The synthesis, structures, electrochemistry, and photophysics of a series of facial (fac) and meridional (mer) tris-cyclometalated Ir(III) complexes are reported. The complexes have the general formula Ir(C'N)(3) [where C'N is a monoanionic cyclometalating ligand; 2-phenylpyridyl (ppy), 2-(p-tolyl)pyridyl (tpy), 2-(4,6-difluorophenyl)pyridyl (46dfppy), 1-phenylpyrazolyl (ppz), 1-(4,6-difluorophenyl)pyrazolyl (46dfppz), or 1-(4-trifluoromethylphenyl)pyrazolyl (tfmppz)]. Reaction of the dichloro-bridged dimers [(C'N(2)Ir(mu-Cl)(2)Ir(C'N)(2)] with 2 equiv of HC( wedge )N at 140-150 degrees C forms the corresponding meridional isomer, while higher reaction temperatures give predominantly the facial isomer. Both facial and meridional isomers can be obtained in good yield (>70%). The meridional isomer of Ir(tpy)(3) and facial and meridional isomers of Ir(ppz)(3) and Ir(tfmppz)(3) have been structurally characterized using X-ray crystallography. The facial isomers have near identical bond lengths (av Ir-C = 2.018 A, av Ir-N = 2.123 A) and angles. The three meridional isomers have the expected bond length alternations for the differing trans influences of phenyl and pyridyl/pyrazolyl ligands. Bonds that are trans to phenyl groups are longer (Ir-C av = 2.071 A, Ir-N av = 2.031 A) than when they are trans to heterocyclic groups. The Ir-C and Ir-N bonds with trans N and C, respectively, have bond lengths very similar to those observed for the corresponding facial isomers. DFT calculations of both the singlet (ground) and the triplet states of the compounds suggest that the HOMO levels are a mixture of Ir and ligand orbitals, while the LUMO is predominantly ligand-based. All of the complexes show reversible oxidation between 0.3 and 0.8 V, versus Fc/Fc(+). The meridional isomers are easier to oxidize by ca. 50-100 mV. The phenylpyridyl-based complexes have reduction potentials between -2.5 and -2.8 V, whereas the phenylpyrazolyl-based complexes exhibit no reduction up to the solvent limit of -3.0 V. All of the compounds have intense absorption bands in the UV region assigned into (1)(pi --> pi) transitions and weaker MLCT (metal-to-ligand charge transfer) transitions that extend to the visible region. The MLCT transitions of the pyrazolyl-based complexes are hypsochromically shifted relative to those of the pyridyl-based compounds. The phenylpyridyl-based Ir(III) tris-cyclometalates exhibit intense emission both at room temperature and at 77 K, whereas the phenylpyrazolyl-based derivatives emit strongly only at 77 K. The emission energies and lifetimes of the phenylpyridyl-based complexes (450-550 nm, 2-6 micros) and phenylpyrazolyl-based compounds (390-440 nm, 14-33 micros) are characteristic for a mixed ligand-centered/MLCT excited state. The meridional isomers for both pyridyl and pyrazolyl-based cyclometalates show markedly different spectroscopic properties than do the facial forms. Isolated samples of mer-Ir(C( wedge )N)(3) complexes can be thermally and photochemically converted to facial forms, indicating that the me...

The synthesis, electrochemistry, and photophysics of a series of square planar Pt(II) complexes are reported. The complexes have the general structure C(wedge)NPt(O(wedge)O),where C(wedge)N is a monoanionic cyclometalating ligand (e.g., 2-phenylpyridyl, 2-(2'-thienyl)pyridyl, 2-(4,6-difluorophenyl)pyridyl, etc.) and O(wedge)O is a beta-diketonato ligand. Reaction of K(2)PtCl(4) with a HC(wedge)N ligand precursor forms the chloride-bridged dimer, C(wedge)NPt(mu-Cl)(2)PtC(wedge)N, which is cleaved with beta-diketones such as acetyl acetone (acacH) and dipivaloylmethane (dpmH) to give the corresponding monomeric C(wedge)NPt(O(wedge)O) complex. The thpyPt(dpm) (thpy = 2-(2'-thienyl)pyridyl) complex has been characterized using X-ray crystallography. The bond lengths and angles for this complex are similar to those of related cyclometalated Pt complexes. There are two independent molecular dimers in the asymmetric unit, with intermolecular spacings of 3.45 and 3.56 A, consistent with moderate pi-pi interactions and no evident Pt-Pt interactions. Most of the C(wedge)NPt(O(wedge)O) complexes display a single reversible reduction wave between -1.9 and -2.6 V (vs Cp(2)Fe/Cp(2)Fe(+)), assigned to largely C(wedge)N ligand based reduction, and an irreversible oxidation, assigned to predominantly Pt based oxidation. DFT calculations were carried out on both the ground (singlet) and excited (triplet) states of these complexes. The HOMO levels are a mixture of Pt and ligand orbitals, while the LUMO is predominantly C(wedge)N ligand based. The emission characteristics of these complexes are governed by the nature of the organometallic cyclometalating ligand allowing the emission to be tuned throughout the visible spectrum. Twenty-three different C(wedge)N ligands have been examined, which gave emission lambda(max) values ranging from 456 to 600 nm. Well-resolved vibronic fine structure is observed in all of the emission spectra (room temperature and 77 K). Strong spin-orbit coupling of the platinum atom allows for the formally forbidden mixing of the (1)MLCT with the (3)MCLT and (3)pi-pi states. This mixing leads to high emission quantum efficiencies (0.02-0.25) and lifetimes on the order of microseconds for the platinum complexes.

