John N. Moore

Femtosecond time-resolved UV-visible absorption spectroscopy of trans-azobenzene: dependence on excitation wavelength

Femtosecond time-resolved UV-visible absorption spectroscopy has been used to study the photochemistry of trans-azobenzene in n-hexane. Excitation to the S-1(n pi*) state results in transient absorption bands at ca. 400 nm (strong) and 550 nm (weaker) which decay with a lifetime 2.5 +/- 0.2 ps on excitation at 503 nm, close to the S-1 origin, and with an additional fast component of ca. 0.6 ps on excitation at 390 or 420 nm, both well above the S-1 origin. Excitation to the S-2(pi pi*) state results in transient absorption at 400 nm which decays with a dominant component of ca. 0.9 ps and a weaker component of ca. 15 ps; this 400 nm band itself is observed to rise synchronously as a transient band at 475 nm decays with a lifetime of < 200 fs. These results are discussed in terms of the dual mechanism proposed for azobenzene photoisomerization.

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...

Donor–π‐Acceptor Species Derived from Functionalised 1,3‐Dithiol‐2‐ylidene Anthracene Donor Units Exhibiting Photoinduced Electron Transfer Properties: Spectroscopic, Electrochemical, X‐Ray Crystallographic and Theoretical Studies

Steric interactions between the anthraquinoid core and the 1,3-dithiole and dicyanomethylene groups play a key role in determining the physical properties of system 1. The intramolecular charge transfer properties of this donor–π-acceptor species have been explored and cyclic voltammetric data, X-ray crystal structures and ab initio calculations are also reported.

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.

A chemometric analysis of the resonance Raman spectra of mutant carbonmonoxy-myoglobins reveals the effects of polarity

Resonance Raman spectra of 10 carbonmonoxy-myoglobins have been obtained, including sperm whale native, pig wild-type, and the mutants H64L, H64A, V68T, V68N, H64V/V68T, F43W, F46V, and L29F. This series was chosen in order to study the effect of ligand binding pocket polarity on the positions of the v(Fe-CO) and delta (Fe-C-O) bands. Spectra of both 12CO and 13CO isotopic forms have been obtained and a detailed analysis has facilitated the identification of both the ligand-specific bands and six underlying porphyrin bands which are insensitive to this isotopic substitution. Along with a band-fitting analysis of infrared spectra, these resonance Raman data provide a comprehensive evaluation of the vibrations of the FeCO unit. The band positions of the ligand-specific modes are found to depend on the structure of the ligand binding pocket, arising from the strength of back-bonding within the FeCO unit, and clear correlations exist between the v(Fe-CO), delta (Fe-C-O), and v(C-O) band positions which characterize this synergic bonding. The results are consistent with the proposal that the vibration band positions are determined primarily by the electrostatic potential at the ligand. Five discrete band sets are observed for this set of mutants, suggesting that 5 discrete conformations occur.

Effect of metal cations on the photochromic properties of spironaphthoxazines conjugated with aza-15(18)-crown-5(6) ethers

Spironaphthoxazines conjugated with aza-15(18)-crown-5(6)-ether moieties at the 6′-position of the naphthalene fragment (Crown-containing Spironaphthoxazines, CSN) were synthesised and studied for the first time. The addition of Li+ and alkaline earth (Mg2+, Ca2+, Sr2+ and Ba2+) metal cations to CSN solutions results in a hypsochromic shift of the UV absorption band of the spiro form and a bathochromic shift of the absorption band of the merocyanine form in the visible region. In addition, the equilibrium shifts to the merocyanine form, and the lifetime of the photoinduced merocyanine form increases. Analysis of the spectral and kinetic data allows a complexation scheme to be proposed and the stability constants of the resulting complexes to be calculated. According to the results obtained, the complexation with Li+ and alkaline earth metal cations in acetonitrile initially involves the crown ether moiety; the participation of the merocyanine oxygen atom in the complexation process occurs at a high metal cation concentration. The UV-induced isomerisation of CSN into the merocyanine form causes a decrease of the cation binding ability.

The anti-Stokes resonance Raman spectrum of photoexcited S1 trans-stilbene

Abstract Anti-Stokes resonance Raman spectra of S1 trans-stilbene have been measured for vibrational wavenumbers up to 1570 cm−1. Trans-stilbene in n-hexane was excited at 290–310 nm and probed at 580–620 nm, respectively, with 8 ps resolution. All the dominant bands seen in the Stokes spectrum of the S1 state are also seen in the anti-Stokes spectrum. Some anti-Stokes bands, notably those at 1570, 1240, 1180, 980, 720 and 285 cm−1, decrease markedly in relative intensity on a 2~ 10 ps timescale, displaying dynamics similar to those observed for the changes in the band position and bandwidth of several features in the Stokes spectrum.

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.

Investigating the Cusp between the nano- and macro-sciences in supermolecular liquid-crystalline twist-bend nematogens

In this article we report the first known linear liquid-crystalline hexamer and in doing so demonstrate that higher oligomers and main chain polymers, with chemical structures based upon dimers and bimesogens, can exhibit the topical twist-bend ‘nematic’ mesophase. In doing so we find that there is continuation of properties and structures across the spectrum from dimer to polymer and possibly to macroscale objects such as helical flagella. This finding highlights the cross over from nanoscience based mainly on electrostatics and other non-covalent interactions to macroscience based mainly on molecular topology, density of packing, and minimisation of the free energy.