Aggregation-Enhanced Room-Temperature Phosphorescence from Au(I) Complexes Bearing Mesogenic Biphenylethynyl Ligands

Abstract

Gold(I) complexes, enabling to form linear coordination geometry, are promising materials for manifesting both aggregation-induced emission (AIE) behavior due to strong intermolecular Au–Au (aurophilic) interactions and liquid crystalline (LC) nature depending on molecular geometry. In this study, we synthesized several gold(I) complexes with rod-like molecular skeletons where we employed a mesogenic biphenylethynyl ligand and an isocyanide ligand with flexible alkoxyl or alkyl chains. The AIE behavior and LC nature were investigated experimentally and computationally. All synthesized gold(I) complexes exhibited AIE properties and, in crystal, room-temperature phosphorescence (RTP) with a relatively high quantum yields of greater than 23% even in air. We have demonstrated that such strong RTP are drastically changed depending on the crystal-size and/or crystal growth process that changes quality of crystals as well as the aggregate structure, of e.g., Au–Au distance. Moreover, the complex with longer flexible chains showed LC nature where RTP can be observed. We expect these rod-like gold(I) complexes to have great potential in AIE-active LC phosphorescent applications such as linearly/circularly polarizing phosphorescence materials.

Luminescent Behavior of Liquid–Crystalline Gold(I) Complexes Bearing a Carbazole Moiety: Effects of Substituent Bulkiness

“…To further investigate this behavior, POM images were collected to examine the optical texture of Cbz-H (Figure 3). A schlieren texture was obtained at 93 • C, which confirmed that the observed phase for Cbz-H is the N phase [14,16]. In the DSC thermogram, a small endothermic peak was observed at 73 • C during the heating process, and an isotropic phase was achieved at 118 • C. The N phase was observed between these temperatures.…”

“…The C1-Au-C2 bond angle is ~180 • [21], and molecular packing gives a Au-Au bond distance of 6.4 Å, which suggests the absence of aurophilic interactions between the molecules. We have previously reported the structure of a similar Au(I) complex that has less-hindered 4-ethynylbiphenyl and alkyl isocyanide ligands [14]. This complex also crystallized in triclinic space group Pī, and the C1-Au-C2 bond angles were the same as in Cbz-H, i.e., ~180 • ; however, the intermolecular Au-Au distance was 3.46 Å, which confirms the presence of intermolecular aurophilic interactions in the crystal.…”

“…For instance, previously synthesized Au(I) complexes with isocyanides and phenylacetylene ligands exhibited interesting AIE properties [16]. Furthermore, our group reported a biphenyl ring system with an LC phase, where luminescence was observed in both the crystal and LC phases [14].…”

“…Owing to the essential contribution of molecular aggregation to luminescence properties, aggregated structures play an important role in luminescence behavior [5][6][7][8][9][10]. Liquid crystals (LCs) are a unique class of soft materials that flow like liquids and possess a long-range orientational order similar to that of crystals [7,[11][12][13][14]. LCs have the potential to control the aggregated structure of luminescent materials.…”

Synthesis and Properties of Room-Temperature Phosphorescent Liquid Crystal Copolymers with Linearly Polarized Luminescence Characteristic

“…Luminescent liquid crystals (LLCs) combine the luminescence properties and ordered structures of liquid crystals. − LLCs are mainly divided into two categories: fluorescent liquid crystals − and phosphorescent liquid crystals. In recent years, room-temperature phosphorescent (RTP) liquid crystals have attracted increasing attention because of their important application prospects in phosphorescent organic light-emitting diodes (OLEDs), polarized light emitting materials, anticounterfeiting materials, and other fields. − Early research on RTP liquid crystal materials mainly focused on heavy metal complexes. − This is because the metal–ligand charge transfer can cause effective spin–orbit coupling (SOC), thereby enhancing the intersystem crossing (ISC) process to populate triplets. It is inevitable that these heavy metal complex RTP materials are usually toxic, environmentally unfriendly, and expensive, and therefore, it is highly desirable to develop pure organic RTP liquid crystal materials.…”