Synthetic control of excited-state properties in cyclometalated Ir(III) complexes using ancillary ligands

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)(2)(-) and (ppy)(2)Ir(NCS)(2)(-) (ppy = N,C-2 '-2-phenylpyridyl). The (tpy)(2)Ir(PPh2CH2)(2B)Ph-2 and [(tpy)(2)Ir(CN-t-BU)(2)](CF3SO3) 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) angstrom) and (tpy)(2)Ir(PPh2CH2)(2)BPh2 (2.047(9) and 2.057(9) angstrom) are longer than their counterparts in (tpy)(2)Ir(acac) (1.982(6) and 1.985(7) angstrom). 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-pi and Ir-d orbitals, while the lowest unoccupied molecular orbital is localized primarily on the pyridyl-g 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 ((LC)-L-3) on the cyclometalating ligand admixed with (MLCT)-M-1 (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 (MLCT)-M-1 state results in less (MLCT)-M-1 character mixed into the T, state, which in turn leads to an increase in the emission energy. The increase in emission energy leads to a linear decrease in In(k(nr)) (k(nr) = nonradiative decay rate). Decreased (MLCT)-M-1 character in the T, state also increases the Huang-Rhys factors in the emission spectra, decreases the extinction coefficient of the T, 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 (epsilon) on (1/Delta E)(2) has been demonstrated, where Delta E is the energy difference between the (MLCT)-M-1 and (LC)-L-3 transitions. A value of 200 cm(-1) for the spin-orbital coupling matrix element < LC vertical bar H-SO vertical bar(MLCT)-M-1 >) of the (tpy)(2)Ir(LL ') complexes can be deduced from this linear relationship. The (fppy)(2)vertical bar r(LL ') complexes with corresponding ancillary ligands display similar trends in excited-state properties.