Laurence C. Abbott

Photoisomerization of a Capped Azobenzene in Solution Probed by Ultrafast Time-Resolved Electronic Absorption Spectroscopy

Ultrafast time-resolved electronic absorption spectroscopy has been used to study the photochemistry of trans-azobenzene and trans-1, a derivative in which azobenzene is capped by an azacrown ether, on UV excitation to the S2(ππ*) state. Excitation of trans-1 results in transient absorption which decays with a dominant component of lifetime ca. 2.6 ps and in bleaching of the ground-state UV absorption band which recovers on a similar time scale. In contrast, excitation of trans-azobenzene results in transient absorption which decays with a dominant component with a shorter lifetime of ca. 1 ps, and in bleaching which recovers on a much longer time scale of ca. 18 ps. The recovery of the ground-state UV absorption band is not complete in either case, and the ultrafast data indicate that the quantum yield of trans-to-cis photoisomerization of 1 is approximately twice that of azobenzene. These observations demonstrate that the restricted rotational freedom of the phenyl groups in trans-1 has a significant ef...

Reductive Reaction Mechanisms of the Azo Dye Orange II in Aqueous Solution and in Cellulose: From Radical Intermediates to Products

Reductive reaction mechanisms of the azo dye Orange II (Acid Orange 7) in aqueous solution have been studied from radical intermediates through to the final products using a combination of nanosecond time-resolved UV-visible absorption spectroscopy, steady-state photolysis, and HPLC techniques. The dye is reduced by photochemically produced 2-hydroxy-2-propyl radicals at a near-diffusion-controlled rate (k2 = 2.2 x 10(9) dm3 mol(-1) s(-1)) to give the dye radical anion, which then disproportionates (k3 = 2.6 x 10(8) dm3 mol(-1) s(-1)) to re-form the parent dye and a hydrazine. The hydrazine decomposes to 4-aminobenzenesulfonate and a naphthylimine species, which hydrolyses to give 1,2-naphthoquinone; this naphthoquinone and 4-aminobenzenesulfonate react to give a species that reacts further in the presence of air to form an indophenol dye. The reduction of Orange II has also been studied in cellophane, where the rate constant for dye reduction by 2-hydroxy-2-propyl radicals is approximately two orders of magnitude lower than that in aqueous solution.

Spectroscopic studies of Direct Blue 1 in solution and on cellulose surfaces: effects of environment on a bis-azo dye

The bis-azo dye Direct Blue 1 (Chicago Sky Blue 6B) has been studied in solution and on cellophane and cotton surfaces using NMR, resonance Raman, infrared, and UV–visible spectroscopy. The data indicate that Direct Blue 1 is present as the hydrazone tautomer in all of these media, with distinct changes in the spectra with medium showing that the dye is affected by interactions with its environment. In DMF and DMSO solutions, the dye is present as a monomer that is internally hydrogen-bonded. It is also monomeric at low concentrations in aqueous solution, with subtle changes in the Raman spectra from those in DMF and DMSO being attributed to external hydrogen bonding with water. Direct Blue 1 is also present as a hydrazone monomer at low concentrations on cellophane and cotton: the Raman spectra indicate that there is hydrogen bonding with cellulose, and the UV–visible spectra indicate that it experiences an apolar environment which is attributed to its adsorption onto cellulose surfaces.

Resonance Raman and UV–visible spectroscopy of black dyes on textiles

Resonance Raman and UV-visible diffuse reflectance spectra were recorded from samples of cotton, viscose, polyester, nylon, and acrylic textile swatches dyed black with one of seven single dyes, a mixture of two dyes, or one of seven mixtures of three dyes. The samples generally gave characteristic Raman spectra of the dyes, demonstrating that the technique is applicable for the forensic analysis of dyed black textiles. Survey studies of the widely used dye Reactive Black 5 show that essentially the same Raman spectrum is obtained on bulk sampling from the dye in solution, on viscose, on cotton at different uptakes, and on microscope sampling from the dye in cotton threads and single fibres. The effects of laser irradiation on the Raman bands and emission backgrounds from textile samples with and without dye are also reported.

Dyes in Liquid Crystals: Experimental and Computational Studies of a Guest–Host System Based on a Combined DFT and MD Approach

Practical applications of guest–host liquid crystal systems are critically dependent on the alignment of the guest species within the liquid crystal host. UV/Vis absorption spectroscopy shows that the 1,5-dihydroxy-2,6-bis-(4-propylphenyl)-9,10-anthraquinone dye aligns within the E7 nematic host, giving an experimental dichroic ratio of 9.40 and dye order parameter of 0.74. This alignment was modelled by using a combination of density functional theory (DFT) and molecular dynamics (MD) computational approaches that do not require the input of experimental data. Time-dependent DFT calculations show that the electronic transition dipole moment is highly aligned with the long molecular axis of the dye. Fully atomistic MD simulations show that the long axis of the dye is less highly aligned within the E7 host, indicating that this contribution limits the overall dye alignment and, thereby, the potential practical applications of this particular system. Importantly, this study demonstrates an experimental and combined DFT and MD computational approach that may be applied generally to guest–host systems, providing a potential route to their rational design.

