Published in “Angewandte Chemie International Edition“ (Online Publication, April 24, 2014).

Assoc Prof. Wakamiya, A., Mr. Nishimura, H., Prof. Murata, Y.

(Structural Organic Chemistry, Division of Synthetic Chemistry)

Assist Prof. Fukushima, T. and Prof. Kaji, H.

(Molecular Materials Chemistry, Division of Environmental Chemistry)

The development of excellent charge-transporting materials with high charge carrier mobility is a crucial issue to achieve high performance in organic-device applications. In order to achieve high charge-carrier mobility using small organic molecules, it is important to control their molecular orientation in the solid state. As general π-conjugated skeletons for charge-transporting materials, planar structures or propeller-like structures are used for crystalline or amorphous materials, respectively.
While progress has been made in understanding the relationship between molecular orientation and charge-transporting property in the crystalline state, the conformations of individual molecules as well as of their aggregates in amorphous film have not been determined unambiguously, rendering precise molecular design strategies difficult. Accordingly, the elucidation of the exact relationship between the molecular orientation in the crystalline and amorphous state remains a challenge in the design of advanced charge-transporting materials.

Figure 1. Molecular design concept for organic semiconducting materials using quasiplanar skeleton.

As new π-conjugated skeleton to control molecular orientation in the solid states, the research groups focused on quasiplanar skeleton. As model compounds, they designed and synthesized partially bridged triphenylamine derivative, in which three phenyl groups are constrained in a quasiplanar fashion with two oxygen-bridges (Figure 1). X-ray crystallography revealed that these compounds form one-dimensional on-top π-stacking aggregates in the crystalline state, due to quasiplanar-skeleton. Time-resolved microwave conductivity (TRMC) measurements showed that high levels of anisotropic charge transport were induced in the direction of the π-stacking. Furthermore, even in the vacuum deposited amorphous films, these compounds retained some of the face-on π-stacking, thus facilitating an out-of-plane carrier mobility (Figure 2).

Figure 2. The structures of oxygen-bridged triphenylamine derivatives and their one-dimensional on-top π-stacking structure in the crystalline state. Anisotropic charge carrier mobility in the amorphous films of these compounds is also shown.

These results would provide new guideline for molecular design of organic semiconducting materials used in the amorphous state. The striking feature of the charge mobility in the amorphous state exhibited by these quasiplanar triarylamine model compounds are expected to be extraordinarily beneficial for charge-transporting materials in OLEDs, organic solar cells, and perovskite-based solar cells, all of which require high levels of out-of-plane charge transport.

This work was partially supported by JST and the Collaborative Research Program at the ICR, Kyoto Univ. Support was also received from the Funding Program for World Leading Innovative R&D on Science and Technology (FIRST Program).

Synchrotron single-crystal X-ray analysis was carried out with BL38B1 of SPring-8 with the approval of JASRI (2012A1448, 2012B1319, and 2013A1489). 2D-GIXD experiments were performed at BL19B2 of SPring-8 with the approval of JASRI (2013A1634). TRMC measurements were conducted by Assoc Prof. Akinori Saeki and Prof. Shu Seki at Osaka University. The authors thank Assoc Prof. Takahiro Sasamori at KUICR and Dr. Itaru Osaka at Riken for X-ray crystallography and 2D-GIXD, respectively.

Wakamiya, A.; Nishimura, H.; Fukushima, T.; Suzuki, F.; Saeki, A.; Seki, S.; Osaka, I.; Sasamori, T.; Murata, M.; Murata, Y.; Kaji, H., “On-Top π-Stacking of Quasiplanar Molecules in Hole-Transporting Materials: Inducing Anisotropic Carrier Mobility in Amorphous Films”, Angewandte Chemie International Edition, 23, 5800 (2014).