The π−π Stacked Geometries and Association Thermodynamics of Quinacridone Derivatives Studied by ¹H NMR

The π−π stacked associations of three N,N‘-di(n-butyl) quinacridone derivatives, widely used dopants in organic light-emitting diodes, with different sizes of substituents were investigated in solution at various temperatures by ¹H NMR spectroscopy. The π−π stacked geometries were estimated by both the magnitudes of peak shifts with concentration and the directions of peak shifts induced by polar solvents. Two patterns of geometries with different π−π interaction strengths were found to coexist in solution for all the three samples. In both of the patterns, the preferential orientation of the stacking is the approach of the carbonyl groups on one molecule to the nitrogen atoms on the stacked partner, which makes the π-deficient aromatic atoms interact with both π-rich and π-deficient aromatic atoms of the stacked partner to maximize the electrostatic complementarity. Differently, whereas the molecules in one pattern are face-to-face stacked in a parallel fashion and slip two rings relative to one another along with the long axis of the conjugated ring systems, the molecules in the other are either face-to-face stacked in an antiparallel fashion with slight slipping between layers or stacked in a turning fashion. Both association constants obtained by fitting the dilution curves and thermodynamic parameters obtained from van't Hoff analyses revealed unexpectedly three thermodynamic processes of aggregations for all the three samples in the temperature region of 298−213 K. The size of substituents on the outer aromatic rings significantly influences the π−π stacked structures and association thermodynamics.