The catalytic effects of several acids and metal salts on the cis-trans isomerization of azobenzene in aqueous ethanol were examined kinetically. The effect of perchloric acid is apparently due to H⁺ ions: a catalytic mechanism involving the formation of the conjugate acid of azobenzene has been postulated to interpret these results. To account for the much higher catalytic activity found for hydrochloric acid, an additional path, involving catalysis by undissociated HCl molecules has been proposed. Acetic acid was found to be inactive. Of the metal salts examined only those of Cu⁺⁺ shoved pronounced catalytic activity, which is interpreted in terms of a catalytic mechanism involving coordination of cupric ions with the azo group. Simultaneous coordination of Cu⁺⁺ and H⁺ has been proposed to account for the high catalytic activity of cupric salts in the presence of acids. / Science, Faculty of / Chemistry, Department of / Graduate

No description available.

Azobenzene derivatives have been shown to act as a molecular switch when ex- posed to an applied electric field. Many applications require the switching molecule to be adsorbed on a surface. The behavior of chemisorbed N-(2-mercaptoethyl)-4- phenylazobenzamide on a Au(111) surface has been investigated using a mean-field theoretical approach for azobenzene in alkylthiol monolayers and density functional theory calculations at the zero-density limit. Azobenzene switching in monolayers was found to be dependent on surface coverage, as well as the strength and polarity of an electric field. In the zero-density regime, azobenzene derivatives adopted paral- lel and upright geometries for both trans and cis isomers. Charged states for upright, adsorbed structures were also analyzed and were found to lower the isomerization energy barrier. / Graduate / 0485

Molecular photoswitches are attracting much attention lately mostly because of their possible applications in nano technology, and their role in biology. One of the widely studied representatives of photochromic molecules is azobenzene (AB). With light, by a static electric field, or with tunneling electrons this specie can be "switched" from the flat and energetically more stable trans form, into the compact cis form. The back reaction can be induced optically or thermally. Quantum chemical calculations, mostly based on density functional theory, on the AB molecule, AB derivatives and related systems are presented. All the calculations were done for isolated species, however, with implications for latest experimental results aiming at the switching of surface mounted ABs. In some of these experiments, it is assumed that the switching process is substrate mediated, by attaching an electron or a hole to the adsorbate forming short-lived anion or cation resonances. Therefore, we calculated also cationic and anionic ABs in this work. An influence of external electric fields on the potential energy surfaces, was also studied. Further, by the type, number and positioning of various substituent groups, systematic changes on activation energies and rates for the thermal cis-to-trans isomerization can be enforced. The nature of the transition state for ground state isomerization was investigated. Applying Eyring's transition state theory, trends in activation energies and rates were predicted and are, where a comparison was possible, in good agreement with experimental data. Further, thermal isomerization was studied in solution, for which a polarizable continuum model was employed. The influence of substitution and an environment leaves its traces on structural properties of molecules and quantitative appearance of calculated UV/Vis spectra, as well. Finally, an explicit treatment of a solid substrate was demonstrated for the conformational switching, by scanning tunneling microscope, of a 1,5-cyclooctadiene (COD) molecule at a Si(001) surface, treated by a cluster model. At first, we studied energetics and potential energy surfaces along relevant switching coordinates by quantum chemical calculations, followed by the switching dynamics using wave packet methods. We show that, in spite the simplicity of the model, our calculations support the switching of adsorbed COD, by inelastic electron tunneling at low temperatures. / Um den technologischen Fortschritt zu gewährleisten, ist man in vielen technischen Gebieten auf der Suche nach neuen und leistungsfähigeren Materialien. In der Computer- und Informationstechnologie folgte daraus die stetige Miniaturisierung von Bauelementen. Molekulare Photoschalter sind häufig an biologischen Prozessen beteiligt und äußerst vielversprechend, auf diesem Gebiet Anwendung zu finden. Ein sehr umfangreich studiertes photochromes Molekül ist Azobenzol (AB). Diese Spezien können durch Licht, statische elektrische Felder oder elektronisches Tunneln von der energetisch stabilen trans Form zur geometrisch kompakten cis Form "geschaltet" werden. Die Rückreaktion kann optisch oder thermisch erfolgen. In dieser Arbeit werden vorwiegend auf der Dichtefunktionaltheorie beruhende quantenchemische Rechnungen von AB, AB-Derivaten und verwandten Systemen vorgestellt. Alle Rechnungen betrachten isolierte Moleküle, werden jedoch in Zusammenhang mit neuesten experimentellen Ergebnissen zu oberflächengebundenen AB-Schaltern gestellt. In einigen dieser Experimente wird angenommen, dass der Schaltprozess substratvermittelt erfolgt, indem dem Adsorbat ein Elektron zugeführt oder entzogen und so eine kurzlebige anionische oder kationische Resonanz erzeugt wird. Daher werden sowohl ionische AB berechnet als auch der Einfluss eines externen elektrischen Feldes auf die Potentialhyperfläche studiert. Weiterhin können Aktivierungsenergie und Reaktionsrate der thermischen cis-trans-Isomerisierung durch Art, Anzahl und Position verschiedener Substituenten variieren. Die Natur des Übergangszustandes wird daher intensiv erforscht. Mit Hilfe der Theorie des Übergangszustandes nach Eyring werden Reaktionsraten prognostiziert, welche gut mit experimentellen Daten übereinstimmen. Daneben wird die thermische Isomerisierung in einem Lösungsmittel unter Verwendung des polarizable continuum model untersucht, da der Einfluss des Substituenten und die Anwesenheit einer Umgebung zu Veränderungen der strukturellen Eigenschaften der Moleküle und dem quantitativen Verlauf der berechneten UV/Vis-Spektren führen. Abschließend wird unter expliziter Einbeziehung eines festen Substrates das elektronisch getriebene konformale Schalten von 1,5-Cyclooctadien (COD) an einer Si(001)-Oberfläche demonstriert. Zunächst wird die Energetik und die Potentialhyperfläche entlang der relevanten Schaltkoordinaten durch quantenchemische Rechnungen ermittelt und das Schaltverhalten durch Wellenpaketmethoden beschrieben. Trotz der Einfachheit wird gezeigt, dass ein derartiges Modell das elektronische Schalten von adsorbiertem COD bei niedrigen Temperaturen gut beschreibt.

