Synthesis and Aggregation-induced Emission of Tetraphenylethylene Derivatives Based on Hexakisphenylbenzene Backbone

Abstract

A series of tetraphenylethylene (TPE) derivatives based on hexakisphenylbenzene backbone (HPB-nTPE, n=2, 4, 6) were synthesized by cobalt carbonyl-catalyzed cyclotrimerization of alkynes with 1,2-bis(4-(tert-butyl)phenyl)ethyne and 1,2-bis(4'-(1,2,2-triphenylvinyl)-(1,1'-biphenyl)-4-yl)ethyne (TPE-TPE) as starting materials. By varying the ratio of the starting materials, HPB-nTPE (n=2, 4, 6) with 2, 4 or 6 TPE substituents were obtained. The structures of HPB-nTPE (n=2, 4, 6) were characterized by IR, 1 H NMR and MALDI-TOF mass spectra. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) show that the decomposition temperatures of HPB-nTPE are all above 440 ℃, indicative of high thermal stability of HPB-nTPE. The photophysical properties of the tetrahydrofuran solution, the aggregate and the neat solid of HPB-nTPE were investigated. The aggregates of HPB-nTPE (n=2, 4, 6) were obtained in tetrahydrofuran/water mixed solvents with various volume fractions of water, which was confirmed by the scanning electron microscope (SEM). HPB-nTPE (n=2, 4, 6) show typical aggregation induced emission properties, almost non-emissive in tetrahydrofuran solu- tion, but strongly emissive in the aggregate or solid state. The highest fluorescence quantum yields for HPB-nTPE (n=2, 4, 6) in aggregate state are 0.37, 0.36 and 0.37, respectively, which are more than 400 times higher than that in tetrahydrofuran. The quantum yields of the neat solid of HPB-nTPE (n=2, 4, 6) are determined to be 0.39, 0.36 and 0.36, which are ca. 1.8 times of that of the neat solid of tetraphenylethylene. Both restrictions of the intramolecular rotation of phenyl rings and the inter/intramolecular π-π stacking of the TPE chromophores caused by the star-shape rigidity structure of hexakisphenylben- zene suppress the nonradiative transition, consequently, giving higher fluorescence quantum yield of the aggregates and the neat solid of HPB-nTPE in comparison with TPE. This work is of importance for developing highly emissive organic materi- als in solid states. Keywords tetraphenylethylene; hexakisphenylbenzene; aggregation induced emission; nanoaggregate