The triplet state of organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs

Mononuclear Ir(III)-polyimine complexes show outstanding luminescence properties, i.e., high intensities, lifetimes in the μs time range, and emission wavelengths that can be tuned so as to cover a full range of visible colors, from blue to red. We discuss the approaches for the use of ligands that afford control on luminescence features. Emphasis is placed on subfamilies of cyclometalated complexes, whose recent enormous expansion is motivated by their potential for applications, including that as phosphorescent dopants in OLEDs fabrication. The interplay of the different excited states associated with the luminescence, usually of MLCT and/or LC nature, is examined and the possible detrimental role of MC levels toward the luminescence properties is outlined. Ir(III)-polyimine moieties can be incorporated within multicomponent arrays where they can play as photoactive and/or electroactive units in photoinduced energy and electron transfer processes. The field is reviewed with attention to the processes of light collection and conversion into chemical energy.

Photochemistry and Photophysics of Coordination Compounds: Iridium

Higher efficiency in the end-use of energy requires substantial progress in lighting concepts. All the technologies under development are based on solid-state electroluminescent materials and belong to the general area of solid-state lighting (SSL). The two main technologies being developed in SSL are light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs), but in recent years, light-emitting electrochemical cells (LECs) have emerged as an alternative option. The luminescent materials in LECs are either luminescent polymers together with ionic salts or ionic species, such as ionic transition-metal complexes (iTMCs). Cyclometalated complexes of IrIII are by far the most utilized class of iTMCs in LECs. Herein, we show how these complexes can be prepared and discuss their unique electronic, photophysical, and photochemical properties. Finally, the progress in the performance of iTMCs based LECs, in terms of turn-on time, stability, efficiency, and color is presented.

Luminescent ionic transition-metal complexes for light-emitting electrochemical cells.

This article presents general concepts that have guided important developments in our recent research progress regarding room-temperature phosphorescent dyes and their potential applications. We first elaborate the theoretical background for emissive metal complexes and the strategic design of the chelating C-linked 2-pyridylazolate ligands, followed by their feasibility in functionalization and modification in an aim to fine-tune the chemical and photophysical properties. Subsequently, incorporation of 2-pyridylazolate chromophores is illustrated in the synthesis of the highly emissive, charge-neutral Os, Ru, Ir, and Pt complexes. Insights into their photophysical properties are gained from spectroscopy, relaxation dynamics, and theoretical approaches, from which the lowest-lying excited states, competitive radiative decay, and radiationless processes are then analyzed in detail. In view of applications, their potentials for OLEDs have been evaluated. The results, in combination with the fundamental basis, give a conceptual design contributed to the future advances in the field of OLEDs.

Phosphorescent Dyes for Organic Light‐Emitting Diodes

Phosphorescent iridium(III) complexes are being widely explored for their utility in diverse photophysical applications. The performance of these materials in such roles depends heavily on their excited-state properties, which can be tuned through ligand and substituent effects. This concept article focuses on methods for synthetically tailoring the properties of bis-cyclometalated iridium(III) materials, and explores the factors governing the nature of their lowest excited state.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/chem.200600618

Synthetically Tailored Excited States: Phosphorescent, Cyclometalated Iridium(III) Complexes and Their Applications

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) A) and (tpy)2Ir(PPh2CH2)2BPh2 (2.047(9) and 2.057(9) A) are longer than their counterparts in (tpy)2Ir(acac) (1.982(6) and 1.985(7) A). 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...

