Zujin
Zhao
*a,
Bairong
He
a and
Ben Zhong
Tang
*ab
aState Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China. E-mail: mszjzhao@scut.edu.cn
bDepartment of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China. E-mail: tangbenz@ust.hk
First published on 14th July 2015
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.
Through systematic experimental measurements and theoretical calculations, it has been rationalized that the restriction of intramolecular motions (RIM), including rotation, vibration, stretching, etc., is highly responsible for the AIE effect.2 In the solution state, active molecular motions serve as a relaxation channel for the excited state to decay nonradiatively, whereas in the aggregated state, these motions are suppressed greatly due to the spatial constraint, which blocks the nonradiative decay channel and promotes the radiative decay of the excited state.
Based on the understanding of the AIE mechanism, various interesting AIE systems have been developed to date, including siloles,1,3 cyanostilbenes,4 tetraphenylethenes,5,6 anthracene derivatives,7 diphenyldibenzofulvenes,8 pyrrole and diketopyrrolopyrrole derivatives,9,10 phosphole and phosphindole oxides,11 pyrazine derivatives,12 ESIPT molecules,13 organoborons,14o-carborane dyes,15 metal complexes,16 and other luminogenic molecules.17 Amongst these AIE-active luminogens, propeller-like siloles, such as 1,1-dimethyl-2,3,4,5-tetraphenylsilole (DMTPS), 1-methyl-1,2,3,4,5-pentaphenylsilole (MPPS) and 1,1,2,3,4,5-hexaphenylsilole (HPS) (Scheme 1), are the archetypes. They have high thermal stability and photostability, and can exist in acidic, neutral and weakly basic aqueous media. The propeller-like conformation endows these siloles with good solubility in common organic solvents, which facilitates purification and processing. Whereas they are almost nonfluorescent when molecularly dispersed in good solvents, they are good light emitters when fabricated into nanoaggregates (Fig. 1), crystals or solid films. In addition, siloles are also considered as a new kind of σ*–π* conjugated material with low-lying LUMO energy levels, deriving from the effective interaction between the σ* orbital of the exocyclic silicon–carbon bond and the π* orbital of the butadiene segment. Such a unique electronic structure imparts high electron affinity and fast electron mobility to siloles, and enables them to function as electron transporters in optoelectronic devices.18 Attracted by the AIE motif and interesting electronic features, numerous functional materials have been prepared by adopting silole as the luminescent core, which exhibit promising applications in organic light-emitting diodes,19 fluorescent bioprobes,20 chemosensors,21,22etc.
Fig. 1 Fluorescence photographs of solutions and suspensions of hexaphenylsilole (HPS; 20 μM) in THF/water mixtures with different fractions of water. Reproduced with permission from ref. 2c. Copyright (2014) Wiley-VCH. |
Owing to the bright prospects of siloles in materials science and biotechnology, the development of new silole derivatives with altered emission colors and desired functionalities draws intense current research interest. Since the structure determines the properties, better understanding the structure–property relationship can guide the rational design of silole-based materials through molecular engineering endeavors in the future.23 A better understanding is conducive to a deeper mechanistic decipherment of the AIE phenomenon, and is meaningful for the advancement of new AIE systems and exploitation of their potential applications. Chemical modifications can be facilely carried out on the 1,2,3,4,5-positions of a silole ring, which offers abundant access to chemical diversity and specific properties. Although plenty of silole derivatives have been synthesized and utilized in various research fields, and some clues to the substitution effects on the photophysical properties and AIE activities of siloles have appeared, comprehensive and systematic studies on the structure–property relationships of AIE-active silole systems are rarely conducted. To depict a clear cut picture of the impacts of substituents on the optical properties of AIE-active siloles, in this review article, we will elaborate on typical siloles with different emission wavelengths and efficiencies. It is demonstrated that substituents at the 2,5-positions determine the emission wavelength of a silole to a large degree, while those at the 3,4-positions are crucial to keeping the AIE properties. Whereas groups at the 1-position hardly exert any effects on the emission of a silole in the solution state, their influence on the solid-state emission properties can't be neglected. After the discussion on the structure–property correlation, the recent advances of new siloles with representative applications in the fields of chemosensors, fluorescent bioprobes and bioimaging, chiral supramolecular self-assembly, and organic light-emitting diodes are presented.
By comparing the optical properties and crystal structures of these silole molecules, it becomes clear that substituents at the 3,4-positions are connected in a twisted manner to the silole ring to reduce steric congestion, regardless of whether there are substituents at the 2,5-positions. They are also involved in steric repulsion with the substituents at the 2,5-positions. Without phenyl rings at the 3,4-positions, the phenyl rings at the 2,5-positions can conjugate with the silole ring much better, resulting in a more planar and rigid conformation.
