Electronic spectra of the cyclometalated complexes M(2-thienylpyridine)(2)with M = Pd, Pt: A theoretical study

The optical spectra of the cyclometalated complexes Pd,Pt(thpy)(2), with th py(-), the deprotonated form of 2-(2-thienyl)pyridine, have been calculated using a multiconfigurational second-order perturbation treatment based on a complete active space reference wave function, i.e., the CASSCF/CASPT2 me thod. The calculations were performed on optimized structures within C-2v s ymmetry, obtained using density functional theory (DFT). The structure and lowest excited states of free protonated thpyH were also calculated. On the basis of the calculated results for excitation energies and oscillator str engths, the most important bands in the experimental absorption spectra of the complexes were assigned as excitations to states of symmetry (1)A(1) or B-1(2), all of mixed charge-transfer/ligand-centered character. The lowest triplet excited state was calculated to be the a (3)A(1) state, closely fo llowed by the B-3(2) state. Both excitations were found to be almost purely ligand-centered, with a very limited charge-transfer contribution in Pd(th py)(2) and an only slightly larger contribution in Pt(thpy)(2). The differe nces between the position and character of the singlet excited states in th e absorption spectra of both molecules are also discussed. The results are round to be consistent with the trends obtained from previous experimental measurements.