Phase transition of θ-(BDT-TTP)2Cu(NCS)2

We present spectroscopic evidence for the charge disproportionation below the phase transition of the crystal of θ-(BDT-TTP) 2 Cu(NCS) 2 , This compound is a highly correlated quasi-2D organic conductor. Below the phase transition, the electronic transition in the mid-infrared region shifts to high-frequency side and the C=C stretching α g modes ν 2 , ν 3 , and ν 4 , which are sensitive to the charge on BDT-TTP, split into two groups. At 5 K these groups are reasonably assigned to charge-poor and charge-rich BDT-TTP molecules.