Photoactive nanodevices for potential biological applications

Biological systems are complex and highly organized architectures governed by noncovalent interactions, which are responsible for molecular recognition, self-assembly, self-organization, adaptation and evolution processes. These systems provided the inspiration for the development of supramolecular chemistry, that aimed at the design of artificial multicomponent molecular assemblies, namely supramolecular systems, properly designed to perform different operations: each constituting unit performs a single act, whereas the entire supramolecular system is able to execute a more complex function, resulting from the cooperation of the constituting components. Supramolecular chemistry deals with the development of molecular systems able to mimic naturally occurring events, for example complexation and self-assembly through the establishment of noncovalent interactions. Moreover, the application of external stimuli, such as light, allows to perform these operations in a time- and space-controlled manner. These systems can interact with biological systems and, thus, can be applied for bioimaging, therapeutic and drug delivery purposes. In this work the study of biocompatible supramolecular species able to interact with light is presented. The first part deals with the photophysical, photochemical and electrochemical characterization of water-soluble blue emitting triazoloquinolinium and triazolopyridinium salts. Moreover, their interaction with DNA has been explored, in the perspective of developing water-soluble systems for bioimaging applications. In the second part, the effect exerted by the presence of azobenzene-bearing supramolecular species in liposomes, inserted both in the phospholipid bilayer and in the in the aqueous core of vesicles has been studied, in order to develop systems able to deliver small molecules and ions in a photocontrolled manner. Moreover, the versatility of azobenzene and its broad range of applications have been highlighted, since conjugated oligoazobenzene derivatives proved not to be adequate to be inserted in the phospholipid bilayer of liposomes, but their electrochemical properties made them interesting candidates as electron acceptor materials for photovoltaic applications.

Azo-propofols: photochromic potentiators of GABA(A) receptors

Shine and rise! GABA(A) receptors are ligand-gated chloride ion channels that respond to γ-aminobutyric acid (GABA), which is the major inhibitory neurotransmitter of the mammalian central nervous system. Azobenzene derivatives of propofol, such as compound 1 (see scheme), increase GABA-induced currents in the dark form and lose this property upon light exposure and thus function as photochromic potentiators. Compound 1 can be employed as a light-dependent general anesthetic in translucent tadpoles.

A novel open channel blocker of GABA-A receptors

GABA-A receptors are chloride ion channels composed of five subunits, mediating fast synaptic and tonic inhibition in the mammalian brain. 19 different subunit isoforms have been identified, with the major receptor type in mammalian adult brain consisting of α1, β2, and γ2 subunits. GABA-A receptors are the target of numerous sedating and anxiolytic drugs such as benzodiazepines. The currently known endogenous ligands are GABA, neurosteroids and the endocannabinoid 2- arachidonoyl glycerol (2-AG). The pharmacological properties of this chloride ion channel strictly depend on receptor subunit composition and arrangement. GABA-A receptors bind and are inhibited by epileptogenic agents such as picrotoxin, and cyclodiene insecticides such as dieldrin. We screened aromatic monovalent anions with five-fold symmetry for inhibition of GABA-A receptors. One of the anions, PCCPinhibited currents elicited by GABA with comparable potency as picrotoxin. This inhibition showed all characteristics of an open channel block. The GABA-A receptor ion channel is lined by residues from the M2 membrane-spanning segment. To identify important residues of the pore involved in the interaction with the blocking molecules PCCP-, a mutation scan was performed in combination with subsequent analysis of the expressed mutant proteins using electrophysiological techniques. In a second project we characterised a light-switchable modulator of GABA-A receptors based on propofol. It was my responsibility to investigate the switching kinetics in patch clamp experiments. After its discovery in 1980, propofol has become the most widely used intravenous general anaesthetic. It is commonly accepted that the anaesthesia induced by this unusually lipophilic drug mostly results from potentiation of GABA induced currents. While GABA-A receptors respond to a variety of ligands, they are normally not sensitive towards light. This light sensitivity could be indirectly achieved by using modulators that can be optically switched between an active and an inactive form. We tested an azobenzene derivative of propofol where an aryldiazene unit is directly coupled to the pharmacophore. This molecule was termed azopropofol (AP2). The effect of AP2 on Cl- currents was investigated with electrophysiological techniques using α1β2γ2 GABA-A receptors expressed in Xenopus oocytes and HEK-cells. In the third project we wanted to investigate the functional role of GABA-A receptors in the liver, and their possible involvement in cell proliferation. GABA-A receptors are also found in a wide range of peripheral tissues, including parts of the peripheral nervous system and non-neural tissues such as smooth muscle, the female reproductive system, liver and several cancer tissues. However their precise function in non neuronal or cancerous cells is still unknown. For this purpose we investigated expression, localization and function of the hepatocytes GABA-A receptors in model cell lines and healthy and cancerous hepatocytes.

