Gene-Hsiang Lee

Systematic Investigation of the Metal-Structure–Photophysics Relationship of Emissive d10-Complexes of Group 11 Elements: The Prospect of Application in Organic Light Emitting Devices

A series of new emissive group 11 transition metal d(10)-complexes 1-8 bearing functionalized 2-pyridyl pyrrolide together with phosphine ancillary such as bis[2-(diphenylphosphino)phenyl] ether (POP) or PPh(3) are reported. The titled complexes are categorized into three classes, i.e. Cu(I) complexes (1-3), Ag(I) complexes (4 and 5), and Au(I) metal complexes (6-8). Via combination of experimental and theoretical approaches, the group 11 d(10)-metal ions versus their structural variation, stability, and corresponding photophysical properties have been investigated in a systematic and comprehensive manner. The results conclude that, along the same family, how much a metal d-orbital is involved in the electronic transition plays a more important role than how heavy the metal atom is, i.e. the atomic number, in enhancing the spin-orbit coupling. The metal ions with and without involvement of a d orbital in the lowest lying electronic transition are thus classified into internal and external heavy atoms, respectively. Cu(I) complexes 1-3 show an appreciable metal d contribution (i.e., MLCT) in the lowest lying transition, so that Cu(I) acts as an internal heavy atom. Despite its smallest atomic number among group 11 elements, Cu(I) complexes 1-3 exhibit a substantially larger rate of intersystem crossing (ISC) and phosphorescence radiative decay rate constant (k(r)(p)) than those of Ag(I) (4 and 5) and Au(I) (6-8) complexes possessing pure π → π* character in the lowest transition. Since Ag(I) and Au(I) act only as external heavy atoms in the titled complexes, the spin-orbit coupling is mainly governed by the atomic number, such that complexes associated with the heavier Au(I) (6-8) show faster ISC and larger k(r)(p) than the Ag(I) complexes (4 and 5). This trend of correlation should be universal and has been firmly supported by experimental data in combination with empirical derivation. Along this line, Cu(I) complex 1 exhibits intensive phosphorescence (Φ(p) = 0.35 in solid state) and has been successfully utilized for fabrication of OLEDs, attaining peak EL efficiencies of 6.6%, 20.0 cd/A, and 14.9 lm/W for the forward directions.

Ruthenium(II) Sensitizers with Heteroleptic Tridentate Chelates for Dye‐Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) are a promising technology with the potential to harvest sunlight at low cost. Specifically, DSSCs based on either organometallic dyes or organic push-and-pull dyes adsorbed on nanocrystalline TiO2 photoanodes have attracted intensive research interest in the past two decades. The use of ruthenium(II)-based sensitizers is still the most attractive approach, because their photophysical and electrochemical properties can be systematically fine-tuned to achieve optimal material characteristics. Key examples of Ru sensitizers for DSSCs include the wellestablished N3 and N719 dyes and their functionalized derivatives. 6] These dyes are commonly assembled by incorporation of at least one 4,4’-dicarboxy-2,2’-bipyridine chelate (Scheme 1), on which the carboxy anchors allow efficient electron injection into the TiO2 nanoparticles. The accompanying thiocyanate ancillary ligands are pointed away from the TiO2 surface, thus providing intimate contact to the I /I3 redox couple in the electrolyte and subsequently triggering rapid regeneration of the oxidized sensitizer. The lower-lying absorption bands are attributed to metal-toligand charge transfer (MLCT) and display absorption thresholds close to 800 nm, unmatched by the majority of organic dyes known to date. For optimizing Ru sensitizers, it has been reported that, upon introduction of 4,4’,4’’-tricarboxy-2,2’:6,2’’-terpyridine (H3tctpy), the lowest energy transition could be extended toward the near-IR region, thus affording the panchromatic sensitizer known as N749, or black dye [nBu4N]3[Ru(Htctpy)(NCS)3]. [7] Although this design seems to show satisfactory sensitization up to 900 nm, it still possesses two major deficiencies. One is the inferior absorptivity in the visible region, attributed to the lack of effective auxochrome. The second is the presence of three thiocyanate ligands, which not only reduce the synthetic yield owing to coordination isomerism but also deteriorate the lifespan of the as-fabricated cell units, because the weak Ru NCS dative bonding causes notable dye decomposition during operation. In conjunction with current endeavors, we described one panchromatic dye PRT4, which possesses a styrene-substituted pyridyl pyrazolate (pypz) auxochrome. Its recorded solar-cell performance supersedes our best reference cells using N749, thus providing the first success for our research initiative. Herein we present a breakthrough design employing both the H3tctpy anchor and a newly designed tridentate ancillary ligand; the latter is also targeted for replacing the last remaining thiocyanate in our PRT4 dye as well as all three thiocyanates in N749 sensitizer. Thiophene derivatives or other auxochrome units were tethered to the central pyridyl group in an attempt to increase the light-harvesting capability. In this synthetic protocol (Scheme 2), the functionalized 2,6-bis(5-pyrazolyl)pyridine chelate was first prepared using a multistep process starting from chlorination of chelidamic acid, except for the synthesis of parent 2,6-bis(5-pyrazolyl)pyridine, which employed the commercially available 2,6diacetylpyridine. Bis-tridentate Ru complexes were then obtained from addition of the relevant 2,6-dipyrazolylpyridine derivative with source complex [Ru(tectpy)Cl3] in ethanol solution (tectpy = 4,4’,4’’-triethoxycarbonyl2,2’:6’,2’’-terpyridine). The crude ethoxycarbonyl Ru products were purified on a silica gel column and eluted with a mixture of hexane and ethyl acetate. To confirm their molecular structure, single-crystal X-ray analysis of one derivative (TF-2OEt; TF = thiocyanate-free) was carried Scheme 1. Ru sensitizers N3, N719, N749, and PRT4.

