Preparação de compósitos de VACNT/PMMA via ATRP e sua caracterização

Mestrado em Ciência de MateriaisVertically aligned carbon nanotubes (VACNT) were synthesized using thermal chemical vapour deposition (CVD) technique and modified with oxygen-containing functional groups (hydroxyl, carbonyl and carboxyl) using an Ar:O2 (97:3) plasma. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and scanning electron microscopy (SEM) have confirmed that those functional groups have been successfully grafted from the surface of the nanotubes. The plasma treatment was also found to remove significant amounts of amorphous carbon produced and deposited on the forests during the CVD process. The aligned carbon nanotube forests were further modified with an atom transfer radical polymerization (ATRP) initiator to grow poly(methyl methacrylate) (PMMA) chains via an in situ controlled/living radical polymerization. The resulting VACNT/PMMA composite was analyzed using Raman, Fourier transform infrared (FT-IR) and proton nuclear magnetic resonance (1H-NMR) spectroscopies, and scanning transmission electron microscopy (STEM). The TEM image mode and the 1H-NMR results clearly showed the presence of a polymer matrix surrounding the vertically aligned CNT forests. In addition, compressive tests carried out for both pristine VACNT and VACNT/PMMA composite have showed a higher compressive strength of the composite material than the pristine. This resulted from the denser structural arrangement of the vertically aligned CNTs due to the zipping effect observed for CNTs in solution processing methods and due to the presence of the polymer matrix, which gives extra support for the VACNTs.Nanotubos de carbono alinhados verticalmente (VACNT) foram preparadas por deposição química em fase vapor (CVD) em parede quente e posteriormente submetidos a um tratamento com plasma de uma mistura Ar:O2 (97:3) de modo a obter grupos hidroxilo, carbonilo, carboxílo à superfície. Esta modificação superficial dos nanotubos foi confirmada através das espectroscopias de XPS e de Raman, bem como por microscopia electrónica de varrimento. Os resultados obtidos mostraram ainda que através do tratamento de plasma se remove uma quantidade significativa de material grafítico que fora depositado durante o processo CVD. Após o tratamento com plasma dos VACNT, procedeu-se ao ancoramentp do iniciador de polimerização radicalar por transferência e átomo (ATRP) à sua superfície. Seguidamente procedeu-se à preparação de cadeias de poli(metacrilato de metido) (PMMA) a partir da superfície. O compósito obtido VACNT/PMMA foi caracterizado pelas espectroscopias de Raman, de FT-IR e 1H-RMN e ainda por microscopia de transmissão electrónica em varrimento (STEM). Os resultados obtidos a partir de imagens em modo TEM e do RMN-1H confirmaram a presença da matriz polimérica em torno dos VACNT. Além disso, os testes de compressão realizados tanto para as florestas originais, como para o compósito, revelaram que este último apresentou uma resistência à compressão superior que resulta da compactação (zipping) sofrida pelos tubos durante o processamento em solução, bem como da presença da matriz polimérica a qual confere uma maior estabilidade dimensional e mecânica à floresta de CNTs

Synthesis and Photochemical Characterisation of Photoactive Compounds for Molecular Electronics

