Luminogenic polymers with aggregation-induced emission characteristics
Introduction
Organic and polymeric luminophores are promising materials for applications in various areas, such as light-emitting diodes [1], [2], [3], [4], [5], [6], plastic lasers [7], [8], [9], and fluorescent chemosensors and bioprobes [10], [11], [12], [13], [14], [15], [16]. To respond to the demand, scientists have synthesized a large number of luminescent materials. Many of them have been found to be highly emissive in dilute solutions, with fluorescence quantum yields reaching unity. For most practical applications, the luminescent materials have to be used in the solid state (e.g. as thin films), where the luminophores tend to form aggregates. However, it is known that aggregation of organic luminophores often leads to partial or even complete quenching of their light emissions. This aggregation-caused quenching (ACQ) effect has limited the scope of technological applications of the luminophoric molecules. To alleviate the ACQ effect in the condensed phase, various chemical, physical, and engineering approaches have been developed. For example, branched chains, bulky cyclics, spiro kinks, and dendritic wedges have been covalently attached to aromatic rings to impede aggregate formation [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. Luminogens have also been physically passivated via surfactant encapsulation, doped into matrices of nonconjugated transparent polymers such as poly(methyl methacrylate), and blended with different inorganic or organic or polymeric materials [29], [30], [31], [32], [33], [34]. Although various approaches have been taken to interfere with luminogen aggregation, the attempts have met with only limited success and they, however, are often accompanied by severe side effects. The steric effects of bulky cyclics, for example, can twist the conformations of the chromophoric units and thus partially jeopardize the electron conjugation in the luminophores. The nonconjugated encapsulates and the transparent matrices used in the physical processes are nonemissive and insulating, and can dilute the luminophore density and obstruct the charge transport in electroluminescence (EL) devices. The spatial distribution of the fluorophore dopants in a doped film suffers from temporal instability: the luminogens dispersed in the polymer matrices gradually migrate together over time, eventually emerging phase separation and forming large aggregates [2].
As aggregation is an inherent process when luminogenic molecules are in the condensed phase, it would be useful to develop a system in which aggregation plays a constructive, rather than destructive role, in the light-emitting processes of luminogens [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51]. In their search for efficient luminescent materials, Tang and co-workers were attracted by a group of organometallic molecules called siloles. A silole molecule named hexaphenylsilole (HPS or 1) is shown in Fig. 1 as an example, whose electronic structure looks extensively conjugated. However, the silole molecules were found to be virtually non-luminescent when molecularly dissolved in good solvents, but became highly emissive when aggregated in poor solvents or fabricated into thin solid films. They coined the term of “aggregation-induced emission (AIE)” for this phenomenon because the silole molecules were induced to emit by aggregate formation [52], [53]. Since then, many organic fluorogens have been found to show the AIE effect [54], [55], [56], [57], [58], [59], [60], [61], [62] and behave like HPS. For example, a dilute THF solution of tetraphenylethene (TPE or 2) is practically non-luminescent (Fig. 1), but addition of a poor solvent, such as water, induces aggregation and results in light emission.
Some luminogenic materials exhibit “turn-on” photophysical properties when utilized as chemo- and biosensors due to metal complexation, hydrogen-bond formation, electrostatic interaction, chemical reaction, etc. [63], [64], [65], [66]. What is the working principle for the AIE effect? A number of possible mechanistic pathways, including conformational planarization, J-aggregate formation, twisted intramolecular charge transfer (TICT), and restriction of intramolecular rotation (RIR), have been proposed for the AIE phenomenon. Through a series of externally and internally modulated experiments and theoretical studies [67], [68], [69], [70], [71], [72], [73], [74], [75], [76], [77], [78], [79], Tang and co-workers rationalized that RIR is the main cause for the AIE effect of their systems [62], [79]. According to fundamental physics, any molecular motion will consume energy. As depicted schematically in Fig. 1, the phenyl rings of HPS and TPE can undergo dynamic intramolecular rotations against the silole and olefin cores. In the solution state, such rotation is active, which serves as relaxation channel for the excited state to decay. Whereas, in the aggregated state, this rotation is restricted due to physical constraints on the molecular packing, blocking the nonradiative path and activates the radiative decay.
Most of the AIE systems developed so far are low-molecular-weight molecules. For practical applications, the luminogens have to be fabricated into thin solid films by expensive techniques, such as vacuum sublimation and vapor deposition, which are not well suited to the manufacture of large-area flat-panel devices. One way to overcome this processing disadvantage is to make polymers, which have high molecular weight and good film-forming capacity, and can be fabricated into large-area thin solid films and devices at ambient conditions by simple processes such as spin-coating, static casting, and ink-jet printing [80].
Conjugated polymers have been utilized in light-emitting diodes, fluorescent chemosensors and bioprobes, and solid-state lasers. Some of them, however, also encounter the ACQ problem in the condensed phase, which has consequently decreased their device performances. Preparation of linear and hyperbranched polymers with AIE or aggregation-enhanced emission (AEE) features may help to solve the problem and meanwhile impart polymers with new properties and practical applications. Although the research on this topic is still in its infancy, many works with excellent results have been achieved. In this review, we will summarize the recent progress on the preparation of AIE/AEE-active linear and hyperbranched polymers and their potential applications.