As discussed above, the substituents at the 2,5-positions influence the emission wavelength greatly. However, it is still not clear whether aromatic substituents at these positions are really necessary to make a silole AIE-active. To gain deeper insight into the impact of these substituents on the emission properties of siloles, a series of flexible groups, dimethyl(phenyl)silyl and methyldiphenylsilyl, were introduced to the 2,5-positions of the silole ring (Scheme 3).25 The absorption maxima of these siloles are in the range of 311–320 nm, which are shorter than that of DPS, but longer than that of TPS. They are non-fluorescent in solution, but show a deep blue emission in the aggregated form or as solid films (Fig. 2 and 3). Their crystals exhibit bluish-violet light in the range of 413–421 nm, and their films show deep blue emissions peaking at 442–448 nm, which are much bluer than those of 2,3,4,5-tetraphenylsiloles. The weak interaction between the σ orbital in the silicon atom and the π orbital on the silole ring enables the silole derivatives to emit in the short-wavelength region. The ΦF values of these siloles in THF solution were measured to be lower than 0.01%, while those of the solid films were enhanced to 12–21%, demonstrating their AIE characteristics.
Fig. 2 PL spectra of BS4 in THF/water mixtures with different water fractions (fw), excited at 330 nm. Reproduced with permission from ref. 25. Copyright (2013) Elsevier. |
Fig. 3 Fluorescence photographs of (a) THF and aqueous solutions, (b) powders and (c) crystals of BS4, taken under the illumination of a UV lamp (365 nm). Reproduced with permission from ref. 25. Copyright (2013) Elsevier. |
Using flexible silyl groups as the substituents at the 2,5-positions can indeed prolong the conjugation of the molecule to some degree, and preserve the AIE properties of the siloles. Since the phenyl rings in the silyl groups are connected with the silole rings via a sp3 hybridized silicon bridge, the orbitals constituting the HOMOs and LUMOs of the siloles are barely distributed on these phenyl rings. The quenching effect of the rotations of these phenyl rings on the silole center is very weak. Therefore, the rotation of phenyl rings at the 3,4-positions should be mainly responsible for the faint emission in the solution state.
To further confirm the quenching effect of the phenyl rings at the 3,4-positions, the photophysical properties of a series of siloles substituted with rod-like, π-conjugated (trialkylylsilyl)ethynyl groups, were investigated (Scheme 4).26 The presence of ethynyl groups effectively elongates the conjugation of the siloles, and the HOMOs and LUMOs are delocalized along the whole molecular backbone comprised of the silole ring and 2,5-ethynyl groups. The absorption maxima of these siloles are in the range of 393–403 nm, being red-shifted obviously in comparison with those of a silole with flexible silyl substituents (311–320 nm), and even those of HPS (366 nm) and MPPS (363 nm). The good conjugation across the 2,5-positions of these siloles is expected to give efficient emission. However, their emissions in solution are virtually faint, with low ΦF values of 0.24–1.67%. The rotation of the phenyl rings at the 3,4-positions should have quenched the emission efficiently. In the solid state, their emissions are enhanced greatly, offering high ΦF values up to 99.9%, which are much higher than those of the flexible siloles BS1–4 (12–21%), demonstrating that a rigid, well-conjugated backbone is conducive to light emission from the silole. These results also indicate that rotatable phenyl rings at the 3,4-positions of a silole ring are essential, the presence of which can keep the silole weakly fluorescent in solution, but phenyl rings at the 2,5-positions of a silole are expendable.
The peripheral alkyl silyl groups impact little on the absorption properties of siloles, and exert almost no steric hindrance of the rotation of the phenyl rings at the 3,4-positions in the solution state, due to their remote positions. Hence, these siloles show close emission wavelengths ranging from 485 to 491 nm in the solution state. However, the alkyl silyl groups can regulate well the emission wavelengths and efficiencies of siloles in their aggregated states (Fig. 4). For example, the solid of triisopropylsilyl-endcapped TIPSES-2 shows an emission peak at 489 nm and an excellent ΦF value of 99.9%, while TMSES-2 substituted with trimethylsilyl gives a red-shifted emission at 542 nm and a decreased ΦF value of 33.3%. Substituents with a branched structure can not only prevent the close π–π stacking of the molecules but also effectively restrict the IMR processes in the aggregated state (Fig. 5). Hence, siloles with triisopropylsilyl groups show relatively shorter emission wavelengths but much higher emission efficiencies among these siloles.
Fig. 4 Fluorescence photos of powders of TMSES-2 (left), TESES-2 (middle) and TIPSES-2 (right), taken under the illumination of a UV lamp (365 nm). Reproduced with permission from ref. 26. Copyright (2009) Wiley-VCH. |
Fig. 5 (a) Packing arrangement of TMSES-1 in the crystalline state, with C–H⋯π hydrogen bonds marked by dashed lines. (b) Top and (c) side views of adjacent molecules of TIPSES-1 in the crystalline state. Reproduced with permission from ref. 26. Copyright (2009) Wiley-VCH. |
On the other hand, although substituents at the 1-position of silole rings affect little the AIE effect, they indeed have influences on the absorption maxima due to their inductive effects. The substituents at the 1,1-positions also exert apparent effects on the emission properties of siloles in the solid state. Small-sized groups allow the silole molecules to be arranged more closely, leading to redder emission colors and lower emission efficiencies.
Replacing the trialkylylsilyl groups with triphenylsilyl ones generates similar silole derivatives (Scheme 5) with AIE characteristics.27 While their emission efficiencies in the solution state are comparable to those of siloles with trialkylylsilyl groups, their solid-state emission efficiencies are lower (18.1–34.6%) than those of siloles with branched trialkylsilyl groups, such as the triisopropylsilyl ones (57.1–99.9%). This is because although the large-sized triphenylsilyl groups can prevent close stacking of the central silole ring, their phenyl rings could alternatively lead to π–π stacking in the aggregated state, which decreases the emission efficiency. The crystalline packing of these molecules has indicated the high possibility of intermolecular interactions in the solid state (Fig. 6).
