Effectively half-filled Mott insulator

It should be added that the expansion of the tr-conjugating system and/or the introduction of Se atoms with greater polarizability (as shown in BETS or TTPs) reduce the effective on-site Coulomb energy U. For example, the V values for the TTP-type donors estimated from electrochemical and optical measurements are considerably small. (TTM-TTP)l3 is a 1 1 salt with a highly one-dimensional half-filled band. Although 1 1 salts were believed to be non-metal due to the Coulomb interaction (Mott-insulator), this salt shows metallic behavior down to about 160 K. This system is regarded as a small-f/ case of a one-dimensional conductor with an inherently half-filled band [27]. 

M(dmit)2 molecule is 1/2-, frequently observed value, the conduction band is the half-filled (anti-bonding) HOMO band. This two-dimensional HOMO band has narrow width as a result of the strong dimerization (the effective band width is correlated to the interdimer transfer integrals). Therefore, the system tends to become a Mott insulator. 

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