited states and intracluster electron transfer as a function of sulphur atom substitution. Photodepletion is strong for all clusters (I-·2-TU, I-·4-TU and I-·2,4-TU) and is dominated by electron detachment processes. For I-·4-TU and I-·2,4-TU, photodecay is accompanied by formation of the respective molecular anions, 4-TU- and 2,4-TU-, behaviour that is not found for other nucleobases. Notably, the I-·2TU complex does not fragment with formation of its molecular anion. We attribute the novel formation of 4-TU- and 2,4-TU- to the fact that these valence anions are significantly more stable than 2-TU-. We observe further similar behaviour for I-·4-TU and I-·2,4-TU relating to the general profile of their photodepletion spectra, since both strongly resemble the intrinsic absorption spectra of the respective uncomplexed thiouracil molecule. This indicates that the nucleobase chromophore excitations are determining the clusters' spectral profile. In contrast, the I-·2-TU photodepletion spectrum is dominated by the electron detachment profile, with the near-threshold dipole-bound excited state being the only distinct spectral feature. We discuss these observations in the context of differences in the dipole moments of the thionucleobases, and their impact on the coupling of nucleobase-centred transitions onto the electron detachment spectrum.

Photodissociation dynamics of the iodide-uracil (I(-)U) complex

Photofragment action spectroscopy and femtosecond time-resolved photoelectron imaging are utilized to probe the dissociation channels in iodide-uracil (I(-) ⋅ U) binary clusters upon photoexcitation. The photofragment action spectra show strong I(-) and weak [U-H](-) ion signal upon photoexcitation. The action spectra show two bands for I(-) and [U-H](-) production peaking around 4.0 and 4.8 eV. Time-resolved experiments measured the rate of I(-) production resulting from excitation of the two bands. At 4.03 eV and 4.72 eV, the photoelectron signal from I(-) exhibits rise times of 86 ± 7 ps and 36 ± 3 ps, respectively. Electronic structure calculations indicate that the lower energy band, which encompasses the vertical detachment energy (4.11 eV) of I(-)U, corresponds to excitation of a dipole-bound state of the complex, while the higher energy band is primarily a π-π(∗) excitation on the uracil moiety. Although the nature of the two excited states is very different, the long lifetimes for I(-) production suggest that this channel results from internal conversion to the I(-) ⋅ U ground state followed by evaporation of I(-). This hypothesis was tested by comparing the dissociation rates to Rice-Ramsperger-Kassel-Marcus calculations.

UV laser photoactivation of hexachloroplatinate bound to individual nucleobases in vacuo as molecular level probes of a model photopharmaceutical

UV excitation of mass-selected hexachloroplatinate–nucleobase clusters provides detailed insight into the photophysics and photochemistry of a model DNA photopharmaceutical.