NMR Study On The Intermolecular π-π Interaction Of Quinacridone Derivatives

N, N'-alkylated quinacridone derivatives show very pronounced photovoltaic and photoconductive activity. It is widely used as a dopant in electroluminescent devices for their good solubility and excellent stability. However, the intramolecular Ï€-Ï€ interaction could result in the decrease in the emission efficiency of electroluminescent devices due to quenching or non-radiative pathways. The Ï€-Ï€ interaction can be regulated by introduction of different sizes of substituents on aromatic rings of quinacridone derivatives. Smaller intramolecular Ï€-Ï€ interaction of the quinacridone derivatives can provide a larger range of the doping concentration, which is an advantage for the making of the devices.In addition to the stacking of quinacridone derivatives in electroluminescent devices, a wide array of phenomena in chemistry and biology are found to be relative to the n-n interactions. Despite the fact that the importance of Ï€-Ï€ interactions is widely recognized, there is no full understanding of their origins and geometrical preferences.Therefore, the study of the Ï€-Ï€ interactions of the quinacridone derivatives is significant not only in the design of molecules that can lead to high emission efficiency of the luminescent materials and improve their lifetime, but also in understanding on the nature of Ï€-Ï€ interaction.In this thesis, we reported the study of the Ï€-Ï€ interaction of three N-alkylated quinacridone derivatives with different sizes of substituents on the aromatic rings: 5, 12-N, N'-di-butyl-quinacridone (DBQA), 5, 12-N, N'-di-butyl-1, 3, 8, 10-tetra-methyl-quinacridone (TM-DBQA) and 5, 12-N, N'-di-butyl-2, 9-di-t-butyl-quinacridone (D^t Bu-DBQA), inorganic solvents by NMR.We found that all of ¹H NMR signals for the three compounds shifted upfield with increasing concentrations in CDCl₃ at the temperature of 298K, suggesting that the molecular aggregation due to n-n stacking 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 sample concentration higher than 0.499 mM for DBQA and 5.00 mM for TM-DBQA and D'Bu-DBQA, respectively. Each of the two groups of signals shifted upfield with increasing sample concentrations, only the signals with larger chemical shifts changed with concentration more slowly than the signals with smaller chemical shifts. This indicates that the two groups of signals represent two oligomers with different conformations of Ï€-Ï€ stack. From the ROESY spectra at 213K, in which the in-phase cross peaks between the two groups of signals relative to the diagonal peaks are produced, we can infer that the two kinds of oligomers exchange with each other.We found a turning point from the dependence of chemical shift on temperature. Moreover, the more dilute the sample concentration is, the more evident the turning point becomes. This indicates that there is a process of the configure transition from high temperature to low temperature. The transition conformation may be more incompact and extended in more dilute sample concentration, which leads to the larger chemical shifts. For the samples with different sizes of substituents, the temperature turning points are little different. The larger the substituent, the lower the temperature turning point.With addition of DMSO-d₆ in CDCl₃, obvious upfield chemical shift of H6 and HI resonances and downfield chemical shift of H4 resonances were observed for the three samples. The addition of CD3OD and CD3CN in the CDCl₃ solution of samples showed the similar results. This implies that these quinacridone derivatives interact with each other through offset stacked forms in which the oxygen atoms on the carbonyls of one molecule are close to the nitrogen atoms of the other molecules. The interaction between the lone-pair electron of nitrogen and Ï€ electron 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 the Ï€-Ï€ interaction between the molecules stacked offset. The upfield shifts of H6 and H1 and the downfield shifts of H4, or the solvophobicity effect, are proportional to the dielectric constants of the polar solvents added.The association constants (K) were determined by assuming that monomer-dimer equilibrium is the predominant process of the self-association. 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 plot, using K values determined in the temperatures ranging from 213 K to 298 K.The van't Hoff plots for the three samples in CDCl₃ display a turning region near 258K. Below this turning region, the self-associations are driven by enthalpy, so do above the turning region. While in the turning region, the increasing contribution of entropy to the self-association was observed with increasing size of substituents on the aromatic rings. For DBQA in CDCl₃, the transition process of Ï€-Ï€ interaction is driven by...