Exploring Ambipolar Semiconductor Nature of Binary and Ternary Charge-Transfer Cocrystals of Triphenylene, Pyrene, and TCNQ

Applications of organic cocrystal systems to obtain semiconductor materials with low band gap, balanced electron and hole carrier mobility, low cost, solution processability, air stability, and easy preparative route have been widely sought after in recent years. Herein, we describe two organic donor-acceptor cocrystals (T2TC1)center dot toluene and T1P1TC2 comprising pyrene (P), triphenylene (T) as the pi-donors, and tetracyanoquinodimethane (TCNQ) (TC) as the pi-acceptor exhibiting significant ambipolar semiconductor nature with charge carrier mobility values in the range 0.01-0.03 cm(2) V-1 s(-1). Both the cocrystals possess mixed D-A stack comprising triphenylene and TCNQ molecules, whereas the other triphenylene or pyrene molecule is inserted between adjacent mixed DA stacks. The cocrystals are characterized with appropriate band gap (1.5-2.5 eV) and lower lying lowest unoccupied molecular orbital level (-4.1 to -4.3 eV) for ambipolar charge transport, low preparation cost, solution processability which make them ideal organic semiconductor materials for practical application. Theoretical studies show that high ambipolar semiconductor nature is a result of synergism between two principal charge carrier transfer pathways in cocrystal system viz. superexchange and direct paths owing to the unique supramolecular features of cocrystals (T2TC1)center dot toluene and T1P1TC2.