Electronic Properties of 1,5-Diaminonaphthalene:Tetrahalo-1,4-benzoquinone Donor-Acceptor Cocrystals

We have investigated the electronic properties of four charge-transfer cocrystals involving 1,5-diaminonaphthalene (DAN) as donor and fluoranil (FA), chloranil (CA), bromanil (BA), and iodanil (IA) as acceptors. While DAN-FA, DAN-CA, and DAN-BA crystallized in a mixed-stack fashion, DAN-IA crystallized with segregated stacks. For the mixed-stack cocrystals, electronic-structure calculations using density functional theory predict large electron-hole couplings with small effective masses, which strongly suggests that these DAN-XA cocrystals are suitable for charge-transport applications. Among the four cocrystals, DAN-CA crystallized in a noncentrosymmetric space group; according to our computational analysis, it is predicted to be weakly ferroelectric with a second-order electrical susceptibility (x((2))) similar to that of urea. The ionicities (rho) of the cocrystals calculated using Mulliken population compare well with the experimental results. The couplings between donor and acceptor molecules in DAN-IA are very small, leading to a very small rho. This is not typical for a system with a segregated-stack packing motif, indicating that hydrogen and halogen bondings can have a strong impact on the structure-property relations in cocrystals.