Multifunctional Pyridine-incorporated Nanographenes Built On A Peri-fusion Approach

The electronic structure of nanographene fragments(NGs)is closely connected to their edge topologies,heteroatom dopping and substituentpatterns,whose precise control is a continuous research hotspot in the fields of organic electronics and spintronics.Without altering the number of πelectrons and aromaticity,"pyridinic" N-insertion can fine-tune the π-electron system of NGs and thus modulate the optical,electronic and magnetic properties.To date,versatile N-embedding NGs with various edge-topologies have been developed,but N-doped NG molecules with sizes of over 1 nm are still scarce due to the substantial synthetic challenges.Meanwhile,N-atoms incorporation might also give rise to certain exotic characteristics for NGs,such as symmetry breaking,stabilization of the edge states,and manipulation of potential topological phases,which has long been obscured.Open-shell NGs have exhibited unique electronic spin states due to their weak-bonding or unpaired π-electrons,and thus can act as excellent candidates for applications such as information store,organic field-effect transistors(OFETs),spin electronic devices,and even quantum calculation.The conventional approaches to design open-shell configurated radicaloid materials mainly relies on the extension of zigzag edges and construction of proaromatic structures,thus their preparations are typically involved with harsh synthesis conditions,hard functional modification,high reactivity and limited tolerance of high-spin state,thus greatly hampering the development and application of topological radical materials.Therefore,there is a highly urgent need to develop novel design strategy to maintain the stability of πconjugated radicals while achieving the precise spin confinement.Moreover,considering that almost all the open-shell NGs hold an inferior luminescence efficiency,devising of NGs with high fluorescence quantum yield would be of high interest to facilitating the further development of muitifunctional topological materials.According to abovementioned research background,the major contents of this dissertation are summarized as following:1.Two quaterrylene molecules AQR-a and AQR-b bearing doubly precise N-dopping at the zigzag edge terminals were designed and synthesized,and the effect of N-embedded positions on their physiochemical properties was systematically studied.The results indicated that N-doping might,to certain extent,break the intrinsically symmetry-forbidden transition and activate the radiative transition pathway,thus leading to energetically increased fluorescence quantum yields as compared to the pristine quaterrylene.Acidbase titration experiments further demonstrated the distinct N-embedded positions could lead to the diverse dipole moments of two NGs so that influence their acid-responsive properties.2.By means of peri-fusion of small perylenes onto a 1,6-anthrazoline segment,two stair-like N-doped narrow nanographenes named as ANG-a and ANG-b were designed and synthesized through an atom-precisely solution synthesis pathway.Strikingly,despite their similar backbone structures,the investigations uncovered that ANG-a was a definitely closed-shell molecule while ANG-b possessing a more planar configuration exhibited a typical diradical characteristic.Unlike the conventional diradicaloids,the open-shell property of ANG-b was assigned to the topological phase discontinuities resulted from its unique intramolecular junction structures,where the robust spin-polarized interface states were appeared driven by Coulomb repulsion.Compared with ANG-a,the exotic interface state endowed ANG-b with versatile properties including the acid-base tolerance,multi-redox properties,narrow band gap and interfacially amplified aromatic pattern.We creatively connected the electronic structure,topological phase,and physical properties in this work,providing a new pathway to construct the open-shell topologicalπ-systems.3.Via the peri-fusion strategy,N-doped nanographene molecules QTA-a and QTA-b were designed and synthesized.A systematic investigation to their optical,electronic and magnetic properties disclosed that both NGs exhibited strongly near-infrared absorbance(～800 nm)and narrow electrochemical band gap.Due to the narrower band gap and large steric repulsion at four coveedges,both QTA-a and QTA-b displayed the hybridized electronic structure consisting of open-shell state and closed-shell state.Intriguingly,both NGs exhibited the NIR emission(～900 nm)with high fluorescence quantum yields and the bimodal emission character varying with solvent polarity.