Spectroscopic and photochemical studies of xanthene and azo dyes on surfaces : cellophane as a mimic of paper and cotton

Abstract Steady-state UV–visible absorption spectra of a xanthene dye, Acid Red 52, and an azo dye, Direct Blue 1, have been studied in aqueous solution, on paper ( Acid Red 52 ) or cotton ( Direct Blue 1 ), and in cellophane films. Distinct spectral changes are observed when these dyes are deposited from solution on to paper or cotton, and the respective spectral changes are replicated on incorporation into a cellophane film. Dye photofading has also been studied in these media: the photofading of Acid Red 52 and Direct Blue 1 in solution was found to be very slow, with a low quantum yield; the photofading of each dye was significantly faster on paper or on cotton; and the photofading of each dye in cellophane was found to be significantly faster than in solution and similar to that on paper or cotton. Similar spectral changes were observed during the photofading of Acid Red 52 in cellophane and on paper, and of Direct Blue 1 in cellophane and on cotton. These similarities, both in steady-state spectra and in photofading properties, show that cellophane can be used as a good mimic for studying dye fading on paper or cotton; loading into cellophane is straightforward and it has the additional advantage, for spectroscopic studies, of being transparent.

Structure and Reactivity of Thiazolium Azo Dyes: UV–Visible, Resonance Raman, NMR, and Computational Studies of the Reaction Mechanism in Alkaline Solution

UV-visible absorption, resonance Raman, and (1)H NMR spectroscopy, allied with density functional theory (DFT) calculations, have been used to study the structure, bonding, and alkaline hydrolysis mechanism of the cationic thiazloium azo dye, 2-[2-[4-(diethylamino)phenyl]diazenyl]-3-methyl-thiazolium (1a), along with a series of six related dyes with different 4-dialkylamino groups and/or other phenyl ring substituents (2a-c, 3a-c) and the related isothiazolium azo dye, 5-[2-[4-(dimethylamino)phenyl]diazenyl]-2-methyl-isothiazolium (4). These diazahemicyanine dyes are calculated to have a similar low-energy structure that is cis, trans at the (iso)thiazolium-azo group, and for which the calculated Raman spectra provide a good match with the experimental data; the calculations on these structures are used to assign and discuss the transitions giving rise to the experimental spectra, and to consider the bonding and its variation between the dyes. UV-visible, Raman, and NMR spectra recorded from minutes to several weeks after raising the pH of an aqueous solution of 1a to ca. 11.5 show that the dominant initial step in the reaction is loss of diethylamine to produce a quinonimine (ca. hours), with subsequent reactions occurring on longer time scales (ca. days to weeks); kinetic analyses give a rate constant of 2.6 × 10(-2) dm(3) mol(-1) s(-1) for reaction of 1a with OH(-). UV-visible spectra recorded on raising the pH of the other dyes in solution show similar changes that are attributed to the same general reaction mechanism, but with different rate constants for which the dependence on structure is discussed.

Considerations in the determination of orientational order parameters from X-ray scattering experiments

ABSTRACT An assessment of the data processing and analysis methods used to obtain the second- and fourth-rank orientational order parameters of liquid crystals from X-ray scattering experiments has been carried out, using experimental data from four extensively studied alkyl-cyanobiphenyls and calculated data generated from two general types of theoretical orientational distribution function. The application of a background subtraction and two different baseline correction methods to the scattering profiles is assessed, along with three different methods to analyse the processed data. The choice of baseline correction method is shown to have a significant effect: an offset to zero overestimates the order parameters from the experimental and calculated data sets, particularly for lower order parameters arising from broad distributions, whereas an offset to a value estimated from regions of low scattering intensity provides experimental values close to those reported from other experimental techniques. By contrast, the three different analysis methods are shown generally to result in relatively small absolute differences between the order parameters. We outline a straightforward general approach to experimental X-ray scattering data processing and analysis for uniaxial phases that results in order parameters that match well with those reported using other experimental techniques. Graphical Abstract

Resonance Raman Spectroscopy of Photolabile Transition Metal Carbonyls: Controlled Photoalteration with Continuous Wave Lasers to Record Spectra of Reactants or Transient Species

A method has been developed that enables resonance Raman spectra of photolabile species in solution to be recorded under conditions where the level of photoalteration is controlled: a low level enables reactant spectra to be recorded, whereas a high level enables the spectra of short-lived transient species to be recorded in real time using continuous-wave (CW) lasers and standard Raman detection equipment. The design includes a sealed flow system, enabling air-sensitive species to be studied under an inert atmosphere. A simple theoretical model has been developed to aid the interpretation of experimental results, and its applicability is demonstrated. Controlled photoalteration and its theory are demonstrated with 413.1-nm excitation of carbonmonoxymyoglobin (MbCO), which generates deoxymyoglobin (deoxy-Mb) on photolysis, and for which the spectra of both species are well established. The methods have also been applied to two air-sensitive, photolabile transition metal carbonyls using 514.5-nm wavelength excitation: for Cp2Mo2(CO)6 (Cp = η5–C5H5), increasing levels of photoalteration result only in a decrease in the parent band intensities, relative to the solvent bands; for Cp2Fe2(CO)4, increasing levels of photoalteration result in the appearance of additional bands that are assigned to the transient species CpFe(μ–CO)3FeCp, formed following the loss of a CO ligand.