Crystal Structures And Optoelectronic Functions Of Phenyl-Substituent Distyrylbenzene Derivatives

In recent years, the investigations of optoelectronic functional organic crystals have been paid great attention in the field of organic optoelectronic materials. Compared with amorphous thin film materials, organic single crystals have the characteristics of high stability, high ordered structure, and high carrier mobility, which make them attractive candidates for optoelectronic devices. Single crystal with definite structure provides a model for us to investigate the basic intermolecular interactions (supramolecular interaction), and the relationship between molecular stacking modes and optoelectronic performance (luminescence and carrier mobility).trans-Distyrylbenzene (trans-DSB) is one of the simple and typical model compound for poly(p-phenylene vinylene) (PPV) type materials, which has been widely used in polymeric light-emitting diodes. trans-DSB has deep blue emission in dilute solution with very high photoluminescence efficiency nearly up to 100%; while in solid state (film or crystal), the emission spectrum is red-shifted and the quantum efficiency decreased dramatically (<10%). In the crystal, trans-DSB molecules tend to pack with parallel stacking image, in which the'H-stacking'is formed for the transition dipole direction is along the molecular long axis. In the H-aggregate, the inter dipole interaction induce the energy level of the lowest excited state splitting into two energy levels, and the lower one is optically forbidden. After the molecule in H-aggregate absorbs a photo and get to the excited state, it will relax to the lowest energy level rapidly, which make the H-aggregate has very low quantum efficiency. The emission spectrum is red shifted because the 0-0 transition is forbidden based on the parallel inter dipole interaction. However, in some other molecular dipole stacking mode, such as J-aggregate, the lowest splitting energy level due to the inter dipole interaction is optically allowed. Thus the emission spectrum is red shifted, and the photoluminescence efficiency is very high. There is also another molecular dipole stacking mode that is called as'X-aggregate', in which the inter dipole interaction is very weak. The emission properties of the X-aggregate are similar to the dilute solution and of course the quantum efficiency is very high.The molecular dipole stacking in the crystal of trans-DSB is H-stacking, which is deficient for luminescence efficiency. It is possible to get highly efficient crystalline materials by adjusting the molecular dipole stacking mode. Based on this information, we added phenyl substituents on the backbone of trans-DSB to avoid the parallel molecular stacking, and furthermore to induce multi- molecular stacking modes.During the synthesis of diphenyl-distyrylbenzene using Wittig reaction, we accidentally found that the phenyl substituents make the reaction have highly stereo-isomer selection and the cis-isomer is nearly up to 100%. After we deposit the result solution into methanol, the pure cis-compound can be obtained easily. The pure trans-isomer can be prepared by reflux the p-xylene solution of cis- isomer using a little iodine as catalyzer. We studied the structural and physical properties of the cis- and trans-compounds, and found that the differences between then are similar to the most cis- and trans- isomers.The crystal structures of the cis- and trans-compounds were examined in detail. The trans-DPDSB (2,5-diphenyl-1,4-distyrylbenzene with two trans double bonds) tends to form two different polymorphs under different conditions, which have X-stacking and J-stacking respectively. The two polymorphs have very high photoluminescence efficiency. The value for X-stacking crystal is up to 80% and the value for J-stacking crystal is 48%.Strong intermolecularπ-πinteractions are benefit for charge transport between adjacent molecules. Although X-stacking is very good for high photoluminescence efficiency, it is deficient for charge transport because the intermolecularπ-overlap is small. While in J-stacking, it is easy to form strong intermolecularπ-πinteractions, and then it is possible to combine high luminescence efficiency and high charge-carrier mobility in one crystal. In the single crystal of trans-DFPDSB (2,5-di-(3,5-difluoro-phenyl)-1,4-distyrylbenzene with two trans double bonds), there is strongπ-πinteractions and the molecular dipole stacking is typical J-stacking. The photoluminescence efficiency of the trans-DFPDSB crystal is 49%, and the calculated hole mobility is 0.88 cm2/Vs.In the cis-DPDSB (2,5-diphenyl-1,4-distyrylbenzene with two cis double bonds) crystals, it is impossible to form face-to-face parallel molecular stacking for the torsional molecular conformations. Three different polymorphs of cis-DPDSB can be formed with the assistant of C-H···πinteractions. cis-DPDSB has very low quantum efficiency in solution due to the occurrence of the photochemical processes; while in crystal the efficiency is as high as 50%, which can be attributed to the stabilized excited state by intermolecular interactions.