La nécessité de construire des édifices moléculaires complexes s'accompagne d'un besoin de mise au point d'outils synthétiques nouveaux et toujours plus performants. Dans ce contexte, le développement de systèmes photocommutables constitue un moyen privilégié pour la modulation de la réactivité chimique avec un excellent contrôle spatial et temporel. Les complexes organométalliques incorporant des ligands photochromes constituent des candidats de choix pour de telles applications, combinant la réactivité riche du centre métallique avec les propriétés de photocommutation du ligand. Ces travaux de thèse portent sur l'utilisation d'azobenzènes en tant que ligands pour la formation de complexes photocommutables de type (η6-arène)ruthénium. L'étude du comportement d'azobenzènes diversement substitués a conduit à la synthèse d'une nouvelle famille de complexes porteurs d'un ligand o-sulfonamide azobenzène dont les propriétés de photo-isomérisation ainsi que la réactivité ont été étudiées. La coordination de dérivés phosphorés à ces composés peut être modulée sous irradiation lumineuse, et cette propriété a été exploitée pour le photo-déclenchement de la réaction d'aza-Morita-Baylis-Hillman.Dans une seconde partie de ces travaux, nous avons synthétisé une famille d'azobenzènes cycliques pontés par un acétal présentant un processus nouveaux de photo-isomérisation à trois espèces, qui permet l'interconversion sous irradiation lumineuse entre des diastéréoisomères conformationnels stables avec une bonne sélectivité. Ces nouveaux composés offrent des perspectives prometteuses pour le contrôle conformationnel par la lumière. / The growing need to create sophisticated molecules and building blocks requires the development of increasingly more efficient synthetic tools. In this context, photoswitchable systems appear as an ideal means for controlling chemical reactivity with excellent spatial and temporal resolution. Organometallic complexes incorporating photochromic ligands represent a promising class of compounds for such applications as they combine both the versatile reactivity of the metal center with the photoswitchable properties of the ligand. This work investigates the use of azobenzenes as ligands for the synthesis of photoswitchable (η6-arene)ruthenium complexes. The study of the behavior of azobenzenes diversely substituted led to the synthesis of a new family of complexes bearing an o-sulfonamide ligand which photo-isomerization properties were investigated. The coordination of these complexes to phosphorus derivatives could be modulated upon irradiation and this property was applied to the light-triggered aza-Morita-Baylis-Hillman reaction.The second part of this work describes the synthesis and study of a new family of ketal-substituted bridged azobenzenes which display an uncommon three-species photo-isomerization pattern, triggering an interconversion between stable conformational diastereomers with a good selectivity. These compounds offer promising perspectives for conformational control by light.

