Dielectric properties derived from proton tautomerism in crystals

6 Dielectric properties derived from proton tautomerism in crystals 

If a material can be electrically polarized under an external electric field and the polarization inverts in response to the alternating electric field, the material is designated as a dielectric. The origins of the polarity inversion are classified as shown in Fig. 20 (a) electric polarization, which is caused by the displacement of electrons relative to the nucleus (b) ionic polarization caused 

From the viewpoint of the dielectric properties of organic molecular crystals, the proton tautomerism of the s-cis form of 3-hydroxyenone [40] is intriguing, because the horizontal component of the dipole moment of a tautomeric 

The mechanism of the polarity inversion of tautomeric molecules is totally different from the orientation polarization of conventional organic dielectrics, such as camphor and poly(vinylidene fluoride), and the dielectric response of this new type of dielectric should be much faster. Furthermore, a significant contribution of the proton-tunnelling mechanism to the proton tautomerism is frequently observed. Consequently, the dielectric property derived from proton tautomerization should have a high chance of being related to quantum phenomena. 

Since the proton tautomerism of naphthazarin [13] or 9-hydroxy-phenalenone [6] can be investigated precisely in an isolated system, it is intriguing to see how these tautomeric molecules behave as dielectrics in the solid state. Incidentally, if the tautomers keep the centrosymmetry during the tautomerization process, the dielectric response cannot be detected, because the changing directions of the dipole moments coupled with the tautomerization, even though they invert, cancel. 

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