Phosphorescent aryl cyclometalated platinum(II) complexes containing para-substituted pyridine ligands

This investigation introduces a new series of cyclometalated platinum(II) complexes of the general formula of [Pt p-MeC6H4)(bzq)(4-X-py)], (bzq = 7,8-benzoquinoline; X = CHO, 1a; CN, 2a; Me, 3a; and NMe2, 4a). The complexes were characterized by NMR spectroscopy, ESI-HRMS, single-crystal X-ray diffraction, UV-Vis absorption and photoluminescence spectroscopies. Similar to normal cyclometalated platinum(II) complexes, their UV-Vis spectra are divided into two distinct regions, i.e., high-energy and low-energy regions, with a border wavelength of 370 nm. The functional groups slightly affect the absorption spectra in both regions. In this regard, using TD-DFT (time-dependent density functional theory) calculations, the electronic transitions in these regions were studied. These complexes are bright green to yellow emitters with relatively structured emission bands. The emission quantum yields of 3a and 4a, with electron-donating functional groups on the pyridine ligand, are significantly larger than those of 1a and 2a, which contain electron-withdrawing groups. Theoretical studies on the triplet excited states support the experimental observation that, in 3a and 4a, the cyclometalated ligand as the main chromophoric moiety has a determining role in the phosphorescence, whereas in 1a and 2a, it does not contribute significantly to the emission.