Abstract
Nonconventional luminescent materials have been rising stars in organic luminophores due to their intrinsic characteristics, including water-solubility, biocompatibility and environmental friendliness, and have shown potential applications in diverse fields. As an indispensable branch of nonconventional luminescent materials, polysiloxanes which consist of electron-rich auxochromic groups, have exhibited outstanding photophysical properties due to the unique silicon atom. The flexible Si-O bonds benefit the aggregation, and the empty 3d orbitals of Si atom can generate coordination bonds like N → Si and O → Si, altering the electron delocalization of the material and improving the luminescent purity. Herein, we review the recent progress in luminescent polysiloxanes with different topologies and discuss the challenges and perspectives. With an emphasis on the driving force for the aggregation and the mechanism of tuned emissions, the role of Si atoms played in the nonconventional luminophores is highlighted. This review may provide new insights into the design of nonconventional luminescent materials and expand their further applications in sensing, biomedicine, lighting devices, etc.
Introduction
Aggregation-induced emission (AIE) photoluminescent (PL) materials are of vital importance in organic luminescent materials due to their antithetic emission behaviors to aggregation-caused quenching (ACQ) luminophores and wide applications in organic light-emitting diode (OLED),[1] bioassay,[2,3]bioimaging,[4,5] circularly polarized luminescence,[6,7] liquid crystal,[8,9]anti-couterfeiting[10-12] and therapy.[13,14] Coined by Tang and coworkers in 2001, AIE is a unique phenomenon that nonemissive luminophores are induced to emit upon aggregation, which settled the ACQ puzzle and experiences booming development since then.[15,16]
Traditional AIE luminophores generally contains π conjugates, for example tetraphenylethene (TPE),[17]hexaphenylsilole (HPS),[18]10,10′,11,11′-tetrahydro-5,5′-bidibenzo[a,d][7]-annulenylidene (THBA),[19]etc. They are highly emissive in aggregated state owing to the activated radiative transition caused by the restriction of intramolecular motions (RIM).[20.21]
As an indispensable branch of AIE luminophores, nonconventional luminophores, such as poly(amido amine) (PAMAM),[22] polyethylenimine (PEI),[23] poly(amino esters) (PAE),[24] polyureas,[25]polysiloxanes[26] and nonaromatic small molecules,[27]are nonaromatic or small aromatic systems, and have aroused increasing attention.[28] In the absence of large π conjugates, these nonconventional AIE luminophores are consisted of electron-rich auxochromic groups, leading to potential biocompatibility, water-solubility and easy preparation. According to clustering-triggered emission (CTE) mechanism, the emission of the nonconventional AIE luminophores comes from the electron delocalization generated by the lone-pare electrons in electron-rich groups.[29]Guided by the CTE mechanism, a growing number of synthetic nonconventional AIE luminophores are developed.[30] Among these emerging luminophores, AIE polymers with intrinsic emissions have attracted great interests.
Silicon is an important abundant element of group Ⅳ with no reported connate toxicity, which provides application potential in biological and medical. The first reported organosilica luminescent material dated back to 1998, when Lianos et al. synthesized a nonconventional sol-gel silica network.[31] After that, various polysiloxanes with different topologies have been developed.[32]As an integral part of organic-inorganic hybrid polymers, hyperbranched polysiloxanes combine the advantage of polysiloxanes and hyperbranched structures, thereby process wide temperature resistance, multifunctionality, low viscosity, etc.[32] As for now, the most popular methods of synthesize hyperbranched polysiloxanes are hydrosilylation and hydrolysis polycondensation. However, the former needs expensive catalyst, and the later suffer from poor control of the hydrolysis process.
In 2015, our group developed a new way to synthesis hyperbranched polysiloxanes, that is transesterification polycondensation reaction.[26] Using this method, various hyperbranched polysiloxanes (abbreviated as HBPSi) that contains Si-O-C chain segment and employing Si as branching point were synthesized from various siloxanes, dihydric alcohols or dicarboxylic acids. Different from traditional polysiloxanes consisted of Si-O-Si chain segment, the bond angle of Si-O-C chain segment is 120 º, which is less than conventional Si-O-Si segment (130 º) and greater than C-O-C (110 º) and C-C-C (109 º) segments, thus renders HBPSis good flexibility and rigidity simultaneously. The good flexibility of organic chains contributes to the aggregation of HBPSis, and the rigidity of inorganic chains suppresses the rotation of chain segment, leading to AIE-active fluorescence. Due to the unique bond angle of Si-O-C chain segment, HBPSis exhibit extraordinary photophysical features. For example, most HBPSis can emit bright monochromatic or even multicolour fluorescence with high purity, due to the presence of abundant heteroatom-bearing groups and N → Si coordination bonds.