Fig. 6 Illustration of π–π interactions between triphenylsilyl groups of adjacent molecules of TPSES-1. Reproduced with permission from ref. 27. Copyright (2011) Wiley-VCH. |
Fig. 7 (a) Absorption spectra of 2,5- and 3,4-BTPEMTPS in THF solutions and (b) the PL spectra of amorphous films of 2,5- and 3,4-BTPEMTPS. Reproduced with permission from ref. 28. Copyright (2010) American Chemical Society. |
Although both silole isomers have quite different electronic structures, they are found to inherit the AIE attributes from MPPS and TPE. They show slightly higher emission efficiencies in solution than MPPS due to the enhanced steric congestion. However, the ΦF values (0.34% and 0.38% for 2,5- and 3,4-BTPEMTPS, respectively) remain low in absolute terms, because the almost unrestricted IMR process of the multiple phenyl rotors on the periphery of the molecules can quench the light emission. As solid films, 2,5-BTPEMTPS and 3,4-BTPEMTPS show good ΦF values of 51.2% and 46.9%, respectively. On the other hand, 2,5-BTPEMTPS shows better thermal stability than 3,4-BTPEMTPS, and also functions better in OLEDs.
Fig. 8 Chemical structures of isopropyl-substituted HPS derivatives, and their fluorescence photographs in THF solutions, taken under an illumination of a UV lamp (365 nm). Reproduced with permission from ref. 32. Copyright (2005) American Chemical Society. |
Previous studies have disclosed that the IMR process of the TPE unit can effectively quench the emission of planar fluorescent chromophores (PFCs) in TPE-PFC adducts.33 Therefore, the emission quenching effect of the IMR process in silole systems is also envisioned. A series of PFCs that have good emission properties, such as pyrene, anthracene, naphthalene and fluoranthene, were incorporated onto the 2,5-positions of the silole ring to test the quenching effect of phenyl rotors at the 3,4-positions (Scheme 7). The results revealed that these new siloles are much weaker emitters in the solution state than PFCs existing individually, as evidenced by the much lower ΦF values (1.2–12%) than those of the PFCs.34 This clearly demonstrates that the good emissions of the PFCs have been quenched by the IMR process. However, these ΦF values are still higher than those of 2,3,4,5-tetraphenylsiloles (<0.1%), because the good emissions of PFCs compensate for the emission quenching effect of the IMR process to some degree.
On the other hand, the conjugation of PFCs also plays an important role in determining the emission efficiency in the solution state. For example, the silole with anthracene groups (AnDMS) shows a much higher ΦF value (12%) than that with naphthalene groups (NpDMS, ΦF = 2.2%) in the solution state, because anthracene has more extended conjugation than naphthalene. The siloles with naphthalene groups show aggregation-enhanced emission (AEE) but those with anthracene show no apparent AIE or AEE effects (Fig. 9). Theory calculation results suggest that AnDMS and NpDMS are more rigid than DMTPS, due to better conjugation. The reorganization energies for the excited states are increased from AnDMS (2258 cm−1) to NpDMS (3387 cm−1) and to DMTPS (4920 cm−1),35 implying that the lowest nonradiative energy loss by structural relaxation from the excited state to ground state occurs in AnDMS. Thanks to the more rigid conformation, siloles with anthracene possess higher radiative decay rates but lower nonradiative decay rates than siloles with naphthalene, which leads to higher ΦF values in solution.
Fig. 9 PL spectra of (A) NpMPS and (B) AnMPS in THF/water mixtures with different water fractions (fw), excited at 350 nm. Insets: photos of NpMPS and AnMPS in THF/water mixtures (fw = 0 and 99%) taken under the illumination of a UV lamp (365 nm). Reproduced with permission from ref. 35. Copyright (2014) The Royal Society of Chemistry. |
If the connection between a silole ring and the PFCs becomes more twisted, due to steric congestion, the emission efficiency decreases in solution. For example, although pyrene itself is similar to anthracene in terms of effective conjugation and ΦF value, siloles with pyrene groups show much lower ΦF values (2.6% and 3.9% for PyDMS and PyMPS, respectively) than those with anthracene (12% and 11% for AnDMS and AnMPS, respectively). From the crystal structure of PyMPS, it can be seen that the PyMPS molecules adopt a highly twisted conformation due to steric congestion. The dihedral angles between the pyrenes and the silole ring are 89.81° and 72.36°, indicating that electronic communication between both segments becomes difficult.37 Concerning AnDMS and AnMPS, the anthracene and silole moieties can become relatively more coplanar, and thus, better conjugated. The different molecular conformations account for the apparent difference in the emission efficiencies of these silole derivatives.
