Infrared spectra of tetramerized organic semiconductors PrPht(TCNQ)2 and PrQuin(TCNQ)2 — the problem of ag phonon mode splitting

Polarized infrared reflectance spectra from single crystals of two organic charge-transfer salts PrPht(TCNQ)2 and PrQuin(TCNQ)2 (where TCNQ = tetracyano-pquinodimethane, PrPht = N-propylphtalazinium, PrQuin = N-propylquinoxalinium) are studied over the frequency range 400–7000 cm−1 at room temperature (in both compounds the TCNQ molecules are arranged to form linear stacks with the tetramer repeat unit). Kramers-Kronig analysis of the reflectance is used to determine the optical properties. Some vibrational lines related to the coupling of ag phonon modes of the TCNQ molecule with electrons are split. The origin of ag mode splitting is discussed. Moreover, the electronic spectra (up to 20 000 cm−1) are measured at room temperature using the KBr pellet technique. The charge-transfer bands in tetramerized TCNQ salts are analysed.