Infrared and Raman spectroscopic study of BDA-TTP [2,5-bis(1,3-dithian-2-ylidene)–1,3,4,6-tetrathiapentalene] and its charge-transfer salts

Infrared and Raman spectra in the frequency range of 1200–1600 cm−1 were observed using BDA-TTP and (BDA-TTP)CuCl2 crystals. The C =C stretching and CH2 bending modes in this frequency region were assigned based on quantum chemical calculation of the normal modes by the density functional theory (DFT) method. The three C =C stretching modes of BDA-TTP showed a significant low-frequency shift upon oxidation. One of the Raman-active C =C stretching modes is strongly coupled with the charge-transfer excited state. Vibrational analysis was applied to β-(BDA-TTP)2I3. The infrared-active C =C stretching mode strongly suggests that the insulating state of β-(BDA-TTP)2I3 is characterized as a dimer-Mott state below 150 K.