In this modern age of technology, communication is highly reliant on computing devices such as mobile phones and computers. The advent of artificial intelligence (AI) and internet of things (IoT) as well as the widespread use of user-generated contents (UGC) are demanding more computing power and large data storage. However, traditional silicon based semiconductor technology is struggling to fulfill the required infrastructure due to fundamental physics and fabrication challenges. To overcome these challenges, molecular electronics has been proposed as potentially viable solution. With judicious design, organic molecules can be synthesized with tailored properties that can emulate the functions of conventional electronics devices such as diodes, switches and transistors. To this end, photochromic molecules attract significant attention since they can be switched with light/voltage between a less conducting and highly conducting forms. This property can be exploited to perform logic operations similar to transistors. This thesis explore the potential of norbornadiene-quadricyclane (NBD-QC)-based photochromic system, to serve as a switch for molecular electronics application. Four NBD derivatives, terminated with a thiol and thiophene groups, to enable tethering between gold electrodes, were synthesized. The compounds photochemical and photophysical properties were investigated using absorption and fluorescence spectroscopy. The results showed the compounds ability to switch between the NBD form and QC form upon photoirradiation. Moreover, the compounds were found to exhibit intrinsic emission. In particular, the long conjugated NBD form were found to be highly emissive, FF= 49%. Moreover, it was discussed that the emission can be tuned by the use of light, this makes them a potential candidate for optical memory device application. To test the robustness of the switching, more than 100 switching cycles were performed in solution and little or no degradation was observed, particularly under inert atmosphere. Additionally, the charge transport through the molecules were studied as well, using Scanning Tunneling Microscope-Break Junction (STM-BJ) technique. The results showed higher conductance values for the NBD forms and lower conductance values for the QC forms. Furthermore, we tested the potential of 2-nitrobenzyl-based photocleavable protection group (PPG) to release terminal alkynes on plasmonic surfaces by selective light activation. The terminal alkynes may then react, for example, with azido groups embedded on nanoparticles to create a dimer linked by a single molecule. By using the tools of template self-assembly the dimers can be aligned and placed on electrodes made by lithography. Initial findings showed promising result moving us closer to create single molecule devices based on parallel fabrication

Release of Terminal Alkynes via Tandem Photodeprotection and Decarboxylation of o -Nitrobenzyl Arylpropiolates in a Flow Microchannel Reactor

Photocleavable protecting groups (PPGs) offer a complementary protection paradigm compared to traditional protection groups. Herein, an o-nitrobenzyl (NB) PPG was employed to protect a variety of arylpropiolic acids. Upon a cascade of light-triggered photodeprotection in a microchannel reactor (residence times of 100-500 s), followed by Cu-catalyzed decarboxylation at 60 \ub0C, the NB-protected arylpropiolic acid afforded a terminal alkyne. This terminal alkyne was further reacted in situ with an azide via click chemistry to yield a 1,2,3-triazole in a one-pot reaction. Furthermore, the effect of different substituents (methyl, vinyl, allyl, and phenyl) at the benzylic position on the rate of photodeprotection was studied. The quantum yields of photolysis for the benzylic-substituted esters were determined to be as high as 0.45 compared to the unsubstituted ester with a 0.08 quantum yield of photolysis

A Memristive Element Based on an Electrically Controlled Single-Molecule Reaction

A single-molecule memory element is electrically-controlled using two distinct reaction mechanisms as reported by K. Moth-Poulsen, J. Hihath, and co-workers (DOI:10.1002/anie.202002300). By using separate electrically-controllable reactions for the forward and reverse reactions the bistable norbornadiene-quadricyclane system can be set in either state. The device can be switched through multiple cycles when a square-wave voltage signal is applied to the molecule. Each state has a unique conductance value allowing the system to act as a switch or memory device

Turn-off mode fluorescent norbornadiene-based photoswitches

Single-molecule fluorescence emission of certain positive photochromic systems such as diarylethenes have been exploited for biological imaging and optical memory storage applications. However, there is a lack of understanding if negative photochromic systems can be used for such type of applications. Hence, to explore the potential of negative photochromic molecules for possible optical memory storage applications, we have here synthesized and studied a series of four norbornadiene-quadricyclane (NBD-QC) photoswitching molecules. These molecules feature either linearly conjugated or cross-conjugated pi-electron systems. Upon photoisomerization, the UV-vis absorption spectra of the molecules revealed a strong blue shift in the QC-form, with a photoisomerization quantum yield close to 80% for the cross-conjugated systems. In contrast, a strong intrinsic emission (up to Φf= 49%) for the linearly conjugated compounds in the NBD form was observed. Upon light-induced isomerization, the emission was completely turned off in the QC-form in all the compounds studied. Further, the robustness of the system was evaluated by performing several switching cycles. Under nitrogen, the emission can be turned off and recovered with almost no loss of emission. We also show that the QC-form can be photochemically triggered to convert back to the NBD-form using a low energy UV light (340 nm), allowing an all optical conversion to both species. The demonstrated properties can make the NBD-QC system attractive for potential applications such as optical memory storage devices