The introduction of large PFCs not only enhances the emission efficiencies of the siloles but also provokes obvious changes in the PL spectra in the solution state. Some siloles carrying large-sized PFCs, such as pyrene, exhibit blue emission bands (Fig. 10), which are ascribed to the emission of the pyrene moieties, in addition to the green emission band from the silole ring with certain π-extension. This can be interpreted as the IMR process leading to inefficient energy transfer from the PFC to the low-lying silole core in the solution state. The blue emission bands vanish when the molecules become more rigid in the aggregated state, where the IMR process is restricted and the electronic communication between the segments becomes dominant.36
Fig. 10 PL spectra of PyMPS in THF/water mixtures with different water fractions (fw), excited at 350 nm. Inset: photos of PyMPS in THF/water mixtures (fw = 0 and 99%) taken under the illumination of a UV lamp (365 nm). Reproduced with permission from ref. 37. Copyright (2014) Wiley-VCH. |
Unlike 2,3,4,5-tetraphenylsiloles that show similar emission wavelengths in both the solution and aggregated states because of the propeller-like conformation that hampers close π–π stacking of the molecules, siloles with large PFCs give rise to red-shifted emission spectra in the aggregated state.37 For example, NpMPS shows an emission maximum of 515 nm as a solid film, being close to that obtained in THF solution (508 nm), with a ΦF value of 37%. AnMPS, however, gives a much redder emission at 559 nm, with a lower ΦF value of 14%. Since naphthalene is smaller than anthracene in volume, the propeller-like molecular conformation can effectively prevent the close packing of the naphthalene moieties in the aggregated state. Therefore, NpMPS experiences less π–π stacking interactions, and affords stable emissions. Similarly, the silole with pyrene groups (PyMPS) also gives a large red-shift as a solid film in comparison with that in the solution state. Additionally, its solid state emission efficiency (22%) is much lower than those of siloles with naphthalene and other small aromatic groups. The packing manner of PyMPS in the crystalline state manifests as the formation of multiple intermolecular π–π stacking interactions. Many sets of four pyrene moieties are piled up face to face, and the distance between the planes of two neighboring pyrene rings is 3.405 or 3.573 Å (Fig. 11). These multiple π–π stacking interactions induced by this kind of packing manner should result in spectral red-shifts and decreased emission efficiencies.
Fig. 11 Packing pattern of PyMPS in the crystalline state. Reproduced with permission from ref. 37. Copyright (2014) Wiley-VCH. |
Fig. 12 PL spectra of (A) 5-BTDMS and (B) 2-BTDMS in THF/water mixtures with different water fractions (fw). Concentration: 10 μM; excitation wavelengths: 370 nm for 5-BTDMS and 420 nm for 2-BTDMS. Insets: photos of (A) 5-BTDMS and (B) 2-BTDMS in THF/water mixtures (fw = 0 and 95%), taken under the illumination of a UV lamp (365 nm). Reproduced with permission from ref. 38. Copyright (2015) Wiley-VCH. |
The theoretical calculation results reveal that 2-BTDMS has a higher HOMO energy level but a lower LUMO one than 5-BTDMS (Fig. 13), corresponding to a narrower energy band gap of 2.91 eV compared with that of 5-BTDMS (3.58 eV), further validating that 2-BTDMS has better conjugation than 5-BTDMS. This is actually understandable because the thiophene ring is smaller than the phenyl ring in size. 2-BTDMS is less sterically demanding and can adopt a more coplanar conformation, which facilitates the π-electron delocalization. In the transition from the ground state (S0) to the first singlet excited state (S1), 2-BTDMS experiences a much smaller structural alteration than 5-BTDMS. 2-BTDMS also has a much smaller total reorganization energy of the S1 (2549 cm−1) than that of 5-BTDMS (4640 cm−1), revealing that the blocking of the nonradiative channel, the internal conversion process from S1 to S0, enhances the emission of 2-BTDMS in solution. The contribution of the low frequency modes that represent the rotational motions of the aromatic rings to the total reorganization is suppressed in 2-BTDMS in comparison with that of 5-BTDMS (Fig. 14). This also suggests that the restriction of rotational motion accounts for the good emission efficiency of 2-BTDMS in solution, and again validates the restriction of intramolecular rotation as a factor contributing to the AIE phenomenon.
Fig. 13 Calculated molecular orbital amplitude plots and energy levels of the HOMOs and LUMOs of 5-BTDMS and 2-BTDMS. Reproduced with permission from ref. 38. Copyright (2015) Wiley-VCH. |
Fig. 14 The calculated reorganization energy versus the normal mode wavenumber for isolated molecules of (A) 5-BTDMS and (B) 2-BTDMS. Reproduced with permission from ref. 38. Copyright (2015) Wiley-VCH. |
The lower emission efficiencies and red-shifted emission wavelengths are caused by the well-conjugated planar backbones in 2-BTDMS and 2-BTMPS, which are prone to forming intermolecular interactions in the aggregated state. 5-BTDMS and 5-BTMPS which possess more twisted conformations can prevent strong intermolecular interactions and emit efficiently in the aggregated state. The negative effect of intermolecular interactions and the positive effect of restriction of rotational motion in the aggregated state compete against each other, and determine the emission behaviors of the molecules to a great degree.
The results presented above clearly suggest that the IMR process of the phenyl rings at the 3,4-positions can quench the light emission of luminescent chromophores attached to the silole ring. However, good conjugation between the luminescent chromophores and the silole ring, and the good emission of the substituents, can compensate for the emission quenching effect of the IMR process to some extent, and thus lead to a higher emission efficiency than siloles with small aromatic groups in the solution state. The PL emissions of siloles in the aggregated state, however, are lowered and red-shifted compared with those of 2,3,4,5-tetraphenylsiloles, due to the alternative quenching effect of the intermolecular interactions between the large PFC moieties and/or coplanar molecular backbones. Therefore, a wise choice of substituents, as well as connection position and pattern, are important in molecular design in order to achieve the desired emission color with high emission efficiency.
