This dissertation describes research on the synthesis and characterization of extended heptazine–based, graphite–like carbon nitride materials (CNx), as well as molecular heptazine (C6N7) derivatives. Spurred on by recent triazine to heptazine conversion studies, a structural examination was performed on an amorphous nitrogen–rich carbon nitride material formed via the rapid and exothermic self-propagating decomposition of a triazine (C3N3) precursor, trichloromelamine (TCM). The thermally stable and insoluble CNxHy product was determined to be composed of heptazine repeat units. This conclusion was supported by 13C solid state NMR and isolation of molecular heptazine anions after base hydrolysis (structural deconstruction) of the CNxHy material. Modifications to the decomposition of TCM were explored. Introduction of a solid template (NaCl or silica) led to morphological changes in the TCM–CNx product, observed by scanning electron microscopy. It was found that the sodium salts, NaBr and NaN3, led to chloride exchange with TCM. The use of mixtures of NH4Cl and NaN3 also showed changes in the morphology of the material, while leading to slight changes in the IR spectra. A series of reactions between NaBH4 and TCM yield novel thermally stable boron carbon nitride (BCN) materials. Reactions between TCM and Li2C2 or aromatic organic solids led to CNx materials with increased carbon contents. Crystalline metal–heptazine precipitates were generated by cation exchange reaction with the base hydrolysis product of TCM–CNx, potassium cyamelurate. A structure solution was attempted for the crystalline copper cyamelurate salt, KCu[C6N7O3]·4H2O. Neutral molecular heptazines were also synthesized; these species included 2,5,8–tribromo–s–heptazine (TBH), 2,5,8–triphenyl–s–heptazine (TPH), 2,5,8–tris(diisopropylamino)–s–heptazine (TAmH), and 2–bis(trimethylsilyl)amido–5,8–dichloroheptazine (DCAH). These materials were sublimable and showed interesting optical absorption and emission properties. A polymeric heptazine material was synthesized by thermal decomposition of DCAH. Several attempts were made to synthesize polymeric materials from heptazine precursors. Extended solids with C6N8 and C9N7 stoichiometry were made through solid state metathesis reactions between trichloroheptazine and either lithium nitride or lithium carbide. Powder X–ray diffraction indicated that salt formation was occurring during these reactions and products had the desired stoichiometry by elemental analysis. It was generally observed that CNx materials containing excess carbon displayed increased thermal stability when compared to pure CNx.

Thesis (Ph.D.)--University of Iowa, 2009. / Thesis supervisor: Edward G. Gillan. Includes bibliographical references (leaves 184-194).

Le nitrure de carbone graphitique (gCN) est un semi-conducteur organique ayant dernièrement attiré l'attention par sa capacité à photocatalyser la séparation de l'eau. Il a récemment été montré que le gCN était un polymère basé sur le cycle heptazine C6N7, mais son arrangement tridimensionnel reste encore très peu connu. En effet, sa faible solubilité empêche l'utilisation des techniques de caractérisation classiques, et le terme gCN recouvre en réalité une large gamme de composés différents, selon les conditions de synthèse utilisées (choix du précurseur, température…). L'obtention de modèles moléculaires, de structures maîtrisées et bien définies, serait donc d'une grande aide dans la compréhension du lien structure-propriétés. Ceci est le but des travaux présentés dans ce manuscrit. La réactivité du chlorure de cyaméluryle, un précurseur monomérique, a été étudiée, et un protocole de substitution sélective quantitative par les amines secondaire aliphatique a été déterminé. L'utilisation de synthèses par déprotonation ou par activation thermique ont permis l'obtention de deux dimères et d'un trimère linéaire solubles. Les oligomères synthétisés ont été caractérisés par de nombreuses techniques (diffraction des rayons X, RMN, IR, absorption UV-vis, fluorescence, électrochimie), et les valeurs obtenues ont été corroborées à celle obtenues par DFT. De façon générale, une diminution des énergies des transitions électronique est observée quand la taille de chaîne augmente, et l'application de méthodes d'extrapolation suggère que les oligomères