In-Solution Synthesis of Polycyclic Aromatic Hrdrocarbons Containing Heptalenes

Professor Liu, Junzhi   (Principal Investigator (PI))

36

2022-01-01

666015

In-Solution Synthesis of Polycyclic Aromatic Hrdrocarbons Containing Heptalenes

heptagon-heptagon pair, heptalene, non-alternant topologies, pi-conjugated systems, polycyclic aromatics

Chemical Sciences

Physical Sciences (P)

17304021

General Research Fund (GRF)

2021

On-going

1 The aim of this research proposal is to establish the structure-property relationships of heptalene-embedded polycyclic aromatic hydrocarbons (PAHs) and graphene nanoribbons (GNRs). The PI will address the trends in the band gaps and electronic structures caused by increasing the number of heptalene units and different topologies, in terms of optical/electronic properties and close-shell/open-shell characteristics, etc. We will also explore the synthetic feasibility of incorporating heptalene in π-extend systems such as in GNRs. Moreover, this work will also utilize the proposed PAHs and GNRs in different electronic applications, such as organic transistors and nonlinear optics. To achieve this goal, the PI will fully exploit the knowledge and experiences related to the solution-phase synthesis of PAHs to fabricate heptagon-heptagon pairs (heptalene units) and the precise incorporation of these units in graphene nanostructures – ranging from 0D PAHs to extended 1D GNRs (Fig. 1, Part II Section 2(c)). To this end, the specific objectives are described in the following sections. 2 Objective 1: Synthesis of close-shell PAHs with heptalenes. The first objective will be to synthesize PAH 1 containing one heptalene and compound 2 with two heptalenes (Fig. 1), which will help to elucidate the effects of the number of heptalene units on their structural and physical properties. The solution processability of these heptalene-embedded PAHs should allow the study of their optoelectronic properties in organic solvents. The growth of single crystals of 1 and 2 will allow us to determine parameters such as bond length, aromaticity, and geometry. In addition to the number of heptalenes, the effect of different topologies will also be investigated. For example, a novel PAH 3 containing two contiguous heptalene units will be synthesized (Fig. 1). We can then investigate the local modulation of the physical and chemical properties of the heptalene-embedded PAHs. 3 Objective 2: Synthesis of nitrogen-doped PAH with heptalenes. The introduction of heteroatoms into the lattice of sp2-carbon framework presents an effective strategy to tune the intrinsic physicochemical properties of PAHs, such as the chemical reactivity, energy gaps, and redox behavior. Bottom-up synthesis of well-defined nitrogen-doped PAHs (N-PAHs) represents a new era in the development of novel PAHs with improved electron affinities and luminescence behavior. Although significant efforts have been made in the past decade, the introduction of N atoms into non-hexagons has remained limited. In this project, the electron-rich pyrrole unit will be explored as a key building block to synthesize PAH 4 bearing two N-doped heptagons (Fig.1). The optical and electronic properties, and aromaticity of 4 will be compared with the heptalene-embedded PAHs in objective 1 to evaluate the effects of introducing N atoms into seven-membered rings. 4 Objective 3: Synthesis of open-shell PAH containing heptalene units. Compared to the close-shell molecules discussed in objective 1, open-shell PAHs have a range of unique electronic and magnetic properties due to the spin-polarized states. Most recent works have focused on the open-shell PAHs with six-membered rings. In this objective, we will focus on the synthesis of heptalene-embedded PAH with open-shell characteristic. For instance, PAH 5 with two heptalene units will be developed (Fig. 1). According to Clar’s sextet rule, 5 should possess significant tetraradical characteristics as a result of gaining aromatic stabilization energy (5-1, Fig. 1). It will be rare to have spins located on the heptagons, which is largely unexplored in this field. The synthesis of such a model compound is expected to lead to the unprecedented synthesis of extended open-shell PAHs with non-alternant topologies. 5 Objective 4: Synthesis of heptalene-embedded graphene nanoribbons. The motivation for this objective relates to the need to evaluate the effects of periodic heptalenes in terms of the structural, optical, and physical properties of π-expanded polymers. In this objective, the heptalene units will be introduced into structurally well-defined GNRs. The implementation of non-hexagonal rings into a honeycomb lattice represents a viable, albeit less explored route to tune the intrinsic optical and electronic properties of GNRs. Towards this end, GNR-1 and GNR-2 with periodically repeating heptalene units will be synthesized (Fig. 1), and the structural and electronic properties will be assessed to evaluate the effects of different topologies on GNRs. 6 Objective 5: Spectroscopic studies and device applications based on the achieved PAHs and GNRs. The photophysical properties of the resultant PAHs will be well-studied by nano/femtosecond time-resolved spectroscopy (with Prof. David Phillips from the department), such as their excited state nature and dynamics, etc. To establish clear structure-property relationships for the application of heptalene-embedded PAHs, the relevant electronic devices will be fabricated through collaborations with partners of the PI. For example, the designed heptalene-embedded PAHs and GNRs are expected to possess low band gaps, making these materials potential candidates for the development of organic field‐effect transistors devices with substantially improved hole/electron mobility or balanced ambipolar charge-carrier transport properties. Open-shell model compounds can also be used to understand spin and electron density dependence in PAHs as potential ""organic magnets"" for spintronic applications.