Triscyclometalated Iridium(III) Fluoro-Substituted Carbene Complexes: Character of Emitting Triplet States and Excited State Dynamics

We report the spectra of UV-vis absorption, emission and excitation for fluoro-substituted triscarbene iridium complexes fac-tris(1-(4-fluorophenyl)-3-methylimidazolin-2-ylidene-C,C-2) iridium(III), fac-Ir(fpmi)(3), and fac-tris(1-(2,4-fluorophenyl)-3-methyl imidazolin-2-ylidene-C,C-2 ) iridium(III), fac-Ir(dfpmi)(3). The decay of emission and transient absorption curves at 266-nm excitation in the singlet metal-to-ligand charge-transfer (MLCT) band region are recorded. Using quantum chemical calculations-density functional theory (DFT) and time-dependent DFT (TD-DFT) method B3LYP, we obtained their optimized geometries, electronic structures, energies of vertical transitions and oscillator strengths. Both complexes emit in deep blue/near UV which has the highest emission energy for iridium complexes. These emission bands display vibronic structure and a small rigidochromic shift in the low-temperature. From the measured lifetime data and the emission quantum yield, we obtain a radiative rate constant k(r) 2.2-2.4 x 10(4) s(-1) for the emitting state and assign the emitting state to (3)-* for these complexes. In subpicosecond transient absorption, we observed an ultrafast rise implying that crossing rate from (MLCT)-M-1 to this ligand-centered T-1/T-2 (3)-* state is faster than our temporal resolution (300 fs)(-1). Two components appear with time constants 0.7-1.5 ps and 15-47 ps and are assigned to vibrational cooling/solvation and geometry relaxation, respectively.