Remarkably Short Metal–Metal Bonds: A Lantern‐Type Quintuply Bonded Dichromium(I) Complex

The field of quadruply bonded dinuclear complexes in which two metal atoms are embraced by eight ligands has been considered mature. The bonding and electronic structures of these compounds have been well understood, ever since the discovery of the first dimetal species containing a quadruple bond, [Re2Cl8] 2 , over 40 years ago. The quest for thermally stable and isolable dinuclear complexes with higher bond orders is one aim of chemists in this field. From the synthetic point of view, quintuply bonded dinuclear complexes have become the focus in the past few years. On the theoretical side, the hypothetical RMMRmolecules (M= Cr, Mo, W, U; R=H, F, Cl, Br, CN, Me) have been highlighted, particularly the trans-bent structures. These feature a quintuple bond between two metal atoms, and for RCrCrR, the calculated Cr–Cr bond lengths are in the range of 1.64–1.78 4. Experimentally, a recent landmark advance was the isolation of a quintuply bonded dichromium(I) complex supported by two monodentate bulky carbyl ligands [Ar’CrCrAr’] (Ar’=C6H3-2,6-(C6H3-2,6iPr2)2), [12] in which, coincidentally, a trans-bent (C2h) geometry is adopted and the two chromium atoms share five electron pairs in five bonding molecular orbitals according to computational analyses. 14] Moreover, D2h-symmetric [Cr2(m-h L)2] ( L=N,N’-bis(2,6-diisopropylphenyl)-1,4-diazadiene) was recently reported to feature a very short Cr–Cr distance of 1.8028(9) 4 and calculated to display some degree of fivefold bonding. We have been interested in the pursuit of low-coordinate and multiply bonded dinuclear complexes since our first report on the unconventional quadruply bonded dimolybdenum complex [Mo2{m-h -(DippN)2SiMe2}2] (Dipp= 2,6iPrC6H3), in which each Mo atom is ligated by only two nitrogen atoms. Accordingly, we were interested in the preparation of multiply bonded dinuclear complexes supported by ancillary ligands which can minimize the metal– ligand p-bonding interaction and maximize the metal–metal interaction. Inspired by the hypothetical eclipsing molecule M2L6, proposed by Hoffmann et al., [17] in which M M could be a quintuple bond, we set out to explore the possibility of synthesizing such complexes. Here we report the use of amidinate ligand ArNC(H)NAr (Ar= 2,6-C6H3(CH3)2) to stabilize mixed-valent Cr2 3+ complex [Cr2(ArNC(H)NAr)3] (2) with formal Cr Cr bond order of 4.5 and its one-electron reduced Cr2 2+ species [Cr2(ArNC(H)NAr)3] (3 with formal Cr Cr bond order of 5.0). Both of these species have very short Cr Cr bonds; the bonding in these two compounds was studied theoretically. Reduction of dichromium bis(amidinate) dichlorido complex [{Cr(thf)}2(m-Cl)2{m-h -(ArNC(H)NAr)}2] (1), [18] in which the Cr–Cr distance is 2.612(1) 4, with KC8 gave redbrown mixed-valent dichromium tris(amidinate) compound 2 in 47% yield (Scheme 1). Compound 2 is paramagnetic, as

