Abstract
We recorded the UV-vis absorption, emission and excitation spectra for complexes fac-tris(1-(4-fluorophenyl)-3-methylimidazolin -2-ylidene-C,C2') iridium(III), fac-Ir(fpmi)3, and fac-tris(1-(2,4–fluorophenyl)-3-methyl imidazolin-2-ylidene-C,C2')iridium (III), fac-Ir(dfpmi)3 and their ligands 1-(4-fluorophenyl)-3-methyl-1H-imidazol-3-ium iodide (H2fpmi+I-), 1-(2,4- difluorophenyl)-3-methyl-1H-imidazol-3-ium iodide (H2dfpmi+I-), 1-(4- fluorophenyl)-3-methyl-1H-imidazol-3-ium hexafluorophosphate (H2fpmi+PF6-) and 1-(2,4-difluorophenyl)-3-methyl-1H-imidazol-3-ium hexafluorophosphate (H2dfpmi+PF6-). Using theoretical calculations-density functional theory (DFT) and time-dependent DFT (TD-DFT), we obtained their optimized structures and assigned the emission bands at 320-440 nm and 370-500 nm to 1π-π* and 3π-π* for the ligands and complexes, respectively. For these blue-emission complexes the lowest emission bands are from 3π-π* instead of the triplet metal-ligand charge transfer state. This explains the low photoemission quantum yield for these complexes. We also recorded the Raman and IR spectra. From comparing with the results of DFT calculations, we assigned the vibrational spectra for fac-Ir(fpmi)3, fac-Ir(dfpmi)3, fac-Ir(ppz)3 and FIrpic. We assigned six Ir-ligand stretching, ν(Ir-C) and ν(Ir-C:) to Raman lines 286/303 and 332 cm-1 for fac-Ir(fpmi)3, and 283/299 and 377 cm-1 for fac-Ir(dfpmi)3, ν(Ir-N) and ν(Ir-C) to 229/243 and 270/321 cm-1 for fac-Ir(ppz)3, and ν(Ir-Npic), ν(Ir-Nph), ν(Ir-O) and ν(Ir-C) to 267, 282, 304 and 321 cm-1 for FIrpic. For fac-Ir(dfpmi)3 the ν(Ir-C:) band is blue-shifted 45 cm-1 than that in fac-Ir(fpmi)3. For FIrpic ν(Ir-N) and ν(Ir-C) are also blue-shifted than those for fac-Ir(ppy)3 showing strong Ir-ligand bonding.