L'orientation et la propriété de mémoire de forme des polymères cristallins liquides à chaînes latérales covalents et supramoléculaires

In many studies of the side-chain liquid crystalline polymers (SCLCPs) bearing azobenzene mesogens as pendant groups, obtaining the orientation of azobenzene mesogens at a macroscopic scale as well as its control is important, because it impacts many properties related to the cooperative motion characteristic of liquid crystals and the trans-cis photoisomerization of the azobenzene molecules. Various means can be used to align the mesogens in the polymers, including rubbed surface, mechanical stretching or shearing, and electric or magnetic field. In the case of azobenzene-containing SCLCPs, another method consists in using linearly polarized light (LPL) to induce orientation of azobenzene mesogens perpendicular to the polarization direction of the excitation light, and such photoinduced orientation has been the subject of numerous studies. In the first study realized in this thesis (Chapter 1), we carried out the first systematic investigation on the interplay of the mechanically and optically induced orientation of azobenzene mesogens as well as the effect of thermal annealing in a SCLCP and a diblock copolymer comprising two SCLCPs bearing azobenzene and biphenyl mesogens, respectively. Using a supporting-film approach previously developed by our group, a given polymer film can be first stretched in either the nematic or smectic phase to yield orientation of azobenzene mesogens either parallel or perpendicular to the strain direction, then exposed to unpolarized UV light to erase the mechanically induced orientation upon the trans–cis isomerization, followed by linearly polarized visible light for photoinduced reorientation as a result of the cis–trans backisomerization, and finally heated to different LC phases for thermal annealing. Using infrared dichroism to monitor the change in orientation degree, the results of this study have unveiled complex and different orientational behavior and coupling effects for the homopolymer of poly{6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate} (PAzMA) and the diblock copolymer of PAzMA-block- poly{6-[4-(4-cyanophenyl) phenoxy]hexyl methacrylate} (PAzMA-PBiPh). Most notably for the homopolymer, the stretching-induced orientation exerts no memory effect on the photoinduced reorientation, the direction of which is determined by the polarization of the visible light regardless of the mechanically induced orientation direction in the stretched film. Moreover, subsequent thermal annealing in the nematic phase leads to parallel orientation independently of the initial mechanically or photoinduced orientation direction. By contrast, the diblock copolymer displays a strong orientation memory effect. Regardless of the condition used, either for photoinduced reorientation or thermal annealing in the liquid crystalline phase, only the initial stretching-induced perpendicular orientation of azobenzene mesogens can be recovered. The reported findings provide new insight into the different orientation mechanisms, and help understand the important issue of orientation induction and control in azobenzene-containing SCLCPs. The second study presented in this thesis (Chapter 2) deals with supramolecular side-chain liquid crystalline polymers (S-SCLCPs), in which side-group mesogens are linked to the chain backbone through non-covalent interactions such as hydrogen bonding. Little is known about the mechanically induced orientation of mesogens in S-SCLCPs. In contrast to covalent SCLCPs, free-standing, solution-cast thin films of a S-SCLCP, built up with 4-(4’-heptylphenyl) azophenol (7PAP) H-bonded to poly(4-vinyl pyridine) (P4VP), display excellent stretchability. Taking advantage of this finding, we investigated the stretching-induced orientation and the viscoelastic behavior of this S-SCLCP, and the results revealed major differences between supramolecular and covalent SCLCPs. For covalent SCLCPs, the strong coupling between chain backbone and side-group mesogens means that the two constituents can mutually influence each other; the lack of chain entanglements is a manifestation of this coupling effect, which accounts for the difficulty in obtaining freestanding and mechanically stretchable films. Upon elongation of a covalent SCLCP film cast on a supporting film, the mechanical force acts on the coupled polymer backbone and mesogenic side groups, and the latter orients cooperatively and efficiently (high orientation degree), which, in turn, imposes an anisotropic conformation of the chain backbone (low orientation degree). In the case of the S-SCLCP of P4VP-7PAP, the coupling between the side-group mesogens and the chain backbone is much weakened owing to the dynamic dissociation/association of the H-bonds linking the two constituents. The consequence of this decoupling is readily observable from the viscoelastic behavior. The average molecular weight between entanglements is basically unchanged in both the smectic and isotropic phase, and is similar to non-liquid crystalline samples. As a result, the S-SCLCP can easily form freestanding and stretchable films. Furthermore, the stretching induced orientation behavior of P4VP-7PAP is totally different. Stretching in the smectic phase results in a very low degree of orientation of the side-group mesogens even at a large strain (500%), while the orientation of the main chain backbone develops steadily with increasing the strain, much the same way as amorphous polymers. The results imply that upon stretching, the mechanical force is mostly coupled to the polymer backbone and leads to its orientation, while the main chain orientation exerts little effect on orienting the H-bonded mesogenic side groups. This surprising finding is explained by the likelihood that during stretching in the smectic phase (at relatively higher temperatures) the dynamic dissociation of the H-bonds allow the side-group mesogens to be decoupled from the chain backbone and relax quickly. In the third project (Chapter 3), we investigated the shape memory properties of a S-SCLCP prepared by tethering two azobenzene mesogens, namely, 7PAP and 4-(4'-ethoxyphenyl) azophenol (2OPAP), to P4VP through H-bonding. The results revealed that, despite the dynamic nature of the linking H-bonds, the supramolecular SCLCP behaves similarly to covalent SCLCP by exhibiting a two-stage thermally triggered shape recovery process governed by both the glass transition and the LC-isotropic phase transition. The ability for the supramolecular SCLCP to store part of the strain energy above T[subscript g] in the LC phase enables the triple-shape memory property. Moreover, thanks to the azobenzene mesogens used, which can undergo trans-cis photoisomerization, exposure the supramolecular SCLCP to UV light can also trigger the shape recovery process, thus enabling the remote activation and the spatiotemporal control of the shape memory. By measuring the generated contractile force and its removal upon turning on and off the UV light, respectively, on an elongated film under constant strain, it seems that the optically triggered shape recovery stems from a combination of a photothermal effect and an effect of photoplasticization or of an order-disorder phase transition resulting from the trans-cis photoisomerization of azobenzene mesogens.