Quintuply‐Bonded Dichromium(I) Complexes Featuring Metal–Metal Bond Lengths of 1.74 Å

The construction of a metal–metal quintuple bond has long been a challenge for chemists, since a large number of quadruple-bonded dinuclear complexes have been reported and their bonding and electronic structures extensively investigated and well understood. On the basis of theoretical work, many possible structures could be capable of accommodating a metal–metal quintuple bond, in contrast to the strict requirement of having twometal atoms embraced by eight ligands in a tetragonal geometry for a metal–metal quadruple bond. Of particular interest is that all of these model structures display a common feature: a low-coordinate environment around metal centers. From a practical point of view, the first quintuple-bonded dichromium complex [Ar’CrCrAr’] (Ar’=C6H3-2,6-(C6H3-2,6-iPr2)2, Cr Cr= 1.8351(4) ), which adopts a trans-bent geometry, was reported by Power and co-workers in 2005. More recently, in 2007, Theopold and co-workers reported an interesting dichromium complex supported by a-diimines, [Cr2(m-h -{C(H)N(C6H3-2,6-iPr2)}2)2], which was shown by computations to exhibit some degree of Cr Cr quintuplebond character. Since our first report on the characterization of an unconventional quadruple-bonded dimolybdenum complex [Mo2{m-h -(DippN)2SiMe2}2], where each Mo atom is ligated by only two nitrogen donors, we have been interested in the pursuit of low-coordinate and multiply-bonded dinuclear complexes. We recently characterized a mixed-valent dichromium complex stabilized by three amidinate ligands, [Cr2{ArNC(H)NAr}3] (Ar = 2,6-C6H3(CH3)2), and its oneelectron reduction partner [Cr2{Ar NC(H)NAr}3] , which exhibited the shortest metal–metal bond length of 1.7397(9) . Qualitatively, the latter is believed to incorporate a Cr Cr quintuple bond. In view of Power s and Theopold s complexes, wherein both Cr centers were coordinated by two donor atoms, we set out to prepare dichromium bis(amidinato) complexes, from which metal–metal quintuple bonds are expected. Herein we report a series of complexes of the form [Cr2{m-h -ArNC(R)NAr}2], which all exhibit very short Cr Cr quintuple-bond lengths of approximately 1.74 . Amidinate ligands, featuring substituents of different bulk, are used to stabilize these Cr Cr quintuple bonds. Prior to the synthesis of the target molecules, four green mononuclear complexes, 1a–d, of the form [CrCl2(thf)2{h ArNC(R)NAr}] were prepared in good yields by treatment of [CrCl3(thf)3] or CrCl3 with lithiated amidines (Scheme 1, see

Iridium(III) Complexes of a Dicyclometalated Phosphite Tripod Ligand: Strategy to Achieve Blue Phosphorescence Without Fluorine Substituents and Fabrication of OLEDs†