The unique silicon atom endows polysiloxanes outstanding photophysical properties. Thus, the understanding of polysiloxanes, especially the role of Si atoms played in these luminophores, would benefit in unveiling the emission mechanism of nonconventional luminophores and expanding their applications. In this review, we summarize recent progress in luminescent polysiloxanes with different topologies, including hyperbranched polysiloxanes, linear polysiloxanes, POSS-based polymers, etc (Scheme 1), with a focus on the driving force for the aggregation and the mechanism of tuned emissions.
1. Aggregation of polysiloxanes
1.1 Photophysical properties of hyperbranched polysiloxanes
The observation of fluorescence from HBPSi can date back to 2016, when our group synthesized aliphatic tertiary amine-containing hyperbranched polysiloxanes (TAHPSis) through the polycondensation reaction of tetraethoxysilane, triethanolamine, N-methyldiethanolamine or diethylene glycol.[26,33] Contrary to the generally believed oxidation mechanism, TAHPSis can emit bright blue fluorescence under 365 nm UV light without further oxidation or acidification, and their fluorescence intensities were enhanced with increasing concentrations, showing typical AIE characteristics (Figure 1A). Investigations of TAHPSis reveal that the aggregation of hydroxyl groups is responsible for their luminescence.
The results of TAHPSis revealed that oxidation of aliphatic amines may not be the intrinsic interpretation of luminescence mechanism, and we infer that the PL behavior of HBPSi strongly depends on the aggregation of functional groups. With the appearance of terminal hydroxyl groups, strong inter/intramolecular hydrogen bonds are formed, leading to aggregated electron-rich clusters. These aggregated electron-rich clusters could serve as the luminescent center and generate strong PL under ambient conditions. To further verify our hypothesis, HBPSis with terminal hydroxyl, amine and epoxide groups were synthesized and their PL behaviors were systematically studied (Figure 1B).[34,35] We found that HBPSis with terminal hydroxyl groups are highly fluorescent, while almost no fluorescence is observed when the hydroxyl groups are blocked byt -butyl acetoacetate. The connection with t -butyl acetoacetate disturbed the hydrogen bonding of hydroxyl groups, leading to limited aggregation of HBPSi and the formation of luminescent center. Epoxide group could also promote the aggregation of HBPSi and enhances their luminescence. It works with hydroxyl group synergistically in HBPSi and make a quantum yield (QY) of 4.61%.[35]
Another critical factor that influences the molecular aggregation is the steric hindrance. To clarify the effect of steric hindrance, we synthesized two HBPSis with similar structures. With the same terminal hydroxyl and amine groups,[34,36] the fluorescence intensity, lifetime and QY of HBPSi carrying neopentyl glycol moiety is higher than those of another HBPSi containing 2-methyl-1,3-propanediol moiety. This could be attributed to the larger steric hindrance of neopentyl glycol. On the one hand, its larger steric hindrance would restrict the aggregation of HBPSi, going against the formation of luminescent center. On the other hand, the larger steric hindrance may restrict the molecular motion, favoring the fluorescence. With the hydrogen bonds promoted aggregation of terminal hydroxyl and amine groups, the larger steric hindrance of neopentyl glycol benefits more from the restricted molecular motion than the restricted aggregation, leading to enhanced PL properties.
With the empty 3d orbital of Si atoms, coordination bonds can be formed between electron-rich atoms and Si atoms. In 2015, Feng et al. reported the N → Si coordination bonds in Si-assisted polymers.[37] Similarly, the N → Si coordination bonds can also promote the aggregation of HBPSi. XPS revealed the existence of N → Si bonds in HBPSis, and density functional theory (DFT) further conformed the molecular aggregation enhanced by N → Si coordination bonds.[38] Besides N atoms, electron-rich atoms, such as S and O atoms, can form coordination bonds with empty 3d orbitals of silicon atoms, generating d-d orbital splitting. The electron in split d orbitals can also absorb UV energy and generate radiative decay, which is beneficial to the fluorescence of HBPSis.
Theoretical calculations reveal that the bond angle of Si-O-C is about 120 º, which is between Si-O-Si (130 º) and C-O-C (110 º) (Figure 1C). As a consequence, the Si-O-C chain segment integrates the good flexibility of organic chains and the rigidity of inorganic chains, which further promotes the aggregation of molecules. Our group reported the long carbon chain integrated HBPSi which used 1,6-hexanediol as the reactant.[39] As shown in Figure 1D, the carbon chain in 1,6-hexanediol is more flexible than previously reported dihydric alcohols and it facilitates the formation of compact aggregates together with the Si-O-C chain segment. DFT results confirm that more hydrogen bonds and intra/intermolecular O···O and O···N interactions contribute to the overlap of electron clouds of electron-rich atoms, thus leading to the generation of through-space conjugation (TSC). With the satisfying flexibility and rigidity generated by Si-O-C chain segment and the long carbon chain in 1,6-hexanediol, the QY of the synthesized HBPSi reached 17.88%.
