Controlled Charge Transfer in Single Walled Carbon Nanotubes using Organic Molecules via Nondestructive Covalent Pre-Functionalization

Abstract

This thesis proposes a novel approach for controlling and optimizing the optoelectronic properties of single-walled carbon nanotubes (SWCNTs) through functional groups for covalent functionalization and, ultimately, building photonic devices at the nanoscale. The carbon nanotube community demonstrated through decades of experiments the exceptional optoelectronic properties of SWCNTs. Semiconducting SWCNTs specifically have transitions that allow near infrared (NIR) fluorescence experiments for imaging purposes. To safeguard the fluorescence properties of the nanotube, Setaro et al. developed the first nondestructive covalent π-preserving functionalization (NCF) of SWCNTs using electron poor aromatic nitrenes (based on trichlorotriazine, called triazine in the following) via a [2+1] cycloaddition reaction in 2017. With additional spiropyran photoswitching groups, they could reversibly shift the Fermi level and facilitate charge transfer without introducing defect states that localize excitons, but preserving their diffusion along the tube. The functionalization approach required to replace the chlorine atoms of the grafted triazine already on the nanotube sidewall with molecular species (e.g. the spiropyran switch). The number of attached switches on the triazine could not be precisely controlled, making it challenging to obtain reproducibly a specific concentration of charges transferred between the nanotube and the organic molecule for optimal optoelectronic features. In this thesis, I worked on changing the synthetic approach of the molecular species attached on the SWCNTs in a fundamental way. Instead of attaching the targeted molecular species onto triazine on the SWCNTs, I developed a pre-functionalization strategy, in which I designed novel triazine derivatives with the desired chemical species replacing one or two chlorine atoms before grafting them onto the tubes. In this way, a successful grafting of the triazine onto the SWCNTs automatically ensures the presence and number of desired chemical species attached within the functional hybrids. I synthesized, characterized, and studied a defined set of charge transfer agents (CTAs) with a cyanuric chloride (triazine) backbone and varying substituents number and orientation that can withdraw or donate charges to the SWCNTs. The correlation between the optical properties and functionality of these CTAs was studied through experiments and simulations. Using the NCF approach, I covalently functionalized SWCNTs with these CTAs without destroying the sp2 crystalline network of the tube. Depending on the structural design of the CTAs, I observed the Fermi level position of the SWCNTs shift up or down by donating or withdrawing charges, as observed by Raman and XPS spectroscopy and confirmed by DFT predictions. As a result, the fluorescence intensity of the nanotubes is either quenched or enhanced, as evidenced by photoluminescence experiments. Effect of the reaction temperature used in the covalent functionalization strategy was also examined. Using only triazine molecule on SWCNTs, the impact of the functionalization temperature was found to directly influence the density of triazine molecules covalently attached to the SWCNTs, with 1 triazine molecule to 24 carbon atoms of the SWCNT at 150 oC and 1 triazine molecule to 155 carbon atoms of SWCNT at 25 oC. Across this temperature range, Raman studies at 638 nm laser pump show no increase of defectivity in the nanotube’s sp2 π network, as indicated by ID/IG ratios, while safeguarding the emission of different nanotube species in the photoluminescence spectra. The last part of this thesis builds upon the same pre-functionalization strategy and involves the stepwise synthesis of a novel photoactive triazine-spiropyran molecular complex, (1'-(4,6-dichloro-1,3,5-triazin-2-yl)-3',3'-dimethyl-6-nitrospiro[chromene-2,2'-indoline]), by attaching electron poor triazine to the nitrogen position of indoline moiety. Single crystal XRD and other spectroscopic analysis confirm the successful synthesis of triazine-spiropyran complex. UV absorption experiments show that triazine substituent strongly impacts the optical response of the complex during isomerization. Protonation followed by UV irradiation resulted in decreased absorptivity and appearance of a new absorption band at 390 nm. Subsequently, I covalently functionalized the triazine-spiropyran complex onto the SWCNTs, with Raman ID/IG ratios (measured using a 638 nm laser pump) indicating the preservation of the nanotube’s sp2 π-conjugated network. Activation of the triazine-spiropyran functionalized SWCNTs using UV photons induces reversible change in the nanotube’s EF, leading to ~52 % emission quenching, which is reversed upon thermal activation.