Fine-Tuning of Luminescence Properties of Cyclometalated Platinum(II) Complexes via Aminopyridine Derivatives

The present work describes two families of cycloplatinated­(II) complexes [PtR­(dfppy)­(L)], dfppy = 2-(2,4-difluorophenyl)­pyridinate, R = Me and p-MeC6H4 and L = pyridine, py, and aminopyridine derivatives, x-NH2-py, x = 2, 3, and 4. The complexes show similar UV–vis spectra possessing intense bands in the 250–400 region, which are assigned as π–π* transitions and mixed metal-to-ligand charge transfer (MLCT)/π–π*. The MLCTs are observed as weak broad bands in the range 400–500 nm. Meanwhile, structured emission bands are observed for the complexes in the range 450–600 nm with quantum efficiencies up to 0.7, wherein the p-MeC6H4 derivatives show higher emission strengths compared to their Me-containing counterparts. Moreover, the NH2 position on the pyridine significantly affects the emission efficiency of the Me derivatives and conversely does not show a large impact on the emission intensity of the p-MeC6H4-containing complexes. DFT and TD-DFT calculations reveal that 3π–π* (dfppy)/3MLCT are responsible for the emissions in the Me complexes. However, in the p-MeC6H4 complexes, the emissions originate from 3L″LCT and 3MLCT (L = dfppy, L″ = p-MeC6H4).