Functionalized Graphene Nanoribbons: Synthesis And Potential Applications

GNRs are defined as graphene strips with a width<10 nm while maintaining a length/width ratio of>10.Quantum confinement effects impart GNRs with semiconducting properties,namely with a finite bandgap that critically depends on the ribbon width,edge structure.And,GNRs have small size,unique optical properties,large surface area and superior mechanical properties.They are attracting increasing attention as highly promising candidates for a wide range of potential applications in the fields of optics,electronics,magnetism,quantum physics and biosciences,and so on.However,the aggregation effect caused by strong?-?interaction has never been well excluded,which will obscure its intrinsic physical properties and expected applications.Functional modification of structurally precise GNRs using groups with special structure and properties or design of new GNRs with new architecture is of great significance for exploring their basic properties,supramolecular self-assembly behaviors and potential applications in liquid phase.In order to surmount these problems mentioned above,a series of GNRs functionalized with dendritic polymers,linear polymers or rigid bulky group were designed and synthesized,and their solution dispersibility,optical properties and potential applications were explored.The results are summarized as follows:?1?We present the solution synthesis of dendronized GNRs,which consist of a structurally defined backbone grafted with cone-shaped benzyl ether-type dendrons of different generations?denoted as GNR-G1,2 and 3?.The GNR backbones possess an arm-chair edged structure with a uniform width of 1.7 nm and an average length of 30nm.The side alkylcarboxyl active groups on the periphery of the backbones allow the grafting of benzyl ether-type dendrons?G1-G3?with hydroxyl groups through an esterification reaction.The three-dimensional?3D?dendrons of large molecular volumes,which may alleviate the?-?stacking of GNR backbones,render the GNRs with excellent dispersibility in common solvents such as tetrahydrofuran?THF?.The maximum concentration of the GNR-G1,2 and 3 dispersion in THF reaches 1.5,2.4and 3 mg/mL?for the GNR backbone unless otherwise mentioned?,respectively.However,no fluorescence emission was observed in solution for these three samples,indicating that the GNRs are still in aggregation states,which has been confirmed by TEM,AFM and THz tests.The dendronized GNRs aggregate into ultralong nanowires,1D helices,or short nanofibers in THF,depending on the dimension of the dendrons.The formation of the superstructures leads to near-infrared?NIR?absorption with a maximum at 685 nm for GNR-G1 and GNR-G2 in THF,and at 652 nm for GNR-G3.Ultrafast photoconductivity measurements reveal that GNR-G2 helices exhibit much longer free carrier?3.5 ps?and exciton lifetime?several hundred ps?than those of GNR-G1 nanowires and GNR-G3 short nanofibers.This discrepancy unveils the considerable effect of supramolecular structures on the optoelectronic properties of GNRs in the liquid phase,making them promising candidates for optoelectronics applications.?2?We prepared three kinds of defect free GNRs grafted with hydrophilic dendromer,linear polymer and functionalized monosaccharide?galactose and mannose?,named as GNR-DPE,GNR-PEO,GNR-man and GNR-gal.These four samples show good dispersibility in water with the help of sonication and Triton-100?0.2%,V/V?.The excellent dispersibility of these samples offers opportunities for exploration of their physical properties and applications in the aqueous phase.We examined their UV-Vis absorption,photothermal conversion performance,biotoxicity and photothermal imaging properties in aqueous phase.Benefiting from the predominant absorption in the NIR region,the aqueous dispersions of the four different samples were irradiated with a NIR laser??=808 nm?and the system temperatures were measured by thermal imaging.These dispersions?50?g/mL?reached temperatures above the photoablation limit of 50?,after treatment with a NIR laser(1 Wcm^-2)for only 2 minutes.GNR-PEO,GNR-gal and GNR-man dispersions have lower biotoxicity and cell viability can be maintained above 83%with a concentration of 40?g/mL.GNR-PEO samples show good photothermal killing ability to cancer cells.Moreover,The GNR-man and GNR-PEO samples show good thermal imaging properties in vivo with a concentration of 0.25 mg/mL.?3?We designed and synthesized a new dichloro-substituted oligophenylene monomer decorated with pending Diels-Alder cycloadducts of anthracenyl units and N-n-hexadecyl maleimide?AHM?.The AHM side groups have radius of?0.5 nm,which is larger than the interlayer spacing of graphite??0.34 nm?.Then,two polymers named PP-AHM-1 and PP-AHM-2 were prepared by AA-type Yamamoto polymerization of the dichloro-substituted oligophenylene monomer.PP-AHM-1 and PP-AHM-2 were transformed into GNR-AHM-1 and GNR-AHM-2 with a para-armchair edge structure as well as a width of 1.0-1.7 nm and calculated average lengths of 5 and 58 nm,by employing intramolecular cyclodehydrogenation in CH₂Cl₂ solution using FeCl₃ as the Lewis acid and oxidant.Thanks to the bulky AHM,GNR-AHMs show superior dispersibility in many common organic solvents?e.g.tetrahydrofuran,THF?with unprecedentedly high concentrations?up to 5 mg/mL?,GNR-AHMs have intense feature absorption limited to the region below 750 nm.Most importantly,a linear relationship between PL intensity and concentration?<0.1 mg/mL?demonstrates that the single-ribbon character of GNR-AHM were achieved at low concentrations of<0.1mg/mL in liquid phase;their photophysical properties were studied,for the first time,by ultrafast spectroscopy coupling with two-dimensional electronic spectroscopy.GNR-AHM-1 and GNR-AHM-2 exhibits PL quantum yield of 1.5%,9.1%and PL lifetime of 6.4 ns,8.7 ns,respectively.Ultrafast TA experiments shows that even the 58nm long GNR-AHM-2 behaves like a small organic molecule in solution,with the electronic structure of a vibronic progression,which provides solid evidence for the single dispersion of GNR-AHM.This breakthrough paves the way for understanding the physical properties of individual GNRs in liquid phase and for exploring their potential applications.