Structure, electronic, and photophysical properties of platinum(ii) biphenyl complexes containing 2,2’-bipyridine and 1,10-phenanthroline derivatives
Platinum(II) biphenyl diimine complexes have been synthesized and characterized. Single crystal X-ray structures from seven complexes show two types of configurations about the platinum coordination sphere in the solid state: X and B (butterfly). The two configurations give different circular dichroism spectra (CD) based on their C2 or Cs symmetry. The metal-to-ligand charge transfer (MLCT) bands which occur at ~440 nm are transitions from d orbitals on the platinum metal center to * orbitals of the diimine ligands. This agrees with the time dependant density functional theory (TDDFT) calculations where the lowest unoccupied molecular orbitals (LUMO) are located on diimine ligands. Emission spectra of the complexes can be divided into three groups according to different emission excited states: group I, ligand centered transitions (LC); group II, ligand field transitions (LF) and group III, metal-to-ligand charge transfer (MLCT). Emission profiles from the groups are different from one another: LC transitions have vibronic structures; LF transitions have sharp peaks without vibronic structure and MLCT transitions have a broad peak without vibronic structure. Linear free energy correlations were found between emission maxims, reduction potentials and emission lifetimes with Hammett sp values. The diimine with electron donating substituents have the higher energy emission maxima and the longer emission life time. The diimine with electron withdrawing substituents has the lower energy emission maxima and the shorter emission life time.