Reply to Comment on Polymorphism in the 1:1 Charge-Transfer Complex DBTTF-TCNQ and Its Effects on Optical and Electronic Properties
In the article “Polymorphism in the 1:1 Charge-Transfer Complex DBTTF-TCNQ and Its Effects on Optical and Electronic Properties”,1 we reported on two polymorphs of the charge-transfer complex dibenzotetrathiafulvalene–7,7,8,8-tetracyanoquinodimethane (DBTTF-TCNQ). The X-ray photoelectron spectroscopy (XPS) measurements performed on each crystal type confirmed the donor:acceptor (D:A) stoichiometry for the previously known α-polymorph and identified it as the same (1:1) for the newly discovered β-polymorph. Selected area electron diffraction confirmed the uniqueness of the crystal structure of each polymorph, though the thin nature of the β-crystals precluded the complete determination of the crystal structure. Nonetheless, a combination of spectroscopic methods, including optical absorption, XPS, infrared (IR) spectroscopy, and density functional theory (DFT) allowed us to propose a crystal structure for the β-polymorph. The structural differences between the two polymorphs manifested themselves in a differing value of the degree of charge transfer between the donor and acceptor species, ρ, as determined via shifts in the agυ4 mode measured using Raman spectroscopy,2 and differing electrical properties. Additionally, when the crystals were used to fabricate field-effect transistors, the α-polymorph was found to be ambipolar with higher electron mobility than a hole mobility, while the β-polymorph was also found to be ambipolar, but with a higher hole mobility than electron mobility.