¹H NMR Study on the Self-Association of Quinacridone Derivatives in Solution

Abstract

The pi-stacking structures and self-association thermodynamics of N, N'-di(n-alkyl) quinacridone derivatives (n-alkyl QAs) with various substituents on the side aromatic rings and different length of n-alkyl chains are investigated in organic solvents by (1)H NMR spectroscopy. The stacking geometries are built based on both the magnitudes and directions of peak shifts with concentration and solvent polarity. The intermolecular interaction between nitrogen atoms and oxygen atoms dominates the general geometrical preferences of the stacking in which the molecules are face-to-face arranged in a parallel and an antiparallel fashion, respectively. The stacking structures are little affected by the length of the n-alkyl chains but are regulated in an allowed range by the size and properties of the substituents. The association processes of all the n-alkyl QAs are enthalpically favorable at 298 K, while the relative stability of these n-alkyl QAs assemblies is governed mainly by the entropy of the association processes. The introduction of larger substituents and longer n-alkyl chains disfavors the association of the n-alkyl QAs, while the binding of the halogen atoms on the side aromatic rings is favorable to the association. The relative strength of the stacking interaction for the substituted n-alkyl QAs has not obvious correlation with the electron-donating or electron-withdrawing nature of the substituents, while it is well associated to the dispersion energy and repulsive exchange energy. The different entropy-enthalpy compensation of the halogen-substituted n-alkyl QAs from others may suggest different association mechanism for the two types of n-alkyl QAs.

π–π interaction of quinacridone derivatives

“…The least fractions of the parallel displaced geometries were found in DX-QA-C 4 dimers. 49 The comparison between the interaction energies of the two types of configurations gives similar conclusion. The data in Table 1 show that the energy difference between PD and r-APD/APD configurations of DF-QA-C 1 dimer is much larger than those of other dimers.…”

“…The theoretically predicted geometries of the QA dimers are in good agreement with the NMR results of N,N 0 -di(n-alkyl) quinacridone derivatives. 49 In the NMR study of these quinacridone derivatives, we observed two types of dimeric geometries coexisting in solution, corresponding to the parallel displaced and antiparallel displaced configurations. Theoretical calculation even predicted correct rotation direction of one monomer relative to the partner in the r-APD configuration of DM-QA-C 1 dimer (corresponding to D t Bu-QA-C n in ref.…”

“…48,49 We found that the geometries of the QA dimers are little affected by the lengths of the n-alkyl chains. To simplify calculation and save computational costs, we used different N,N 0 -dimethyl quinacridone derivatives as model molecules which are denoted as QA-C 1 , TM-QA-C 1 , DF-QA-C 1 , and DM-QA-C 1 (Fig.…”

“…Theoretical calculation even predicted correct rotation direction of one monomer relative to the partner in the r-APD configuration of DM-QA-C 1 dimer (corresponding to D t Bu-QA-C n in ref. 49 ). Interestingly, although the potential energies of PD dimers are higher than those of r-APD dimers, the PD geometries were still observed at lower temperatures by the NMR experiments.…”

“…We have previously studied the p-stacking geometries and association thermodynamics of N,N 0 -di(n-alkyl) quinacridone derivatives in solution by NMR. 48,49 The p-p interactions of N,N 0 -di(n-alkyl) quinacridone derivatives possessing larger p-conjugated units could offer stronger driving forces to compel molecules stacking as a faceto-face fashion in both crystal and solution structures, [48][49][50] which enable us to use these compounds as models to study the faceto-face interactions and get further understanding on the nature of p-p interactions. Moreover, both stacking manner and substituent property of quinacridone derivatives affect their luminescent color and charge transport property.…”