Functional π-Gelators and Their Applications

This review focuses on classifying different types of long wavelength absorbing BODIPY dyes based on the wide range of structural modification methods that have been adopted, and on tabulating their spectral and photophysical properties. The structure–property relationships are analyzed in depth with reference to molecular modeling calculations, so that the effectiveness of the different structural modification strategies for shifting the main BODIPY spectral bands to longer wavelengths can be readily compared, along with their effects on the fluorescence quantum yield (ΦF) values. This should facilitate the future rational design of red/NIR region BODIPY dyes for a wide range of different applications.

Structural modification strategies for the rational design of red/NIR region BODIPYs

The possibility to change the molecular assembled structures of organic and organometallic materials through mechanical stimulation is emerging as a general and powerful concept for the design of functional materials. In particular, the photophysical properties such as photoluminescence color, quantum yield, and emission lifetime of organic and organometallic fluorophores can significantly depend on the molecular packing, enabling the development of molecular materials with mechanoresponsive luminescence characteristics. Indeed, an increasing number of studies have shown in recent years that mechanical force can be utilized to change the molecular arrangement, and thereby the optical response, of luminescent molecular assemblies of π-conjugated organic or organometallic molecules. Here, the development of such mechanoresponsive luminescent (MRL) molecular assemblies consisting of organic or organometallic molecules is reviewed and emerging trends in this research field are summarized. After a brief introduction of mechanoresponsive luminescence observed in molecular assemblies, the concept of "luminescent molecular domino" is introduced, before molecular materials that show turn-on/off of photoluminescence in response to mechanical stimulation are reviewed. Mechanically stimulated multicolor changes and water-soluble MRL materials are also highlighted and approaches that combine the concept of MRL molecular assemblies with other materials types are presented in the last part of this progress report.

/pdf/mechanoresponsive-luminescent-molecular-assemblies-an-39keab9a91.pdf

Mechanoresponsive Luminescent Molecular Assemblies: An Emerging Class of Materials

A Perspective is presented on the history and current understanding of molecular gels and the factors that must be considered to characterize them. The abilities of the most important structural, dynamic, and rheological tools available currently to provide the information necessary to follow the formation of a molecular gel from its initial sol phase and then to define it at different distance and time scales are discussed. Approaches to determining a priori when a molecule will gelate a selected liquid, as well as possible methodologies for overcoming current limitations in understanding molecular gels, are presented. Finally, some of the many potential and realized applications for these materials are enumerated.

The past, present, and future of molecular gels. What is the status of the field, and where is it going?

Mechanochromic luminescent metal complexes have recently attracted considerable interest as smart materials for use in the fields of luminescence switches, mechanosensors, mechanohistory indicators, security papers, optoelectronic devices, and data storage. In this review, recent advancements in metal complex mechanochromic luminescence are summarized. The majority of the reported mechanochromic luminescent metal complex systems are discussed, including Zn(II), Au(I), Pt(II), Ag(I), Cu(I), Al(III), and Ir(III) complexes. Aggregation-induced emission (AIE) complexes are described based on the existence of a structural relationship between AIE compounds and mechanochromism. It is expected that AIE complexes may become important alternative sources of mechanochromic luminescent complexes.

Recent advances in mechanochromic luminescent metal complexes

Four-coordinate organoboron compounds with rigid π-conjugated structures are intensely luminescent and have high carrier mobility which enables them to be applied in optoelectronics including organic light-emitting diodes (OLEDs), organic field-effect transistors, as well as photoresponsive, sensory and imaging materials. Various chelate ligands and boron moieties have been explored to construct proper electronic structures and suitable molecular arrangements, which play important roles on the photophysical and electronic properties of the four coordinate boron compounds. These efforts have produced a number of fascinating molecules, some of which have exhibited high performance as light emitting materials. In this article, we provide an overview of the progress in the molecular construction of four-coordinate organoboron compounds with an emphasis on their applications in OLEDs.

Four-coordinate organoboron compounds for organic light-emitting diodes (OLEDs).