Two-photon photo-activated functional molecules play key roles in a growing number of technological and biological applications as the availability of high powered, tuneable lasers increases. The development of new, strongly two-photon absorbing functional chromophores is key to the advancement of these technologies. A series of photoresponsive compounds including organic chromophores and transition metal coordination complexes were designed, synthesised and characterised by NMR, mass spectrometry, elemental analysis and X-ray crystallography. Photophysical characterisation of the compounds including electronic absorption spectroscopy, luminescence spectroscopy, luminescence lifetime studies and two-photon absorption (2PA) experiments were performed. Density functional theory (DFT) calculations were performed and supporting data are presented. Of particular interest is the use of photochromic materials to address the increasing demand for high-density data storage employing two-photon absorption for 3-dimensional control of excitation. Towards this, a series of four photochromic "push-pull" azobenzenes of donor-pie-acceptor structure containing a para-ethoxy donor substituent have been synthesized with varying strength of the acceptor group (para-NO2, CN, CO2Et, F). Kinetic studies of their one-photon photoisomerisation and thermal cis-trans isomerisation were performed in solution and in polymer films, and their non-linear two-photon trans-cis photoisomerisation was demonstrated. Luminescent metal centres offer the possibility of a fluorescent read-out mechanism for optical data storage, modulated by a photochromic ligand, which may quench fluorescence in one isomer and not the other. Two photochromic pyridyl azobenzene ligands were designed and synthesised and coordinated to Re(I), Pt(II) and Ru(II) polypridyl metal centres. The trans-cis photochromism of the ligands and complexes was investigated by one- and two-photon absorption. Luminescence modulation was demonstrated via photochromism of the coordinated ligand L2 for the Re(I) tricarbonyl bipyridyl complex ReL2 which displayed enhanced emission intensity in the cis form.2PA is also of interest for biological applications due to the longer, less damaging wavelengths used at which biological tissue is more transparent. Photodynamic therapy (PDT) is a non-invasive clinical treatment for cancers where a photosensitizer is optically excited to generate cytotoxic singlet oxygen, which kills cells in the area under irradiation. Platinum group transition metal complexes have been investigated for this purpose due to their strong 2PA properties and accessible low-lying triplet states, which make them efficient triplet-triplet energy transfer agents. A new cyclometallated Ir(III) complex bearing a fluorenyl phenanthroline ligand and a related bimetallic Ir(III)-Ru(II) complex were synthesised and assessed as two-photon singlet oxygen sensitizers. The monometallic Ir(III) complex was also shown to demonstrate PDT action by altering the morphology of C6 Glioma cells under 740 nm two-photon excitation.

We report on a new class of nonionic, photosensitive surfactants consisting of a polar di(ethylene oxide) head group attached to an alkyl spacer of between two and eight methylene groups, coupled through an ether linkage to an azobenzene moiety. Structural changes associated with the interconversion of the azobenzene group between its cis and trans forms as mediated by the wavelength of an irradiating light source cause changes in the surface tension and self-assembly properties. Differences in saturated surface tensions (surface tension at concentrations above the CMC) were as high as 14.4 mN/m under radiation of different wavelengths. The qualitative behavior of the surfactants changed as the spacer length changed, attributed to the different orientations adopted by the different surfactants depending on their isomerization states, as revealed by neutron reflection studies. The self-assembly of these photosensitive surfactants has been investigated by light scattering, small angle neutron scattering, and cryo-TEM under different illuminations. The significant change in the self-assembly in response to different illumination conditions was attributed to the sign change in Gaussian rigidity, which originated from the azobenzene photoisomerization. / Singapore-MIT Alliance (SMA)