Monitoring of CO2 gas has practical implications for environmental protection because CO2 gas is an important component of gas mixtures from many natural and anthropogenic processes and exerts significant influences on the global climate and human wellbeing. Although several methods have been applied for sensing CO2 gas, the monitoring of high-concentration CO2 gas basically remains untouched. Recently, Tang et al. developed a new approach to detect high-concentration CO2 gas using HPS dissolved in an amine solvent of dipropylamine (DPA) (Fig. 15).42 The solution is non-fluorescent without CO2 gas, but a strong emission is observed when CO2 gas is bubbled through the solution. Since CO2 can react with DPA to yield a carbamate ionic liquid (CIL), the viscosity of the solution is increased greatly. The IMR process in HPS is thus suppressed and radiative decay is promoted, making the solution of HPS highly emissive. As more CO2 gas is bubbled through the solution, more CIL is formed, which gives rise to enhancement in emission intensity. The PL intensity correlates linearly with the CO2 gas level (Fig. 15C), which provides an inexpensive and visually discernable method for the quantitative detection of CO2 gas with applications as diverse as predicting volcanic eruptions and alerting to dangerous environmental conditions.
Fig. 15 (A) Photograph showing the discharge of CO2 gas from a power plant into the atmosphere. (B) Scheme showing the formation of carbamate ionic liquid (CIL) by the reaction of CO2 with dipropylamine (DPA). (C) Plot of the emission intensity of HPS versus CIL fraction in the DPA/CIL mixture. Reproduced with permission from ref. 42. Copyright (2010) American Chemical Society. |
Fig. 16 Schematic illustration of Net-TPS-PEI-DMA as a pH-responsive light-up nanoparticle probe for targeted cancer cell imaging. Reproduced with permission from ref. 43. Copyright (2014) The Royal Society of Chemistry. |
Specific bioprobes with a fluorescence turn-on response were also achieved by integrating ligands such as the cyclic arginine-glycine-aspartic acid tripeptide (cRGD), which target integrin αvβ3 receptors, into 2,3,4,5-tetraphenylsiloles (Fig. 17).44,45 Due to the AIE attributes of the 2,3,4,5-tetraphenylsilole core, the probe is almost non-fluorescent in solution. When the probes are specifically bonded to the target analyte, integrin αvβ3, the IMR process in the silole core is restricted by the interactive forces between them, and the strong emission of the silole is turned on. The probe maintains an “off” state when a protein has no specific interaction with TPS-2cRGD. The probe is thus able to quantitatively detect integrin αvβ3 in solution and discriminate integrin αvβ3-positive cancer cells from integrin αvβ3-negative ones.45 The binding process between cRGD and integrin αvβ3 on the cells can even be imaged in a real-time manner. The specific bonding interaction between cRGD and integrin αvβ3 ensures the high selectivity of the probe, which is often difficult for many probes to achieve based on a functional mechanism involving nonselective electrostatic and hydrophobic interactions between the AIE luminogens and biomolecules.
Fig. 17 Discrimination of (A) integrin αvβ3 from (B) other proteins via a specific cRGD-integrin interaction. Reproduced with permission from ref. 44. Copyright (2012) American Chemical Society. |
Liu et al.46 also successfully applied 2,3,4,5-tetraphenylsilole in the design of a chemotherapeutic Pt(IV) prodrug with a special focus on monitoring drug-induced cell apoptosis in situ (Fig. 18). The cyclic arginine glycine aspartic acid (cRGD) tripeptide for targeting integrin αvβ3 over-expressed cancer cells and a caspase-3 enzyme specific Asp-Glu-Val-Asp (DEVD) peptide were combined to construct the prodrug. The Pt(IV) prodrug can be reduced to active Pt(II) inside the cells and simultaneously releases the cell apoptosis sensor composed of 2,3,4,5-tetraphenylsilole and DEVD. The generated Pt(II) causes apoptosis of the cancer cells and activates caspase-3, which further cleaves the DEVD sequence within the apoptosis sensor. Therefore, the hydrophobic interaction between the 2,3,4,5-tetraphenylsilole segments leads to aggregate formation, which turns on light emission.46 This prodrug system with AIE characteristics allows the early evaluation of its therapeutic response in cancer cells with high sensitivity.
