(Invited) Synthesis and Characterizations of Highly Fluorescent Nanographene Molecules
Authors: Narita Akimitsu 
Published: July 2022

Large polycyclic aromatic hydrocarbons (PAHs) correspond to nanostructures of graphene and have attracted increasing research attentions as nanographene molecules during the last decade. In contrast to the zero-bandgap graphene, nanographene molecules have open energy gaps that are dependent on their size, symmetry, and edge structure. Various nanographene molecules have been synthesized, demonstrating intriguing optical, electronic, and magnetic properties, which render them promising for a wide range of applications from optoelectronic and energy devices to spintronics and bioimaging. In particular, nanographene molecules have a high potential as fluorescent materials for light-emitting devices and bioimaging. However, large nanographene molecules with smaller optical energy gaps typically show low solubility due to the strong aggregation, and stable and processable nanographenes with red or near infrared (NIR) emission have been limited. We have synthesized dibenzo[hi,st]ovalene (DBOV) as a nanographene molecule with a combination of armchair and zigzag edges, which demonstrated strong red emission and stimulated emission [1]. DBOV derivatives with mesityl groups and/or other substituents showed good solubility in common organic solvents, allowing for facile processing. We have subsequently established regioselective bromination of DBOV, enabling the edge-functionalization to install various functional groups for the fine-tuning of the optical and electronic properties [2]. For example, fluoranthene imide groups were attached to DBOV, which led to red-shifted emission with larger Stokes shift [3]. Moreover, π-extension of DBOV was achieved through Pd-catalyzed alkyne benzannulation, affording circumpyrene, or a nanographene molecule with multiple zigzag edges [4]. Nitrogen-doped DBOV was also synthesized, displaying acid- and metal-sensitive fluorescence and potential for fluorescent imaging and sensing applications [5]. We have more recently also synthesized a wider variety of functionalized DBOVs as well as novel nanographene molecules with red to NIR emission, providing further insights into the structure-property relationship of such graphene nanostructures.

References

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[3] Paternò, G. M.; Chen, Q.; Muñoz-Mármol, R.; Guizzardi, M.; Bonal, V.; Kabe, R.; Barker, A. J.; Boj, P. G.; Chatterjee, S.; Ie, Y.; Villalvilla, J. M.; Quintana, J. A.; Scotognella, F.; Müllen, K.; Díaz-García, M. A.; Narita, A.; Lanzani, G., Excited states engineering enables efficient near-infrared lasing in nanographenes. Mater. Horiz.
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[4] Chen, Q.; Schollmeyer, D.; Müllen, K.; Narita, A., Synthesis of Circumpyrene by Alkyne Benzannulation of Brominated Dibenzo[hi,st]ovalene. J. Am. Chem. Soc.
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[5] Jin, E.; Yang, Q.; Ju, C.-W.; Chen, Q.; Landfester, K.; Bonn, M.; Müllen, K.; Liu, X.; Narita, A., A Highly Luminescent Nitrogen-Doped Nanographene as an Acid- and Metal-Sensitive Fluorophore for Optical Imaging. J. Am. Chem. Soc.
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