Benjamin J. Coe

Trans-effects in octahedral transition metal complexes

Abstract An extensive survey of the occurrences and origins of both structural trans-effects (STEs) and kinetic trans-effects (KTEs) in octahedral d-transition metal complexes is presented. This allows the identification of general STE classes into which the majority of common ligands fit: (a) very large STE ligands (STE vs. Cl − >ca. 0.20 A): SiR 3 − , NO − , N 3− , O 2− , S 2− , RC 3− ; (b) large STE ligands (ca. 0.20>STE vs. Cl − >ca. 0.10 A): H − , R − , η 1 -alkenyl, η 1 -Ph, RCO − , RN 2 − ; (c) moderate STE ligands (ca. 0.10 A>STE vs. Cl − >0.00 A): CO, CN − , CNR, η 1 -acetylide, R 2 C, NO 2 − , NS + , RN 2 + , SO 3 2− , RSO 2 − , PR 3 , P(OR) 3 , RNH − , RS − , η 1 -thiones. The NO + ligand best illustrates the mutual nature of STEs, since it shows moderate STEs when trans to π-acceptor ligands, negligible STEs when trans to purely σ-donor ligands, and inverse STEs when trans to π-donors. STEs can sometimes show a marked dependency upon the electronic properties of the complexed metal centre, e.g. π-accepting RNC and PR 3 ligands generally give moderate STEs, but in d 0 complexes their STEs are weaker than that of Cl − . This may be attributed to an absence of π-back-bonding in such complexes. Also, the STEs of π-donating RN 2– ligands show an extremely wide variation which partially correlates with the metal d-configuration. The relationship between STEs and KTEs depends upon ligand substitution mechanisms, and because such reactions in octahedral complexes are generally dissociatively activated, there is often a close correlation between STEs and KTEs. For example, N 3− causes very large STEs and KTEs, whilst SO 3 2− gives moderate STEs and large KTEs. Since both of these ligands cause STEs primarily via powerful electron donation, the ground state destabilisations implied by STEs are likely to be accompanied by stabilisation of the electron-deficient five-co-ordinate transition states. By contrast, π-acceptor ligands such as CO or RNC generally exert moderate STEs, but cause pronounced delabilisation of trans metalligand bonds due to destabilisation of transition states.

Molecular Materials Possessing Switchable Quadratic Nonlinear Optical Properties

ReversibleswitchingofquadraticNLOresponses can be achieved in organic and metallo-organic molecular compounds by application of various external stimuli. Strategies for the molecular engineering of switchable NLO materials are outlined (an example is depicted), and practical demonstrations of the switching of NLO effects, such as SHG, are discussed.

METAL COMPLEXES FOR MOLECULAR ELECTRONICS AND PHOTONICS

Within the fields of coordination and organometallic chemistry there has recently been much interest in the design, synthesis and study of novel complexes having potential for applications in the nascent technologies of molecular electronics and photonics. Here we provide an overview of the current status of research involving organotransition metal complexes in the following important areas: liquid crystalline materials, nonlinear optical materials, molecular wires and switches, chromophore-quencher complexes, dye-sensitized photovoltaic cells and organic light-emitting diodes. Cover-age is selective, generally focusing on highlights and the most recent developments, with the broad aim of conveying the essence and excitement of a group of related research topics at the forefront of modern inorganic chemistry.

Redox-Switching of Nonlinear Optical Behavior in Langmuir−Blodgett Thin Films Containing a Ruthenium(II) Ammine Complex

Alternating Langmuir−Blodgett multilayers containing a metal-to-ligand charge-transfer chromophore based on {RuII(NH3)5}2+ show redox-switching of the second harmonic generation from a 1064 nm laser.

Why hyperpolarizabilities fall short of the fundamental quantum limits.

Quantum sum rules impose limits on the hyperpolarizability, beta. A survey of the largest second-order molecular susceptibilities finds what appears to be a universal gap between the experimental results and the fundamental limits. In this work, we use theory, linear spectroscopy, Raman spectroscopy, and measured values of beta (using hyper-Rayleigh scattering and Stark Spectroscopy) to show that this gap is due to an unfavorable arrangement of excited state energies. The question of whether this result is a universal property of a quantum system or a matter of present paradigms for making molecules is discussed.

