The Study On Intermolecular π-π Interactions Of Quinacridone Derivatives

Early before a half century, it was found that there were strong interactions in the molecules withπelectrons widely. Theπ-πinteractions between aromatic units play a significant role in supramolecular chemistry and biological recognition processes. Although the significance ofπ-πinteractions has been recognized, no commonly used model has been built to interpret the mechanism of the interactions well. The interaction energies involved in aromatic association are small, making it especially difficult to study this phenomenon in solution. Therefore molecules in which theπ-πinteractions are amplified may be valuable for studying aromaticπ-stacking. Quinacridone derivatives, the crystal structures of which were characterized by intermolecularπ-πinteraction, showed a considerable strength ofπ-πinteraction in solution. The self-association of quinacridone derivatives induced detectable change in the 1H and 13C NMR chemical shifts. Therefore, quinacridone derivatives can be used as model molecules for the study ofπ-πinteractions both in solution and solid, and the study on theπ-πinteractions of this type of molecules can help us understand the mechanism of the interactions deeply. Moreover, QAs are widely used organic pigments that display excellent stability and a wide range of luminescent spectrum. High photoluminescent efficiency in dilute solution as well as good electrochemical stability in the solid state has allowed the fabrication of high-performance organic electroluminescent devices (OLEDs) based on QAs. It was demonstrated that the PL and EL properties of quinacridone derivatives depend on the structures and the packing patterns of the molecules. Accordingly, understanding on factors that affect the packing geometry and strength of quinacridone derivatives will also aid rational design and synthesis of new quinacridone derivatives for preparation of high-performance organic optical and electronic materials.Theπ-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 were investigated in organic solvents by 1H NMR spectroscopy.We found that all of 1H NMR signals for the compounds shifted upfield with increasing concentrations in CDCl3 at the temperature of 298K, suggesting that the molecular aggregation due toπ-πstacking interaction occurs. The spectra should be the weighted average between monomer and oligomer resulted from the fast intermolecular exchange on the NMR time scale at 298 K.With the decrease of the temperature, all of the 1H signals of aromatic rings broadened gradually and splitted to two groups at the temperatures of 228 K and 213 K with the higher sample concentrations. Each of the two groups of signals shifted upfield with increasing sample concentrations, only the signals with larger chemical shifts changed (group I) with concentration more slowly than the signals with smaller chemical shifts (group II). This indicates that the two groups of signals represent two oligomers with different conformations ofπ-πstack. The aggregation number N, i.e., the monomeric number included in an aggregate, was estimated and suggesting that the quinacridone derivatives are in equilibrium between monomer and dimer at various temperatures. With addition of CD3CN in CDCl3, obvious upfield chemical shift of H6 and H1/ Me1 resonances and downfield chemical shift of H4 resonances were observed for all the samples studied. The chemical shifts of other protons were less affected by the polar solvent. This implies that 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 (structure I) and an antiparallel (structure II) fashion, respectively. The interaction between the lone-pair electron of nitrogen andπelectron of carbonyl results in both the decrease in the ability of electron-drawing of carbonyl and the decrease in the ability of electron-giving of nitrogen. The addition of the polar solvents in CDCl3 induces the solvophobicity interaction of molecules of the quinacridone derivatives that enhances theπ-πinteraction between the molecules stacked, which gives rise to the upfield shift of H6 and H1 and the downfield shift of H4 relative to the chemical shifts of the corresponding protons in pure chloroform solvent.The order of the peak shiftΔδH of the aromatic protons from both group I (ΔδH-I) and II (ΔδH-II) at 213 K was used to estimate theπ-stacking geometries of the n-alkyl QAs. The stacking structures were little affected by the length of the n-alkyl chains, but regulated in an allowed range by the size and property of the substituents.The association constants (K) of the monomer-dimer equilibrium were determined. The least-square curve fitting to observed chemical shifts at different sample concentrations was carried out to determine K. The thermodynamic parameters for the dimerization were elucidated on the basis of the van't Hoff plots, using K values determined in the temperatures ranging from 213 K to 298 K.The association processes of all the n-alkyl QAs are enthalpically favorable at 298K, 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 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.The strength and geometrical preference of intermolecularπ-πinteractions of n-alkyl QAs and the effects of substituents were also studied by the theoretical computation based on MP2/6-31G level. Four model molecules, namely QA, TM-QA, DM-QA and 2F-QA were used for the calculations. The results were compared with those of NMR measurements.The potential energy curves of four types of configurations, parallel sandwich (PS), anti-parallel sandwich (APS), parallel displaced (PD) and anti-parallel displaced (APD), respectively, for dimers of the model molecules were obtained. The results indicate that the electrostatic interactions of nitrogen atoms of one molecule with oxygen atoms of the partner molecule dominate the general geometrical preferences of dimers. The PS configurations of four model molecules have higher potential energy, while the configurations of the APS, PD and APD types are more stable than PS configuration. The potential energy curves of four model molecules revealed that both the configurations and the binding energy of the model molecule dimers are affected by the spatial hindrance of substituents. The electrostatic effects of substituents have little effect on the configurations of the model molecule dimers, but have an effect on the binding energy.Theoretical results for the PD, APS and APD dimers show that both the substitutions of CH3 with electron-donating nature and F with electron-withdrawing nature cause the dimers to be more stable than the unsubstituted QA dimers (with exception of the PD type of 2F-QA). The larger or more the substitutent groups are, the lower of the total energy. These suggest that the dispersion force may play an important role for the strength ofπ-πinteractions of QAs.We also computed the potential energy curves of r-APD dimers in which the stacking molecules were turned relative to each other. The results indicate that the rotation of the molecules decreases the total energy slightly, and the turning direction depends on the spatial crowding of substituents.The energy of the stable state configurations from the calculation of QAs dimers is lowered according to the order of PS, PD APS, APD/r-APD. The stable state configurations of PD and r-APD/APD/APS are approximately corresponding to the structure I and II proposed based on 1H NMR observations, respectively.