Excited-State Switching between Ligand-Centered and Charge Transfer Modulated by Metal-Carbon Bonds in Cyclopentadienyl Iridium Complexes.

Three series of pentamethylcyclopentadienyl (Cp*) Ir(III) complexes with different bidentate ligands were synthesized and structurally characterized, [Cp*Ir(tpy)L] n+ (tpy = 2-tolylpyridinato; n = 0 or 1), [Cp*Ir(piq)L] n+ (piq = 1-phenylisoquinolinato; n = 0 or 1), and [Cp*Ir(bpy)L] m+ (bpy = 2,2'-bipyridine; m = 1 or 2), featuring a range of monodentate carbon-donor ligands within each series [L = 2,6-dimethylphenylisocyanide; 3,5-dimethylimidazol-2-ylidene (NHC); methyl)]. The spectroscopic and photophysical properties of these molecules and those of the photocatalyst [Cp*Ir(bpy)H]+ were examined to establish electronic structure-photophysical property relationships that engender productive photochemical reactivity of this hydride and its methyl analogue. The Ir(III) chromophores containing ancillary CNAr ligands exhibited features anticipated for predominantly ligand-centered (LC) excited states, and analogues bearing the NHC ancillary exhibited properties consistent with LC excited states containing a small admixture of metal-to-ligand charge-transfer (MLCT) character. However, the molecules featuring anionic and strongly σ-donating methyl or hydride ligands exhibited photophysical properties consistent with a high degree of CT character. Density functional theory calculations suggest that the lowest energy triplet states in these complexes are composed of a mixture of MLCT and ligand-to-ligand CT originating from both the Cp* and methyl or hydride ancillary ligands. The high degree of CT character in the triplet excited states of methyliridium complexes bearing C^N-cyclometalated ligands offer a striking contrast to the photophysical properties of pseudo-octahedral structures fac-Ir(C^N)3 or Ir(C^N)2(acac) that have lowest-energy triplet excited states characterized as primarily LC character with a more moderate MLCT admixture.

Temperature dependence of photophysical properties of a dinuclear C

The temperature dependence (1.7 K < T < 100 K) of emission decay is reported for the first time for a type of di-nuclear Pt complex featuring a metal-metal-to-ligand charge transfer (MMLCT) lowest energy transition that arises from a strong Pt-Pt interaction. The effect of local variation of the host/guest cage in a polymer matrix upon the phosphorescence decay time constants is characterized by the Kohlrausch-Williams-Watts function. The temperature dependence of the average decay time constants is fit by a Boltzmann-type expression to obtain the average zero-field splittings and individual sublevel decay rates of the photoluminescent triplet excited state.

Bidirectional "ping-pong" energy transfer and 3000-fold lifetime enhancement in a Re(I) charge transfer complex.

The synthesis and photophysics of a new Re(I)-carbonyl diimine complex, Re(PNI-phen)(CO)(3)Cl, where the PNI-phen is N-(1,10-phenanthroline)-4-(1-piperidinyl)naphthalene-1,8-dicarboximide is reported. The metal-to-ligand charge transfer (MLCT) emission lifetime was increased approximately 3000-fold at room temperature with respect to that of the model complex [Re(phen)(CO)(3)Cl] as a result of thermal equilibrium between the emissive (3)MLCT state and a long-lived triplet ligand-centered ((3)LC) state on the PNI chromophore. This represents the longest excited state lifetime (τ = 651 µs) that has ever been observed for a Re(I)-based CT photoluminescence at room temperature. The energy transfer processes and the associated rate constants leading to the establishment of the excited state equilibrium were elucidated by a powerful combination of three techniques (transient visible and infrared (IR) absorption and photoluminescence), each applied from ultrafast to the micro/milliseconds time scale. The MLCT excited state was monitored by transient IR using CO vibrations through time intervals where the corresponding signals obtained in conventional visible transient absorption were completely obscured by overlap with strong transients originating from the pendant PNI chromophore. Following initial excitation of the (1)LC state on the PNI chromophore, energy is transferred to form the MLCT state with a time constant of 45 ps, a value confirmed in all three measurement domains within experimental error. Although transient spectroscopy confirms the production of the (3)MLCT state on ultrafast time scales, Förster resonance energy transfer calculations using the spectral properties of the two chromophores support initial singlet transfer from (1)PNI* to produce the (1)MLCT state by the agreement with the experimentally observed energy transfer time constant and efficiency. Intersystem crossing from the (1)MLCT to the (3)MLCT excited state is believed to be extremely fast and was not resolved with the current experiments. Finally, triplet energy was transferred from the (3)MLCT to the PNI-centered (3)LC state in less than 15 ns, ultimately achieving equilibrium between the two excited states. Subsequent relaxation to the ground state occurred via emission resulting from thermal population of the (3)MLCT state with a resultant lifetime of 651 µs. The title chromophore represents an interesting example of "ping-pong" energy transfer wherein photon excitation first migrates away from the initially prepared (1)PNI* excited state and then ultimately returns to this moiety as a long-lived excited triplet which disposes of its energy by equilibrating with the photoluminescent Re(I) MLCT excited state.