Three-Input Molecular Keypad Lock Based on a Norbornadiene-Quadricyclane Photoswitch

Copyright \ua9 2018 American Chemical Society. The photo- and acidochromic properties of a new generation norbornadiene derivative have been harnessed for the realization of a three-input keypad lock, where a specific sequence of inputs induces a unique output. Reversible quadricyclane/norbornadiene photoisomerization is reported, and this rare feature allows the first example of a norbornadiene-based molecular logic system. The function of this system is clearly rationalized in terms of the interconversion scheme and the absorption spectra of the involved species

Investigating New Applications of a Photoswitchable Fluorescent Norbornadiene as a Multifunctional Probe for Delineation of Amyloid Plaque Polymorphism

Amyloid beta (Aβ) plaques are a major pathological hallmark of Alzheimer’s disease (AD) and constitute of structurally heterogenic entities (polymorphs) that have been implicated in the phenotypic heterogeneity of AD pathology and pathogenesis. Understanding amyloid aggregation has been a critical limiting factor to gain understanding of AD pathogenesis, ultimately reflected in that the underlying mechanism remains elusive. We identified a fluorescent probe in the form of a turn-off photoswitchable norbornadiene derivative (NBD1) with several microenvironment-sensitive properties that make it relevant for applications within advanced fluorescence imaging, for example, multifunctional imaging. We explored the application of NBD1 for in situ delineation of structurally heterogenic Aβ plaques in transgenic AD mouse models. NBD1 plaque imaging shows characteristic broader emission bands in the periphery and more narrow emission bands in the dense cores of mature cored plaques. Further, we demonstrate in situ photoisomerization of NBD1 to quadricyclane and thermal recovery in single plaques, which is relevant for applications within both functional and super-resolution imaging. This is the first time a norbornadiene photoswitch has been used as a probe for fluorescence imaging of Aβ plaque pathology in situ and that its spectroscopic and switching properties have been studied within the specific environment of senile Aβ plaques. These findings open the way toward new applications of NBD-based photoswitchable fluorescent probes for super-resolution or dual-color imaging and multifunctional microscopy of amyloid plaque heterogeneity. This could allow to visualize Aβ plaques with resolution beyond the diffraction limit, label different plaque types, and gain insights into their physicochemical composition

Heteroaryl-linked norbornadiene dimers with redshifted absorptions

Development of Molecular Solar Thermal (MOST) systems for harvesting and storing solar energy is based on molecular photoswitches that undergo photoisomerizations to metastable isomers. One challenge is to achieve low-molecular weight molecules that absorb at sufficiently long wavelengths to match the solar spectrum. Here we show that this can be achieved by linking two norbornadiene (NBD) photoswitches to a central heterocycle, thiophene or carbazole, via alkyne appendages. In this approach, the same heteroaryl is used to tune the properties of two photoswitches at the same time, thereby keeping the molecular weight as low as possible. A series of NBD dimers was prepared by Sonogashira coupling reactions, and these compounds showed remarkable redshifted absorptions, with onsets of absorption as high as 468 nm, and thermal half-lives ranging from 44 seconds to 16 hours

A Memristive Element Based on an Electrically Controlled Single‐Molecule Reaction

A single-molecule memory element is electrically-controlled using two distinct reaction mechanisms as reported by K. Moth-Poulsen, J. Hihath, and co-workers (DOI:10.1002/anie.202002300). By using separate electrically-controllable reactions for the forward and reverse reactions the bistable norbornadiene-quadricyclane system can be set in either state. The device can be switched through multiple cycles when a square-wave voltage signal is applied to the molecule. Each state has a unique conductance value allowing the system to act as a switch or memory device