Ferroelectric properties of a triphenylene derivative with polar functional groups in the crystalline state

We studied ferroelectric ordering in a triphenylene derivative embedded with electric dipoles [2,3,6,7,10,11-hexakis (4-octyloxy-benzoyloxy) triphenylene (HOBPT)] in a crystalline state. Experimental results indicate that the ferroelectricity in HOBPT is caused by an ordered orientation of $\mathrm{C}\mathrm{O}$ dipoles. Our experiments also reveal that dielectric anomaly due to ferroelectric–paraelectric phase transition occurs at $380\mathrm{K}$. A photovoltaic effect was observed in an electrically treated thin film of HOBPT. The phenomenon results from a high charge mobility due to the $\pi -\pi$ stack between adjacent molecules as well as an internal electric field derived by the residual polarization.

Figure 1

Chemical structure of 2,3,6,7,10,11-hexakis(4-octyloxy-benzoyloxy) triphenylene (HOBPT) and a schematic view of crystalline HOBPT.

Figure 2

(a) Temperature dependence of dielectric constant ${ϵ}^{*}$ of HOBPT. The frequency dependence of ${ϵ}^{*}$ at the dielectric anomaly point is also shown in the inset. (b) Temperature dependence of pyroelectric coefficient $\beta$ of HOBPT.

Figure 3

(a) Electric field $\left(E\right)$–electric displacement $\left(D\right)$ hysteresis loop of HOBPT at $293\mathrm{K}$. (b) Temperature dependence of coercive field $\left(E_c\right)$ and residual polarization $\left(P_r\right)$ of HOBPT.

Figure 4

Examples of photovoltaic effects of poled HOBPT in a short circuit. A step wise application of DC external electric field (a), irradiation of UV light (b) and a photo-current response (c) are shown.