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 center dot Q, popularly known as TTF center dot 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 center dot Q-based co-crystals are extracted from the Cambridge Structural Database (CSD) and classified into Class1 (D-on-D and A-on-A segregated stacks; F center dot Q, F1 center dot Q-F6 center dot Q, and F center dot Q1), Class2 (-A-D-A-D-A-D- mixed stacks; F6a center dot Q-F11 center dot Q and F center dot Q2), and Class3 [-A-D-A-A-D-A-; Class 3a (F12 center dot Q and F13 center dot Q) and -D-D-A-A-; Class 3b (F14 center dot 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 3D 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 center dot Q-based co-crystals under study. Although the Q-on-Q dimer is the energetically most favored dimer in F center dot Q, the substituents on F capable of forming hydrogen-bonding, C center dot center dot center dot S, and other weak intermolecular interactions result in the greater stability of the F-on-F dimer for F1 center dot Q-F6 center dot Q (except F2 center dot Q). The C center dot center dot center dot S, C-sp center dot center dot center dot S, S center dot center dot center dot N, and pi center dot center dot center dot pi interaction-driven D-on-A dimer is found to be the most stable dimer of all the Class2 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 center dot Q and F6 center dot 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=0.003eV) and metallic (overlapping bands in the Fermi level), respectively, whereas the polymorph of F6 center dot Q belonging to Class2 (F6a center dot Q) displays a semiconductor-type band structure (E-g=0.053eV). F12 center dot Q of Class 3a exhibits a metal-like band structure (E-g=0.001eV). 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.