Assoc Prof. Wakamiya, A., Mr. Satou, M., Prof. Murata, Y.

(Structural Organic Chemistry, Division of Synthetic Chemistry)

Organic photovoltaics (OPVs) have received much attention as a next generation solar cells because of their potential of cost-effective fabrication by printing process. In order to improve the photon-to-electricity conversion efficiency of OPVs, the development of sophisticated organic semiconducting materials with both high carrier mobility and intense and broad light absorption characteristics are strongly required. In the molecular design of such organic materials, construction of rigid and planar skeleton is a promising strategy to realize high carrier mobility, which would be induced by dense packing structure in solid state. However, such rigid and planar molecules generally show low solubility in the common solvent, thus being difficult to be applied to printing process.

Figure 1. Molecular design concept of soluble T-shaped electron-accepting unit (SaT).

As a skeleton having both high planarity in the solid state and high solubility in organic solvents, the research group designed a T-shaped electron-accepting unit composed of thiazole-fused benzothiadiazole (Figure 1).
X-ray diffraction analyses confirmed that the model compounds have highly planar structure to form dense π-stacking aggregate in the crystalline state, due to the intramolecular interaction between sulfur and nitrogen atoms (S···N interactions).
On the other hand, the rotation barriers of connected five-membered rings are estimated to be lower than 10 kcal/mol, which is enough low to rotate in the solution. Owing to this flexibility in the skeleton, these molecules exhibited high solubility in common organic solvents.
In the photophysical properties, these compounds show two absorption bands in visible region. Theoretical calculations indicate that these characteristic bands are assignable to the dual π-π* transitions from the vertically and horizontally delocalized π orbitals to the π* orbital over thiazole-fused benzothiadiazole skeleton, respectively. This result demonstrates that the two-dimensional expansion of the π-conjugation through the fused thiazole moiety renders these compounds ideal candidates to be used as T-shaped π-conjugation junctions.
Electrochemical analyses revealed that these thiazole-fused benzothiadiazole units show high electron-accepting ability compared to fluorine substituted benzothiadiazole. In addition, their electron-accepting ability is tunable by the choice of substituents on the fused thiazole moiety, which would be beneficial for the development of organic semiconducting materials for OPVs.

In summary, novel T-shaped electron-accepting units based on thiazole-fused benzothiadiazole skeletons (SaT) were designed and synthesized. These compounds are highly soluble in solution and exhibit high planarity in the solid state as evident from the structural analysis. The electrochemical and photophysical properties of these benzothiadiazole units demonstrated that thiazole-fusion induces high levels of electron-accepting ability, which can be easily tuned. These thiazole-fused benzothiadiazoles moreover exhibit dual absorption properties based on an effective two-dimensional π-expansion. Further development of D-A materials based on these T-shaped benzothiadiazole building blocks and their device applications in organic electronics are currently in progress in their laboratories.

Acknowledgement

This research was partially supported by the Center of Innovation Program and the PRESTO project from Japan Science and Technology Agency, JST, and JSPS KAKENHI Grant Number 26288093.

These results were introduced in “The Business & Technology Daily News” (26, March, 2014, page 26) and will be published in “Chemistry Letters“, 2014.