The above results reveal that the electron-rich atoms could accelerate the aggregation of HBPSis, leading to unique AIE emissions. DFT predictions suggest that not only hydrogen bonds, the interaction of O and N atoms and N → Si coordination bonds are also be found in the HBPSi aggregates. These inter/intramolecular interactions drive the aggregation of electron-rich atoms, making them clustered in close proximity. The electron clouds of these electron-rich atoms overlap with each other, generating “clustered chromophores” via TSC. With enriched energy levels, the energy gaps between HOMO and LUMO orbitals are reduced, thus benefiting excitation. The abundant hydrogen bonds, O···O and O···N interactions, and N → Si coordination bonds are also conducive to rigidify the molecular conformation, thereby favoring radiative decay. Guided by this idea, more attentions were given to the exploration of structure-promoted aggregation at the molecular level, and multicolor HBPSis were synthesized afterward.
1.1.1 Hyperbranched polysiloxanes with local conjugations
The absence of aromatic structures generated unique advantages of HBPSis, but also results in relatively low QY and short lifetime, and the emission bands are mainly centered in the blue region. Inspired by the classic luminescence theories, carbonyl groups were brought in to fabricate local conjugated HBPSis. By adjusting the aggregation and relative positions of carbonyl groups, the QY of HBPSis can be enhanced to 47.8% and multicolor fluorescence was observed.[40-42]
One simple way to bring in carbonyl groups is to replace the dihydric alcohols with dicarboxylic acids. Together with the Si-O bond that exhibit partial double bond features, local conjugations were formed in HBPSi containing conjugated O=C-O-Si-C=C segment (marked as P1).[43] In comparison, HBPSis contain O=C-O-Si-O-C=O segment (P2) and C-C-O-Si-C=C segment (P3) as reference. The three HBPSis all exhibit strong AIE characteristics, with a QY of up to 43.9% for P1. The largest conjugated segment O=C-O-Si-C=C renders P1 the highest QY among reported non-conjugated fluorescent polymers then. The optimized structural parameters and topology results suggest that P1 molecules are aggregated with TSC due to the strong intermolecular H···O interactions, O → Si coordination bonds and overlapping between carbonyl and C=C groups. Thus, as depicted in Figure 2A, the energy levels are enriched and energy gaps are lowered with the increasing of molecular numbers, favoring the excitation of HBPSis. In the meantime, the strong H···O interactions and abundant O → Si coordination bonds also help to rigidify the molecular conformation, thereby promoting the radiative decay. DFT prediction also revealed that the C=O, C=C, and Si-O groups promote the formation of supramolecular TSC in P1, and coplanar TSC is generated when these groups are located in local conjugated positions (Figure 2B and 2C). The above factors lead to the highest QY of P1 jointly.[43]
Recently, disulfide was integrated with O=C-O-Si-C=C segment to further improve the QY of HBPSi. The electron-rich S atoms favor the formation of space electronic communication and the dipole moment of the disulfide bond intensifies the distortion of the chain segment, which contribute to the molecular aggregation and restricted non-radiative decay (Figure 2D). With the increase in disulfide content, compact aggregates are formed and a QY of 47.8% is achieved.[40]
Another way to bring in carbonyl groups is to employ carbonyl containing silanes. The methacryloxypropyltriethoxysilane and 1,3-propanediol were used to prepare a new HBPSi that contains conjugated C=C-C=O segment.[41] In comparison, a linear structured polysiloxane was synthesized. Experiments suggest that the local conjugated HBPSi has a higher QY than the linear polysiloxane, which are 7.71% and 1.12% respectively. Under varying excitations, the HBPSi can emit distinct colors (Figure 2F). However, the reference linear polysiloxane can only emit weak blur fluorescence. DFT calculations reveal that HBPSis possess more compact aggregation and stronger oscillator strength than the linear polysiloxane due to the large number of terminal hydroxyl groups. As shown in Figure 2E, the multiring TSC is found in the ground state geometry of HBPSi, which leads to different electron delocalization systems in the supramolecular aggregates. Under different excitation wavelengths, the long-wavelength emission may be generated by the big TSC rings consist of the HBPSi main chain, and short-wavelength emission may be caused by the small rings that exist in their side chains. This conjecture was concluded as multiring-induced multicolour emission (MIE), and further supported by other non-conjugated AIE polymers. Conversely, it is more difficult for the linear polysiloxane to form strong multiring TSC, thus render it relatively low QY and weak blue emission.[41]