linéaires sont des bon modèle moléculaire du gCN. / Graphitic carbon nitride (gCN) is an organic semi-conductor which has lately attracted a lot of attention when its photocatalytic properties were highlighted for water splitting. It has been recently shown to be based on the heptazine core, but its three-dimensional structure remains elusive. This is first due to its poor solubility which prevents the use of classical characterization techniques, and second to the fact that changes in synthesis experimental conditions (precursors, temperature…) yield different materials. The synthesis of tailored and well-defined molecular models would therefore certainly be of great interest to better understand the structure-properties relationship of this material. This is the aim of the work presented in this manuscript. The reactivity of cyameluryl chloride, a monomeric precursor, has been studied, and a protocol for a quantitative selective substitution by aliphatic secondary amines has been determined. The use of deprotonation by a strong base or thermal treatment yielded two dimers and one linear trimer. The oligomers have been characterized by several technique (X-ray diffraction, NMR, IR, UV-vis absorption, emission, electrochemistry), and the obtained data were in close agreement to the ones observed in DFT. As a rule of thumb, a decrease of the electronic transition energies is observed for an increasing chain length. The application of extrapolation methods to the experimental data suggests that oligomers are relevant molecular models for gCN.

In the first section of the thesis graphitic carbon nitride was for the first time synthesised using the high-temperature condensation of dicyandiamide (DCDA) – a simple molecular precursor – in a eutectic salt melt of lithium chloride and potassium chloride. The extent of condensation, namely next to complete conversion of all reactive end groups, was verified by elemental microanalysis and vibrational spectroscopy. TEM- and SEM-measurements gave detailed insight into the well-defined morphology of these organic crystals, which are not based on 0D or 1D constituents like known molecular or short-chain polymeric crystals but on the packing motif of extended 2D frameworks. The proposed crystal structure of this g-C3N4 species was derived in analogy to graphite by means of extensive powder XRD studies, indexing and refinement. It is based on sheets of hexagonally arranged s-heptazine (C6N7) units that are held together by covalent bonds between C and N atoms. These sheets stack in a graphitic, staggered fashion adopting an AB-motif, as corroborated by powder X-ray diffractometry and high-resolution transmission electron microscopy. This study was contrasted with one of many popular – yet unsuccessful – approaches in the last 30 years of scientific literature to perform the condensation of an extended carbon nitride species through synthesis in the bulk. The second section expands the repertoire of available salt melts introducing the lithium bromide and potassium bromide eutectic as an excellent medium to obtain a new phase of graphitic carbon nitride. The combination of SEM, TEM, PXRD and electron diffraction reveals that the new graphitic carbon nitride phase stacks in an ABA’ motif forming unprecedentedly large crystals. This section seizes the notion of the preceding chapter, that condensation in a eutectic salt melt is the key to obtain a high degree of conversion mainly through a solvatory effect. At the close of this chapter ionothermal synthesis is seen established as a powerful tool to overcome the inherent kinetic problems of solid state reactions such as incomplete polymerisation and condensation in the bulk especially when the temperature requirement of the reaction in question falls into the proverbial “no man’s land” of classical solvents, i.e. above 250 to 300 °C. The following section puts the claim to the test, that the crystalline carbon nitrides obtained from a salt melt are indeed graphitic. A typical property of graphite – namely the accessibility of its interplanar space for guest molecules – is transferred to the graphitic carbon nitride system. Metallic potassium and graphitic carbon nitride are converted to give the potassium intercalation compound, K(C6N8)3 designated according to its stoichiometry and