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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - NANOGRAPHOUT (Design, synthesis, study and applications of distorted nanographenes)

Teaser

Summary

Graphene is considered as a new starting point for new technologies applicable in different fields. It exhibits unique properties: it is the thinnest and strongest compound known and the lightest material. It is extremely flexible, impermeable to molecules, extremely electrical and thermal conductive and a transparent conductor. Graphene is constituted by a single layer of carbon atoms arranged in a flat hexagonal lattice. However, for many potential applications, such as sensors, energy storage or catalysis, this perfect hexagonal structure is chemically little active and actually it is the presence of intrinsic irregularities what leads to better properties. Understanding the influence of structural imperfections can pave the way for designing defective graphene for particular applications. Among the different production methods of graphene, direct chemical synthesis it is the choice to create small graphene structures with well-defined geometries. Using this approach, we aim to prepare distorted nanographenes containing seven- and higher membered rings into an otherwise planar hexagonal lattice as a new tool for the preparation of innovative materials for organic electronics. These defects induce a curvature in the planar sheet, distorting the structure out of the plane. In particular medium-size rings such as heptagons and octagons induce a saddle-shape curvature in the carbon network. NANOGRAPHOUT focuses on providing a general synthetic method for the construction of a variety of distorted nanographenes with good control on size, shape and the edges of the final compounds as well as number and position of non-hexagonal rings. Combination of defects in also contemplating, as the simultaneously introduction of heptagonal carbocycles with helical moieties. By evaluating the morphology, optical and electronic properties and electron transport of synthesized nanographenes, we aim to establish the first comprehensive study clarifying the influence of defects on the properties of nanographenes. Adding chiroptical response to the semiconductor properties of nanographenes will provide the new devices the added value of their potential application in photonics.

Work performed

We have established a new a versatile method for the preparation of distorted nanographenes containing heptagonal rings in the structure, leading to a curved saddle-shaped compounds. The developed strategy is very simple as in one single step we obtain both the seven-membered carbocycle and the aromatic backbone of the final carbon nanostructure. Simultaneously, we can introduce functional groups at desired positions which allow a subsequent expansion of the structure in a controlled manner. Hence, this new approach to saddle-shaped nanographenes allows the tailor preparation of curved carbon nanostructures. The distortion of planarity caused by the introduction of defects led to relevant (chir)optical properties. In this sense, using our methodology we have presented a distorted ribbon-shaped nanographene that constitutes the first nanographene emitter of circularly polarized luminescence (CPL). Furthermore, the described nanographene is the first organic compound that simultaneously exhibits two-photon absorption and CPL. More recently we have reported a nanographene ribbon fully arranged into a helicoidal shape, with enhanced photophysical responses.

Final results

The controlled introduction of several structural defects into a nanographene structure has led to unprecedented combination of optical properties exhibited by an organic compound, namely two-photon absorption (TPA) and circularly polarized luminescence (CPL). That means that we have presented the first demonstration of an organic compound that can emit chiral luminescence either by excitation with one photon of higher energy or by two photons of half energy. Those results represent the proof-of-concept for the future development of a new photophysical technique, namely two-photon CPL (TPCPL), the non-linear analogue of CPL that would combine the advantages of both responses. Further work on CPL-active distorted nanographenes is expected until the end of the project opening a new set of opportunities for a variety of applications. Development of novel curved carbon nanostructures containing larger carbocycles as octagons or nonagons is also envisioned and subsequent combination of defects.