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R. Tamura et al.
irregular homochiral sheet pairs, and (ii) the homochiral 2D sheets pairs stack to
give partially irregular arrangement in the crystals. As a result, the resulting brittle
deposited mixed crystals contain the less stable parallel stacking sites indicated by
the outside orange arrows (structure III), where the excess homochiral 2D sheet (an
S sheet in the present case) dissolves into the mother liquor to result in the slightly
R-rich crystal (process 3 and structure IV). Thus, it is reasonable to consider that a
large molecular dipole moment of the CPPPA-INA unit and the weak interactions
(only CH/O and CH/π interactions) between homochiral 2D sheets are responsible
for the observed PE phenomenon.
20.6 Conclusions and Prospects
Through the 25 years’ studies on the mechanism and generality of PE, we have
confirmed that PE is attributed to an event of complexity system and applicable to
not only certain kinds of mixed crystals which we discovered for the first time,
but also various quasi-mixed crystals prepared by co-crystallization of common
racemic compounds with achiral coformers. The mechanism of PE has turned out
to be quite flexible depending on the molecular and crystal structures. Since various
compounds which satisfied all the five requirements listed in this article could show a
PE phenomenon with very high probability, we believe that it is easy to find out new
compounds showing PE according to this simple protocol, i.e., the five requirements.
Furthermore, it is noteworthy that SHG measurement of deposited crystals can serve
as a facile method at the first-stage screening to explore new candidate compounds
showing PE or confirm the occurrence of PE for newly found compounds.
Thus, we anticipate that PE can be used as a general chiral resolution method by
simple crystallization, with high flexibility and versatility, and full reproducibility,
similarly to PC of racemic conglomerates.
References
1. Prigogine, I.: The End of Certainty. The Free Press, New York (1977)
2. Prigogine, I., Stengers, I.: Order Out of Chaos: Man’s New Dialogue with Nature. Bantam
Books, New York (1984)
3. Lightman, A.: The Discoveries. Vintage Books, New York (2005)
4. Waldrop, M.M.: Complexity. Simon & Schuster Paperbacks, New York (1992)
5. Kauffman, S.A.: At Home in the Universe. Oxford University Press, New York (1995)
6. Kauffman, S.A.: Investigations. Oxford University Press, New York (2000)
7. Mainzer, K.: Symmetry and Complexity. World Scientific (2005)
8. Mainzer, K.: Thinking in Complexity. Springer, Berlin (2007)
9. Di Bella, S., Ratner, M.A., Marks, T.J.: Design of chromophoric molecular assemblies with
large second-order optical nonlinearities. A theoretical analysis of the role of intermolecular
interactions. J. Am. Chem. Soc. 114, 5842–5849 (1992)
10. Bustamante, C., Liphardt, J., Ritort, F.: The nonequilibrium thermodynamics of small systems.
Phys. Today 58, 43–48 (2005)
R. Tamura et al.
irregular homochiral sheet pairs, and (ii) the homochiral 2D sheets pairs stack to
give partially irregular arrangement in the crystals. As a result, the resulting brittle
deposited mixed crystals contain the less stable parallel stacking sites indicated by
the outside orange arrows (structure III), where the excess homochiral 2D sheet (an
S sheet in the present case) dissolves into the mother liquor to result in the slightly
R-rich crystal (process 3 and structure IV). Thus, it is reasonable to consider that a
large molecular dipole moment of the CPPPA-INA unit and the weak interactions
(only CH/O and CH/π interactions) between homochiral 2D sheets are responsible
for the observed PE phenomenon.
20.6 Conclusions and Prospects
Through the 25 years’ studies on the mechanism and generality of PE, we have
confirmed that PE is attributed to an event of complexity system and applicable to
not only certain kinds of mixed crystals which we discovered for the first time,
but also various quasi-mixed crystals prepared by co-crystallization of common
racemic compounds with achiral coformers. The mechanism of PE has turned out
to be quite flexible depending on the molecular and crystal structures. Since various
compounds which satisfied all the five requirements listed in this article could show a
PE phenomenon with very high probability, we believe that it is easy to find out new
compounds showing PE according to this simple protocol, i.e., the five requirements.
Furthermore, it is noteworthy that SHG measurement of deposited crystals can serve
as a facile method at the first-stage screening to explore new candidate compounds
showing PE or confirm the occurrence of PE for newly found compounds.
Thus, we anticipate that PE can be used as a general chiral resolution method by
simple crystallization, with high flexibility and versatility, and full reproducibility,
similarly to PC of racemic conglomerates.
References
1. Prigogine, I.: The End of Certainty. The Free Press, New York (1977)
2. Prigogine, I., Stengers, I.: Order Out of Chaos: Man’s New Dialogue with Nature. Bantam
Books, New York (1984)
3. Lightman, A.: The Discoveries. Vintage Books, New York (2005)
4. Waldrop, M.M.: Complexity. Simon & Schuster Paperbacks, New York (1992)
5. Kauffman, S.A.: At Home in the Universe. Oxford University Press, New York (1995)
6. Kauffman, S.A.: Investigations. Oxford University Press, New York (2000)
7. Mainzer, K.: Symmetry and Complexity. World Scientific (2005)
8. Mainzer, K.: Thinking in Complexity. Springer, Berlin (2007)
9. Di Bella, S., Ratner, M.A., Marks, T.J.: Design of chromophoric molecular assemblies with
large second-order optical nonlinearities. A theoretical analysis of the role of intermolecular
interactions. J. Am. Chem. Soc. 114, 5842–5849 (1992)
10. Bustamante, C., Liphardt, J., Ritort, F.: The nonequilibrium thermodynamics of small systems.
Phys. Today 58, 43–48 (2005)