Spectroelectrochemical studies of electrochromic diarylethene ionic Liquids : From solution to ionogel based devices

Acord transformatiu CRUE-CSICThis study establishes the photoelectrochromic properties of four diarylethene ionic liquids (DAE-ILs) composed of a diarylethene dicarboxylate (DAE) moiety, as counter-anion, and different organic cations ([Cmim], [CPyr] , [N], [P]). The synthetized ILs viscous fluids maintain the photochemical and electrochemical properties of this well-known family of photoswitches, as well as the typical features of ILs (e.g., low melting point, and high thermal and electrochemical stability). Our strategy of making multi-stimuli-responsive ionic liquids by ionic exchange opens the door to easily tune the physicochemical properties of smart DAE-ILs in function of the selected cation. Finally, those DAE-ILs are used to formulate smart ionogels (IGs), a type of materials that, by showing good conductive properties and retaining the stimuli-responsive behavior of the embedded DAE molecules, allow the fabrication of photoelectrochemical devices

Multi Stimuli-responsive Organic Salts: From preparation to functional device application

The discovery of novel and efficient stimuli-responsive materials that change their properties according to external stimuli and therefore, adjust to our demands become an interesting research topic for several potential applications. The integration of the most promissory stimuli-responsive materials in devices for antiglare car mirrors, smart windows, sunglasses, sensors, among others has been applied. In this thesis, the development and application of stimuli-responsive materials containing ionic photochromic or electrochromic units have been explored. In general, it is possible to incorporate specific scaffolds possessing pH, temperature, and light or electron-transfer stimuli-responsive behaviour in the cation or anion structures. Also, ionic polymers based on these structures have been investigated. In this context, 4,4’-bipyridinium, diarylethenes and flavylium were selected as electrochromic and photochromic units, respectively. For the preparation of the selected organic salts including polymers, different synthetic routes and purification processes have been followed. Detailed characterization of all prepared salts by spectroscopic techniques in order to elucidate their structures and purities has been performed. Thermal, rheological, photochemical and electrochemical properties of some prepared salts were also studied. It is important to focus that the adequate selection of the counter-ions can be crucial to achieve ionic liquids as well as to tune their final properties. The most promissory electrochromic salts based on 4,4’-bipyridinium symmetrical and non-symmetrical di and tetra-cations as well as ionic polymers were tested in liquid, gel or solid electrochromic devices. The substituents from bipyridinium scaffold as well as counter-ions can significantly influence the electrochromic performance in particular its reversibility; stability; colour contrast and transition times. Photochromic Room Temperature Ionic Liquid based on the combination of diarylethene derivative anion with tri-octyl methylammonium ([ALIQUAT]) cation was developed and characterized. Additionally, a new thermal, pH and photo-stimuli responsive co-polymer containing N-isopropylacrylamide and flavylium derivative have been prepared. The chemical versatility of the prepared ionic photo- and electrochromic materials opens excellent perspectives for future applications as efficient and reversible multi stimuli-responsive materials

Dithienylethene optical switches : multicomponent molecular systems

Dithienylethenes are photochromic compounds that interconvert between two distinct states, between colorless open form and colored closed form when irradiation with ultraviolet and visible light, respectively. In this thesis, photochromic dithienylethene switching is employed to investigate multi-functional systems. The synthesis and photochemical characterization of a dithienylethene dimer covalently tethered by a short linker, -SiMe2-, is described as multiswitches for multi-addressable systems. Importantly, it was found that there is no significant intramolecular interaction between the two dithienylethene units despite theirs proximity. Secondly, the photo- and electrochemical properties of self-assembled monolayers of diarylethenes on non-metallic surfaces (quartz and ITO surfaces) are investigated. The state of the modified surface can be read ‘non-destructively’ by electrochemical readout to achieve Read/Write/Erase information storage. Thirdly, the photochemical switching properties of dithienylethene compounds, which can be used to control the electropolymerization properties of bis-terthiophene monomer onto the surface with UV and visible light, respectively, is described. In the open state, electropolymerization yields alkene bridged sexithiophene polymers through oxidative α,α- coupling, while in the closed state the polymerizability is switched off. Moreover, the photoswitchable sexithiophene molecular wires were prepared by employing electrochemical dimerization of dithienylethene monomers. The photochromic dithienylethene units retain their photochemical properties when part of an extended π- conjugated sexithiophene system. Finally, the syntheses of star-shaped dithienylethene substituted hexaphenylbenzenes through a dicobaltoctacarbonyl-catalyzed cyclotrimerization reaction. This reaction represents a direct facile method for the synthesis of hexaphenylbenzene centered multi-dithienylethene systems.

