A TDDFT Computational Study of Platinum Complexes Bound to Nucleobases

The electronic spectra of a number of platinum (II) complexes bound to single nucleobases have been investigated using Time-Dependent Density Functional Theory (TDDFT). The calculated spectra obtained in this work have been benchmarked against recent gas-phase photo-dissociation spectra of platinum complex-nucleobase clusters. UV spectra have been calculated for a range of density functionals and basis sets to determine the best functional-basis set combination for reproducing the experimental spectra.
The first series of TDDFT calculations conducted in this work investigated the electronic transitions of iodide ion-nucleobase clusters and their constituent “monomer” parts (i.e. the isolated iodide anion and isolated nucleobases). Calculations on the I-∙Nu clusters (Nu = Uracil, Thymine or Adenine) and isolated uracil, cytosine, thymine and adenine produced computed UV spectra and the associated electronic transitions were characterised by inspection of respective molecular orbitals. For the nucleobases, these orbitals were revealed to be mainly of ππ* character. The electronic transitions of the I-∙Nu clusters were dominated by excitations involving orbitals localised on the nucleobases.
A second series of studies focused on the electronic transitions of isolated platinum (II) and platinum (IV) cyanide complexes as well as their clusters involving a single water molecule. The excited states of the Pt(CN)4,62-∙H2O complexes were found to involve only platinum localised orbitals. The final set of TDDFT calculations were performed on Pt(CN)4,62-∙M complexes (M = Uracil or Cytosine). The electronic transitions occurring in the Pt(CN)42-∙Uracil and Pt(CN)42-∙Cytosine complexes were found to be of a short-range charge transfer nature. Conversely, the electronic transitions of Pt(CN)42-∙Uracil involved uracil localised orbitals that were of ππ* character.