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

Structurally similar but charge-differentiated platinum complexes have been prepared using the bidentate phosphine ligands [Ph_(2)B(CH_(2)PPh_(2))_(2)], ([Ph_(2)BP_(2)], [1]), Ph_(2)Si(CH_(2)PPh_(2))_(2), (Ph_(2)SiP_(2), 2), and H_(2)C(CH_(2)PPh_(2))_(2), (dppp, 3). The relative electronic impact of each ligand with respect to a coordinated metal center's electron-richness has been examined using comparative molybdenum and platinum model carbonyl and alkyl complexes. Complexes supported by anionic [1] are shown to be more electron-rich than those supported by 2 and 3. A study of the temperature and THF dependence of the rate of THF self-exchange between neutral, formally zwitterionic [Ph_(2)BP_(2)]Pt(Me)(THF) (13) and its cationic relative [(Ph_(2)SiP_(2))Pt(Me)(THF)][B(C_(6)F_(5))_(4)] (14) demonstrates that different exchange mechanisms are operative for the two systems. Whereas cationic 14 displays THF-dependent, associative THF exchange in benzene, the mechanism of THF exchange for neutral 13 appears to be a THF independent, ligand-assisted process involving an anchimeric, η3-binding mode of the [Ph_(2)BP_(2)] ligand. The methyl solvento species 13, 14, and [(dppp)Pt(Me)(THF)][B(C_(6)F_(5))_(4)] (15), each undergo a C−H bond activation reaction with benzene that generates their corresponding phenyl solvento complexes [Ph_(2)BP_(2)]Pt(Ph)(THF) (16), [(Ph_(2)SiP_(2))Pt(Ph)(THF)][B(C_(6)F_(5))_(4)] (17), and [(dppp)Pt(Ph)(THF)][B(C_(6)F_(5))_(4)] (18). Examination of the kinetics of each C−H bond activation process shows that neutral 13 reacts faster than both of the cations 14 and 15. The magnitude of the primary kinetic isotope effect measured for the neutral versus the cationic systems also differs markedly (k(C6H6)/k(C6D6):  13 = 1.26; 14 = 6.52; 15 6). THF inhibits the rate of the thermolysis reaction in all three cases. Extended thermolysis of 17 and 18 results in an aryl coupling process that produces the dicationic, biphenyl-bridged platinum dimers [{(Ph_(2)SiP_(2))Pt}2(μ-η3:η3-biphenyl)][B(C6F5)4]2 (19) and [{(dppp)Pt}2(μ-η^(3):η^(3)-biphenyl)][B(C_(6)F_(5))_(4)]_(2) (20). Extended thermolysis of neutral [Ph_(2)BP_(2)]Pt(Ph)(THF) (16) results primarily in a disproportionation into the complex molecular salt {[Ph_(2)BP_(2)]PtPh_(2)}-{[Ph_(2)BP_(2)]Pt(THF)_(2)}+. The bulky phosphine adducts [Ph_(2)BP_(2)]Pt(Me){P(C_(6)F_(5))_(3)} (25) and [(Ph_(2)SiP_(2))Pt(Me){P(C_(6)F_(5))_(3)}][B(C_(6)F_(5))_(4)] (29) also undergo thermolysis in benzene to produce their respective phenyl complexes, but at a much slower rate than for 13−15. Inspection of the methane byproducts from thermolysis of 13, 14, 15, 25, and 29 in benzene-d6 shows only CH_(4) and CH3D. Whereas CH_(3)D is the dominant byproduct for 14, 15, 25, and 29, CH_(4) is the dominant byproduct for 13. Solution NMR data obtained for 13, its 13C-labeled derivative [Ph_(2)BP_(2)]Pt(^(13)CH_(3))(THF) (13-^(13)CH_(3)), and its deuterium-labeled derivative [Ph_(2)B(CH_(2)P(C_(6)D5)_(2))_(2)]Pt(Me)(THF) (13-d20), establish that reversible [Ph_(2)BP_(2)]-metalation processes are operative in benzene solution. Comparison of the rate of first-order decay of 13 versus the decay of d_(20)-labeled 13-d_(20) in benzene-d_(6) affords k_(13)/k_(13-d20) ~ 3. The NMR data obtained for 13, 13-^(13)CH_3, and 13-d_20 suggest that ligand metalation processes involve both the diphenylborate and the arylphosphine positions of the [Ph_(2)BP_(2)] auxiliary. The former type leads to a moderately stable and spectroscopically detectable platinum(IV) intermediate. All of these data provide a mechanistic outline of the benzene solution chemistries for the zwitterionic and the cationic systems that highlights their key similarities and differences.

Zwitterionic and cationic bis(phosphine) platinum(II) complexes: structural, electronic, and mechanistic comparisons relevant to ligand exchange and benzene C-H activation processes.

