Quasi Chiral Phase Separation in a Two-Dimensional Orientationally Disordered System: 6-Nitrospiropyran on Au(111) Tian Huang, Zhenpeng Hu, Aidi Zhao, Haiqian Wang, Bing Wang, Jinlong Yang, and J. G. Hou* Contribution from the Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China Received September 9, 2006; E-mail: jghou@ustc.edu.cn Abstract: The adsorption and chiral expression of 6-nitrospiropyran (SP6) molecules on a Au(111) surface are studied by scanning tunneling microscopy (STM) in combination with density functional theory (DFT) calculations. Both the chirality and the adsorption orientation of each adsorbed SP6 molecule are determined. The racemic mixture of SP6 enantiomers forms two-dimensional (2D) domains with same close packed positional orders but different internal orientational structures due to the random distribution of two adsorption orientations in each domain. However, all these orientationally disordered 2D domains undergo spontaneous quasi chiral phase separation; the 2D SP6 domains separate into 1D homochiral chains in which the SP6 molecules adopt two orientations randomly. This novel phenomenon is attributed to the preferential formation of the energetic favorable configurations with both the C-H‚‚‚O weak hydrogen bonds and the π-stacking of the two moieties of each SP6 molecule. Introduction The chiral chemistry in two-dimensional (2D) molecules/ substrate systems is of both technological and fundamental importance and has attracted numerous interests in the past decade. 1-5 Constructing a chiral surface is a crucial step for enantioselective heterogeneous catalysis in chemical and phar- maceutical industries. 6-10 On the other hand the fundamental investigation of molecular chirality is more feasible in 2D systems due to the reduction of the spatial freedom. With the help of scanning tunneling microscopy (STM), various chiral phenomena such as chiral resolution, 11-22 chirality amplifica- tion, 23 chiral phase transition, 11,12,24 and loss of chirality 25 have been directly observed at the submolecular scale in 2D molecules/substrate systems. Spontaneous resolution, i.e., the chiral separation of the racemic mixture into enantiomorphic condensates upon crystal- lization, is an intriguing and valuable chiral phenomenon which has been observed in various media such as crystals, 26 liquid crystals, 27 self-assembled helical fibers, 28 and 2D molecules/ substrate systems. 11-22 Previous STM observations revealed that both prochiral 10-14 and chiral 15-22 molecules could undergo spontaneous resolution upon adsorption on surfaces. The enan- tiomorphic condensates that resulted from the segregations included 2D domains, 10,11,15-18 1D molecular chains, 12-14,18-20 and 0D molecular clusters. 14,21,22 But the STM investigations (1) Jannes, G., Dubois, V., Eds. Chiral Reactions in Heterogeneous Catalysis; Plenum: New York, 1995. (2) Baiker, A.; Blaser, H. U. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno ¨ zinger, H., Weitkamp, J., Eds.; VCH: Weinheim, 1997; Vol. 5, pp 2422-2430. (3) Baddeley, C. J. Top. Catal. 2003, 25, 17-28. (4) Verbiest, T.; Elshocht, S. V.; Kauranen, M.; Hellemans, L.; Snauwaert, J.; Nuckolls, C.; Katz, T. J.; Persoons, A. Science 1998, 282, 931-915. (5) Bodenho ¨ fer, K.; Hierlemann, A.; Seemann, J.; Gauglitz, G.; Koppenhoefer, B.; Go ¨pel, W. Nature 1997, 387, 577-580. (6) Ohtani, B.; Shintani, A.; Uosaki, K. J. Am. Chem. Soc. 1999, 121, 6515- 6516. (7) Zhao, X. J. Am. Chem. Soc. 2000, 122, 12584-12585. (8) Lorenzo, M.; Baddeley, C.; Muryn, C.; Raval, R. Nature 2000, 404 (6776), 376-379. (9) Schunack, M.; Lægsgaard, E.; Stensgaard, I.; Johannsen, I.; Besenbacher, F. Angew. Chem., Int. Ed. 2001, 40, 2623-2626. (10) France, C.; Parkinson, B. J. Am. Chem. Soc. 2003, 125, 12712-12713. (11) Vidal, F.; Delvigne, E.; Stepanow, S.; Lin, N.; Barth, J. V.; Kern, K. J. Am. Chem. Soc. 2005, 127, 10101-10106. (12) Bo ¨ hringer, M.; Schneider, W.-D.; Berndt, R. Angew. Chem., Int. Ed. 2000, 39, 792-795. (13) Barth, J. V.; Weckesser, J.; Trimarchi, G.; Vladimirova, M.; De Vita, A.; Cai, C.; Brune, H.; Gu ¨nter, P.; Kern, K. J. Am. Chem. Soc. 2002, 124, 7991-8000. (14) Bo ¨hringer, M.; Morgenstern, K.; Schneider, W.-D.; Berndt, R.; Mauri, F.; De Vita, A.; Car, R. Phys. ReV. Lett. 1999, 83, 324-327. (15) Fang, H. B.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B 1998, 102 (38), 7311-7315. (16) Xu, Q. M.; Wang, D.; Wan, L. J.; Wang, C.; Bai, C. L.; Feng, G. Q.; Wang, M. X. Angew. Chem., Int. Ed. 2002, 41, 3408-3411. (17) Fasel, R.; Parschau, M.; Ernst, K.-H. Angew. Chem., Int. Ed. 2003, 42, 5178-5181. (18) De Feyter, S.; Gesquie `re, A.; Wurst, K.; Amabilino, D. B.; Veciana, J.; De Schryver, F. C. Angew. Chem., Int. Ed. 2001, 40, 3217-3220. (19) Cai, Y.; Bernasek, S. L. J. Am. Chem. Soc. 2003, 125, 1655-1659. (20) Cai, Y.; Bernasek, S. L. J. Phys. Chem. B 2005, 109, 4514-4519. (21) Ku ¨hnle, A.; Linderoth, T. R.; Hammer, B.; Besenbacher, F. Nature 2002, 415, 891-893. (22) Blu ¨ m, M.-C.; C Ä avar, E.; Pivetta, M.; Patthey, F.; Schneider, W.-D. Angew. Chem., Int. Ed. 2005, 44, 5334-5337. (23) Fasel, R.; Parschau, M.; Ernst, K.-H. Nature 2006, 439, 449-452. (24) Weigelt, S.; Busse, C.; Petersen, L.; Rauls, E.; Hammer, B.; Gothelf, K.; Besenbacher, F.; Linderoth, T. Nat. Mater. 2006, 5, 112-117. (25) Zhang, J.; Gesquie `re, A.; Sieffert, M.; Klapper, M.; Mu ¨llen, K.; De Schryver, F. C.; De Feyter, S. Nano. Lett. 2005, 5, 1395-1398. (26) Pasteur, L. Ann. Phys. 1848, 24, 442. (27) Takanishi, Y.; Takezoe, H.; Suzuki, Y.; Kobayashi, I.; Yajima, T.; Terada, M.; Mikami, K. Angew. Chem., Int. Ed. 1999, 38, 2354-2356. (28) Sakurai, S.; Okoshi, K.; Kumaki, J.; Yashima, E. J. Am. Chem. Soc. 2006, 128, 5650-5651. Published on Web 03/09/2007 10.1021/ja066521p CCC: $37.00 © 2007 American Chemical Society J. AM. CHEM. SOC. 2007, 129, 3857-3862 9 3857