Excited-state electronic asymmetry prevents photoswitching in terthiophene compounds

The diarylethene moiety is one of the most extensively used switches in the field of molecular electronics. Here we report on spectroscopic and quantum chemical studies of two diarylethene-based compounds with a non-C3-symmetric triethynyl terthiophene core symmetrically substituted with RuCp*(dppe) or trimethylsilyl termini. The ethynyl linkers are strong IR markers that we use in time-resolved vibrational spectroscopic studies to get insight into the character and dynamics of the electronically excited states of these compounds on the picosecond to nanosecond time scale. In combination with electronic transient absorption studies and DFT calculations, our studies show that the conjugation of the non-C3-symmetric triethynyl terthiophene system in the excited state strongly affects one of the thiophene rings involved in the ring closure. As a result, cyclization of the otherwise photochromic 3,3″-dimethyl-2,2′:3′,2″-terthiophene core is inhibited. Instead, the photoexcited compounds undergo intersystem crossing to a long-lived triplet excited state from which they convert back to the ground state

Doctor of Philosophy

dissertationFor the past 40 years, optical lithography has been the patterning workhorse for the semiconductor industry. However, as integrated circuits have become more and more complex, and as device geometries shrink, more innovative methods are required to meet these needs. In the far-field, the smallest feature that can be generated with light is limited to approximately half the wavelength. This so-called far-field diffraction limit or the Abbe limit (after Prof. Ernst Abbe who first recognized this) effectively prevents the use of long-wavelength photons (>300nm) from patterning nanostructures (<100nm). Even with a 193nm laser source and extremely complicated processing, patterns below ~ 20nm are incredibly challenging to create. Sources with even shorter wavelengths can potentially be used. However, these tend to be much more expensive and of much lower brightness, which in turn limits their patterning speed. Multi-photon reactions have been proposed to overcome the diffraction limit. However, these require very large intensities for modest gain in resolution. Moreover, the large intensities make it difficult to parallelize, thus limiting the patterning speed. In this dissertation, a novel nanopatterning technique using wavelength-selective small molecules that undergo single-photon reactions, enabling rapid top-down nanopatterning over large areas at low-light intensities, thereby allowing for the circumvention of the far-field diffraction barrier, is developed and experimentally verified. This approach, which I refer to as Patterning via Optical Saturable Transitions (POST) has the potential for massive parallelism, enabling the creation of nanostructures and devices at a speed far surpassing what is currently possible with conventional optical lithographic techniques. The fundamental understanding of this technique goes beyond optical lithography in the semiconductor industry and is applicable to any area that requires the rapid patterning of large-area two- or three-dimensional complex geometries. At a basic level, this research intertwines the fields of electrochemistry, material science, electrical engineering, optics, physics, and mechanical engineering with the goal of developing a novel super-resolution lithographic technique

The synthesis, photochemistry & electrochemistry of dithienylethene switches & their organometallic complexes

