Synthetic Control of Excited-State Properties in Cyclometalated Ir(III) Complexes Using Ancillary Ligands

Abstract

The synthesis and photophysical characterization of a series of (N,C^2'-(2-para-tolylpyridyl))_2Ir(LL') [(tpy)_2Ir(LL')] (LL' = 2,4-pentanedionato (acac), bis(pyrazolyl)borate ligands and their analogues, diphosphine chelates and tert-butylisocyanide (CN-t-Bu)) are reported. A smaller series of [(dfppy)_2Ir(LL')] (dfppy = N,C^2'-2-(4',6'-difluorophenyl)pyridyl) complexes were also examined along with two previously reported compounds, (ppy)_2Ir(CN)_2- and (ppy)_2Ir(NCS)_2- (ppy = N,C^2'-2-phenylpyridyl). The (tpy)_2Ir(PPh_2CH_2)_2BPh_2 and [(tpy)_2Ir(CN-t-Bu)_2](CF_3SO_3) complexes have been structurally characterized by X-ray crystallography. The Ir−C_(aryl) bond lengths in (tpy)_2Ir(CN-t-Bu)_2^+ (2.047(5) and 2.072(5) Å) and (tpy)_2Ir(PPh_2CH_2)_2BPh_2 (2.047(9) and 2.057(9) Å) are longer than their counterparts in (tpy)_2Ir(acac) (1.982(6) and 1.985(7) Å). Density functional theory calculations carried out on (ppy)_2Ir(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)_2Ir(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 (^3LC) on the cyclometalating ligand admixed with ^1MLCT (MLCT = metal-to-ligand charge-transfer) character. The different ancillary ligands alter the ^1MLCT state energy mainly by changing the HOMO energy. Destabilization of the ^1MLCT state results in less ^1MLCT 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 ^1MLCT 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 (ε) on (1/ΔE)^2 has been demonstrated, where ΔE is the energy difference between the ^1MLCT and ^3LC transitions. A value of 200 cm^(-1) for the spin−orbital coupling matrix element ‹^3LC|H_(SO)|^1MLCT› of the (tpy)_2Ir(LL') complexes can be deduced from this linear relationship. The (fppy)_2Ir(LL') complexes with corresponding ancillary ligands display similar trends in excited-state properties.

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