Evolution of linear absorption and nonlinear optical properties in v-shaped ruthenium(II)-based chromophores

In this article, we describe a series of complexes with electron-rich cis-{Ru(II)(NH(3))(4)}(2+) centers coordinated to two pyridyl ligands bearing N-methyl/arylpyridinium electron-acceptor groups. These V-shaped dipolar species are new, extended members of a class of chromophores first reported by us (Coe, B. J. et al. J. Am. Chem. Soc. 2005, 127, 4845-4859). They have been isolated as their PF(6)(-) salts and characterized by using various techniques including (1)H NMR and electronic absorption spectroscopies and cyclic voltammetry. Reversible Ru(III/II) waves show that the new complexes are potentially redox-switchable chromophores. Single crystal X-ray structures have been obtained for four complex salts; three of these crystallize noncentrosymmetrically, but with the individual molecular dipoles aligned largely antiparallel. Very large molecular first hyperpolarizabilities beta have been determined by using hyper-Rayleigh scattering (HRS) with an 800 nm laser and also via Stark (electroabsorption) spectroscopic studies on the intense, visible d --> pi* metal-to-ligand charge-transfer (MLCT) and pi --> pi* intraligand charge-transfer (ILCT) bands. The latter measurements afford total nonresonant beta(0) responses as high as ca. 600 x 10(-30) esu. These pseudo-C(2v) chromophores show two substantial components of the beta tensor, beta(zzz) and beta(zyy), although the relative significance of these varies with the physical method applied. According to HRS, beta(zzz) dominates in all cases, whereas the Stark analyses indicate that beta(zyy) is dominant in the shorter chromophores, but beta(zzz) and beta(zyy) are similar for the extended species. In contrast, finite field calculations predict that beta(zyy) is always the major component. Time-dependent density functional theory calculations predict increasing ILCT character for the nominally MLCT transitions and accompanying blue-shifts of the visible absorptions, as the ligand pi-systems are extended. Such unusual behavior has also been observed with related 1D complexes (Coe, B. J. et al. J. Am. Chem. Soc. 2004, 126, 3880-3891).

Combining Very Large Quadratic and Cubic Nonlinear Optical Responses in Extended, Tris-Chelate Metallochromophores with Six π-Conjugated Pyridinium Substituents

We describe a series of nine new complex salts in which electron-rich Ru(II) or Fe(II) centers are connected via pi-conjugated bridges to six electron-accepting N-methyl-/N-arylpyridinium groups. This work builds upon our previous preliminary studies (Coe , B. J. J. Am. Chem. Soc. 2005, 127, 13399-13410; J. Phys. Chem. A 2007, 111, 472-478), with the aims of achieving greatly enhanced NLO properties and also combining large quadratic and cubic effects in potentially redox-switchable molecules. Characterization has involved various techniques, including electronic absorption spectroscopy and cyclic voltammetry. The complexes display intense, visible d --> pi* metal-to-ligand charge-transfer (MLCT) bands, and their pi --> pi* intraligand charge-transfer (ILCT) absorptions in the near-UV region show molar extinction coefficients as high as ca. 3.5 x 10(5) M(-1) cm(-1). Molecular quadratic nonlinear optical (NLO) responses beta have been determined by using hyper-Rayleigh scattering at 800 and 1064 nm and also via Stark (electroabsorption) spectroscopic studies. The directly and indirectly derived beta values are very large, with the Stark-based static first hyperpolarizabilities beta(0) reaching as high as ca. 10(-27) esu, and generally increase on extending the pi-conjugation and enhancing the electron-accepting strength of the ligands. Cubic NLO properties have also been measured by using the Z-scan technique, revealing relatively high two-photon absorption cross sections of up to 2500 GM at 750 nm.

