Substantia. An International Journal of the History of Chemistry 1(2): 95-98, 2017 Firenze University Press www.fupress.com/substantia ISSN 2532-3997 (online) | DOI: 10.13128/substantia-29 Citation: H. Bouas-Laurent, J.-P. Des- vergne (2017) The Master and the Slave. A glance at the social life of molecules. Substantia 1(2): 95-98. doi: 10.13128/substantia-29 Copyright: © 2017 H. Bouas-Lau- rent, J.-P. Desvergne. This is an open access, peer-reviewed article pub- lished by Firenze University Press (http://www.fupress.com/substantia) and distribuited under the terms of the Creative Commons Attribution License, which permits unrestricted use, distri- bution, and reproduction in any medi- um, provided the original author and source are credited. Data Availability Statement: All rel- evant data are within the paper and its Supporting Information files. Competing Interests: The authors declared that no competing interests exist. Feature Article The Master and the Slave. A glance at the social life of molecules Henri Bouas-Laurent, Jean-Pierre Desvergne Institut des Sciences Moléculaires, Université de Bordeaux, 351 cours de la Libération, Bât A12, 33405 Talence Cedex E-mail: h.bouaslaurent@cegetel.net, desvergnejp@gmail.com Abstract. Low energy interactions induce the formation of molecular assemblies that can display a large variety of sizes and shapes such as dimers, oligomers, colloids, gels, helices, cylinders, etc. These grouping modes mimic human relationships, as people generally flock together according to their affinities. Moreover, chemical reactions, undergone under strong energy interactions, that result in bond breaking and forma- tion of new compounds, can also be compared to human behaviour. The fables usually involve animals but rarely molecules to play the role of human beings. In this article, we report a molecular tale where two different 9-substituted anthracene derivatives compete in a photochemical reaction, simulating the behaviour of a master and a slave, respectively. Keywords. Molecular sociology, photochemistry, aromatic endoperoxides, singlet oxy- gen, graduate education. INTRODUCTION Oil and water are known to repel each other. Despite shaking a mixture of the two in a bottle, they quickly form two distinct layers. The substances soluble in oil are called lipophilic (vitamines A, D, E...) whereas those solu- ble in water are said to be hydrophilic (sugars, amino-acids...). Fats cannot be eliminated with pure water, but require the use of surfactants to bring them into solution. These substances have affinity both to oil and water and con- tribute to form a single macroscopic phase.1 Such behaviour results from low-energy intermolecular interactions as compared to much stronger ones governing molecular bonds. Non-covalent intermolecular associations are wide-spread in abiotic as well as in biologi- cal systems and are fundamental for the formation of molecular assemblies. These can be very diverse: dimers, oligomers, cylinders, helices, colloids, liquid crystals, cellular membranes, to cite but a few. They form the basis of “supramolecular chemistry”.2 The affinities between molecules mimic human relationships. An interest- ing example was reported by Green et al. in the form of sergeants and sol- diers.3 Alkylisocyanates copolymers are known to adopt a rigid helical con- 96 Henri Bouas-Laurent, Jean-Pierre Desvergne formation in solution. The authors observed that, in solutions of copolymers formed from chiral and achiral monomers, even a small proportion (ca 1%) of chiral monomers induced a high enantiomeric excess; thus, a small number of chiral motifs (playing the role of ser- geants) can trigger the movements of a large number of achiral motifs (the soldiers). Apart from the above soft interactions, chemi- cal reactions transform starting materials into prod- ucts, through bonds breaking and bond formation. This activity may also be compared to human behaviour. For example, in his third novel “the elective affinities”,4 Goe- the compared acid-base reactions to love affairs between human couples. Amongst other reactions, one of them inspired Cohen et al.5 who compared molecules to wolfs and lambs, like in story tales. Here, two molecules, reacting fiercely with one another in solution and lead- ing to a mixture of products, become mutually inactive when tightly linked to two different polymers (Merrifield resins); a third reactant, in solution, reacts successively with one of them (the wolf ) then with the second (the lamb), to generate a single product with high yield. In this article, we describe a particularly relevant reaction where two different molecules (M and E) com- pete in an addition reaction to a special reactant pro- duced in situ through light irradiation. The reaction, which is reminiscent of human behaviour, is brief ly described below. MATERIALS AND METHODS Formation of endoperoxides 9,10-Endoperoxides “AO2” (Figure 1 ) are formed by a hetero Diels-Alder addition of singlet oxygen (1O2) to anthracenes “A”.6,7,8 Singlet oxygen can be generated by several processes. One of them, photosensitization,9,10,11,12 involves energy transfer from an organic compound in its excited triplet state (T1). Anthracene derivatives can be good singlet oxygen producers when they have high yields of triplet formation.8 The energy transfer from the triplet state and dioxygen leads to singlet oxygen 1O2 which can participate in the addition reaction as shown in Figure 1. The two anthracene derivatives considered here are 9-isopropyl (E) and 9-tertiobutyl (M) anthracene. Although E was found to readily generate 1O2, M was shown to be unable to do so.8 The primarily reached photochemical state is