Synthesis and luminescence of distyrylstilbenes with asymmetrically substituted functionalized dendrons

Highly luminescent distyrylstilbenes (DSBs) bearing either propoxy or oxadiazole-surface-functionalized poly(benzyl ether)-type dendritic wedges at one end have been synthesized by a stepwise Wadsworth-Emmon reaction. We show for the first time that an emissive core bearing asymmetrically substituted surface-functionalized dendrons exhibits more favorable luminescence properties than the symmetrically dendron-substituted core. All of these asymmetrically dendron-substituted DSBs display very similar optical characteristics in both solution and solid state, except for a few nanometers of blue shift in the absorption and emission maxima as compared to the corresponding symmetrically dendron-substituted DSBs. Their solution fluorescence lifetimes and photoluminescence quantum efficiencies were found to be smaller than those of the symmetrically dendron-substituted counterparts, which was attributed to the inefficient shielding of the surface-functionalized dendritic wedges of the asymmetrically dendron-substituted DSBs. However, the energy transfer efficiency of the oxadiazole-surface-functionalized G1-dendron-substituted DSB, 2, reaches 75%, which is the highest among all of the dendritic DSBs synthesized so far. Single-layer light-emitting diodes (LEDs) using dendritic DSB doped poly(N-vinylcarbazole) (PVK) film as an emissive layer with a structure of (ITO/DSB:PVK/A1) have been fabricated and investigated. Importantly, LEDs based on these asymmetrically dendron-substituted DSBs exhibit better device performance than do LEDs based on the corresponding symmetrically dendron-substituted counterparts. Furthermore, there is a remarkable enhancement in device performance, particularly for oxadiazole-surface-functionalized asymmetrically dendron-substituted DSB-based LEDs, when the emissive layer is blended with diphenylamine.