Mike Scudder CHEM 7350 November 15, 2017

d6 Ru(II), Os(II) give long excited-state lifetimes, high luminescent efficiencies (increase likelihood of either an energy or electron-transfer process occurring prior to radiative/nonradiative relaxtion) d6 Ir(III) exhibits longer lifetimes in solution due to heavier SOC

Cyclometalated iridium complexes Ir emitters can harvest both the singlet and triplet electrically generated excitons. Strong phosphorescence shown even at room temperature because the strong SOC increases radiative decay rate Have generated intense interest due to excellent photophysical properties Rational design and selection of the cyclometalating ligand and ancillary ligand provide an opportunity to control the photophysical, electrochemical, and steric properties

Cyclometalated iridium complexes Dye-sensitized solar cells1 Absorption and emission spectra of (ppy)2Ir(acac)2 OLEDs LECs Current density (open squares) and luminance (triangles) vs time at 4V bias in an Ir(ppyPbu3)3:(PF6)33 Exhibit wide color tunability, high phosphorescence Φ Photophysical, electrochemical properties crucial for materials performance

Controlling emission wavelength in Ir complexes Ancillary ligands blueshift the absorption and emission energies by stabilizing HOMO, leaving LUMO unchanged Destabilization of the 1MLCT state results in a decreased 1MLCT character in the lowest excited state, becomes more ligand-localized Thompson et al. Inorganic Chemistry 2005 44 (6), 1713-1727

Controlling kr, knr in iridium complexes Excited-state lifetimes and quantum yields are used to calculate kr, knr kr: based on the amount of metal character in emitting excited state, energy gap between singlet and triplet states knr: intramolecular vibronic coupling and thermal accessibility of metal-centered states Thompson et al. Inorganic Chemistry 2005 44 (6), 1713-1727

Structural modifications of Ir complexes Potential (V vs Fc+/Fc) Current “Dynamic Properties” “Static Properties” knr kr 3A 1A* 1A ISC E phosphorescence hν-E fluorescence hν Both static and dynamic properties sensitive to complex architecture Motivation: Find design approach to allow variation of kr , knr independently of redox potential and emission maximum

Synthetic Approach: Tris-heteroleptic complexes Not emissive at room temp. To date, only C^N ligands with the same chemical core but different substitution patterns have been used to prepare tris-heteroleptic complexes. Ppy=2-phenylpyridine. Ppz=1-phenylpyrazole.

Cyclic voltammetry results (complexes 1-10) Eox (V) 0.43 0.73 0.58 Eox (V) 0.49 0.79 0.64 Eox (V) 0.45 0.75 0.61 0.60 CV: useful experimental techniques to estimate the energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the ground state of Ir complexes Pyridine-containing complexes: show quasi-reversible oxidation and reduction potentials Adding Fluorines to substituent increases oxidation potential by removing electron density from Ir(III) center

Cyclic voltammetry results (isomers) Eox (V) 0.73 0.72 0.76 Each diastereomer contains at least one pyridine group: quasi-reversible redox Replacing acac with pic: shifts both potentials by 150 mV

UV-Vis results Ppz ligand: not emissive at room temperature Direct population of 3A LC 1ππ* Ppz ligand: not emissive at room temperature Replacing ppy with ppz has negligible effect on redox potentials, emission spectra Significant impact on kr, knr MLCT

UV-Vis results Emission maxima dictated by the overall number of fluorine substituents on the complexes Complex 5: Not emissive at room temperature

UV-Vis results Placement of F on ligand: no effect on redox/emission maxima Significant effect on kr, knr

UV-Vis results: tris-heteroleptic isomers Unlike acac, pic is asymmetric and nonchromophoric Resulted in blueshift relative to acac complexes Isomers a and b: very similar absorption spectra Close similarity confirmed by theory

Tris-heteroleptic isomers: density differences Blue represents electron density, red represents hole density First excited state of both isomer has MLCT character, large HOMO-LUMO contribution T1: Combination of MLCT and LC character

Summary A series of bis, tris-heteroleptic cyclometalated iridium (III) complexes synthesized. Use of picolinate as ancillary ligand provided two pairs of diastereomers. Absorption, emission wavelengths and redox properties: controlled by overall structure of complexes. Radiative, non-radiative constants: controlled by specific placement of substituents. Demonstrated it is possible to vary kr, knr without significantly affecting other properties.