FT-IR Micro-Spectroscopic Studies of Several Charge-Transfer Organic Electrical Conductors

A series of infrared reflectance measurements using FT-IR Micro methods were conducted on bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or “ET”) and bis(propylenedithio)tetrathiafulvalene (BPDT or “PT”) salts of the type (ET)2 X, where X = I3, AuCl2−, AuBr2−, AuI2−, I2Br−, IBr2, ICl2, ReO4−, ClO4−, and [Formula: see text]; and (PT)2 X, where X = I3−, IBr2−, I2Br−, and ICl2−. The technique is useful for characterizing, at room temperature, differences between β-, β′- and α′-type structures vs. α-structures. The β-, β′-, and α′-type structures show a vibrational feature at ∼1280 cm−1 which has been assigned as the -C-C-H bending vibration and is absent in the spectrum of the α-type salts. This vibration shifts toward higher frequency in going from the PT to the ET salts, and the results indicate stronger hydrogen bonding and harder lattices in the ET salts as opposed to the PT salts. This frequency shift also appears to be a function of the electronegativity of the halogen in the anion moiety of the charge-transfer salt.