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Authors: Bengasi, Giuseppe
Title: Synthesis and Deposition of Directly Fused Porphyrin Tapes by Chemical Vapour Deposition Approach
Online publication date: 19-Jun-2020
Year of first publication: 2020
Language: english
Abstract: Porphyrins play a key role in biological processes such as photosynthesis and respiration. Inspired by their ubiquitous presence in natural systems, scientists have investigated their implementation in modern technological devices. In the last century, a wide series of studies have demonstrated the outstanding properties of this class of hybrid metal-organic compounds. In 2001, Osuka and co-workers reported for the first time the synthesis of directly fused polymeric porphyrin systems (porphyrin tapes). These systems possess impressive optoelectronic properties including electronic transitions occurring in the infrared, low attenuation factor and two-photon absorption. However, after twenty years from the discovery of porphyrin tapes, only few reports have investigated their use in the production of optoelectronic devices. This is mainly due to their complex synthesis and poor solubility, that reduce the industrial appeal and hinder the formation of smooth thin films required for device production. In 2006, Gleason and co-workers reported the synthesis of conjugated polymers directly from the gas phase in a novel approach named oxidative chemical vapour deposition (oCVD). In oCVD, the delivery of the chemicals through the gas phase circumvents the problems related to the poor solubility of precursors and products and directly yields conjugated polymer thin films on a given substrate. The technique involves modest temperatures (T≈ 300-500K) allowing the deposition of thin films on many thermostable substrates. Prior to this work, the oCVD technique was limited to the polymerization of small, purely organic monomers. Inspired from these two ground breaking studies on porphyrin tapes and oCVD, this thesis aims to bridge the knowledge acquired in different scientific fields and to fill the technological gap required to integrate porphyrin tapes into devices. This thesis also aims to develop a deep understanding on the reactivity of porphyrins towards oxidative coupling in the gas phase. The use of metalloporphyrins as monomers in oCVD can be considered as improvement for both the oCVD and the porphyrin tapes synthesis fields. While the oCVD of metalloporphyrin would allow obtaining smooth thin films of porphyrin tapes with controlled thickness, the use of metal-containing monomers such as metalloporphyrins is unprecedented and would allow to introduce new functionalities in the films obtained by oCVD. In addition, the synthesis of porphyrin thin films represents an important step towards the production of smart devices for catalysis, sensing and nonlinear optics. A custom built oCVD reactor was built to tackle the challenge of using solid and poorly volatile monomers in oCVD. The reactor allowed the sublimation of metalloporphyrins with free meso and beta positions in the presence of FeCl3 as oxidant to obtain porphyrin thin films on sensitive substrates. Both chemical and physical properties of the films were investigated. High Resolution Mass Spectrometry (HRMS), UV/Vis/NIR spectroscopy and Gel Permeation Chromatography (GPC), were used to characterize the chemical composition of the synthetized film. Two-point probe conductivity measurements, Conductive Atomic Force Microscopy (C-AFM) and Helium Ion Microscopy (HIM) probed the film morphology, topography and conductive properties. Density Functional Theory (DFT) and Electron Paramagnetic Resonance (EPR) spectroscopy unravelled a conceivable reaction mechanisms of nickel(II) porphyrins during the oxidative coupling. Secondary Ion Mass Spectrometry (SIMS) of the films prepared using different oxidants confirmed that FeCl3 yields the best results for the oCVD of porphyrins. The effect of the size of the porphyrin substituents on the film composition and properties was investigated. Since no solubilizing substituents are required on the porphyrin in oCVD, simple small phenyl substituents can be used for the synthesis of porphyrin tapes. This aspect represents the most important step forward in this work towards applications, as small substituents improve the electric properties of the films by decreasing the hindrance between different polymer chains. Different metal cations such as FeIII, CoII, NiII, CuII, ZnII, PdII in the porphyrin core during the oCVD and in the film show, as in solution based studies, that the metal center influences the regioselectivity of the oxidative coupling steps. Thanks to cyclic voltammetry and DFT, information on the position and symmetry of the frontier orbitals were obtained and related to the regioselectivity of the reaction. Finally, the metal center appears to be crucial, since it helps avoiding acid-base reactions which stop the oxidative coupling and hence the polymerization
DDC: 540 Chemie
540 Chemistry and allied sciences
Institution: Johannes Gutenberg-Universität Mainz
Department: FB 09 Chemie, Pharmazie u. Geowissensch.
Place: Mainz
URN: urn:nbn:de:hebis:77-diss-1000035922
Version: Original work
Publication type: Dissertation
License: In Copyright
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Extent: 200 Seiten
Appears in collections:JGU-Publikationen

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