Improvement of Luminescence Quantum Efficiency by Intermolecular Interaction

Abstract

Due to their unique photophysical properties, highly fluorescent compounds are currently of great interest for material science applications, such as photo-electronics, organic light-emitting diodes (OLEDs) and chemical sensors. In particular, efforts have focused on the improvement of fluorescence quantum yields (ΦF), since the ΦF of emitting materials and the device efficiency in OLEDs are generally related. For this purpose, several approaches have been suggested including the attachment of anthracene and/or pyrene units having high ΦF to the molecular framework, the extension of π-conjugation and the introduction of electronwithdrawing groups (F, CN, etc) as substituents. Among such approaches, the introduction of electron-withdrawing groups to the molecular backbone has received greater attention, because the molecules containing these groups have shown unusual and high fluorescence quantum efficiency through the intermolecular interactions. Such phenomenon is often called aggregation induced emission (AIE). During our ongoing efforts on the development of luminescent materials, we have observed that fluoreneand anthracene-based compounds linked by a perfluorocyclopentene core unit have high fluorescence quantum efficiency as compared to 9,10-diphenylanthracene (DPA). Our interest in the synthesis of highly fluorescent materials containing a perfluorocyclopentene core unit, which can provide binding sites for hydrogen atoms of adjacent molecules via intermolecular interactions, has prompted us to investigate the syntheses and photophysical properties of their derivatives. Here, we report on the syntheses, crystal structure and optical properties of a series of V-shaped molecules containing the linker of an electron-withdrawing moiety. In addition, the effect of electron-withdrawing substituents on fluorescence quantum efficiency has been systematically evaluated at the molecular level. Scheme 1 outlines the synthesis of the series of benzothiophene derivatives. Compound 1 and 2 were prepared by the addition of octafluorocyclopentene to the corresponding lithiated compound, prepared from the reaction of a small excess of n-BuLi and the bromoarene at −78 C. The standard work up and crystallization from CH2Cl2/hexane produces compounds as colorless or pale yellow solids in moderate yields (45-55%). In addition, to evaluate the effect of a perfluorocyclopentene ring on structural features and photoluminescence, we have synthesized an aromatic analogue of compound 3, through Pd-mediated Suzuki coupling. The structures of 1-3 have been characterized by H-NMR, C-NMR and elemental analyses, including Xray diffraction analysis of 2. Yellow crystals of 2 that were suitable for X-ray analysis were obtained by the slow evaporation in CH2Cl2 and hexane. The crystal structures and selected bond lengths and angles of 1 are presented in Figure 1. As shown in the Figure, two benzothiophene groups connected by the hexafluorocyclopentene linker are twisted with respect to each other. Note that the hexafluorocyclopentene linker and one (S1 ring) of two benzothiophene rings are arranged a nearly coplanar manner with a dihedral angle of 5.44, while the other (S2 ring) is highly tilted toward the hexafluorocyclopentene core with a dihedral angle of 71.96. This observation is very uncommon in diarylethene derivatives