proposed crystal structure. Reaction of the intercalate with aqueous solvents triggers the exfoliation of the graphitic carbon nitride material and – for the first time – enables the access of singular (or multiple) carbon nitride sheets analogous to graphene as seen in the formation of sheets, bundles and scrolls of carbon nitride in TEM imaging. The thus exfoliated sheets form a stable, strongly fluorescent solution in aqueous media, which shows no sign in UV/Vis spectroscopy that the aromaticity of individual sheets was subject to degradation. The final section expands on the mechanism underlying the formation of graphitic carbon nitride by literally expanding the distance between the covalently linked heptazine units which constitute these materials. A close examination of all proposed reaction mechanisms to-date in the light of exhaustive DSC/MS experiments highlights the possibility that the heptazine unit can be formed from smaller molecules, even if some of the designated leaving groups (such as ammonia) are substituted by an element, R, which later on remains linked to the nascent heptazine. Furthermore, it is suggested that the key functional groups in the process are the triazine- (Tz) and the carbonitrile- (CN) group. On the basis of these assumptions, molecular precursors are tailored which encompass all necessary functional groups to form a central heptazine unit of threefold, planar symmetry and then still retain outward functionalities for self-propagated condensation in all three directions. Two model systems based on a para-aryl (ArCNTz) and para-biphenyl (BiPhCNTz) precursors are devised via a facile synthetic procedure and then condensed in an ionothermal process to yield the heptazine based frameworks, HBF-1 and HBF-2. Due to the structural motifs of their molecular precursors, individual sheets of HBF-1 and HBF-2 span cavities of 14.2 Å and 23.0 Å respectively which makes both materials attractive as potential organic zeolites. Crystallographic analysis confirms the formation of ABA’ layered, graphitic systems, and the extent of condensation is confirmed as next-to-perfect by elemental analysis and vibrational spectroscopy. / Die vorliegende Arbeit befasst sich mit der Synthese und Charakterisierung neuer Allotropen und Nanostrukturen von Karbonitriden und berührt einige ihrer möglichen Anwendungen. Alle gezeigten, ausgedehnten, kovalent verbundenen Karbonitridgerüste wurden in einem ionothermalen Syntheseprozess – einer Hochtemperaturbehandlung in einem eutektischen Salzgemisch als ungewöhnlichem Lösungsmittel – aus einfachen Präkursormolkülen erzeugt. Der Kondensationsmechanismus folgt einer temperaturinduzierten Deaminierung und Bildung einer ausgedehnten, aromatischen Einheit; des dreifach substituierten Heptazines. Die Dissertation folgt vier übergreifenden Themen, beginnend mit der Einleitung in Karbonitridsysteme und der Suche nach einem Material, welches einzig aus Kohlenstoff und Stickstoff aufgebaut ist – einer Suche, die 1834 mit den Beobachtungen Justus von Liebigs „über einige Stickstoffverbindungen“ begann. Der erste Abschnitt zeigt die erfolgreiche Synthese von graphitischem Karbonitrid (g-C3N4); einer Spezies, welche auf Schichten hexagonal angeordneter s-Heptazineinheiten beruht, die durch kovalente Bindungen zwischen C- und N-Atomen zusammengehalten werden, und welche in einer graphitischen, verschobenen Art und Weise gestapelt sind. Der zweite Abschnitt berührt die Vielfalt von Salzschmelzensystemen, die für die Ionothermalsynthese geeignet sind und zeigt auf, dass die bloße Veränderung der Salzschmelze eine andere Kristallphase des graphitischen Karbonitrides ergibt – das g-C3N4-mod2. Im dritten Abschnitt wird vom Graphit bekannte Interkallationschemie auf das g-C3N4 angewendet, um eine Kalliuminterkallationsverbindung des graphitischen Karbonitirdes zu erhalten (K(C6N8)3). Diese Verbindung kann in Analogie zum graphitischen System leicht exfoliiert werden, um Bündel von Karbonitridnanoschichten zu erhalten, und weist darüberhinaus interessante optische Eigenschaften auf. Der vierte und letzte Abschnitt handelt von der Einführung von Aryl- und Biphenylbrücken in das Karbonitridmaterial durch rationale Synthese der Präkursormoleküle. Diese ergeben die heptazinbasierten Frameworks, HBF-1 und HBF-2 – zwei kovalente, organische Gerüste.