Photophysical properties of the series fac- and mer-(1-phenylisoquinolinato-N∧C2')(x)(2-phenylpyridinato-N∧C2')(3-x)iridium(III) (x = 1-3).

The photophysical properties of tris-cyclometalated iridium(III) complexes have been probed by chemical and geometric variation through the series fac- and mer-Ir(piq)(x)(ppy)(3-x) (x = 1-3; piq = 1-phenylisoquinolinato-N(∧)C(2'), ppy = 2-phenylpyridinato-N(∧)C(2')). The phosphorescent decays were recorded in solution at 295 K and in polymer films from 2 to 295 K. In the heteroleptic complexes, emission occurs based solely on the piq ligand(s), at least by the nanosecond time scale, as its excited states are the lowest energy. Because fac-Ir(piq)(3) and fac-Ir(ppy)(3) possess practically the same oxidation potential, comparison of photophysical properties through the series fac-Ir(piq)(x)(ppy)(3-x) (x = 1-3) revealed the effects of having one, two, or three emissive piq ligands with no confounding effects from differences in electron withdrawing or donating properties between the spectator ppy ligands and the piq ligands. Effects of placement of piq ligands in different coordination geometries were elucidated by comparisons to the mer series.

E-type delayed fluorescence of a phosphine-supported Cu2(mu-NAr2)2 diamond core: harvesting singlet and triplet excitons in OLEDs.

A highly emissive bis(phosphine)diarylamido dinuclear copper(I) complex (quantum yield = 57%) was shown to exhibit E-type delayed fluorescence by variable temperature emission spectroscopy and photoluminescence decay measurement of doped vapor-deposited films. The lowest energy singlet and triplet excited states were assigned as charge transfer states on the basis of theoretical calculations and the small observed S(1)-T(1) energy gap. Vapor-deposited OLEDs doped with the complex in the emissive layer gave a maximum external quantum efficiency of 16.1%, demonstrating that triplet excitons can be harvested very efficiently through the delayed fluorescence channel. The function of the emissive dopant in OLEDs was further probed by several physical methods, including electrically detected EPR, cyclic voltammetry, and photoluminescence in the presence of applied current.

Highly luminescent tetradentate bis-cyclometalated platinum complexes: design, synthesis, structure, photophysics, and electroluminescence application.

