Mechanisms of nucleation and growth in the formation of charge transfer nanocrystals

Nanocrystals of the charge transfer compound perylene:7,7′,8,8′-tetracyanoquinodimethane (TCNQ) were created using the reprecipitation method, a simple technique for inducing crystallization in solution by the rapid addition of a miscible non-solvent. The nanocrystal size was followed in situ using dynamic light scattering, showing rapid formation of crystals with sizes on the order of 100 nm followed by much slower growth. The complexation of the electron donor perylene and electron acceptor TCNQ was monitored in situ through the appearance of the charge transfer band in the infrared. Both 1:1 and 3:1 cocrystal stoichiometries could be obtained, but the ratio of the components did not always result in the same stoichiometry in the nanocrystals. In particular, a sample prepared with equal molar amounts of the constituents contained both 1:1 and 3:1 forms, as well as pure perylene and TCNQ nanocrystals. This sample also demonstrated non-classical nucleation, with solid particles forming immediately, but longer range order appearing later.