OPTICAL STUDY OF CHARGE-TRANSFER SALTS WITH ORGANIC DONORS ISSUED FROM TTF AND TETRACYANOMETALLATE PLANAR DIANIONS PT(II)(CN)4, NI(II)(CN)4

The class of mixed-valence one-dimensional systems based on tetrathiafulvalene (TTF) and its derivatives and inorganic anions of tetracyanometallates such as Krogmann's salts have been two of the most extensively studied low-dimensional conducting materials. A new series of conducting radical-cation salts has been recently synthesized including both types of compounds with a general formula (Dx)2+C2- (D = TTF, TMTTF, TMTSF, BEDT-TTF; C = Ni(CN)4, Pt(CN)4). The visible and IR absorption spectra of these compounds have been examined. Two main types of results are observed and presented: (i) first, the charge-transfer bands characteristic of a mixed-valence system are observed around 3400 cm-1 for every salt (except TTF5[Pt(CN)4]2[(CH3CN)2 ] for which this 'A band' occurs at 5700 cm-1); (ii) secondly, vibronic lines which are ag modes, usually IR-forbidden, are detected in all compounds; besides, the temperature dependency studied for both BEDT-TTF salts seems to be representative of a phase transition occurring around 200- 250 K. These optical properties are explained in relation to the specific structural organization observed in these new materials, in particular with the presence of centrosymmetric pentamers in a TTF salt. © 1990.