Unraveling the charge transfer variation of tetrathiafulvalene-based organic crystals through fragment charge difference calculation

Organic crystals assembled by a well-established family of electron donors, tetrathiafulvalene (TTF)-based molecules, hold great potential for electronics, smart materials, and superconductors. Combining with Marcus' theory and first-principles calculations, we have adopted a fragment charge difference (FCD) method to investigate the charge transfer properties of the TTF-based crystals. Our FCD predictions are highly consistent with those obtained from a well-accepted site energy correction method. We have demonstrated the significant influence of both structure and chemistry on the charge transfer properties using polymorphs, i.e., α-phase tetrathiafulvalene (1) versus β-phase tetrathiafulvalene (2), and crystals with homologous molecular packings, i.e., 1 versus dithiophene-TTF (3). We have also introduced multiple factors to provide further insights into the variation in charge transfer properties of the TTF-based crystals, including energy gap (∆E), centroid distance (ri), orbital distribution correction factor (Hs), and reorganization energy (λ). By taking advantage of our analysis, we have rationalized high mobility in hexamethylene-TTF (4) and low mobility in bis(ethylenedithio)-TTF (5). Our multiple-factor evaluation could support an approach to designing electrically conducting TTF-based materials and provide a method to estimate charge transfer properties effectively.