Abstract
Amphiphilic ionic perylenediimides (AIPDIs) with well‐defined structures have been widely studied, which involve abundant non‐covalent interactions. Among these interactions, electrostatic interactions serve as the primary force that may be followed by other non‐covalent interactions like π–π stacking. Taking advantage of these tunable interactions between simple AIPDI‐building blocks, AIPDIs are widely used for constructing increasingly complex structures at varying scales. Besides, AIPDIs with outstanding photochemical stability exhibit high fluorescence quantum yields (FQYs) in aqueous solution, because hydrophilic substituents of AIPDIs can shield the inner perylene chromophores and weaken π–π stacking, contributing to the improvement of water solubility and the suppression of aggregation‐caused quenching (ACQ). AIPDIs with excellent water solubility, strong FQYs and desired interactions with charged components in cells and tissues hold great promise for various biomedical applications, which can be concluded in three hierarchical levels, which is in vitro, live cell and tissue.
Keywords
- amphiphilic ionic perylenediimides
- fluorescence
- self‐assembly
- biomedical applications
1. Introduction
Amphiphilic ionic self‐assembly (AIS) is an important method to construct complex nanostructures by using non‐covalent interactions of simple ionic molecular building blocks. Because of the well‐defined structures, amphiphilic ionic molecules (AIMs) are studied widely in the field of AIS. Various nanostructures are obtained by the self‐assembly of AIMs and the relationship between the morphologies and parameters was discovered by theoretical studies [1–8]. AIMs are designed and fabricated to form various nanostructures such as helical fibres, nanotubes, nanospheres and so on [9–12]. Among these works, Zhang and Eisenberg studied the effects of hydrophobic/hydrophilic ratio, chemical structure, temperature, concentration and solvent on the self‐assembly behaviours of AIMs [13]. AIMs self‐assemblies endow AIMs with enhanced stabilities, improved water solubility and increased fluorescence intensity, making them promising candidates in biomedical applications [10, 14–21]. As an attractive chromophore, perylene‐3,4,9,10‐tetracarboxylic acid diimides (PDIs) have been generally used in organic electronic and photovoltaic fields for their broad absorption range, high extinction coefficients, high fluorescence quantum yields (FQYs) as well as outstanding stability. In particular, amphiphilic ionic perylenediimides (AIPDIs) are found to form tunable supramolecular‐ordering constructions by a well‐known non‐covalent force, that is, electrostatic interaction, which is operative in AIS. This electrophorus supramolecular process was firstly introduced by Faul and Antonietti [22], who found water‐soluble molecular building blocks self‐assembled with oppositely charged surfactants by electrostatic interactions as the primary force. This self‐assembly approach has been successfully applied to AIPDIs for their hierarchical organization in aqueous solution and bulk solid state as well. AIPDIs exhibit good water solubility, strong fluorescence and desired interactions with charged components in cells and tissues. Moreover, AIPDIs emit fluorescence above 500 nm, which effectively avoid the interference of the autofluorescence from organism. All the advantages of AIPDIs make them excellent candidates for biomedical materials. In this chapter, we describe self‐assembly behaviours of AIPDIs and focus on their chemical structures, self‐assembly studies and biological applications.
2. Structures and self‐assemblies of AIPDIs
In general, AIPDIs are synthesized by the modification with ionic substituents in the bay region, ortho or imide positions of PDIs (Figure 1A). The modifications in the imide positions of PDIs with ionic substituents will improve solubility and minimally affect the optical properties. By contrast, substituents in the bay region will lead to a twist of perylene core and changes in optical properties such as significant bathochromic shift. These modification strategies of PDIs in the imide position and bay region have attracted great interests in self‐assembly behaviours because various intermolecular forces are introduced in this way. Besides, it will affect electronic and optical properties but not contort the two naphthalenes when selectively functionalized in the ortho positions of PDIs.
The fluorescence intensity and water solubility of modified PDIs are significantly improved compared with the unsubstituted PDIs [23]. Typically, the charged substituents can serve as the shell of PDI core, resulting in core‐shell structure of AIPDIs (Figure 1B). In addition, the ionic substituents could provide electrostatic forces between the PDI molecules themselves or other charged guests. Intermolecular π‐π stacking of PDIs could be attenuated by steric hindrance and electrostatic repulsion forces, leading to good water solubility and strong fluorescence of dye molecules.
