Synthesis, Characterization, and Physical Properties of Cyclometalated Iridium(III) Complexes with 2-Phenylthiophene or 2-Phenylfuran Ligands

Tris-cyclometalated Ir(III) complexes bearing (CS)-S-boolean AND or (CO)-O-boolean AND ligands have been synthesized and characterized, and their redox behavior, absorption spectra, and luminescence properties are reported. The four Ir((CS)-S-boolean AND)- and Ir((CO)-O-boolean AND)-type complexes studied here are mer-Ir(dfppz)(2)(ptp), mer-Ir(ppy)(2)(ptp), mer-Ir(dfppz)(2)(pf), and mer-Ir(ppy)(2)(pf) [dfppz = 1-(2,4-difluorophenyl)pyrazole; ppy = 2-phenylpyridine; ptp = 2-phenylthiophene; pf = 2-phenylfuran]. While no (CS)-S-boolean AND or (CO)-O-boolean AND cyclometalation took place using conventional synthetic methods, transmetalation of the chloro-bridged dimeric iridium complexes [(dfppz)(2)Ir(mu-Cl)(2)(dfppz)(2)] and [(ppy)(2)Ir(mu-Cl)(2)(ppy)(2)] with the organozincs of 2-phenylthiophene or 2-phenylfuran afforded the four anticipated complexes in good yields. X-ray structural analyses on complexes Ir(dfppz)(2)(ptp) and Ir(dfppz)(2)(pf) were established to confirm their molecular structures. The Ir-C (ptp) (2.121(13) angstrom) and Ir-C (pf) (2.08(3) angstrom) bonds are much longer than the other two Ir-C (,dfppz) bonds in their respective structures and are also markedly longer than the value of 2.051(2) angstrom reported for the Ir-C bond trans to a phenyl group in mer-Ir(ppz)(3), implying weaker binding of 2-phenylthiophene and 2-phenylfuran to Ir(III) compared to typical (CN)-N-boolean AND ligands. All of the complexes show reversible oxidation between 0.21 and 0.70 V, measured relative to an internal ferrocene reference. While no room-temperature emission was observed, the four Ir(III) complexes gave emissions upon lowering the temperature to 77 K. The broad, single-band emissions observed from the two ptp-based complexes are assigned to a metal-centered excited state, in accordance with the DFT calculations. The two pf-based complexes showed emissions with the shape of the emission profiles dependent upon the excitation wavelength. Spectral resolution was partially achieved by direct excitation into the lower energy state. The multiple emissions are discussed as originating mainly from two thermally nonequilibrated excited states: one metal-centered state and one pi-pi state localized on 2-phenylfuran.