DOI: 10.1002/chem.201302622 Organic Chemistry of Graphene: The Diels–Alder Reaction Pablo A. Denis* [a] Introduction Graphene, the “rising star” of the nanotechnological revolu- tion, [1] is currently being considered as a key material for improving silicon-based electronic compounds. [2] However, it is well-known that the lack of a band gap renders such a goal difficult to achieve. A plethora of methods have been proposed to open up a band gap in graphene and to improve the “ON/OFF” ratio in graphene-based field-effect transis- tors. Among them, chemists have proposed the covalent functionalization of the sp 2 framework of graphene as a viable approach to fine-tune its electronic properties. Some of the chemical processes that have been proposed in- clude: 1) radical-addition reactions, such as hydrogena- tion [3–5] and fluorination; [6–7] 2) organometallic chemistry, [8] by forming complexes with 3d transition metals and ben- zene; 3) organic chemistry of graphene. Thanks to the vast number of well-developed organic reactions, several proce- dures have been employed to functionalize graphene by using organic reagents. Without attempting to review all of them, we highlight the addition of aryl-diazonium salts, [9–12] alkylation, [13–14] 1,3-dipolar cycloaddition, [15–20] [2+2] cycload- dition, [21–23] [2+1] cycloaddition, [24–25] and the recently pro- posed reactions in which graphene can play the role of a diene or dienophile. [26–27] The first report on the diene and dienophile character of graphene was published by Sarkar et al. [26] In this work, these authors were able to reversible modify the electronic properties of graphene by the addition of dienes and dienophiles. The dienophiles that were investi- gated included tetracyanoethylene (TCNE) and maleic an- hydride (MA), whilst the selected dienes were 9-methylan- thracene (9MA) and 2,3-dimethoxy-1,3 butadiene (DMBD); highly oriented pyrolytic graphite (HOPG), exfoliated gra- phene, and epitaxial graphene were used. The powerful di- enophile TCNE was attached to graphene at room tempera- ture but, when heated at 100 8C, the reverse reaction oc- curred and the composite decomposed. On the addition of MA, the reactions required heat to proceed. Exfoliated gra- phene reacted at 130 8C, whilst epitaxial graphene needed less heat and the reaction took place at 70 8C. The cycload- dition products were stable up to 150 8C because the retro Diels–Alder reaction became dominant at this temperature. In terms of the performance of graphene as a dienophile, DMBD was attached to epitaxial graphene by heating at 50 8C. Such modification increased the electrical resistance by 60%. The second diene that was employed was 9MA, which was successfully attached by heating in p-xylene at Abstract: Herein, by using dispersion- corrected density functional theory, we investigated the Diels–Alder chemistry of pristine and defective graphene. Three dienes were considered, namely 2,3-dimethoxy-1,3-butadiene (DMBD), 9-methylanthracene (9MA), and 9,10- dimethylanthracene (910DMA). The dienophiles that were assayed were tet- racyanoethylene (TCNE) and maleic anhydride (MA). When pristine gra- phene acted as the dienophile, we found that the cycloaddition products were 47–63 kcal mol 1 less stable than the reactants, thus making the reaction very difficult. The presence of Stone– Wales translocations, 585 double vacan- cies, or 555-777 reconstructed double vacancies did not significantly improve the reactivity because the cycloaddition products were still located at higher energy than the reactants. However, for the addition of 910DMA to single vacancies, the product showed compa- rable stability to the separated reac- tants, whereas for unsaturated armchair edges the reaction was extremely favor- able. With regards the reactions with dienophiles, for TCNE, the cycloaddi- tion product was metastable. In the case of MA, we observed a reaction product that was less stable than the reactants by 50 kcal mol 1 . For the re- actions between graphene as a diene and the dienophiles, we found that the most-promising defects were single va- cancies and unsaturated armchair edges, because the other three defects were much-less reactive. Thus, we con- clude that the reactions with these above-mentioned dienes may proceed on pristine or defective sheets with heating, despite being endergonic. The same statement also applies to the di- enophile maleic anhydride. However, for TCNE, the reaction is only likely to occur onto single vacancies or unsatu- rated armchair edges. We conclude that the dienophile character of graphene is slightly stronger than its behavior as a diene. Keywords: cycloaddition · density functional calculations · Diels–Al- der reactions · dienes · graphene [a] Dr. P. A. Denis Computational Nanotechnology DETEMA Facultad de Química, UDELAR, CC 1157 11800 Montevideo (Uruguay) Fax: (+ 589) 229241906 E-mail : pablod@fq.edu.uy Chem. Eur. J. 2013, 19, 15719 – 15725  2013 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 15719 FULL PAPER