Section snippets
Polyacetylenes
As evidenced by the 2000 Nobel Prize in Chemistry, polyacetylene is an archetypal conjugated polymer [81], [82], [83] and exhibits a metallic conductivity upon doping. Little work has been done on the development of light-emitting polyacetylenes because the pristine polymers are non-luminescent. Replacement of the hydrogen atoms in its repeat units by appropriate substituents can fine-tune its electronic properties and generate mono- and disubstitued polyacetylenes [84], [85], [86]. During the
Polyphenylenes
Hyperbranched polymers have drawn much attention in recent years because they enjoy the advantages of readily synthetic access, unique molecular structure, microscopic processibility, and periphery functionalization, etc. [101], [102], [103]. Tang and co-workers worked on the preparation of hyperbranched polyphenylenes with AIE/AEE features [104], [105]. They first investigated the silole-containing poly(1,1-silolylphenylene) 13. The polymer was synthesized in a high yield via Ta-catalyzed
Polytriazoles
Since the first report in 2002, much attention has been placed on the copper-catalyzed azide-alkyne cycloaddition [112], [113]. This reaction is so powerful that it has been referred to as “click chemistry” and hailed for its features which exhibit a number of remarkable advantages, including high efficiency, regioselectivity, atom economy, and tolerance to functional groups, and has found widespread applications in a great diversity of areas [114], [115]. The utility of the click reaction in
Poly(phenyleneethynylene)s
To expand the variety of AIE-active polymers, Tang and co-workers recently prepared conjugated poly(phenyleneethynylene) containing TPE or silole units [129], [130]. The inset in Fig. 22 shows an example of such polymer (32), which was synthesized by the Sonogashira coupling of 1,2-bis(4-iodophenyl)-1,2-diphenylethene with 1,2-bis(4-ethynylphenyl)-1,2-diphenylethene. Polymer 32 is soluble in common organic solvents, such as THF, chloroform, and DCM. It is somewhat emissive in THF and its
Poly(phenylenevinylene)s
Tang and co-workers investigated the synthesis of conjugated poly(phenylenevinylene)s bearing TPE luminogens (Fig. 25) [129], [130]. Polymers 36 and 37 possess moderate molecular weights (4,400 and 11,800, respectively) and are soluble in common organic solvents.
Fig. 26 shows the photographs of 37 in THF/water mixtures; 37 emits green light in THF albeit in low efficiency. Gradual addition of water into its THF solution has, however, progressively intensified its emission; in other words, 37 is
Poly(thienylazulene)s
The azulene-containing conjugated polymers have been widely investigated because azulene possesses non-alternating aromatic structure and unusual physical properties, as well as anomalous fluorescence from the second excited state to the ground state [136]. A team led by Han and Lai reported that poly{1,3-bis[2-(3-alkylthienyl)]azulene} and poly{1,3-bis[2-(3-alkoxythienyl)]azulene} with ∼40–50 repeat units can form size-tunable nanoparticles through chain aggregation [137]. The nanoparticles
Polysilole
Since siloles are AIE-active [138], their polymers with linear and hyperbranched structures are also anticipated to show similar emission behaviors. Sohn and co-workers had synthesized polysilole 40 with a moderate molecular weight by reduction of 1,1-dichlorotetraphenylsilole (39) with lithium in THF (Fig. 30) [139], [140], [141].
Polysilole 40 is weakly emissive at ∼520 nm when molecularly dissolved in THF (Fig. 31). The PL intensity remains almost unchanged when less than 40% water is added to
Polyolefins
The polymers discussed in the previous sections possess conjugated structures. In this section, we consider electronically saturated polymers such as polyolefins featured with AIE/AEE characteristics.
Chi and Xu designed and synthesized a group of carbazole-substituted triphenylethenes and found that they are AIE-active with strong blue light emission and high thermal stability [142]. Recently, they succeeded in preparing a polystyrene 42 containing one of these luminogens in a high yield with a
Poly(acylhydrazone)s
Dynamers are dynamic polymers and are synthesized by reversibly connecting monomers through covalent or noncovalent bonds [151]. Such polymers are capable of varying their constituent, length, and sequence, enabling modification of their mechanical and optical properties by changing the feed components during polymerization or by adding another monomer after polymerization. During the extension of their dynamer research to biopolymers, Lehn and co-workers found that the biodynamers with
Conclusions and outlook
Recent research efforts on the design and synthesis of AIE/AEE-active luminogenic polymers with linear and hyperbranched structures are summarized in this review. The general strategy to synthesize such polymers is by attaching propeller-like AIE-active moieties, such as TPE and silole, as pendants to the polymer backbones or utilizing these units as building blocks for main-chain polymers. The change in the emission behavior from AIE characteristic in low-molecular-weight molecules to AEE
Acknowledgements
This work was supported by the National Science Foundation of China (20634020, 50703033, 20974098, and 20974028), the National Basic Research Program of the Ministry of Science and Technology of China (2009CB623605) and the Research Grants Council of Hong Kong (603509, 601608, HKUST13/CRF/08, and HKUST2/CRF/10). A.J.Q. acknowledges China Postdoctoral Science Foundation (20081461) and B.Z.T. thanks Cao Guangbiao Foundation of Zhejiang University for support.
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