Structure, Stoichiometry, and Charge Transfer in Cocrystals of Perylene with TCNQ-F_x

Abstract

Semiconductor charge transfer (CT) cocrystals are an emerging class of molecular materials which combines the characteristics of the constituent molecules in order to tune physical properties. Cocrystals can exhibit polymorphism, but different stoichiometries of the donor-acceptor (DA) pair can also give different structures. In addition, the structures of the donor and acceptor as pristine compounds can influence the resulting cocrystal forms. We report a structural study on several CT cocrystals obtained by combining the polyaromatic hydrocarbon perylene with 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its fluorinated derivatives having increasing electronegativity. This is achieved by varying the amount of fluorine substitution on the aromatic ring, with TCNQ-F2 and TCNQ-F4. We find structures with different stoichiometries. Namely, the system perylene:TCNQ-F0 is found with ratios 1:1 and 3:1, while the systems perylene:TCNQ-Fx (x = 2, 4) are found with ratios 1:1 and 3:2. We discuss the structures on the basis of the polymorphism of perylene as pure compound, and show that by a judicious choice of growth temperature the crystal structure can be in principle designed a priori. We also analyze the structural motifs taking into account the degree of charge transfer between the perylene donor and the TCNQ-Fx acceptors and the optical gap determined from infrared (IR) spectroscopy. This family of materials exhibits tunable optical gaps in the near-IR (NIR), promising applications in organic optoelectronics

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

“…In the present context, the series of 1:1 CT crystals of perylene with TCNQF x (Table 3) is particularly interesting [75,76]. In this series the HOMO-LUMO gap and degree of CT are tuned by increasing the electron affinity of the Acceptor by the number of fluorine atoms, while keeping the Donor constant.…”

“…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.…”

“…In this series the HOMO-LUMO gap and degree of CT are tuned by increasing the electron affinity of the Acceptor by the number of fluorine atoms, while keeping the Donor constant. The crystals are not isomorphous, but the type of HOMO-LUMO overlap is the same along the series, so the hopping integral can be considered approximately constant, between about 0.25 and 0.35 eV [76]. In these conditions the optical gap and the ionicity change monotonically with TCNQF x electron affinity.…”

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

“…We therefore use the TCNQF 2 ring stretching vibrations, as singled out and calibrated in Ref. [15]. The frequencies of the two safely identified charge sensitive modes are reported directly in Fig.6.…”

“…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.…”