Design of organic electroactive materials

Research topics

■ Materials development of new electroactive molecules based on
"protonic-defect induction" strategy

■ Single-component pure organic metal, TED

PI

Yuka Kobayashi
Molecular electronics, Solid-state chemistry, Ab initio calculations
Principal Researcher, NIMS (2010-)
Associate Professor, Waseda University (-2010)
Assistant Professor, The University of Tokyo (-2007)
PhD. The University of Tokyo (2001)
KOBAYASHI.Yuka[at]nims.go.jp

Access

Sengen-site (Headquarters) blank
1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047 JAPAN

 

 

 

Background of research

Organic compounds are generally insulators unless special molecular design for charge carrier injection are adopted, so carrier generation method is very important for the synthesis of organic electroactive materials.

Recently we found a new carrier generation method utlizing a salt-bridge network, where protonic defects are partially included and oxidize coexisting electron-donor molecules such as tetrathiafulvalene (TTF) without electrochemical oxidation or the addition of external dopants. The typical example is TTFCOONH4. The electroactive materials derived from this method are in a separate category from other charge-injected compounds, CT complexes and conducting polymers.

This method can provide new electroactive materials , different from CT complexes and conducting polymers, so this is a third strategy for preparing charge-injected organic compounds. (J.Am.Chem.Soc.2009,131,9995)

Mechanism of the "protonic-defects induction doping"

The inclusion of protonic defects sponteneously takes place in the salt formation between -COO- and -NH3+R in an organic solvent, and the amount determines the doping level of the materials. We found that the doping level is controlled only by the selection of organic solvent in the crystallization, where the doping level correlates with the self-dissociation parameters (pKSH) of the organic solvent. This verifies that protonic defects are the intrinsic dopant in this doping system.(Syn.Met.2012,162,531)

Effect of mobile dopant in semiconductors

The electronic properties of the doped materials are truly unique, especially for semiconductors. We found that the dopant (= protonic defects ) is mobile in salt-bridge networks through the migration of ammonium protons even in dry air at room temperature. The mobility of the dopant can enhance thermopower in TTFCOONH4 and TTFCOONH3Ph, respectively (J.Mater.Chem.A,2013,1,5089, Eur JIOC,2014), where the thermopower values of the proton forms are both twice as large as those of deuterium forms, (TTFCOOND4 and TTFCOOND3Ph). Moreover, the isotope effect in the subsitution of ammonium protons can also be observed in negative magnetoresistance at room temperature in single crystalline forms, TTFCOONH3Ph and TTFCOOND3Ph. (Solid State Commun.2013,165,27)

Metallization of salt-bridge materials

The charge-injected salt-bridge TTF-based materials are all semiconductors with activation-type transport. We designed a new donor molecule with extended pai-conjugated moiety, tetrathiapentalenecarboxylic acid, TTPCOOH. (Tetrahedron Letters,2012,53,3277), and treated it with ammonium solution to obtain a doped salt. The ammonium salt, (TTPCOO)2NH4, exhibits particularly high electrical conductivity with 13 S/cm at room temperature. The weak temperature dependence of magnetic susceptibility and T-linear behavior of relaxation time of conducting electrons in solid-state NMR has demonstrated that the achievement of metallization of salt-bridge materials for the first time. This compound shows a stable metallic state even at 4 K. As far as we know, this is the third organic metal, after CT complexes and conducting polymers. (Chem.Comm.2014,50,7111)

Single-component pure organic metals

It is well known that pure organic molecules are insulators due to large on-site Coulomb repulsion than electron transfer, namely Mott-Hubard insulators.

Tetrathiafulvalene-extended dicarboxylate (TED) is the first molecule, which overcame the Mott-Hubbard gap. TED exhibits metallic conduction in itself until low temperatures at ambient pressure (Nature Mat. 2017,16, 109). This is due to a molecular packing in crystal with efficient overlap of molecular orbitals in two dimension (Chem. Sci. 2020, 11, 11699).

Owing to the high conducticity and high chemical stability of the single-component conductors, Solar cells with TED can keep high conversion efficiency for a long time compared with doped-polymer based solar cells(ACS Appl. Energy Mater.2019,2,2637).

Further application use such as thermoelectric and spintronics devices is expected.

Future applications of research

Light, abundant, low-toxicity, low-cost organic materials with outstanding physical properties would have a meaningful impact on the creation of new technology strongly contributing to the development of next generation society.

Research into the applications of the doped materials for energy-conversion and spintronics devices is now in progress.

Publications

Organic electroactive materials

Original papers, 2009-

Other topics (Selected papers)

Patents

Conferences (Invited talks)