Organic light-emitting diodes (OLEDs) based on heavy transition-metal complexes are playing a pivotal role in next generation of, for example, flat panel displays and solid-state lighting. The readily available, Os-, Pt-, and in particular Ir-based phosphorescence complexes grant superior advantage over fluorescent materials. This is mainly due to heavyatom-induced spin–orbit coupling, giving effective harvesting of both singlet and triplet excitons. However, tuning of phosphorescence over the entire visible spectrum still remains a challenge. Particularly, designing new materials to show higher energy, such as deep-blue emission—with an ideal CIEx,y coordinate (CIE = Commission Internationale de L Eclairage) of (0.14, 0.09)—encounters more obstacle than the progress made for obtaining green and red colors. Representative blue phosphors are a class of Ir complexes possessing at least one cyclometalated 4,6-difluorophenyl pyridine {(dfppy)H} ligand, known as FIrpic, FIr6, FIrtaz, and others. The majority of blue phosphors showed inferior color chromaticity with a sum of CIEx+y values being much greater than 0.3 or with single CIEy coordinate higher than 0.25. Such inferior chromaticity, in part, has been improved upon adoption of carbene-, triazolyl-, and fluorine-substituted bipyridine (dfpypy) based chelates. The above urgency prompted us to search for better and new blue phosphors. We produced a class of 2-pyridylazolate chelates possessing very large ligand-centered p–p* energy gap, as evidenced by the blue-emitting Os complexes. Subsequently, room-temperature blue phosphorescence was also visualized for the respective heteroleptic Ir complexes, particularly for those dubbed “nonconjugated” ancillary chelate(s). The nonconjugated ligands so far comprise a benzyl substituted pyrazole, an N-heterocyclic carbene, phosphines, and other ingenious molecular designs. Herein, we report the preparation of a novel class of heteroleptic Ir complexes by incorporation of tripodal, facially coordinated phosphite (or phosphonite), denoted as the P^C2 chelate, for serving as the ancillary, together with the employment of 2-pyridyltriazolate acting as blue chromophore. The reaction intermediate, which possesses an acetate chelate, was isolated and characterized to establish the synthetic pathway. The tridentate P^C2 ancillary chelate offers several advantages: 1) Good stabilization of complex and necessary long-term stability in application of for example, emitting devices. 2) The strong bonding of phosphorous donors is expected to destabilize the ligand field d–d excited state, thus minimizing its interference to the radiative process from the lower lying excited state. 3) P^C2 inherits profound and versatile functionality (see below) capable of fine-tuning the electronic character. As a result, highly efficient blue phosphorescence is attained with good OLED performance. Treatment of a mixture of [IrCl3(tht)3] (tht = tetrahydrothiophene) with an equimolar amount of triphenylphosphine (PPh3), triphenylphosphite {P(OPh)3}, and an excess of sodium acetate resulted in a high yield conversion (> 80%) into [Ir(P^C2)(PPh3)(OAc)] (1a); P^C2 = tripodal dicyclometalated phosphite (Scheme 1). Subsequent replacement of acetate in 1a with chelating 3-tert-butyl-5-(2-pyridyl)triazo-