deactivated much too fast and no triplet state can be formed; therefore M cannot act as a sensitizer.13,14 Thus, irradiation of E in solution in the presence of dioxygen generates the endoperoxide “EO2” in high yield. Under the same experimental con- ditions, M is not transformed into “MO2” and remains unchanged. Competition reaction The irradiation through Pyrex of an equimolecular mixture of E and M in solution in dioxygen-saturated ether, at room temperature initially leads to the exclusive formation of the endoperoxide of M (MO2), Figure 2. Then, after the complete transformation of M, the reac- tion of E with singlet oxygen begins, leading entirely to the formation of EO2.8 The sequence is illustrated in the following sketches: (1), (2) and (3) of Figure 3. One observes that M, the most crowded molecule, is considerably more reactive with 1O2 than E. This is called a steric acceleration, due to a relief of strain in the acti- vated complex that is product-like. The same phenom- enon was also noted for other anthracene derivatives.15 The thermal and photochemical stability of the two adducts during the reaction attests to its irreversibility. 8,16 CONCLUSIONS Master and Slave The above molecular behaviour is suggestive of human attitudes. Singlet oxygen might be compared to food, pro- duced only by E, the slave. As soon as the food is available, O2 A R O O R 9 10 AO2 endoperoxide Figure 1. Addition of singlet oxygen to 9-alkylanthracenes (hetero Diels-Alder reaction). O O CH3 CH3 CH3 CH3 CH3 CH3 + 1O2 Figure 2. Developed formulae of M and MO2 respectively, suggest- ing the steric overcrowding due to the tertiobutyl substituent. 97The Master and the Slave. A glance at the social life of molecules M (the master) rushes at it in a gluttonous manner until he has eaten his fill. When he has gorged himself, then the poor slave is allowed to eat. The story stops there because after the meal, both master and slave are in a peaceful longstanding state. The nasty master is not punished, in contrast to what generally happens in fairy tales. One could imagine other scenarios: 1) instead of an equal number of E and M, an excess of M would lead to an accumulation of MO2; thus a sin- gle slave would work for many masters. This would delay the meal time of E. 2) if the irradiation is stopped after M’s hunger is satisfied, then in the dark, E would no longer produce the food and be doomed to be starving for ever. Today, slavery has been abolished on earth. Howev- er, it seems that some inactive people are prompt to eat what others have strained to produce. This observation might be extended to relationships between countries. It could be argued that human beings have com- mon ancestors in the Mendeleev table, especially car- bon, hydrogen, oxygen, etc., and that their reptilian brain keeps traces of their molecular constitution; this might partly explain their shocking deeds. However, the extreme complexity of human behaviours cannot be traced back to simple chemical reactions. Let this short tale contribute to inspire to everybody with a humble attitude. ACKNOWLEDGEMENTS The authors are grateful to Dr Esther Oliveros for her precious information about singlet oxygen and Prof. Dr Herbert Dreeskamp for perceptive comments. We warmly thank Dr Dario Bassani for valuable linguistic assistance. REFERENCES 1. A. Lattes, I. Rico, La Sociologie Moléculaire et les Ten- sioactifs, Pour la Science, mars 1992, N° 173. 2. J.-M. Lehn, Supramolecular Chemistry; Concepts and Perspectives, VCH, Weinheim, 1995. Version fran- çaise: La Chimie Supramoléculaire. Concepts et Per- spectives, De Boeck Université, 1997. 3. M.M. Green, M.P. Reidy, R.J. Johnson, G. Darling, D.J. O’Leary, G. Wilson, J. Amer. Chem. Soc. 1989, 111, 452. 4. Johann Wolfgang Goethe, “Die Wahlverwandtschaf- ten” (The elective affinities), 1809. We thank professors Henning Hopf (Braunschweig) and Herbert Dreeskamp (Bonn) for pointing out this information. 5. B.J. Cohen, M.A. Kraus, A. Patchornik, J. Amer. Chem. Soc. 1981, 103, 7620. 6. J. Rigaudy, Pure Appl. Chem. 1968, 16, 169. 7. D.O. Cowan, R.L. Drisko, Elements of Organic Photo- chemistry, Plenum Press, New York, 1976, Chap. 2, pp. 69-71. 8. R. Lapouyade, J.-P. Desvergne, H. Bouas-Laurent, Bull. Soc. Chim. Fr., 1975, 2137. Under our photooxidation experimental conditions no photodimer was observed. 9. A. Braun, M.T. Maurette, E. Oliveros, Technologie Photochimique, Presses Polytechniques Romandes, Lausanne, 1986 and Photochemical Technology, J. Wiley, Chichester, 1991, chap.11, pp. 445-499. 10. F. Wilkinson, H. P. Helman, A.B. Ross, J. Phys. Chem. Ref Data 1955, 24, 663. 11. M.C. DeRosa, R. J. Crutchley, Coord. Chem. Rev., 2002, 233-234, 351. 12. E.L. Clennan, A. Pace, Tetrahedron 2005, 61, 6665. 13. H. Güsten, M. Mintas, L. Klasinc, J. Amer. Chem. Soc., 1980, 102, 7936. 14. B. Jahn, H. Dreeskamp, Ber. Bunsenges. Phys. Chem. 1984, 88, 42. 15. N. Lahrahar, P. Marsau, J. Rigaudy, H. Bouas-Lau- rent, J.-P. Desvergne, Austr. J. Chem. 1999, 52, 213. Figure 3. (1) A solution of M and E in equal amounts is irradiated under dioxygen bubbling; E produces singlet oxygen (the food) in contrast to M, which is unable to do so. (2) M swallows and con- sumes all the food available. (3) M being entirely transformed into MO2, E can in turn eat the food and generate EO2. 98 Henri Bouas-Laurent, Jean-Pierre Desvergne 16. H.D. Brauer, R. Schmidt, Photochromism, Molecules and Systems (Eds: H. Dürr and H. Bouas-Laurent) Elsevier, Amsterdam, 2003, chap. 15, pp. 631-653. This chapter describes the formation and decomposi- tion of endoperoxides. Some of them, such as heteroco- erdianthrone (lifetime 900 years at 20°C) are particu- larly thermally stable.