Graduate

This thesis presents the first example of reversible photoregulation of the binding of a protease, α-chymotrypsin, to a surface. A modular approach is used involving the azobenzene photoswitch group, a surface linker and an enzyme binding group. This approach is designed to be easily extended to the photoregulation of binding of other proteases to surfaces by use of enzyme binding groups selective to these proteases. Chapter one gives a brief outline of some of the important areas involved in to this work, including molecular switches, proteases and surface modification. Chapter two describes the synthesis of azobenzene-containing boronate esters designed as photoswitch inhibitors of α-chymotrypsin. Boronate esters were prepared containing the aminophenylboronate group or the peptidomimetic borophenylalanine group for enzyme binding and a range of substituents designed for enzyme affinity and/or surface attachment. Syntheses primarily involved peptide coupling reactions and azobenzene formation by condensation of nitrosobenzenes and anilines. Coupling reactions were successfully carried out using EDCI or isobutyl chlorofomate in several cases where other reagents gave unacceptable decomposition. Chapter three describes the syntheses and HPLC stability studies of derivatives of a noncovalent α-chymotrypsin inhibitor. Several dipeptide-based compounds containing either an amide group for surface attachment or an azobenzene group for photoswitching were prepared, primarily using peptide coupling reactions. Each compound was incubated with α-chymotrypsin to assess its stability, and all were found by HPLC monitoring to be stable to α-chymotrypsin catalysed hydrolysis. Chapter four describes syntheses of azobenzene-containing trifluoromethylketones and α-ketoesters designed as photoswitch inhibitors of α-chymotrypsin. Trifluoromethylketones/α-ketoesters containing amine groups for surface attachment were prepared, primarily using peptide coupling reactions, but could not be isolated due to the incompatibility of the electrophilic ketone and primary amine groups. Trifluoromethylketones/α-ketoesters containing terminal alkynes for surface attachment were prepared either by the attachment of an alkyne substituent group to a symmetrical azobenzene core or by Pd-catalysed reaction of a protected alkyne with an azobenzene having a halide substitutent. Chapter five describes syntheses of sulfur-containing surface linkers for use in surface attachment of the photoswitch inhibitors described in chapters 2-4. A range of compounds containing disulfide or protected thiol groups for surface attachment and azide or carboxylic acid groups for inhibitor attachment were prepared. Syntheses primarily involved coupling of functionalised alcohols/amides to carboxylic acid-containing disulfides/thioacetates. Selected linkers were attached to azobenzenes by amide coupling or azide-alkyne cycloaddition for surface attachment, photoswitching and/or enzyme assay. Azide-alkyne cycloaddition yields were initially poor, but were improved by use of stoichiometric amounts of copper catalyst. Chapter six describes UV/vis photoisomerisation studies and enzyme assays carried out to assess enzyme photoswitching of the compounds described in chapters 2-5. The trifluoromethylketones and α-ketoesters described in chapter 4 gave the best results, with moderate inhibition of α-chymotrypsin (µM affinity constants) and up to 5.3 fold changes in inhibition on UV/vis irradiation. Many of the boronate esters described in chapter 2 were found to inhibit α-chymotrypsin, but were somewhat unstable to irradiation. The dipeptide-based compounds described in chapter 3 were inactive against α-chymotrypsin. Good photoisomerisation was obtained for an azobenzene containing a symmetrical disulfide surface linker and poor photoisomerisation was obtained for an azobenzene containing a lipoic acid surface linker. Chapter seven describes surface attachment of selected photoswitch inhibitors and studies of photoregulated enzyme binding to the resultant functional surfaces. Self assembled monolayers (SAMs) of disulfides were formed on gold surfaces and characterised by electrochemistry and contact angle measurements. Binding of α-chymotrypsin to SAMs containing a photoswitch inhibitor was detected by quartz crystal microbalance (QCM), but was found to be largely irreversible. An alkyne-containing photoswitch inhibitor was attached to a surface plasmon resonance (SPR) chip in a two step procedure involving generation of an azide modified surface followed by azide-alkyne cycloaddition. Binding of α-chymotrypsin to the resultant modified surface was detected by SPR and successfully regulated by UV/vis irradiation. Chapter eight provides conclusions for the work described in this thesis and suggests future directions. Chapter nine gives experimental details for the work described in this thesis.

This thesis is concerned with the structural analysis of dyestuffs in their natural solid state by the application of solid-state nuclear magnetic resonance. These dysetuffs are all derived from the phenylazobenzene group, but tautomerism can produce structural changes, which have so far been uncharacterised in the solid-state for many of the dyestuffs currently under investigation. The information obtainable from (^13)C and (^15)N chemical shifts, both isotropic and anisotropic will be applied in this structure determination. Under magic-angle spinning the anisotropic nature of solid-state interactions is partially averaged or removed. The rotational resonance technique will be presented, which reintroduces the homonuclear dipolar interaction allowing dipolar coupling constants to be measured. Second-order effects arising from the (^14)N quadrupole interaction broaden spin-1/2 lines (RDC) in such a manner that bond lengths can be determined. This RDC analysis will be applied to a series of hydrazone structures to determine the (^15)N-(^14)N bond length within the hydrazone linkage. Finally, the two-dimensional magic-angle turning experiment will be discussed and applied to both the (13)C and (^15)N nuclei for a range of dyestuffs to show that accurate shielding tensor information can be obtained from large molecules.