Theoretical Studies Of Magnetic Coupling Interaction Of Graphene Patches And Its Derivatives

Over the past few years, carbon-based nanomaterials have aroused intense attention of chemists, almost dominating the whole field of nano-science and nano-technology. Especially, the discovery of graphene in2004shocked scientists once again. Graphene, a truly two-dimensional flat monolayer of carbon atoms tightly packed into a honeycomb lattice, is the thinnest known material in the universe and the strongest ever measured. It possesses excellent charge transmission mobility, wonderful photoelectric performance, the abnormal quantum Hall effect and zero band gap, and so on, which provides possibility of designing nanoscale devices by modulating elecreonic properties of graphene. Despite that, there exist limitations: linear oligoacenes only larger than hexacene might possess diradical character in their ground states and higher acenes have difficulty in experiment syntheses due to high instability; elongation of the zigzag edge results in increase of diradical character, but those with long armchair edge only have weak and even have no diradical character. Therefore, many efforts have been made to modify graphene patches and its derivatives. Thus, we devote to modify them and obtain some meaningful achievements. The main results are related as follows.1. In the present knowledge, higher acenes could exhibit diradical character while short acenes are closed-shell systems. In this work, two classes of multi-Zn-expanded oligoacenes from benzene to pentacene are computationally designed through introducing a Zn array into acene rings in two ways:acene-chain axial versus single-ring quasi-transversal direction. Combined density functional theory and CASSCF calculations predict the structural and electronic properties as well as the magnetic coupling interaction for them. Results show us that all these multi-Zn-expanded oligoacenes have the open-shell broken-symmetry (BS) singlet diradical ground states, in contrast with the common fact that their parent oligoacenes are closed-shell systems and only acenes larger than octacene have open-shell BS singlet diradical ground states. In addition, magnetic coupling interactions of these multi-Zn-expanded oligoacenes are determined through the magnetic exchange constant, J, from which we know that multi-Zn-expanded oligoacenes in the nZn-acene series possess strong antiferromagnetic character while those in the2Zn-acene series can exhibit tunable magnetic property. Obviously, the results offer the first theoretical prediction that the multi-Zn introduction into the acene ring(s), forming the Zn-expanded oligoacenes, can lead them to nearly perfect diradical structures and relatively strong antiferromagnetic character. This work provides a strategy to design perfect singlet diradicals from oligoacenes or their derivatives.2. Inspired by a promising modification mode which could lead to diradical character for oligoacenes and the electronic mediation effect of transition metal zincs, we utilize the similar modified way to small nano-sized graphene patches. Three classes of multi-Zn-expanded graphene patches in different shapes are computationally designed through introducing a Zn chain into the corresponding middle benzenoid chain. The relevant structures and properties were investigated by means of quantum calculations. The results reveal that all multi-Zn-expanded graphene patches are thermodynamically stable. Besides, molecules of nnn-quasi-linear and nnn-slightly bent series have the open-shell BS singlet diradical ground states, whereas those of n(n+1)n series possess quintet tetraradical as their ground state and become open-shell BS singlet tetraradicals when they are in a higher energy state. And their J values indicate that these multi-Zn-expanded graphene patches show variable degrees of antiferromagnetic or ferromagnetic characteristics. In summary, this work could help people toward a further understanding of different spin-state character of multi-Zn-expanded graphene patches in different shapes, and provides theoretical evidences of the practical application in electromagnetic field for graphene-based magnetic materials.3. As is known, graphene oxides are a series of inevitable and automatic intermediates. Therefore, further understanding the mechanism for the oxidative breakup of graphene sheets is very desirable towards the bulk production and practical electronics applications of graphene. Providing that oligoacenes are considered as the simplest finite-sized models of graphene with saturated edges, and further, in order to simplify the research system and to explore the influence of various factors on their properties more obviously, we choose the oligoacene dioxides that contain only two carbonyl groups as our target systems. Taking1,5-dioxidized naphthalene as a starting point, three series of oligoacene dioxides are considered as follows:1) middle insertion by1-2benzene rings;2) single-side expansion using1-2benzene rings;3) double-side expansion using two benzene rings. Our computational results reveal that oligoacene dioxides in the middle insertion series have a triplet ground state with a certain degree of ferromagnetic character; whereas those in both the single-side and the double-side expansion series have open-shell BS singlet diradical ground states and strong antiferromagnetic character, except for their common origin naphthalene-1,5-dioxide whose ground state is triplet and which is also viewed as the origin of the middle insertion series. Our work not only helps to understand the oxidation mechanism of graphene sheets at an atomistic level, more importantly, it also provides a reasonable strategy of modifying the graphene oxide synthesis route even designing novel graphene-oxide-based magnetic materials.4. In a previous article, the authors reported that the introduction of alkali metal ions into2-(3,5-dinitrophenyl)-1,3-dithiane carbanion could lead to two kinds of isomers which possess different ground states. Motivated by this, we extend that target system by expanding the π-conjugated structures and introducing Lewis acids with different acidities. To simplify the research system, we only use the common Li+, Na+, K+, and some simple polar molecules as Lewis acids. On the basis of DFT and CASSCF calculations, we obtain a conclusion that ring-expansion does not change its open-shell BS singlet diradical ground state, but decreases its antiferromagnetic character, whereas introduction of Lewis acids can lead to different ground states (triplet versus singlet) and different magnetism, depending on the binding sites of the Lewis acid. For systems involving Lewis acid, they show closed-shell singlet ground states with no magnetism when the cation locates near the anionic center of the1,3-dithiane ring, but convert to triplet ground states with ferromagnetic character when the cation combines with one nitro group of the3,5-dinitrophenyl-based π-conjugation-expanded fragment. These findings give evidences that we could modulate the magnetisms of2-(3,5-dinitrophenyl)-1,3-dithiane-based carbanions through ring-expansion and Lewis acid-binding ways. Such conclusion extends the study of that previous report, and might open a new strategy for designing magnetism-tunable building blocks for novel electromagnetic materials.5. People always consider pentacene as perfect closed-shell systems. In our work, we we present a combined first-principles calculation and ab initio molecular dynamics simulation study of one pentacene molecule and results reveal that some configurations possess a certain degree of diradical character while others do not. By analysis, we speculate that the pentacene molecule might exist in two vibration modes: diradical mode and non-diradical mode. As is known, molecules do thermal motion constantly. When the pentacene molecule falls into the diradical zone, it will show a certain degree of diradical character periodically, although the diradical percentages are small and various. While when the pentacene molecule is in the non-diradical zone, there exists no diradical character at all. The conversion and alternation between diradical zone and non-diradical zone are controlled by molecular structural changes caused by thermal motion of molecules. In all, we successfully prove that pentacene possesses potential diradical character, which is very different from the common view that pentacene is a complete closed-shell system and makes the possible praetical applications of pentacene much broader.In summary, the field of functional modification of graphene patches and its derivatives is booming. Along with the further exploration of various properties of such modified systems, we believe their applications in electromagnetic devices, molecular wires, sensors, etc. will be realized in the near future.