Self‐Assembled Arrays of Single‐Walled Metal–Organic Nanotubes

Many recent advances in the field of metal–organic framework (MOF) materials have been reported, not only from the standpoint of the potential applications, ranging from gas storage to catalysis and drug delivery, but also because of their intriguing architectures and framework topologies. 2] Conceptually, endless structures can be produced by assembling judiciously selected molecular building blocks. Just as the notable saying in crystal engineering goes “the limits are mainly in our imagination”, any conceivable MOF might be obtained in the future, although it is all currently imagination. Since the first discovery of carbon nanotubes (CNTs) by Iijima in 1991, discrete hollow tubular structures such as various CNTs and other synthetic nanotubes (SNTs) prepared from inorganic, organic, or biological precursors have been successfully developed, because they possess useful functionalities and can serve as molecular capillaries, sieves, and biological models. In theory, the curling-up or rollingup mechanism of topology transformations from 2D flat sheets to 1D hollow tubes is achievable. Thanks to effective design and synthesis strategies, many porous MOFs with various interesting network topologies have been reported over the past decade. Compared with the focus on CNTs and SNTs, it is surprising that significantly less effort has been directed to the preparation of metal–organic nanotubes (MONTs). In particular, discrete MONT structures are extremely rare to date. As part of our ongoing efforts in the design and synthesis of functional crystalline materials, 8d, 10] we wish to report herein on a unique type of MOF of [{[Cd(apab)2(H2O)]3(MOH)·G}n] (MAS-21–23, M I = Cs, K, Na, respectively; for MAS-22, G = 18 H2O·6C2H5OH·3C4H8O; apab = 4-amino-3[(pyridin-4-ylmethylene)amino]benzoate; MAS = materials of Academia Sinica), all of which consist of a large singlewalled metal–organic nanotube of [{Cd(apab)2(H2O)}3n] (MONT-A1) with an exterior wall diameter of up to 3.2 nm and an interior channel diameter of 1.4 nm. These MONTs are held together by alkaline cations to form 3D nanotubular supramolecular arrays (Figure 1). To the best of our knowledge, a single-walled MONT with such a large diameter is unprecedented. Compounds MAS-21–23 were synthesized by reaction of cadmium perchlorate, 4-amino-3-[(pyridin-4-ylmethylene)amino]benzoic acid (Hapab), and MOH (M = Cs, K, and Na, respectively) in an EtOH/THF/H2O solvent diffusion system at 4 8C through a single-step, self-organization process (Scheme 1). The appropriate choice of an organic ligand with specific functional groups and geometry is a major factor in achieving these large nanotube-based structures. The multifunctional Schiff base ligand of Hapab was designed deliberately and possesses a bending angle of 1208 between the pyridyl and carboxylate groups. Unlike similar bananashaped organic linkers, the use of the apab scaffold favors the formation of a tubular structure, rather than a spherical network.

Two Linear Undecanickel Mixed-Valence Complexes: Increasing the Size and the Scope of the Electronic Properties of Nickel Metal Strings**

The importance of one-dimensional (1D) transition-metal complexes stems from their ability to provide a fundamental understanding of metal–metal interactions and electron transport along an extended metal-atom chain (EMAC), and from the perspective of taking advantage of their specific properties for potential applications, such as molecular metal wires and switches. A series of string complexes of oligo-apyridylamino ligands ranging from 3 to 9 core metal atoms has been synthesized and characterized by Cotton s group and our group. Attempts to characterize such very long EMACs with high electron conductivity were hindered by the synthetic difficulties rapidly increasing with the size of the metal chain. We synthesized [Ni9(m9-peptea)4Cl2] ten years ago, but all attempts to characterize a longer chain of Ni atoms have, to date, been unsuccessful, owing to very low yields and to the instability of the target compound, probably because of the high flexibility of large pyridylamino ligands. Recently we developed a new family of ligands by substituting rigid and potentially redox active naphthyridine (na) groups for the pyridine (py) rings. Naphthyridinemodulated ligands stabilize nickel ions in a low oxidation state, giving rise to mixed-valent [Ni2(napy)4] 3+ units (napy= naphthyridine). Using this strategy, we obtained a series of stable, low-oxidation-state-nickel string complexes combining mixed-valency, a property important in the development of novel electronic materials, with an enhanced electron mobility, which is able to increase the conductance of molecular metal wires. We report a new tetranaphthyridyltriamine ligand, N-(2(1,8-naphthyridin-7-ylamino)-1,8-naphthyridin-7-yl)-N-(1,8naphthy-ridin-2-yl)-1,8-naphthyridine-2,7-diamine (H3tentra) and two undecanickel complexes of the deprotonated tentra trianion, [Ni11(tentra)4Cl2](PF6)4 (1) and [Ni11(tentra)4(NCS)2](PF6)4 (2). The ligand H3tentra was synthesized on the basis of Buchwald s palladium-catalyzed procedures by the crosscoupling of bis(2-chloro-1,8-naphthyridin-7-yl)amine and 2amino-1,8-naphthyridine. Undecametallic complex [Ni11(tentra)4Cl2](PF6)4 (1) was obtained by the reaction of anhydrous NiCl2 with the H3tentra ligand in an argon atmosphere employing naphthalene as solvent and tBuOK as a base to deprotonate the amine groups. The thiocyanate species (2) was obtained from 1 by an axial ligand exchange reaction. The crystal structures of 1 and 2 are shown in Figure 1 and the Supporting Information Figure 1S, respectively. Both 1 and 2 are tetracationic molecules associated each with four PF6 counterions. They crystallize in unusually large cells, with one dimension exceeding 50 . The Ni11 chain of 1 and 2 is linear and wrapped in a helical manner by four tentra trianions. In both complexes, the atoms of the axial ligands are collinear with the Ni11 axis; the molecular lengths are 27.7 and 32.4 for 1 and 2, respectively. These are the longest EMAC complexes reported to date. The nature of the axial ligand does not significantly affect the metal–metal bond length, and no obvious structural change is observed for compound 2 with respect to 1. Therefore, we will only analyze the structure of 1 in detail. Selected bond lengths for 1 are displayed in Figure 1c together with the corresponding values obtained from geometry optimization at the DFT/B3LYP level. Molecule 1 consists of eleven nickel atoms in a linear chain with the Ni-Ni-Ni bond angles in the range of 179–1808. The N-Ni-Ni-N torsion angles for adjacent nickel are between 13.0 and 18.78, much smaller than those in oligo-a-pyridylamino ligand EMAC complexes (ca. 22.58). Metal–metal distances usually decrease from the end to the center of the chain in both nickel and cobalt EMACs of oligo-a-pyridyl[*] Dr. R. H. Ismayilov, Dr. W.-Z. Wang, Dr. G.-H. Lee, S.-A. Hua, Prof. Dr. S.-M. Peng Department of Chemistry, National Taiwan University 1, Sec. 4, Roosevelt Road, Taipei, 106 (Taiwan, ROC) Fax: (+886)2-8369-3765 E-mail: smpeng@ntu.edu.tw

