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

Abstract

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 K 2PtCl 4. 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 ( τ P = 12.4 μs) and enhanced phosphorescence quantum yield ( Φ P = 66%) were observed for Pt-3 compared to ppyPt(acac) ( τ P = 2.4 μs and Φ P = 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 O 2 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 O 2 sensing material among the complexes studied herein, with quenching constant of K SV = 0.0238 Torr −1.