2-(2′-Hydroxyphenyl)benzoxazole (HBO) and 2-(2′-hydroxyphenyl)benzothiazole (HBT) reacted with triphenylborane produced two rigid π-conjugated fluorescent cores 1 (BPh2(BOZ), BOZ = 2-(benzo[d]oxazol-2-yl)phenol) and 2 (BPh2(BTZ), BTZ = 2-(benzo[d]thiazol-2-yl)phenol). Comparisons of photophysical properties and calculations between para- and meta-diphenylamine-substituted derivatives 5 (BPh2(para-NPh2-BTZ)) and 7 (BPh2(meta-NPh2-BTZ)) demonstrated that functionalization at the para-position of the rigid core is effective in tuning the electronic structure and hence the photophysical properties of this type of boron-chelate complex. Simple modification of these frameworks by introducing various amine groups at the para-position allows the synthesis of strongly fluorescent materials 3 (BPh2(para-Cz-BTZ), Cz = 9H-carbazol-9-yl), 4 (BPh2(para-NPh2-BOZ), NPh2 = diphenylamino), 5, and 6 (BPh2(para-NMe2-BTZ), NMe2 = dimethylamino). The emission colors of these newly synthesized complexes together with the parent complexes 1 and 2 covered a wide range from deep blue to saturated red in both solution and the solid state. Crystal structure analysis discloses that two phenyl groups attached to the boron atom effectively keep the luminescent ring-fused π-conjugated skeletons apart, making these fluorophores highly emissive in solid forms (ΦF = 0.36–0.71). Organic light-emitting diodes employing these boron complexes as emitters not only keep the full-color tunable emission feature but also show high electroluminescent (EL) performance; for instance, the greenish-blue device based on 2 showed the highest efficiency of 7.8 cd A−1 and the yellow light-emitting device based on 4 exhibited the highest brightness (31 220 cd m−2) among the boron-containing emitters reported so far.

/pdf/construction-of-full-color-tunable-and-strongly-emissive-1z2r5b94p9.pdf

Construction of full-color-tunable and strongly emissive materials by functionalizing a boron-chelate four-ring-fused π-conjugated core

Four phenanthroimidazole derivatives (a: 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole; b: 2-phenyl-1-p-tolyl-1H-phenanthro[9,10-d]imidazole; c: 1-phenyl-2-p-tolyl-1H-phenanthro[9,10-d]imidazole; d: 1,2-di-p-tolyl-1H-phenanthro[9,10-d]imidazole) were synthesized and their single crystal structures, photophysical, electrochemical and mobility properties were carefully studied. Taking advantage of the thermal stability and the hole transporting (HT) ability, the highly efficient Alq3-based organic light-emitting diodes (OLEDs) have been achieved by employing the compounds a–d as a functional layer between NPB (4,4-bis(N-(1-naphthyl)-N-phenylamino)biphenyl) and Alq3 (tris(8-hydroxyquinoline)aluminium) layers. For the device of [ITO/NPB/d/Alq3/LiF/Al], a maximum luminous efficiency (LE) of 8.1 cd A−1 was obtained with a maximum brightness of 65130 cd m−2, which exhibited much higher efficiency compared to the device with structure of [ITO/NPB/Alq3/LiF/Al]. The results demonstrated not only an alternative idea to design novel HT materials, but also a convenient way to improve the performance of the NPB/Alq3-based devices by introduction of a suitable organic buffer layer.

Phenanthroimidazole-derivative semiconductors as functional layer in high performance OLEDs

Two π-conjugated organoboron complexes 1 and 2 with highly efficient red (632 nm) and deep red (670 nm) solid-state fluorescence have been constructed and qualified as potential non-doped red emitters accompanied by excellent electron-transport ability. X-ray crystal analysis demonstrated that the two side phenyl groups coordinated to each boron atom effectively keep the luminescent units apart. As a result, these red fluorophores are brightly fluorescent in the solid state (fluorescence quantum yields: 0.30 for 1 and 0.41 for 2). In addition, these boron complexes possess good thermal stability and high electron-transport ability. Organic light-emitting diodes employing 1 or 2 as non-doped emitters with simple device configuration exhibit bright red and near-infared electroluminescence.

Brightly fluorescent red organic solids bearing boron-bridged π–conjugated skeletons

Four diboron-bridged ladder molecules 1–4 have been designed and synthesized. X-ray diffraction analysis revealed that the bulky phenyl substituents on boron centers efficiently prevented π stacking of the luminescent ladder unit. Characterizations of these complexes demonstrated that the construction of diboron-containing ladder-type skeletons endowed these materials with good thermal stability, high fluorescence quantum yields, and strong electron affinity. The highly efficient nondoped organic light-emitting diodes using complexes 1 and 2 as electron-transporting emitters exhibited maximum luminance values of 16 930 and 18 060 cd/m2 with turn-on voltages of 3.5 and 2.5 V as well as maximum luminous efficiencies of 6.4 and 5.4 cd/A, respectively.

Boron-Bridged π-Conjugated Ladders as Efficient Electron-Transporting Emitters

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