Fig. 18 Schematic illustration of the targeted theranostic platinum(IV) prodrug with a built-in AIE-active light-up apoptosis sensor for noninvasive in situ early evaluation of its therapeutic response. Reproduced with permission from ref. 46. Copyright (2014) American Chemical Society. |
Previous studies have shown that the nanoparticles of AIE-active siloles can stain cells for one-photon fluorescence cellular imaging and long term tracking.47 However, robust siloles with potential utility in TPF imaging applications are seldomly reported, possibly due to the green fluorescence of most siloles, which is not preferable for TPF imaging. Recently, a red fluorescent silole, (MesB)2DTTPS, carrying (dimesitylboranyl)thiophen-2-yl substituents was prepared (Fig. 19).48 The intramolecular charge transfer process is strengthened because of the good electron-donating ability of the thiophene units, which gives rise to the red fluorescence of the molecule. (MesB)2DTTPS shows weak emission peaking at 568 nm with a ΦF value of 5.6% in THF solution. The organic dots fabricated by encapsulating (MesB)2DTTPS within DSPE–PEG matrix emit a red fluorescence located at 598 nm and give a high ΦF value of 32%, indicative of the AEE attribute. Meanwhile, a large two-photon absorption cross section of 3.43 × 105 GM, and a two-photon action cross section of 1.09 × 105 GM, at an excitation wavelength of 820 nm, are recorded per dot, revealing their great potential as luminescent reagents for TPF imaging. The (MesB)2DTTPS dots are biocompatible, and can be applied in the one-photon and two-photon fluorescence targeted imaging of MCF-7 breast cancer cells after functionalization of the dot surfaces with a cell penetration peptide. The dots also function well in the TPF in vivo real-time visualization of blood vessels in mouse muscle (Fig. 20) and ear skin in a vivid and noninvasive manner, providing abundant spatiotemporal information about the three-dimensional network of blood vascular structures.
Fig. 19 Chemical structure of (MesB)2DTTPS and schematic illustration of organic dots based on (MesB)2DTTPS. |
Recently, Tang et al. prepared two silole derivatives, CS-1 and CS-2, consisting of an AIE-active 2,3,4,5-tetraphenylsilole core and chiral pendants (sugar and valine) connected to each other by a triazole group (Scheme 9).49 Both luminogens turned out to be high-performance CPL-active materials with large ΦF and gem values in their aggregated states. They show neither circular dichroism (CD) signals nor fluorescence in the solution state. However, as the aggregate forms, obvious CD signals and intense fluorescence are recorded. CS-1 tends to emit right-handed circular polarized (RCP) light when aggregated as nanoparticles in suspension, fabricated into thin films, or dispersed as domains in a polymeric matrix, as well as when assembled into a regular micropattern (Fig. 21), with large gem values of up to −0.32. In addition, these molecules can undergo a self-assembly process to form varied nanostructures. CS-1 can self-assemble into right-handed helical nanostructures, such as nanoribbons and superhelical ropes (Fig. 22). The interesting CD signals and circular polarized emissions are considered to be caused by the chirality transfer from the chiral groups to the silole core, which forces the silole core to be helically arranged in a preferred screw manner. In the solution state, the silole molecules are conformationally random due to the IMR process, which suppresses the CPL signals.
Fig. 21 Plots of (a) (IL − IR) and (b) the CPL dissymmetry factor gemversus wavelength for CS-1 existing in different formats: DCM solution, DCM/hexane (v/v 1/9) mixture (suspension), neat cast film from a DCE solution of 2 mg mL−1, dispersion in a polymer matrix (10 wt% in PMMA), and a fabricated micropattern by evaporation of DCE solution in microfluidic channels; gem = 2(IL − IR)/(IL + IR), where IL and IR denote the left- and right-handed emission intensities, respectively. In DCM and the DCM/hexane mixture, the concentration was 2 × 10−4 M and the excitation wavelength was 325 nm. Reproduced with permission from ref. 49a. Copyright (2012) The Royal Society of Chemistry. |
Fig. 22 Characterisation of the microstructures of the assemblies of CS-1. (a) SEM images of the aggregates of CS-1 in the DCM/hexane mixture (v/v 1/9). Concentration: 2 × 10−4 M. Inset: enlarged area as indicated. (b and c) TEM images of the aggregates of CS-1 in the DCM/hexane mixture (v/v 1/9). Reproduced with permission from ref. 49a. Copyright (2012) The Royal Society of Chemistry. |
CS-2, which bears valine groups, also shows aggregation-induced CD and PL. Left-handed circular polarized (LCP) light was detected from the casted film, which was not detected in the solution state. The average gem value is about −0.5 in the spectral window range of 420–620 nm. The presence of chiral amino acid groups also exerts an asymmetric force field on the silole core and induces a helical molecular conformation when the IMR process is suppressed in the condensed phase. CS-2 is assembled into left-handed nanostructures by the slow evaporation of THF solvent. Addition of a poor solvent, water, to the THF solution of CS-2 can also cause left-handed nanostructures, which vary from thin nanofibers to thicker loops as the fraction of water increases (Fig. 23). But the addition of hexane to the THF solution can give rise to handedness inversion of the nanomaterials, and afford right-handed nanofibers, suggesting that the polarity of the mixtures impacts the self-assembling behaviors of the molecules.
Fig. 23 AFM images of the helical assemblies formed by CS-2 upon the evaporation of its different THF/water mixtures. The water content is (A and B) 5%, (C and D) 20%, (E) 80%, and (F) 90%. Reproduced with permission from ref. 49b. Copyright (2014) The Royal Society of Chemistry. |
Recently, with the great endeavors of molecular engineering, efficient light-emitting materials for OLEDs have been created from silole derivatives. By the incorporation of fluorene-based substituents, including bare fluorene, 9,9-dimethylfluorene, 9,9-diphenylfluorene and 9,9′-spirobifluorene, a series of efficient solid-state light emitters have been generated (Scheme 10).52 These silole derivatives show absorption maxima in the range of 388–402 nm, and main emission peaks in the range of 497–515 nm, accompanied by blue emission bands (409–439 nm) stemming from the fluorene groups. The ΦF values in solution are slightly enhanced from 2.5 to 5.4% as the volume of the substituents increases. The films of these silole derivatives fluoresce solely at 521–535 nm, and the blue emission peaks disappear, due to the restriction of the IMR process and rigidified molecular backbones. The solid films are also highly emissive, giving high ΦF values of 50–88%.