The preparation, photophysics, and solid state structures of octahedral organometallic Ir complexes with several different cyclometalated ligands are reported. IrCl3.nH2O cleanly cyclometalates a number of different compounds (i.e., 2-phenylpyridine, 2-(p-tolyl)pyridine, benzoquinoline, 2-phenylbenzothiazole, 2-(1-naphthyl)benzothiazole, and 2-phenylquinoline), forming the corresponding chloride-bridged dimers, CwedgeN2Ir(mu-Cl)2IrCwedgeN2 (CwedgeNis a cyclometalated ligand) in good yield. These chloride-bridged dimers react with acetyl acetone (acacH) and other bidentate, monoanionic ligands such as picolinic acid (picH) and N-methylsalicylimine (salH), to give monomeric CwedgeN2Ir(LX) complexes (LX = acac, pic, sal). The emission spectra of these complexes are largely governed by the nature of the cyclometalating ligand, leading to lambda(max) values from 510 to 606 nm for the complexes reported here. The strong spin-orbit coupling of iridium mixes the formally forbidden 3MLCT and 3pi-pi* transitions with the allowed 1MLCT, leading to a strong phosphorescence with good quantum efficiencies (0.1-0.4) and room temperature lifetimes in the microsecond regime. The emission spectra of the CwedgeN2Ir(LX) complexes are surprisingly similar to the fac-IrCwedgeN3 complex of the same ligand, even though the structures of the two complexes are markedly different. The crystal structures of two of the CwedgeN2Ir(acac) complexes (i.e., CwedgeN = ppy and tpy) have been determined. Both complexes show cis-C,C', trans-N,N' disposition of the two cyclometalated ligands, similar to the structures reported for other complexes with a "CwedgeN2Ir" fragment. NMR data (1H and 13C) support a similar structure for all of the CwedgeN2Ir(LX) complexes. Close intermolecular contacts in both (ppy)2Ir(acac) and (tpy)2Ir(acac) lead to significantly red shifted emission spectra for crystalline samples of the ppy and tpy complexes relative to their solution spectra.

The synthesis and photophysical characterization of a series of (N,C 2′ -(2-para-tolylpyridyl)) 2 Ir(LL′) [(tpy) 2 Ir(LL′)] (LL′ ) 2,4-pentanedionato (acac), bis(pyrazolyl)borate ligands and their analogues, diphosphine chelates and tertbutylisocyanide (CN-t-Bu)) are reported. A smaller series of [(dfppy) 2 Ir(LL′)] (dfppy ) N,C 2′ -2-(4′,6′-difluorophenyl)pyridyl) complexes were also examined along with two previously reported compounds, (ppy) 2 Ir(CN) 2and (ppy) 2 Ir(NCS) 2 -(ppy ) N,C 2′ -2-phenylpyridyl). The (tpy) 2 Ir(PPh 2 CH 2 ) 2 BPh 2 and [(tpy) 2 Ir(CN-t-Bu) 2 ](CF 3 SO 3 ) complexes have been structurally characterized by X-ray crystallography. The Ir−C aryl bond lengths in (tpy) 2 Ir(CN-t-Bu) 2 + (2.047-(5) and 2.072(5) Å) and (tpy) 2 Ir(PPh 2 CH 2 ) 2 BPh 2 (2.047(9) and 2.057(9) Å) are longer than their counterparts in (tpy) 2 Ir(acac) (1.982(6) and 1.985(7) Å). Density functional theory calculations carried out on (ppy) 2 Ir(CN-Me) 2 + show that the highest occupied molecular orbital (HOMO) consists of a mixture of phenyl-π and Ir-d orbitals, while the lowest unoccupied molecular orbital is localized primarily on the pyridyl-π orbitals. Electrochemical analysis of the (tpy) 2 Ir(LL′) complexes shows that the reduction potentials are largely unaffected by variation in the ancillary ligand, whereas the oxidation potentials vary over a much wider range (as much as 400 mV between two different LL′ ligands). Spectroscopic analysis of the cyclometalated Ir complexes reveals that the lowest energy excited state (T 1 ) is a triplet ligand-centered state ( 3 LC) on the cyclometalating ligand admixed with 1 MLCT (MLCT ) metal-to-ligand charge-transfer) character. The different ancillary ligands alter the 1 MLCT state energy mainly by changing the HOMO energy. Destabilization of the 1 MLCT state results in less 1 MLCT character mixed into the T 1 state, which in turn leads to an increase in the emission energy. The increase in emission energy leads to a linear decrease in ln(k nr ) (k nr ) nonradiative decay rate). Decreased 1 MLCT character in the T 1 state also increases the Huang−Rhys factors in the emission spectra, decreases the extinction coefficient of the T 1 transition, and consequently decreases the radiative decay rates (k r ). Overall, the luminescence quantum yields decline with increasing emission energies. A linear dependence of the radiative decay rate (k r ) or extinction coefficient ( ) on (1/∆E) 2 has been demonstrated, where ∆E is the energy difference between the 1 MLCT and 3 LC transitions. A value of 200 cm -1 for the spin−orbital coupling matrix element 〈 3 LC|H SO | 1 MLCT〉 of the (tpy) 2 Ir(LL′) complexes can be deduced from this linear relationship. The (fppy) 2 Ir(LL′) complexes with corresponding ancillary ligands display similar trends in excited-state properties.