A study of the phase transition in semiconducting 1:2 tetracyanoquinodimethane charge transfer complexes with dipropylamine derivatives: X-ray structure and polarized reflection spectra

Abstract

The donors dipropylamine (DPA), methyldipropylamine (MDPA), and ethyldipropylamine (EDPA) from 1:2 charge transfer complexes with the acceptor tetracyanoquinodimethane (TCNQ). All crystals in the above series are semiconductors and undergo a first-order phase transition at the temperature around 210 K as obtained from conductivity measurements. The X-ray structure of DPA–(TCNQ)2 was obtained as a function of temperature, and shows a distinct order–disorder transition in the hydrogen-bond network between donors and acceptors. Polarized reflection spectra on single crystals of these compounds were measured as a function of temperature in the frequency range from 400 to 4000 cm−1. A detailed study of the C≡N modes of TCNQ indicates this mode is strongly influenced by the transition due to it being hydrogen bonded to the donors. The reflection coefficients of the IR active Ag modes of TCNQ also undergo a sharp discontinuity at the transition temperature. These results are interpreted as due to a cooperative order–disorder and electronic spin – Peierls transition. Key words: organic semiconductors, X-ray structure, polarized reflection spectra, phase transition, cooperative order–disorder transition, spin–Peierls transition.