Cooperative Recognition of a Copper Cation and Anion by a Calix[4]arene Substituted at the Lower Rim by a β‐Amino‐α,β‐Unsaturated Ketone

We report herein a new ditopic calix[4]arene receptor 25,27-bis-{[4-amino-4-(1-naphthyl)-2-oxo-3-butenyl]oxy}-26,28-dihydroxycalix[4]arene (2) for the simultaneous complexation of anionic and cationic species. The host molecule 25,27-bis{[3-(1-naphthyl)-5-isoxazolyl]methoxy}-26,28-dihydroxycalix[4]arene (1) was synthesised first and was followed by a [Mo(CO)6]-mediated ring-opening reaction to give the target receptor 2. The binding properties of ligands 1 and 2 towards metal ions in CH3CN were investigated by UV/Vis and fluorescence spectroscopies. The results showed that both ligands 1 and 2 were highly selective for Cu(II) ions. Upon titration with Cu(II), the fluorescence of 1 was severely quenched, whereas 2 showed strong fluorescence enhancement because the metal ions help to lock the conformation of the fluorophores. During the complexation of 2 with Cu(II), the Cu(II) was reduced to Cu(I) by the free phenolic OH of 2, whereas the phenol was oxidised by Cu(II), after which it assisted in the trapping of Cu(I). Ditopic behaviour was observed for the complex 2.Cu(I), which showed further enhancement of its fluorescence intensity upon complexation with anions such as acetate or fluoride.

Neutral, panchromatic Ru(II) terpyridine sensitizers bearing pyridine pyrazolate chelates with superior DSSC performance

A new series of neutral, panchromatic Ru(II) terpyridine sensitizers (PRT1-PRT4) exhibit much higher molar extinction coefficients at 400-550 nm and superior DSSC performance in terms of conversion efficiency (eta = 10.05 for PRT4) and stability.

Bis-Tridentate Ir(III) Complexes with Nearly Unitary RGB Phosphorescence and Organic Light-Emitting Diodes with External Quantum Efficiency Exceeding 31%.

A new class of neutral bis-tridentate Ir(III) metal complexes that show nearly unitary red, green, and blue emissions in solution is prepared and employed for the fabrication of both monochrome and white-emitting organic light-emitting diodes, among which a green device gives external quantum efficiency exceeding 31%.