Role of the Trifluoropropynyl Ligand in Blue-Shifting Charge-Transfer States in Emissive Pt Diimine Complexes and an Investigation into the PMMA-Imposed Rigidoluminescence and Rigidochromism.

Square-planar Pt II complexes are of interest as dopants for the emissive layer of organic light-emitting diodes. Herein, the photophysics of three Pt bipyridyl complexes with the strongly e - withdrawing, high-field, 3,3,3-trifluoropropynyl ligand has been investigated. One complex, (phbpy)PtC 2 CF 3 (phbpy = 6-phenyl-2,2'-dipyridyl), has also been characterized by single-crystal X-ray diffraction. All complexes reported are emissive in both RT CH 2 Cl 2 solution (Φ PL = 0.007 to 0.027) and PMMA film (Φ PL = 0.25 to 0.42). The trifluoropropynyl ligand elevates the energy of the MLCT and LL'CT states above that of the IL π-π* state, resulting in IL emission in all cases. The emission energies of the trifluoropropynyl compounds are also blue-shifted relative to the analogous pentafluorophenylethynyl compounds, suggesting that the trifluoropropynyl ligand is one of the most electron-withdrawing alkynyl ligands. Rate constants for radiative and nonradiative deactivation were determined from experimentally determined values of Φ PL and excited-state lifetimes in both solution and PMMA films. The increase in Φ PL upon incorporation into PMMA film (rigidoluminescence) results from a decrease in the rate constant for non-radiative relaxation. Experimental activation energies for excited-state decay in combination with TDDFT are consistent with the rigidoluminescence resulting from an increase in the energy of the non-emissive triplet metal-centered state. Two of the complexes investigated, ( Ph2 bpy)Pt(C 2 CF 3 ) 2 and ( t -Bu2 bpy)Pt(C 2 CF 3 ) 2 , where t -Bu2 bpy = 4,4'-di- tert -butyl-2,2'-dipyridyl and Ph2 bpy = 4,4'-diphenyl-2,2'-dipyridyl, exhibit concentration-dependent excimer emission (orange) along with monomer emission (blue), enabling fine-tuning of the emission color. However, excimer emission was absent in cured PMMA films up to the solubility limit for solution processing of ( Ph2 bpy)Pt(C 2 CF 3 ) 2 in CH 2 Cl 2 , demonstrating the diffusional nature of excimer formation.