Extremely large nonresonant second-order nonlinear optical response in crystals of the stilbazolium salt DAPSH

We report on the extremely large nonresonant quadratic optical nonlinearity of the stilbazolium salt trans-4′-(dimethylamino)-N-phenyl-4-stilbazolium hexafluorophosphate (DAPSH). The phenyl-pyridinium chromophores in DAPSH crystals grown from acetone solution pack with a highly aligned polar order, resulting in a very large birefringence, Δn=1.17±0.06 at λ=0.83 μm and Δn=0.83±0.04 at λ=1.55 μm. More importantly, this leads to an extremely large diagonal quadratic susceptibility with the nonlinear optical coefficient for second-harmonic generation reaching up to d111=290±40 pm/V at 1.907 μm fundamental wavelength, which presents a considerable improvement with respect to the presently best material trans-4′-(dimethylamino)-N-methyl-4-stilbazolium tosyate (DAST) with d111=210±55 pm/V at λ=1.907 μm. The result is in agreement with the preferential packing of the chromophores and the previous studies demonstrating higher microscopic nonlinearity of the chromophores in DAPSH compared to that of DAST.

Synthesis and photophysical properties of kinetically stable complexes containing a lanthanide ion and a transition metal antenna group

A series of bimetallic complexes has been prepared in which an octadentate DOTA-monoamide pocket containing a bound lanthanide ion is linked covalently to a Re(I) or Ru(II) bipyridyl unit via an alkyl spacer group. The transition metal chromophores incorporated in this way act as effective sensitisers for lanthanide-centred luminescence. The rate and efficiency of energy transfer are dependent upon the nature of the spacer group, and upon the nature of the lanthanide acceptor. For the RuNd diad, there is a long rise-time associated with the energy-transfer step, such that energy transfer is rate determining in H(2)O, but not in D(2)O. The results also lead us to suggest that energy transfer may precede formation of the (3)M((Ru/Re))L((bpy))CT state and may be a competitive deactivation pathway for the precursor state ((1)M((Ru/Re))L((bpy))CT).

Nickel(II) and Palladium(II) Complexes of Azobenzene-Containing Ligands as Dichroic Dyes

A large series of complexes has been synthesized with two chelating, Schiff base azobenzene derivatives connected linearly by coordination to a central nickel(II) or palladium(II) ion. These compounds have the general formulas M(II)(OC(6)H(3)-2-CHNR-4-N═NC(6)H(4)-4-CO(2)Et)(2) [M = Ni; R = n-Bu (3c), n-C(6)H(13) (3d), n-C(8)H(17) (3e), n-C(12)H(25) (3f), Ph (3g), OH (3h), C(6)H(4)-4-CO(2)Et (3i). M = Pd; R = i-Pr (4a), t-Bu (4b), n-Bu (4c), n-C(6)H(13) (4d), n-C(8)H(17) (4e), n-C(12)H(25) (4f), Ph (4g)], M(II)[OC(6)H(3)-2-CHN(n-C(8)H(17))-4-N═NC(6)H(4)-4-CO(2)(n-C(8)H(17))](2) [M = Ni (9), Pd (10)], M(II)[OC(6)H(3)-2-CHN(n-C(8)H(17))-4-N═NC(6)H(4)-4-C(6)H(4)-4-O(n-C(7)H(15))](2) [M = Ni (14), Pd (15)], and M(II)[OC(6)H(3)-2-CHN(CMe(2))-4-N═NC(6)H(4)-4-CO(2)Et](2) [M = Ni (17), Pd (18); the CMe(2) groups are connected]. These compounds have been characterized by using various physical techniques including (1)H NMR spectroscopy and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. Single-crystal X-ray structures have been obtained for two pro-ligands and five complexes (3e, 4e, 14, 15, and 17). The latter always show a strictly square planar arrangement about the metal center, except for the Ni(II) complex of a salen-like ligand (17). In solution, broadened (1)H NMR signals indicate distortions from square planar geometry for the bis-chelate Ni(II) complexes. Electronic absorption spectroscopy and ZINDO_S (Zerners intermediate neglect of differential overlap) and TD-DFT (time-dependent density functional theory) calculations show that the lowest energy transition has metal-to-ligand charge-transfer character. The λ(max) of this band lies in the range of 409-434 nm in dichloromethane, and replacing Ni(II) with Pd(II) causes small blue-shifts. Dichroic ratios measured in various liquid crystal hosts show complexation-induced increases with Ni(II), but using Pd(II) has a detrimental effect.