Synthesis and structural stability of metal-organic frameworks

Metal-organic frameworks (MOFs) have attracted significant attention during the past decade due to their high porosity, tunable structures, and controllable surface functionalities. Therefore many applications have been proposed for MOFs. All of them however are still in their infancy stage and have not yet been brought into the market place. In this thesis, the background of the MOF area is first briefly introduced. The main components and the motifs of designing MOFs are summarized, followed by their synthesis and postsynthetic modification methods. Several promising application areas of MOFs including gas storage and separation, catalysis and sensing are reviewed. The current status of commercialization of MOFs as new chemical products is also summarized. Examples of the design and synthesis of two new MOF structures Eu(4,4′,4′′,4′′′-(porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid))·2H2O∙xDMF and Zn4O(azobenzene-4,4’-dicarboxylic acid)3∙xNMP are described. The first one contains free-base porphyrin centers and the second one has azobenzene components. Although the structures were synthesized as designed, unfortunately they did not possess the expected properties. The research idea to use MOFs as template materials to synthesize porous polymers is introduced. Several methods are discussed to grow PMMA into IRMOF-1 (Zn4O(benzene-1,4-dicarboxylate)3, IR stands for isoreticular) structure. High concentration of the monomers resulted in PMMA shell after MOF digestion while with low concentration of monomers no PMMA was left after digestion due to the small iii molecular weight. During the study of this chapter, Kitagawa and co-workers published several papers on the same topic, so this part of the research was terminated thereafter. Many MOFs are reported to be unstable in air due to the water molecules in air which greatly limited their applications. By incorporating a number of water repelling functional groups such as trifluoromethoxy group and methyl groups in the frameworks, the water stability of MOFs are shown to be significantly enhanced. Several MOFs inculding Banasorb-22 (Zn4O(2-trifluoromethoxybenzene-1,4-dicarboxylate)3), Banasorb-24 (Zn4O(2, 5-dimethylbenzene-1,4-dicarboxylate)3) and Banasorb-30 (Zn4O(2-methylbenzene-1,4-dicarboxylate)3) were synthesized and proved to have isostructures with IRMOF-1. Banasorb-22 was stable in boiling water steam for one week and Banasorb-30’s shelf life was over 10 months under ambient condition. For comparison, IRMOF-1’s structure collapses in air after a few hours to several days. Although MOF is a very popular research area nowadays, only a few studies have been reported on the mechanical properties of MOFs. Many of MOF’s applications involve high pressure conditions, so it is important to understand the behavior of MOFs under elivated pressures. The mechanical properties of IRMOF-1 and a new MOF structure Eu2(C12N2O4H6)3(DEF)0.87(H2O)2.13 were studied using diamond anvil cells at Advanced Photon Source. IRMOF-1 experienced an irriversible phase transtion to a nonporous phase followed by amorphization under high pressure. Eu2(C12N2O4H6)3(DEF)0.87(H2O)2.13 showed reversible compression under pressure up to 9.08GPa.

Plasmon-enhanced two-photon-induced isomerization for highly-localized light-based actuation of inorganic/organic interfaces

Two-photon initiated photo-isomerization of an azobenzene moiety adsorbed on silver nanoparticles (Ag NPs) is demonstrated. The azobenzene is linked to a materials-binding peptide that brings it into intimate contact with the Ag NP surface, producing a dramatic enhancement of its two-photon absorbance. An integrated modeling approach, combining advanced conformational sampling with Quantum Mechanics/Capacitance Molecular Mechanics and response theory, shows that charge transfer and image charges in the Ag NP generate local fields that enhance two-photon absorption of the cis isomer, but not the trans isomer, of adsorbed molecules. Moreover, dramatic local field enhancement is expected near the localized surface plasmon resonance (LSPR) wavelength, and the LSPR band of the Ag NPs overlaps the azobenzene absorbance that triggers cis to trans switching. As a result, the Ag NPs enable two-photon initiated cis to trans isomerization, but not trans to cis isomerization. Confocal anti-Stokes fluorescence imaging shows that this effect is not due to local heating, while the quadratic dependence of switching rate on laser intensity is consistent with a two-photon process. Highly localized two-photon initiated switching could allow local manipulation near the focal point of a laser within a 3D nanoparticle assembly, which cannot be achieved using linear optical processes.