Structure-Packing-Property Correlation of Self-Sorted Versus Interdigitated Assembly in TTF⋅TCNQ-Based Charge-Transport Materials

Among the various donor-acceptor (D-A) charge-transfer co-crystals investigated in the past few decades, tetrathiafulvalene-tetracyanoquinodimethane (F⋅Q, popularly known as TTF⋅TCNQ)-based co-crystals have fascinated materials chemists owing to their exceptional conducting and magnetic properties that arise from the packing in crystal structures. Here, crystallographic information files of eighteen F⋅Q-based co-crystals are extracted from the Cambridge Structural Database (CSD) and classified into Class 1 (D-on-D and A-on-A segregated stacks; F⋅Q, F1⋅Q-F6⋅Q, and F⋅Q1), Class 2 (-A-D-A-D-A-D- mixed stacks; F6a⋅Q-F11⋅Q and F⋅Q2), and Class 3 [-A-D-A-A-D-A-; Class 3a (F12⋅Q and F13⋅Q) and -D-D-A-A-; Class 3b (F14⋅Q)] systems according to their packing modes. Hirshfeld surface analysis, PIXEL energy calculations, and quantum theory of atoms in molecules (QTAIM) analysis are performed on the selected multicomponent charge-transfer crystals for the first time, in an attempt to explore the driving forces that give rise to different classes of 3 D crystal packing, which in turn mandates the expedient electronic properties exhibited by the investigated co-crystals. PIXEL calculations reveal that the dispersion energy component makes the maximum contribution to the total lattice energy for most of the F⋅Q-based co-crystals under study. Although the Q-on-Q dimer is the energetically most favored dimer in F⋅Q, the substituents on F capable of forming hydrogen-bonding, C⋅⋅⋅S, and other weak intermolecular interactions result in the greater stability of the F-on-F dimer for F1⋅Q-F6⋅Q (except F2⋅Q). The C⋅⋅⋅S, C_sp ⋅⋅⋅S, S⋅⋅⋅N, and π⋅⋅⋅π interaction-driven D-on-A dimer is found to be the most stable dimer of all the Class 2 co-crystals. Band structure and density-of-state calculations of the representative co-crystals in each class indicate different electronic structures according to the packing arrangement. F⋅Q and F6⋅Q with a high interaction of electronic orbitals between D-on-D and A-on-A in segregated stacks are found to be metal-like (bandgap, E_g =0.003 eV) and metallic (overlapping bands in the Fermi level), respectively, whereas the polymorph of F6⋅Q belonging to Class 2 (F6a⋅Q) displays a semiconductor-type band structure (E_g =0.053 eV). F12⋅Q of Class 3a exhibits a metal-like band structure (E_g =0.001 eV). The fine tuning of chromophores with diverse functional substituents capable of triggering weak intermolecular interactions that give rise to the desired packing and charge-transfer properties has the potential to open floodgates of opportunity for research in the chemistry of materials and fabrication of efficient electronic devices.