Cationic, coordinatively unsaturated metal centers exhibit a
wide range of both stoichiometric and catalytic transformations.
Such species are frequently generated by methide abstraction with
a strong Lewis acid, or alternatively by protonation with an acid
whose conjugate base is noncoordinating or weakly coordinating.3
We are targeting charge neutral, zwitterionic complexes that
display reaction chemistry reminiscent of these reactive metal
centers. Of particular interest is the development of species that
exhibit transformations at X-H bonds, where an X-H bond refers
generally to a C-H or other robust σ bond. A conceptual diagram
illustrating this strategy is shown in Scheme 1.

Benzene C−H Activation at a Charge Neutral Zwitterionic Platinum(II) Complex

A series of divalent, monovalent, and zerovalent nickel complexes supported by the electron-releasing, monoanionic tris(phosphino)borate ligands [PhBP_3] and [PhBP^(iPr)_3] ([PhBP_3] = [PhB(CH_2PPh_2)_3]-, [PhBP^(iPr)_3] = [PhB(CH_2PiPr_2)_3]-) have been synthesized to explore fundamental aspects of their coordination chemistry. The pseudotetrahedral, divalent halide complexes [PhBP_3]NiCl (1), [PhBP_3]NiI (2), and [PhBP^(iPr)_3]NiCl (3) were prepared by the metalation of [PhBP_3]Tl or [PhBP^(iPr)_3]Tl with (Ph_3P)_2NiCl_2, NiI_2, and (DME)NiCl_2 (DME = 1,2-dimethoxyethane), respectively. Complex 1 is a versatile precursor to a series of complexes accessible via substitution reactions including [PhBP_3]Ni(N_3) (4), [PhBP_3]Ni(OSiPh_3) (5), [PhBP_3]Ni(O-p-tBu-Ph) (6), and [PhBP_3]Ni(S-p-tBu-Ph) (7). Complexes 2−5 and 7 have been characterized by X-ray diffraction (XRD) and are pseudotetrahedral monomers in the solid state. Complex 1 reacts readily with oxygen to form the four-electron-oxidation product, {[PhB(CH_2P(O)Ph_2)_2(CH_2PPh_2)]NiCl} (8A or 8B), which features a solid-state structure that is dependent on its method of crystallization. Chemical reduction of 1 using Na/Hg or other potential 1-electron reductants generates a product that arises from partial ligand degradation, [PhBP_3]Ni(η^2-CH_2PPh_2) (9). The more sterically hindered chloride 3 reacts with Li(dbabh) (Hdbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene) to provide the three-coordinate complex [κ^2-PhBP^(iPr)_3]Ni(dbabh) (11), also characterized by XRD. Chemical reduction of complex 1 in the presence of L-type donors produces the tetrahedral Ni(I) complexes [PhBP_3]Ni(PPh_3) (12) and [PhBP3]Ni(CNtBu) (13). Reduction of 3 following the addition of PMe_3 or tert-butyl isocyanide affords the Ni(I) complexes [PhBP^(iPr)_3]Ni(PMe_3) (14) and [PhBP^(iPr)_3]Ni(CN^tBu) (15), respectively. The reactivity of these [PhBP_3]Ni^IL and [PhBP^(iPr)_3]NiI^L complexes with respect to oxidative group transfer reactions from organic azides and diazoalkanes is discussed. The zerovalent nitrosyl complex [PhBP_3]Ni(NO) (16) is prepared by the reaction of 1 with excess NO or by treating 12 with stoichiometric NO. The anionic Ni(0) complexes [[κ^2-PhBP_3]Ni(CO)_2][^nBu_4N] (17) and [[κ^2-PhBP^(iPr)_3]Ni(CO)_2][ASN] (18) (ASN = 5-azoniaspiro[4.4]nonane) have been prepared by reacting [PhBP_3]Tl or [PhBP^(iPr)_3]Tl with (Ph_3P)_2Ni(CO)_2 in the presence of R_4NBr. The photolysis of 17 appears to generate a new species consistent with a zerovalent monocarbonyl complex which we tentatively assign as {[PhBP_3]Ni(CO)}{^nBu_4N}, although complete characterization of this complex has been difficult. Finally, theoretical DFT calculations are presented for the hypothetical low spin complexes [PhBP_3]Ni(N^tBu), [PhBP^(iPr)_3]Ni(N^tBu), [PhBP^(iPr)_3]Ni(NMe), and [PhBP^(iPr)_3]Ni(N) to consider what role electronic structure factors might play with respect to the relative stability of these species.

