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

Li, Jian and Djurovich, Peter I. and Alleyne, Bert D. and Yousufuddin, Muhammed and Ho, Nam N. and Thomas, J. Christopher and Peters, Jonas C. and Bau, Robert and Thompson, Mark E. (2005) Synthetic Control of Excited-State Properties in Cyclometalated Ir(III) Complexes Using Ancillary Ligands. Inorganic Chemistry, 44 (6). pp. 1713-1727. ISSN 0020-1669. https://resolver.caltech.edu/CaltechAUTHORS:20140729-091550683

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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.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1021/ic048599hDOIArticle
http://pubs.acs.org/doi/abs/10.1021/ic048599hPublisherArticle
http://pubs.acs.org/doi/suppl/10.1021/ic048599hRelated ItemSupporting Information
ORCID:
AuthorORCID
Peters, Jonas C.0000-0002-6610-4414
Additional Information:© 2005 American Chemical Society. Received October 7, 2004. Publication Date (Web): February 12, 2005. The authors thank the Universal Display Corporation, the Defense Advanced Research Projects Agency, and the National Science Foundation for their financial support.
Funders:
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Universal Display CorporationUNSPECIFIED
Defense Advanced Research Projects Agency (DARPA)UNSPECIFIED
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Issue or Number:6
Record Number:CaltechAUTHORS:20140729-091550683
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20140729-091550683
Official Citation:Synthetic Control of Excited-State Properties in Cyclometalated Ir(III) Complexes Using Ancillary Ligands Jian Li, Peter I. Djurovich, Bert D. Alleyne, Muhammed Yousufuddin, Nam N. Ho, J. Christopher Thomas, Jonas C. Peters, Robert Bau and, and Mark E. Thompson Inorganic Chemistry 2005 44 (6), 1713-1727
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:47551
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:29 Jul 2014 18:09
Last Modified:03 Oct 2019 06:54

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