Electron Transfer and Aggregate Formation Coinduced Emission Enhancement of 9-Cycloheptatrienylidene Fluorenes in the Presence of Cupric Chloride

A series of 2,7-diaryl-9-cycloheptatrienylidene fluorenes (9-CHFs) were synthesized, and their structures were confirmed by spectroscopic methods. By cooperation with cupric chloride in tetrahydrofuran, 9-CHFs exhibited emission enhancement at locally excited (LE) band and aggregation induced emission at lower energy band when the solution was excited at two different wavelengths. By kinetically tracking the process via absorption, emission, dynamic light scattering, and TEM photography, it could be concluded that electron transfer from 9-CHFs to cupric cation and the aggregate formation coinduced the turn-on luminescence. Based on the study of the fluorescent structure−property relationship, the investigation of anion effect, and the analysis of X-ray diffraction, it was revealed that electrostatic interactions and intermolecular C−H...π interactions resulted in aggregates formation and thereafter restricted the intramolecular free rotation of σ-bonds, which finally increased the quantum yields at two bands. Thus, emission efficiencies of 9 and cupric chloride in THF solution were determined to be 0.55 and 0.48 for LE band emission and lower energy band emission, respectively.