Michael G. Hutchings

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.

Synthesis of a cyclodextrin azo dye [3]rotaxane as a single isomer

Azo coupling between 2,6-dimethylphenol and aqueous 4,4′-bis(diazonio)azobenzene chloride in the presence of α-cyclodextrin yields an azo dye [3]rotaxane as a single stereoisomer, with the 2,3-rims of both cyclodextrins pointing outwards.

Direct synthesis of sulfonated azacalixarenes in water

Successive treatment of cyanuric chloride with two aromatic diamines, at least one of them sulfonated, yields water-soluble sulfonated azacalix[4]arenes which may be isolated by crystallisation. Functionalised azacalixarenes may be made by first displacing two chloro substituents from the cyclisation precursor. Attempted formation of an azacalix[6]arene led to a dimeric species for which two structures may be proposed, one of them an azacalixarene catenane.

Structural characterisation of the photoisomers of reactive sulfonated azo dyes by NMR spectroscopy and DFT calculations

1H NMR spectroscopy coupled with in situ laser irradiation has been used together with density functional theory (DFT) computation to examine the structures of the photoisomers of a series of sulfonated reactive azo dyes. Assignment of 1H NMR spectra acquired at the photostationary state has allowed, for the first time, NMR characterisation of unstable cis isomers of commercially relevant water-soluble azo dyes. Structural features of the two isomeric forms predicted by DFT calculations are clearly reflected in the experimental NMR data. The trans-cis photoisomerisation process could be unambiguously identified in each case, based on the large chemical shift change observed for resonances associated with aromatic protons adjacent to the azo linkage.

Additivity of substituent effects on the visible absorption spectra of some heteroarylazo compounds: the influence of solvent

Abstract Pronounced solvatochromic shifts are observed in the visible spectra of two series of heteroarylazobenzene derivatives based on m -acetylamino- and m -methyl- N,N -diethylaniline, measured in various solvents. The heteroaryl azo component of these dyes can be viewed as a thiophene ring substituted by cyano, nitro, alkoxycarbonyl, or ring aza (to give thiazole and isothiazole derivatives). Free–Wilson analysis has shown that the influence of each of the substituents is constant across all molecules within the two series for a given solvent. However, the substituent increments vary between different solvents and correlate with the Kamlet–Taft dipolarity/polarisability parameter π * of the solvents. There appears to be no intuitive explanation for the relative solvent variation of different substituents, but Onsager theory suggests sensitivity to the relative size and shape of the dyes. Visible absorption maxima are predicted for unknown heteroarylazo compounds related to those studied.

Software review: PISYSTEM for Windows version 3.1. A program for the calculation of excited state properties including colour of π electron-based molecules

PISYSTEM for Windows is a commercial program based on a modified PPP SCF-CI-MO model for the rapid and convenient calculation of excited state properties, especially colour, of π-conjugated systems such as dyes. The use and performance of the program is reviewed.

The effect on the colour of the diarylazo chromogen, and electronic substituent constants, of the N-pyridinium group

Abstract Para- and meta-N-pyridinium-substituted anilines have been synthesised by displacement of dinitroaniline from N-(2,4-dinitrophenyl)pyridinium chloride by p-phenylenediamine or m-acetylaminoaniline in hot DMF. Diazotisation and coupling with N,N-diethylaniline gave azo compounds, with visible absorption maxima: p-pyridinium isomer, 486 nm (neutral), and 504 nm (acid); m-pyridinium isomer, 456 nm (neutral), and 508 nm (acid). Dual-parameter relationships have been derived for a series of substituted azo compounds, connecting their halochromic shifts (difference in visible absorption maximum between neutral and protonated forms) and the electronic substituent constants, σ 1 and σ R . Via these relationships, values of σ 1 = 1.01 and σ R = 0.05 have been derived for the N-pyridinium substituent. Thus, this group is shown to be an exceedingly powerful electron-withdrawer, exerting its effect mainly by inductive/field effects. Its effect on the colour of azo dyes is roughly equivalent to that of the nitro group.

Benzodifurantrione: a stable phenylogous enol.

The first example of a stable phenylogous enol, resulting from an extended keto-enol tautomerization across a benzene ring, is described. The enol has been isolated, and its structure was proven by X-ray crystallography. The equilibrium between the keto- and enol-tautomers has been extensively studied and quantified in solution by NMR and UV-vis spectroscopy. The position of equilibrium showed a linear correlation to the Kamlet-Taft solvatochromic scale for solvent H-bond acceptor strength (beta(OmicronEta)), and the equilibrium was proven to be fully dynamic, obeying first-order equilibrium kinetics. To attempt to explain why enolization occurs, at what surprisingly appears to be the expense of aromatic resonance stabilization, various structural features have been considered and explored further with the aid of MO calculations. Nucleus independent chemical shift (NICS) index of aromaticity calculations for each of the rings comprising both tautomers showed that while the central benzene ring loses aromaticity on enolization, the alpha-keto-lactone ring showed an unexpected and significant antiaromaticity in the keto-tautomer, which is by no means intuitive. The loss of stabilization energy associated with the central benzene ring is, therefore, to a certain degree compensated by removal of the antiaromatic destabilization of the alpha-keto-lactone ring rendering the two structures much closer in energy than would otherwise be expected.

Unexpected bathochromic shifts in the visible absorption spectra of arylazo dyes containing the ortho-N-pyridinium-nitro substituent combination. c

Abstract A combination of 4-nitro and 5-N-pyridinium groups in a donor-substituted azobenzene leads to an unexpected bathochromic shift in visible absorption maximum compared with the 5-chloro precursor, despite increased steric interaction between the two substituents. This behaviour probably derives from an intramolecular polarisation effect of the cationic pyridinium group acting on the adjacent nitro group. Molecular orbital calculations support this conclusion. The visible spectra differences in various solvents can be related to solvent dipolarity as reflected by the Kamlet-Taft π * solvent parameter. There is no relationship with H-bonding parameters.