Excited states of phosphorescent platinum(II) complexes containing NΛCΛN-coordinating tridentate ligands:: Spectroscopic investigations and time-dependent density functional theory calculations

The absorption and emission spectra of the Pt(II) complexes containing N boolean AND C boolean AND N-coordinating tridentate ligands, platinum(II) 1,3-di(2-pyridyl)benzene chloride [Pt(dpb)Cl] and platinum(II) 3,5-di(2-pyridyl)toluene chloride [Pt(dpt)Cl], together with their corresponding free ligands, 1,3-di(2-pyridyl)benzene (dpbH) and 3,5-di(2pyridyl)toluene (dptH), have been analyzed by density functional theory (DFT) for the ground state and time-dependent DFT (TDDFT) for the excited states. T-1(A(1)) and S-1(B-2) of the complexes (in C-2v symmetry) were assigned on the basis of the calculated excitation energies as well as comparison of the experimental spectroscopic properties and the calculated states' characteristics. The calculated excitation energies for T-1 and S-1 of the complexes as well as those for T-1 of the free ligands were in good agreement with their observed values within 600 cm(-1). The d-pi* characters of the excited states were evaluated from the change in electron densities between the ground and excited states by Mulliken population analysis; values of 25% for T-1 and 32% for S-1 were obtained for both complexes. The calculated values of d-pi* character were found to be consistent with the reported emission lifetimes as well as the observed emission energy shifts from the corresponding free ligands. Most spectroscopic properties of the complexes and the free ligands, which include solvatochromic shift, Stokes shifts, methyl substitution shifts, and emission spectra profiles, were well explained from the calculation results.