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

The synthesis and photophysical characterization of a series of (N,C2‘-(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,C2‘-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,C2‘-2-phenylpyridyl). The (tpy)2Ir(PPh2CH2)2BPh2 and [(tpy)2Ir(CN-t-Bu)2](CF3SO3) complexes
have been structurally characterized by X-ray crystallography. The Ir−Caryl bond lengths in (tpy)2Ir(CN-t-Bu)2+ (2.047(5) and 2.072(5) Å) and (tpy)2Ir(PPh2CH2)2BPh2 (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 (T1) 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 T1
state, which in turn leads to an increase in the emission energy. The increase in emission energy leads to a linear
decrease in ln(knr) (knr = nonradiative decay rate). Decreased 1MLCT character in the T1 state also increases the
Huang−Rhys factors in the emission spectra, decreases the extinction coefficient of the T1 transition, and consequently
decreases the radiative decay rates (kr). Overall, the luminescence quantum yields decline with increasing emission
energies. A linear dependence of the radiative decay rate (kr) 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|HSO|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.