New(Polycyclic Aromatic Hydrocarbons)-based Tetraarylethenes: Synthesis And Aggregation-induced Emission Properties

Organic luminescent materials with efficient solid-state emission have attracted much attention due to its potential applications such as in organic light-emitting diodes. However, most of the organic functional materials have intense emission in solvent, but become weak emitter when they are in poor solvents or solid state owning to the intermolecular interactions. The appearance of compounds with aggregation-induced emission(AIE) characteristic in which tetraphenylethene is the representative one, provids an effective method to exploit efficient solid-state emitters. In particular, introducing polycyclic aromatic hydrocarbons(PAHs) into aryl ethenes can enlarge the conjugation of the system, which is the most effective method to improve the solid-state fluorescence. Therefore, in this paper, pyrene, tetrahydropyrene, flurorene and biphenyl were selected as the material to synthesize PAHs-based tetraarylenthenes and their optoelectronic properties were investigated in detail.1. 4,5,9,10-Tetrahydropyrene, fluorene and biphenyl were used to react with oxalyl chloride, and then di(polycyclic aryl)ketones were successfully synthesized. Based on these intermediates, tetra(poylcyclic aryl)ethenes were obtained finally. All the three compounds are weakly fluorescent in solutions, but are highly emissive in the condensed phase, revealing distinct aggregation-induced emission characteristics. The fluorescence quantum yield of tetrahydropyrene-based ethene in solid film is up to 74.1%, which is much higher than that of the two others. Its crystal structure study indicates that there are no p-stacking interactions formed between molecules in the crystal phase. This study reveals that when constructing aggregation-induced emission molecules using substituent groups with expanded p-conjugation, introduction of appropriate steric hindrance on the substituent groups can restrict the formation of excimers effectively, hence achieving efficient solid state emission.2. Based on the 2-position of 4,5,9,10-tetrahydropyrene and pyrene, the vinyl compounds substituted by one or two polycyclic aromatic hydrocarbons were successfully synthesized. The aggregation-induced emission studies reveal that all the four compounds are AIE active. The optical research combining with theoretical calculation proves that the absorption and emission behavior of pyrene-based ethenes are dominated by the moieties of pyrene. It is noteworthy that tetrahydropyrene-based ethenes have higher fluorescence quantum yield than that of pyrene-based ethenes. In particular, the FF value of trans-1,2-diphenyl-1,2-dipyrenylethene reaches 62.6%, whreas the FF value of corresponding pyrene-based ethenes is only 9.7%. These researches further confirm that adopting polycyclic aromatic hydrocarbons with certain steric hinderence to construct aryl ethenes can obtain efficient solid-state fluorescent materials.3. Based on biphenyl and 4,5,9,10-tetrahydropyrene, using oxalyl chloride to obtain aryl chloride derectly has been exploited, and then new aryl ethenes with high molecular mass have been synthesized. The tetrahydropyrene-based compound shows longer absorption and emission wavelengths and also exhibits higher solid-state fluorescence quantum yield, because the p-conjugation of tetrahydropyrene is much greater than that of biphenyl. Aggregation-induced emission studies reveal that the two compounds are AIE active. It is noteworthy that the biphenyl-based compound thanks to its special structure, shows a novel crystal phase transient state during the reseach of aggregation-induced emission.