The Coordination Chemistry of “[BP_3]NiX” Platforms: Targeting Low-Valent Nickel Sources as Promising Candidates to L_3Ni=E and L_3Ni≡E Linkages

The reaction of dimethyldiaryltin reagents Me_(2)SnR_(2) (R ) Ph (1), p-MePh (2), m,m-Me_(2)Ph (3), p-tBuPh (4), p-MeOPh
(5), p-CF_(3)Ph (6)) with BCl_(3) provided a high-yielding, simple preparative route to the corresponding diarylchloroboranes R_(2)BCl (R ) Ph (10), p-MePh (11), m,m-Me2Ph (12), p-tBuPh (13), p-MeOPh (14), p-CF_(3)Ph (15)). In some cases, the desired diarylchloroborane was not formed from an appropriate tin reagent Me_(2)SnR_(2) (R ) o-MeOPh (7), o,o-(MeO)_(2)Ph (8), o-CF_(3)Ph (9)). The reaction of lithiated methyldiaryl- or methyldialkylphosphines with diarylchloroboranes or dialkylchloroboranes is discussed. Specifically, several new monoanionic bis(phosphino)borates
are detailed: [Ph_(2)B(CH_(2)PPh_(2))_(2)] (25); [(p-MePh)_(2)B(CH_(2)PPh_(2))_(2)] (26); [(p-^(t)BuPh)_(2)B(CH_(2)PPh_(2))_(2)] (27); [(p-MeOPh)_(2)B-
(CH_(2)PPh_(2))_(2)] (28); [(p-CF_(3)Ph)_(2)B(CH_(2)PPh_(2))_(2)] (29); [Cy_(2)B(CH_(2)PPh_(2))_(2)] (30); [Ph_(2)B(CH_(2)P{p-^(t)BuPh}_(2))_(2)] (31); [(p-MeOPh)_(2)B- (CH_(2)P{p-^(t)BuPh}_(2))_(2)] (32); [Ph_(2)B(CH_(2)P{p-CF_(3)Ph}_(2))_(2)] (33); [Ph_(2)B(CH_(2)P(BH_(3))(Me)_(2))_(2)] (34); [Ph_(2)B(CH_(2)P(S)(Me)_(2))_(2)] (35); [Ph_(2)B(CH_(2)PiPr_(2))_(2)] (36); [Ph_(2)B(CH_(2)P^(t)Bu_(2))_(2)] (37); [(m,m-Me_(2)Ph)_(2)B(CH_(2)P^(t)Bu_(2))_(2)] (38). The chelation of diarylphosphine derivatives 25-33 and 36 to platinum was examined by generation of a series of platinum dimethyl complexes. The electronic effects of substituted bis(phosphino)borates on the carbonyl stretching frequency of neutral platinum alkyl carbonyl complexes were studied by infrared spectroscopy. Substituents remote from the metal center (i.e. on boron) have minimal effect on the electronic nature of the metal center, whereas substitution close to the metal center (on phosphorus) has a greater effect on the electronic nature of the metal center.

http://authors.library.caltech.edu/47396/7/ic034150xsi20030212_062804_si.pdf

J. Christopher Thomas

Papers

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

Zwitterionic and cationic bis(phosphine) platinum(II) complexes: structural, electronic, and mechanistic comparisons relevant to ligand exchange and benzene C-H activation processes.

Benzene C−H Activation at a Charge Neutral Zwitterionic Platinum(II) Complex

The Coordination Chemistry of “[BP_3]NiX” Platforms: Targeting Low-Valent Nickel Sources as Promising Candidates to L_3Ni=E and L_3Ni≡E Linkages

Bis(phosphino)borates: A New Family of Monoanionic Chelating Phosphine Ligands