Aggregation-induced emissions in the mesogenic BF2 complexes of aroylhydrazines

PL peak intensity of 1a-BF2 (n = 12) versus f_w of THF/H₂O in pure THF (f_w = 0) and THF/water with a specific f_w. The concentration is 10 μM and PL emission was excited at 376 nm. Photos were taken under UV illumination.

Aggregation-induced emission of siloles

Aggregation-induced emission (AIE) is a unique and significant photophysical phenomenon that differs greatly from the commonly acknowledged aggregation-caused emission quenching observed for many π-conjugated planar chromophores. The mechanistic decipherment of the AIE phenomenon is of high importance for the advance of new AIE systems and exploitation of their potential applications. Propeller-like 2,3,4,5-tetraphenylsiloles are archetypal AIE-active luminogens, and have been adopted as a core part in the design of numerous luminescent materials with diverse functionalities. In this review article, we elucidate the impacts of substituents on the AIE activity and shed light on the structure-property relationship of siloles, with the aim of promoting the judicious design of AIE-active functional materials in the future. Recent representative advances of new silole-based functional materials and their potential applications are reviewed as well.

Influences of Conjugation Extent on the Aggregation-Induced Emission Quantum Efficiency in Silole Derivatives: A Computational Study

The photophysical properties of a series of silole derivatives, with hydrogen (TPS), bromine (BrTPS), and conjugated phenyl (HPS), triphenylsilyethynyl (BTPES), and dimethylfluorene (BFTPS) substituents at 2,5-positions in both gas and aggregate phases have been investigated computationally by employing the correlation function rate formalism coupled with a hybrid quantum/molecular mechanics (QM/MM) approach. It is found that the solid-state fluorescence quantum efficiency first increases sharply with the degree of π-conjugation of the 2,5-substituents, then levels off, and finally starts to decrease slightly. This is because the side-group conjugation tends to enhance the radiative decay rate in both gas and solid phases. However, a further increase in conjugation leads to saturation in the radiative decay rate but increases the non-raditiave decay rate due to the decreased energy gap.

Modulation of aggregation-induced emission and electroluminescence of silole derivatives by a covalent bonding pattern

The deciphering of structure-property relationships is of high importance to rational design of functional molecules and to explore their potential applications. In this work, a series of silole derivatives substituted with benzo[b]thiophene (BT) at the 2,5-positions of the silole ring are synthesized and characterized. The experimental investigation reveals that the covalent bonding through the 2-position of BT (2-BT) with silole ring allows a better conjugation of the backbone than that achieved though the 5-position of BT (5-BT), and results in totally different emission behaviors. The silole derivatives with 5-BT groups are weakly fluorescent in solutions, but are induced to emit intensely in aggregates, presenting excellent aggregation-induced emission (AIE) characteristics. Those with 2-BT groups can fluoresce more strongly in solutions, but no obvious emission enhancements are found in aggregates, suggesting they are not AIE-active. Theoretical calculations disclose that the good conjugation lowers the rotational motions of BT groups, which enables the molecules to emit more efficiently in solutions. But the well-conjugated planar backbone is prone to form strong intermoelcular interactions in aggregates, which decreases the emission efficiency. Non-doped organic light-emitting diodes (OLEDs) are fabricated by using these siloles as emitters. AIE-active silole derivatives show much better elecroluminescence properties than those without the AIE characterisic, demonstrating the advantage of AIE-active emitters in OLED applications.

Synthesis, structure, photoluminescence, and electroluminescence of siloles that contain planar fluorescent chromophores

Herein, a new series of siloles that were 2,5-substituted with planar fluorescent chromophores (PFCs), including fluorene, fluoranthene, naphthalene, pyrene, and anthracene, were synthesized and characterized. These compounds showed weak emission in the solution state, owing to active intramolecular rotation (IMR), but the synergistic effect from electronic coupling between the PFC and the silole ring compensated for the emission quenching by the IMR process to some extent, thereby affording higher emission efficiencies than those of 2,3,4,5-tetraphenylsiloles in solution. These new siloles showed enhanced emission efficiencies in the aggregated state. The electroluminescence (EL) color and efficiency of new siloles were sensitive towards the PFC. Siloles containing naphthalene moieties showed green EL emission, whilst those containing anthracene moieties showed orange EL emission. The siloles containing pyrene moieties exhibited yellow EL emission at 546 nm, with a peak luminance of 49000 cd cm(-2) and a high current efficiency of 9.1 cd A(-1).

Aggregation effects on the optical emission of 1,1,2,3,4,5-hexaphenylsilole (HPS): a QM/MM study

We investigate the photophysical property for 1,1,2,3,4,5-hexaphenylsilole (HPS) through combined quantum mechanical and molecular mechanical (QM/MM) simulations. Under the displaced harmonic oscillator approximation with consideration of the Duschinsky rotation effect (DRE), the radiative and nonradiative rates of the excited-state decay processes for HPS are calculated by using the analytical vibration correlation function approach coupled with first-principles calculations. The intermolecular packing effect is incorporated through electrostatic interaction modeled by a force field. We find that from the gas phase to the solid state (i) the side phenyl ring at the 5-position becomes coplanar with the central silacycle, which increases the degree of conjugation, thus accelerating the radiative decay process, and (ii) the rotation of the side phenyl ring at the 2-position is restricted, which blocks the excited-state nonradiative decay channels. Such a synergetic effect largely enhances the solid-state luminescence quantum efficiency through reducing the nonradiative decay rate by about 4 orders of magnitude, leading to the radiative decay overwhelming the nonradiatvie decay. In addition, the calculated solid-phase absorption and emission optical spectra of HPS are found to be in agreement with the experiment.

Conjugation versus rotation: good conjugation weakens the aggregation-induced emission effect of siloles

Incorporation of polycyclic aromatic hydrocarbons into siloles enhances their light emission in solutions but lowers emission efficiency in the aggregated state. The competitive interaction between conjugation and rotation is thus studied.

2,5-difluorenyl-substituted siloles for the fabrication of high-performance yellow organic light-emitting diodes

2,3,4,5-Tetraarylsiloles are a class of important luminogenic materials with efficient solid-state emission and excellent electron-transport capacity. However, those exhibiting outstanding electroluminescence properties are still rare. In this work, bulky 9,9-dimethylfluorenyl, 9,9-diphenylfluorenyl, and 9,9'-spirobifluorenyl substituents were introduced into the 2,5-positions of silole rings. The resulting 2,5-difluorenyl-substituted siloles are thermally stable and have low-lying LUMO energy levels. Crystallographic analysis revealed that intramolecular π-π interactions are prone to form between 9,9'-spirobifluorene units and phenyl rings at the 3,4-positions of the silole ring. In the solution state, these new siloles show weak blue and green emission bands, arising from the fluorenyl groups and silole rings with a certain extension of π conjugation, respectively. With increasing substituent volume, intramolecular rotation is decreased, and thus the emissions of the present siloles gradually improved and they showed higher fluorescence quantum yields (Φ(F) =2.5-5.4%) than 2,3,4,5-tetraphenylsiloles. They are highly emissive in solid films, with dominant green to yellow emissions and good solid-state Φ(F) values (75-88%). Efficient organic light-emitting diodes were fabricated by adopting them as host emitters and gave high luminance, current efficiency, and power efficiency of up to 44,100 cd m(-2), 18.3 cd A(-1), and 15.7 lm W(-1), respectively. Notably, a maximum external quantum efficiency of 5.5% was achieved in an optimized device.