The OLEDs based on these silole derivatives exhibit high performances. The undoped OLED of MFMPS (ITO/NPB (60 nm)/emitter (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)) radiates yellow light at 544 nm (CIE = 0.37, 0.57), and affords the best EL performance with a lowest turn-on voltage (Von) of 3.2 V, and a highest maximum luminance (Lmax) of 31900 cd m−2. The maximum current efficiency (ηC,max), maximum power efficiency (ηP,max) and maximum external quantum efficiency (ηext,max) attained by the device are as high as 16.0 cd A−1, 13.5 lm W−1 and 4.8%, respectively. By increasing the thickness of the TPBi layer to balance the holes and electrons and adding a thin layer of MoO3 to facilitate the hole-injection, an even more efficient OLED (ITO/MoO3 (5 nm)/NPB (60 nm)/MFMPS (20 nm)/TPBi (60 nm)/LiF (1 nm)/Al (100 nm)) was obtained, which shows a Von of 3.3 V, a yellow light peak at 540 nm (CIE = 0.36, 0.57), and excellent luminance, current and power efficiencies of 37800 cd m−2, 18.3 cd A−1 and 15.7 lm W−1, respectively. Impressively, a notable ηext,max of 5.5% was achieved by the device (Fig. 24), which exceeds the theoretical limit for EL devices based on conventional fluorescent materials.
Fig. 24 Plot of the external quantum efficiency vs. luminance of an OLED based on MFMPS. Inset: photo of the OLED. Reproduced with permission from ref. 52. Copyright (2014) Wiley-VCH. |
Thanks to the unique electronic structure of σ*–π* conjugation, siloles possess lower-lying LUMO energy levels, and higher electron affinities than common five-membered heterocycles, such as pyrrole, furan and thiophene, and are considered as a new class of electron-transporting materials. However, as a matter of fact, bare 2,3,4,5-tetraphenylsiloles are still not capable of functioning as electron transporters in OLEDs. The incorporation of other electron-withdrawing groups, such as pyridine, can improve the electron-transporting ability and enable them to function as electron-transporting layers.18a However, those that can simultaneously serve as light-emitting and electron-transporting layers are rarely developed. West et al.53 introduced diquinaldinatoalumino groups onto the 1,1-positions of 2,3,4,5-tetraphenylsiloles to improve their electron-transporting abilities. However, the EL performances of the non-doped devices prepared from the resulting siloles were hardly advanced.
To address this issue, a series of tailored dimesitylboryl functionalized silole derivatives were developed (Scheme 11).54 The grafting of inherently electron deficient bimesitylboryl groups is beneficial in advancing the electron affinity and electron-transporting ability of the molecule. In addition, the branched conformation of the bimesitylboryl group can suppress close π–π stacking, and thus keep the AIE characteristics intact. These silole derivatives are highly emissive as solid films, with emission peaks at 516–526 nm and high ΦF values of 56–62%. The LUMO energy levels of (MesB)2DMTPS, (MesB)2MPPS and (MesB)2HPS are −3.00, −3.06 and −3.10 eV, respectively, which are equal to or even lower than those of commercialized typical electron transporters, such as tris(8-hydroxyquinoline)aluminum (Alq3, −3.0 eV) and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi, −2.7 eV), revealing that they are capable electron-transporting materials for OLEDs (Fig. 25). A similar silole derivative, (MesBF)2MPPS, created by integrating dimesitylboryl substituents into MFMPS, also shows excellent solid-state emission and great potential as an electron transporter.55 The incorporation of other electron deficient substituents, such as diphenylphosphine oxide, can also improve the electron-transporting ability without undermining the solid-state fluorescence efficiency (Scheme 12).
Fig. 25 Plots of the external quantum efficiency vs. luminance of devices based on (MesB)2MPPS. Inset: device configuration and energy levels. Reproduced with permission from ref. 54. Copyright (2014) Wiley-VCH. |
The double-layer OLEDs [ITO/NPB (60 nm)/silole (60 nm)/LiF (1 nm)/Al (100 nm)] fabricated by adopting (MesB)2MPPS or (MesB)2HPS as both light emitter and electron transporter show EL emissions at 520 nm (CIE = 0.30, 0.56) and 524 nm (CIE = 0.33, 0.56), and afford excellent performances, with high EL efficiencies up to 13.9 cd A−1, 4.35% and 11.6 lm W−1. The values are much higher than those attained from the triple-layer devices with an additional TPBi electron-transporting layer (8.4 cd A−1, 2.62% and 4.1 lm W−1). The advanced EL performances of both silole derivatives are attributed to the improved electron-transporting ability of the materials, which balances holes and electrons in the devices, and thus enhances the carrier recombination efficiencies. Thanks to the matched energy levels and electron-transporting abilities of the present siloles, the TPBi layer is actually not necessary, and may even have an adverse effect on device performance because of the contact resistance at the interfaces. These results demonstrate that these dimesitylboryl functionalized siloles are promising bifunctional materials, n-type solid-state light emitters, for the fabrication of high-performance OLEDs with simplified device structures.
