Polymorphism in Core-Chlorinated Naphthalene Tetracarboxylic Diimide Thin Films

Polymorphism within organic semiconductors can play a critical role in device performance, as some packing motifs may be more favorable to charge transport than others. As-evaporated polycrystalline thin-films of core-chlorinated naphthalene tetracarboxylic diimides (NTCDI-1) adopt a triclinic polymorph that is not different from those of single crystals grown via physical-vapor transport. Exposing these thin-films to saturated vapors of select organic solvents, such as those of acetone and chloroform, induces structural transformation; thermally evaporated films convert from the triclinic polymorph to a monoclinic polymorph that was reported for solution-grown single crystals. Isothermal transformations are well described by second-order Avrami kinetics; molecular dynamic simulations give us insight into how solvents induce different kinds of favorable molecule-molecule interactions. Interestingly, the surface energy of the underlying substrate also plays a role in determining the rate of transformation; the rate of transformation is 2x and 4x faster on hexamethyldisilazane modified-Si/SiO$_{2}$ compared to on Si/SiO$_{2}$ and octadecyltrichlorosilane modified-Si/SiO$_{2}$, respectively.