2.1. Self‐assembly of anionic AIPDIs
The self‐assembly of AIPDI without surfactants was first studied by Ford in the 1980s [24]. In this fundamental research, concentration‐dependent UV/vis absorption and fluorescence spectroscopy were studied. It showed that the anionic di(glycyl) PDI derivative
Malik and co‐workers reported a pair of chiral AIPDIs
2.2. Self‐assembly of cationic AIPDIs
The synthesis and optical properties of AIPDIs
Yao and co‐workers [35] synthesized a PID derivative with pyridyl in the bay region. They found that the self‐assembly of amphiphilic AIPDI
2.3. Self‐assembly of AIPDIs in charged surface
The attachment of charged dendritic substituents at the imide positions or in the bay regions of PDI can effectively suppress the aggregation of perylene chromophores and provide hydrophilia. The ionic nature of these dendrimers can self‐assemble on the charged surface of gold or alumina template. Yin, Müllen et al. firstly reported a series of core‐shell structures, by using shape‐persistent polyphenylene dendrimer as a scaffold and a stabilizer. The outer flexible polymer shells contributed to proton‐conducting property [36, 37]. Thus, a core‐shell architecture
Recently, Yin, Müllen et al. reported a series of fluorescent core‐shell AIPDI macromolecules based on polyphenylene dendrimers [39–41]. In order to introduce ionic functionalities such as amine and carboxylic acid, flexible polymer shell was attached to the shape‐persistent cores. As representatives, core‐shell AIPDIs
Templated self‐assembly of the oppositely charged polymers was also investigated. LbL deposition of oppositely charged
3. Biological applications of AIPDIs
Charged components are widely existed in organism, such as the negatively charged extracellular matrixes (ECMs), membranes, DNA and positively charged histone proteins. Therefore, water‐soluble AIPDIs with charges on the surface would interact with the charged biological components through electrostatic forces. Moreover, AIPDIs have good water solubility and strong fluorescence, the fluorescence and absorption maxima of AIPDIs are above 500 nm, which could minimize the background noise from organism auto‐fluorescence. All the advantages make them excellent candidates for biomedical materials. The biological applications of water‐soluble AIPDIs are summarized below including imaging studies of living cells and tissues.
3.1. In vitro studies
Because of the fluorescence nature, the cationic AIPDIs were widely studied in the field of molecular detection in organism. As mentioned above, interactions between AIPDI and biological molecules are the most important part for researches in cells and tissues. Yin et al. have reported a type of fluorescent nanotubes fabricated by oppositely charged
Based on the interaction between probe
3.2. Live cell studies
With good selectivity and sensitivity, fluorescence bioimaging offers microscopic visualization of the organism. Thus, water‐soluble AIPDIs with high fluorescence intensity have attracted great attention in bioimaging.
Yin et al. reported that dendritic AIPDIs
3.3. Tissue imaging
Fluorescence labelling, the process of attaching a reactive probe to a functional group of target molecule with covalent or non‐covalent interactions, is usually used for tissue imaging. Generally, the target structure, such as antibodies, proteins, amino acids and peptides, is charged. Thus, based on the electrostatic interactions with those charged electrolytes, AIPDIs have unique advantages in tissue imaging.
The star polymer
By contrast, the positively charged star polymer
4. Conclusions and perspectives
This chapter gives a brief summary of the structures, self‐assembly and biology applications of AIPDIs. AIPDIs have emerged as promising candidates for AIS and used in bio‐applications. The synthetic strategies of AIPDIs are composed of the introduction of ionic substituents in the bay region, ortho or imide positions of PDIs. AIPDIs were found to form supramolecular‐ordering construction by electrostatic interaction and other synergistic non‐covalent interactions. By adjusting self‐assembly conditions, AIPDIs have been successfully applied to form hierarchical organization in aqueous solution and bulk solid state. The biological applications involve studies
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (21574009 and 51521062), Beijing Collaborative Innovative Research Center for Cardiovascular Diseases, and the Higher Education and High‐quality and World‐Class Universities (PY201605).
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