Crystal growth of new charge-transfer salts based on π-conjugated donor molecules

Abstract

New charge transfer crystals of π-conjugated, aromatic molecules (phenanthrene and picene) as donors were obtained by physical vapor transport. The melting behavior, optimization of crystal growth and the crystal structure is reported for charge transfer salts with (fluorinated) tetracyanoquinodimethane (TCNQ-F x , x=0, 2, 4), which was used as acceptor material. The crystal structures were determined by single-crystal X-ray diffraction. Growth conditions for different vapor pressures in closed ampules were applied and the effect of these starting conditions for crystal size and quality is reported. The process of charge transfer was investigated by geometrical analysis of the crystal structure and by infrared spectroscopy on single crystals. With these three different acceptor strengths and the two sets of donor materials, it is possible to investigate the distribution of the charge transfer systematically. This helps to understand the charge transfer process in this class of materials with π-conjugated donor molecules.

Charge‐Transport Properties of F6TNAP‐Based Charge‐Transfer Cocrystals

“…The nitrile stretching frequencies of cyano‐functionalized acceptors such as F 6 TNAP have been found to be sensitive to the charge on the acceptor molecules, and several previous studies of cocrystals of acceptors including TCNQ, F 4 TCNQ, F 6 TNAP, and 2,3‐dihalo‐5,6‐dicyanobenzoquinones, have used these frequencies to obtain values of ρ. That said, the sensitivity of these stretching frequencies to a variety of local interactions, as well as the acceptor charge, means that derived values of ρ should only be regarded as approximate.…”

Mixed stack charge transfer crystals: Crossing the neutral-ionic borderline by chemical substitution

“…6) to estimate ρ since it is well know that it gives unreliable (generally overestimated) value. In fact, CN antisymmetric stretching suffers from uncertainty about the correct assignment and is subject to extrinsic effects due to the interactions with the surrounding molecules [16,36]. We therefore use the TCNQF 2 ring stretching vibrations, as singled out and calibrated in Ref.…”

“…Such a task has recently started in the field of organic semiconductors, with the aim of engineering systems with different band gaps and degree of CT, in such a way to understand how these parameters affect the semiconducting behavior [14][15][16]. However, this systematic exploration has been so far limited to ms-CT crystals in the proximity of the neutral side (0 ρ 0.2), as a rather weak electron donor has been associated with acceptors of increasing electron affinity.…”

Phenomenology of the Neutral-Ionic Valence Instability in Mixed Stack Charge-Transfer Crystals

“…Table 2), among others, has been found to exhibit such a property, although with limited mobility values [72]. Mainly for these reasons, several studies have appeared aimed at exploring the phase space of ms CT crystals at ambient conditions [73][74][75][76][77], tuning the band gap, and at the same time moving towards the N-I borderline, where the stack dimerizes and becomes potentially ferroelectric.…”