Synthesis and characterization of starburst conjugated molecules with multiple p-n branches for narrow band gap modulation

Well-defined π conjugated molecules have attracted much attention in the field of organic electronics, due to their promising optoelectronic properties, good monodispersity, high purity, excellent reproducibility and facile functionalization. Here, a series of starburst molecules (P1, P2 and P3) with truxene as the core, thiophene unit as p-type electron donating segments, and benzothiadiazole chromophore as the n-type electron accepting segments, have been designed and synthesized. The aim of the systematic investigation of these structural variations was to provide insight into the relationship between the effective conjugation length and the photophysical properties, the intramolecular energy transfer process, which would shed light for the rational design of novel optoelectronic materials for organic electronic devices. The resulting molecules, P1, P2, and P3, possess a series of advantages, including high solubility, broad absorption, low band gap and good thermal stability, etc. The chemical structures of the intermediates and resulting molecules were confirmed by ¹H NMR, ¹³C NMR, GC-MS/MALDI-TOF. P1, P2, and P3 exhibited good thermal stability according to thermogravimetric analysis (TGA) and low-lying HOMO and LUMO energy levels as well as narrow band gaps according to cyclic voltammetry (CV). P1 and P3 possessed amorphous properties, while P2 exhibited crystalline properties as revealed by differential scanning calorimetry (DSC). The electronic properties have also been investigated by DFT calculation. The HOMO/LUMO energy levels and band gaps could be fine tuned by varying the content of p and/or n type moieties. Compared to P1 and P2, the maximum absorption and emission of P3 were obviously red-shifted due to increased effective conjugation with introducing additional thiophene units into the p-n branches and modulating the substituted position of the alkyl group. To investigate the primarily optoelectronic properties of these molecules, OLEDs were fabricated using the following configuration: ITO/PEDOT: PSS (50 nm)/EML (74 nm)/TPBi (30 nm)/LiF (1.3 nm)/Al (80 nm). Maximum luminance of 2908, 4683, 1085 and 4730 cd/m² were demonstrated with maximum current efficiency of 1.18, 0.50, 0.16 and 2.50 cd/A, respectively.