N,N-Di(6-phenylpyridin-2-yl)aniline (L1), N,N-di(6-(2,4-difluorophenyl)pyridin-2-yl)aniline (L2), N,N-di(3-(pyridin-2-yl)phenyl)aniline (L3), N,N-di(3-(1H-pyrazol-1-yl)phenyl)aniline (L4), N,N-di(3-(3-methyl-1H-pyrazol-1-yl)phenyl)aniline (L5), and N,N-di(3-(4-methyl-1H-pyrazol-1-yl)phenyl)aniline (L6) undergo cyclometalation to produce two types of tetradentate bis-cyclometalated platinum(II) complexes: C--N*N(wedge)C platinum complexes 1 and 2 and N--C*C--N platinum complexes 3-6, respectively, where an "X--Y" (X, Y = C or N) denotes a bidentate coordination to the platinum to form a five-membered metallacycle and "X*Y" denotes a coordination to form a six-membered metallacycle. The crystal structures of 1, 3, and 5 were determined by the single-crystal X-ray diffraction analysis, showing distorted square-planar geometry, that is, two C--N coordination moieties are twisted. Complex 5 showed much greater distortion with largest deviation of 0.193 A from the mean NCCNPt coordination plane, which is attributed to the steric interaction between the two 3-methyl groups on the pyrazolyl rings. Density functional theory (DFT) calculations were carried out on the ground states of 1 and 3-6. The optimized geometries are consistent with the crystal structures. The highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of the molecules displayed a localized characteristic with the contribution (18-45%) of the platinum metal to the HOMOs. All complexes are emissive at ambient temperature in fluid with quantum yields of 0.14 to 0.76 in 2-methyltetrahydrofuran. The emission of the complexes covers from blue to red region with lambda(max) ranging from 474 to 613 nm. Excimer emission was observed for 1 and 2 at high concentration of the complexes. The emission lifetime at infinite dilution for 1 and 2 was determined to be 7.8 and 11.4 micros, respectively. Concentration quenching was observed for 3 and 4, but the excimer emission was not observed. The life times for 3-6 were determined to be in the range of micro seconds, but those of 4-6 (3.4-5.7 micros) were somewhat shorter than that of 3 (7.6 micros). The highly structured emission spectra, long life times, and DFT calculations suggested that the emissive state is primarily a (3)LC state with metal-to-ligand charge-transfer (MLCT) admixture. The ZFS of 23 cm(-1) for the emissive triplet state was observed directly by high resolution spectroscopy for 1 in a Shpol'skii matrix, which also suggested an emission from a triplet ligand centered ((3)LC) state with admixture of MLCT character. Complex 1 was incorporated into an organic light-emitting diode (OLED) device as an emitter at 4 wt % in the mixed host of 4,4',4''-tris(N-carbazolyl)triphenylamine (TCTA) and 2,2',2''-(1,3,5-benzenetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBI) and demonstrated excellent performance with maximum external quantum efficiency of 14.7% at the current density of 0.01 mA/cm(-1).

Acridinone/amine(carbazole)-based bipolar molecules: efficient hosts for fluorescent and phosphorescent emitters.

Three acridinone-based molecules ADBP, ACBP, and DABP were synthesized, and their application to the OLED devices was investigated. When used as the host for either the deep blue singlet or the green triplet emitter in OLED devices, the bipolar molecules ADBP and ACBP demonstrated superior performance compared to either DABP or commonly used host CBP, remarkably lowering the drive voltage and improving efficiencies.

Narrow-line and broadband spectra of iridium(III) complexes in a Shpol'skii matrix and an amorphous host.

Narrow-line emission and excitation spectra of the lowest triplet state have been studied in a Shpol'skii host (n-octane) for three cyclometalated IrL(2)acac complexes of current practical interest. The ligands L and acac are the C-deprotonated anions of 2-phenylpyridine, 2-benzothienylpyridine, 1-phenylisoquinoline, and acetylacetone. The occurrence of narrow lines is surprising for such nonplanar structures. Individual triplet sublevels are identified, and a striking intensity effect is observed in a magnetic field. Broadband spectra and lifetime data of the complexes in PMMA yield additional properties of the individual sublevels-radiative and nonradiative decay rates and emission spectra. The data are consistent with the assumption that these individual rates and spectra are approximately independent of temperature. For each complex, the decay rates vary by a factor of > or =20 among the three sublevels, and that of the lowest sublevel is the smallest, but rates of radiative and nonradiative decay generally vary in parallel for all three sublevels. The individual sublevels have significantly different spectra. These regularities appear to be universal characteristics of Ir(III) and Pt(II) complexes. They cannot be explained solely on the basis of the symmetries of the electronic states because the symmetry of the present complexes is too low.

Highly efficient, selective, and general method for the preparation of meridional homo- and heteroleptic tris-cyclometalated iridium complexes.

A highly efficient and general method based on transmetalation with an organozinc reagent is developed for selective preparation of homo- and heteroleptic meridional tris-cyclometalated iridium complexes. The molecular structure of mer-Ir(1-piq)2(ppy) (2) has been determined by a single-crystal X-ray diffraction analysis. The emission properties of a series of meridional complexes are reported.