By a similar method, p-type solid-state luminescent materials that function remarkably in double-layer OLEDs were also achieved.56 The silole carrying the triphenylamine groups, 1,1-dimethyl-2,5-bis(4-diphenylaminophenyl)-3,4-diphenylsilole (DMTPS-DPA), shows not only a high solid-state fluorescence efficiency (74%) but also a good hole-transporting ability.19a The device where DMTPS-DPA functions as both the light emitter and hole-transporter (ITO/DMTPS-DPA (80 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm)) radiates a yellow EL emission at 548 nm (CIE = 0.40, 0.57), shows a low turn-on voltage of 3.1 V, and has a high luminance up to 14038 cd m−2. The maximum current, power and external efficiencies of the device were 7.60 cd A−1, 6.94 lm W−1 and 2.26%, respectively. These data are close to those attained from the device with an additional NPB hole-transporting layer, demonstrating the good bifunctionality of DMTPS-DPA. The hole-transporting ability of the triphenylamine groups is dominant in DMTPS-DPA, while the intrinsic low-lying LUMO energy level of the silole core facilitates electron injection, and thus, exciton recombination.
The working mechanism behind the AIE phenomenon of propeller-like siloles is rationalized to be the restriction of IMR process of the aromatic rotors against the silole ring stator. The rotors at the 3,4-positions contribute little to determining the absorption and emission wavelengths, but they are virtually crucial for the AIE activities of siloles, whose rotational motions efficiently deactivate the excited states of the molecules in a nonradiative manner, and thus, lead to weak emissions in the solution states. Without them, the steric congestion of the molecule is reduced, and the conjugation between substituents at the 2,5-positions and the central silole ring is improved, making a better conjugated, more rigid molecular backbone. The emission efficiency in solution is thus enhanced and the AIE effect disappears. The substituents at the 2,5-positions control the effective conjugation lengths and electronic structures of siloles, and thus impact the emission wavelengths and efficiencies remarkably. The good conjugation between the large planar fluorescent groups at the 2,5-positions and the silole ring compensates for the emission quenching effect caused by the rotational motion, and enhances the emission efficiency in solution. However, the emission can be red-shifted and decreased in the aggregated state because large planar groups are prone to forming strong π–π intermolecular interactions. The collective effect of both factors gives rise to no or faint AIE features. The substituents located at the 1-position of the silole ring and those weakly conjugated to the molecular backbone exert no obvious effect on the emission properties of the siloles in solution, but they have apparent impacts on the emission behaviors in the aggregated state via intermolecular interaction modulation.
Guided by the AIE mechanism deciphering and the structure–property correlation understanding, desired emission behaviors and functionalities can be readily realized by simple chemical modifications. A large variety of new silole-based functional materials have been developed, with tunable emission colors and good fluorescence efficiencies. The new silole derivatives have shown great potential in chemosensors, biological probes and imaging, chiral supramolecular self-assembly, OLEDs, etc. Thanks to the excellent AIE properties, these silole derivatives have advantages in a wide array of high-tech applications. For example, they can be used to fabricate high-performance non-doped OLEDs, and the complicated and troublesome doping technique for alleviating the ACQ problem of many conventional fluorescent and phosphorescent light emitters can be avoided. High solid-state emission efficiencies and good electron-transporting abilities are highly favored in non-doped OLEDs. In bioimaging applications, photobleaching is a common problem with organic luminescent reagents, which is unfavorable for long-term tracking. Fabricating luminescent dyes into small dots can improve the photostability of the materials. However, in most cases, this method will cause the adverse effect of emission quenching. In this regard, AIE-active siloles are perfect candidates for preparing fluorescent dots, in which both high photostability and bright fluorescence can be facilely achieved.
One defect that should be pointed out is that the emission color of most siloles is in the green to yellow region, and efficient blue emission has hardly been achieved yet. Even the archetypal 2,3,4,5-tetraphenylsiloles only give strong green emissions in the aggregated state, which is not as interesting as red emissions for applications in bioscience and OLEDs. Weakening the conjugation of the molecule can move the emission to the blue region, but may cause a decrease in emission efficiency. Efficient red emission is envisioned to be more readily achievable by the endeavors of molecular engineering. In view of this, the development of functional siloles with long-wavelength emissions should be a promising direction for research, which is still in an infant stage.
Current achievements are anticipated to trigger more interest and ideas from researchers and to further promote and broaden applications of AIE-active siloles in various environmental, biological and optoelectronic fields. By structure–property relationship understanding and AIE mechanism deciphering, the AIE studies can extend from peculiar propeller-like siloles to more common fluorescent systems, such as green fluorescent proteins, bacterial luciferase fluorophores, coumarin derivatives, etc. In addition to the restriction of intramolecular rotation, restriction of other kinds of intramolecular motions, such as vibration or twisting, may play a role in these new AIE systems. These fluorophores have diverse molecular structures from natural products to artificial compounds containing pure hydrocarbons or heteroatoms or metal ions. Benefiting from the intriguing AIE nature, they have a prosperous future in diverse research frontiers.
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