Tuning the emission property of carbazole-caped cyclometalated platinum(II) complexes and its application for enhanced luminescent oxygen sensing

Carbazole-caped cyclometalated platinum(II) complexes aryl-R-ppyPt(acac) (where ppy = 4-phenylpyridine, acac = acetylacetonato, aryl = carbazole and R is linker) were synthesized. The carbazole group is attached to the ppy ligand via α-diketo moiety (Pt-1) and (Pt-2) or C−C single bond (Pt-3), by Sonogashira or Negishi coupling reactions. We found that the ethynylene bonds of the C^N ligands were oxidized to α-diketo or methylene-keto structure during the metalation with K2PtCl4. Emissions beyond 550 nm were observed for Pt-1, with electron-withdrawing α-diketo moiety attached to ppy ligand, compared to the emission at 486 nm for the parent complex ppyPt(acac). Extended phosphorescence lifetime (τP = 12.4 μs) and enhanced phosphorescence quantum yield (ΦP = 66%) were observed for Pt-3 compared to ppyPt(acac) (τP = 2.4 μs and ΦP = 15%), we attribute the enhanced phosphorescence property to the electron-donating carbazole substituent. With density functional theoretical calculations (DFT), we found that the carbazole moiety is involved in the HOMO (Pt-3), the α-diketo moiety is involved in the LUMO (Pt-1), thus the energy gaps between the HOMO and LUMO in both cases were decreased and red-shifted emission is expected, compared to ppyPt(acac). The different emission properties of 1,2-dione containing complexes (Pt-1 and Pt-2) and the Pt-3 were rationalized by the spin density surface analysis of the complexes. The luminescent O2 sensing properties of these complexes were studied in solution and in polymer films, for which fast response time (3.3 s) and recovery time (3.7 s) were observed. Two-site model fitting indicated that complex Pt-3 is the most sensitive O2 sensing material among the complexes studied herein, with quenching constant of KSV = 0.0238 Torr−1.

ppyPt(acac) complexes containing carbazole subunits were prepared, which show intense RT phosphorescence in 500–700 nm range. The red-shifted emission is attributed to either the elevated HOMO energy level or the decreased LUMO energy level. Extended phosphorescence lifetimes (12.4 μs) were observed and were used for enhanced luminescent O2 sensing.Figure optionsDownload as PowerPoint slideHighlights
► The emission property of the Pt(II) complexes was improved by modification of the ligand.
► Phosphorescence quantum yield of 66% and lifetime up to 12.4 μs were observed.
► DFT calculations were used for rationalization of the different emission properties of the complexes.
► Luminescent oxygen sensing property was improved with our new complexes.