Chapter one presents a detailed literature survey on dithienylcyclopentene switches, describing their synthesis, properties and applications. The photochromic and electrochromic effects of functionalising such switching units with metal complexes, is then discussed. Finally, an introduction to cobalt carbonyl complexes is given, with an outline of the photochemical and electrochemical properties of such carbonyl complexes, as described in the literature. Chapter two describes the synthetic procedures employed to prepare a number of dithienyl-perhydro- and perfluoro-cyclopentene switches, substituted with thienyl and ferrocenyl moieties. The methods used to generate their corresponding Co2(CO)6 and Co2(CO)4dppm complexes are also described. 1H, 13C, 19F and 31P NMR techniques were employed to analyse the resulting products. Elemental analysis and infra-red spectroscopy were also utilised, where applicable, to further characterise the final pure compounds and the results are detailed in this chapter. Chapter three describes the photochemical properties found for the thienyl-based dithienylcyclopentene switches and their corresponding cobalt carbonyl complexes. The photochromic properties of the thienyl-based switches were monitored in the UVvis absorption spectra and the 1H NMR spectra. Their thermal stability, fatigue resistance and fluorescent properties were also investigated. Furthermore, the effects of incorporating cobalt carbonyl moieties onto these thienyl-based switches, on their photocyclisation processes, are reported in this chapter. Chapter four reports the photochromic behaviour of the ferrocenyl-based dithienylcyclopentene switches, as observed in the UV-vis and 1H NMR. Investigations into the thermal stability, fatigue resistance and fluorescent properties of these switches were carried out. The effects on the photochromic properties of the ferrocenyl-based switches, following the introduction of Co2(CO)6 and Co2(CO)4dppm moieties, are also described in this chapter. Chapter five focuses on the electrochemical properties of the thienyl-based dithienylcyclopentene switches. Electrochemically induced cyclisation/ cycloreversion processes were investigated through cyclic voltammetry and UV-vis spectroelectrochemistry techniques. Similar experiments were carried out for the cobalt carbonyl derivatives in order to examine the effects of the presence of the metal carbonyls on the oxidative ring-closing/opening abilities of the thienyl-based switches. The effects of the oxidation processes on the cobalt carbonyl centres were also studied by monitoring the changes of the carbonyl stretches in the infra-red spectra. Chapter six details the electrochemical behaviour of the ferrocenyl-based dithienylcyclopentene switches. The oxidative and reductive processes of these switches were monitored in the cyclic voltammograms and UV-vis spectra of these switches in order to investigate if ring-opening/closing can be induced by electrochemical means. Their Co2(CO)6 and Co2(CO)4dppm complexes were also subjected to similar experiments in order to determine the effects of introducing such metal carbonyl complexes on the electrochemical switching behaviour of these ferrocenyl-based switches. IR spectroelectrochemical techniques were employed to examine the effects of oxidation processes on the cobalt carbonyl centres. Chapter seven presents an overall conclusion of the results obtained, with an emphasis on a comparison between the effects of the thiophene substituents and the ferrocene units, and the prospects of future work in this area is discussed. All publications are presented in the appendix

Controlling the Cycloreversion Reaction of a Diarylethene Derivative Using Sequential Two-Photon Excitation

Diarylethenes (DAE) are a class of photochromic molecular switches that convert between two structural isomers upon excitation with light. A great deal of research has been dedicated to elucidating the mechanisms of the reversible electrocyclic reactions to make optical memory devices with DAE compounds, but details of the fundamental reaction mechanism after one- or two-photons of light is still lacking. The primary DAE discussed in this dissertation is 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene (DMPT-PFCP), which is a model compound for studying the fundamental reaction dynamics using one- and two-photon excitation experiments. Pump-probe spectroscopy was used to study the low one-photon quantum yield cycloreversion reaction of DMPT-PFCP by changing the excitation wavelength, solvent, and temperature to describe the dynamics on the ground- and excited states. However, the primary goal of this work was to use sequential two-photon excitation with fs laser pulses to map out the cycloreversion reaction dynamics for DMPT-PFCP compound on the first and higher excited states. The cycloreversion quantum yield was selectively increased using sequential two-photon excitation, where after promotion to the S1 state, a second excitation pulse promotes the molecules to an even higher excited state. The mechanism of increasing the yield by promoting the molecules to a higher excited state was explored using pump-repump-probe (PReP) spectroscopy. The PReP experiments follow the excited-state dynamics as the molecules sample different regions of the S1 potential energy surface. The projection of the S1 dynamics onto the higher excited states showed that by changing the secondary excitation wavelength and the delay between excitation pulses, the cycloreversion quantum yield was selectively controlled. Future studies to obtain the specific modes involved in the ring-opening reaction coordinate on the excited-state would further improve our knowledge of the cycloreversion reaction and therefore improve the efficiency of the sequential two-photon excitation process to make very efficient optical memory devices using DAE compounds