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 thpy(-), 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 method. The calculations were performed on optimized structures within C-2v symmetry, 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 strengths, 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 followed 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(thpy)(2) and an only slightly larger contribution in Pt(thpy)(2). The differences between the position and character of the singlet excited states in the absorption spectra of both molecules are also discussed. The results are round to be consistent with the trends obtained from previous experimental measurements.