Properties of phenylazomethine dendrimers and their derivatives

Generally, dendrimers have a tree-like structure comprising core and dendron parts. The core is positioned at the center of the dendritic structure, and the dendrons are connected to the core in a branching shape. Therefore, the size, molecular weight, and shape of a dendrimer change depending on the number of dendrons on the core. The size also depends on the number of branching repetitions, referred to as the generation. A wide range of dendrimers has been investigated, including major types of polyamideamines (PAMAMs) [1,2,3,4] and polypropyleneimines [5,6,7].

Dendritic polyphenylazomethines (DPAs) [8, 9], which are generally called phenylazomethine dendrimers, have been developed as template dendrimers with a π-conjugated skeleton derived from imine and benzene moieties. DPAs have various features, including a semirigid side-chain structure and dendrons with metal coordination sites (imine sites) and π-conjugated structures. These features have been exploited for various functions of dendrimers, accompanied by variations in DPAs. The number of DPA variants has been expanded by changing the core and the number of generations (Fig. 1) [10]. Cores designed with two or four bonding sites have been reported. In particular, the fourth-generation DPA with a tetraphenylmethane (TPM) core, TPM-DPAG4, is often used in the synthesis of metal clusters. In addition, PyDPAG4, which has a TPM-DPAG4 skeleton with a pyridine site in the core, has also been reported [11]. By changing the numbers of coordinating parts of DPAs prepared for coordination with metal salts, the atomicity in the cluster can be controlled. PyDPAG4 is an example that enables the precise accumulation of 13 atoms to achieve the solution-phase synthesis of superatoms [12]. The phenylazomethine dendrimer with a zinc tetraphenylporphyrin core (Zntpp-DPAG4) has the inner space which is larger than that of TPM-DPAG4 or PyDPAG4 due to the expansion of the core part. In addition, the coordination feature of the central porphyrin part enables it to function as a shape recognition sensor [13]. The details are provided in the paragraph below. In addition to the core part, reports on the modification of the peripheral part with -NH2, -OH, and other moieties have also been published [14, 15]. This modification is useful for controlling the solubility and surface charges of dendrimers. The branched phenylazomethine dendron is also introduced into linear polymers [16]. Moreover, various functional groups, such as ferrocene, are also linked at the periphery of the dendrimers. These ferrocene groups are reported to modulate the charge of the redox potential by assembling metal salts on the dendrons [14]. Combination with carbazole parts induces a high Hall transport ability [17]. In this review, we focus on the functionalities of DPAs for photoluminescence, shape-recognition sensors, and atomicity-controlled subnanoreactors, along with metal integration and superatom synthesis.

Fig. 1
figure 1

Typical examples of DPAs and their functionalities. Characteristic parts are marked in red

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Photoluminescence properties of DPAs

When considering the application of DPAs for light-emitting materials, their relatively rigid (semirigid) shape, which enables easy film formation, is advantageous. Unlike soft dendrimers such as PAMAMs, the spherical structure of DPAs is maintained even on the substrate; therefore, the closest packing on a substrate is expected [8]. As an electronic property, a unique Hall transport function of DPAs has already been reported [18, 19]. In DPA systems, the Hall transport ability can be tuned by metal assemblies. Their characteristics for application to electroluminescence devices are improved by the formation of metal-salt complexes. Because of these features, a carbazole moiety has been introduced into DPAs to improve their Hall transport and photoreceptor activity. As a result, a dendrimer consisting of an inner phenylazomethine skeleton encapsulated by an outer carbazole skeleton exhibits a particularly high Hall transport ability (Fig. 1, DPA-carbazole hybrid dendrimer) [17]. Furthermore, a dendrimer consisting of the carbazole skeleton developed thermally activated delayed fluorescence (TADF) by placing the triazole ring on the core [20]. TADF materials are expected to be used as organic electroluminescence devices because they convert electrical energy into light energy with high efficiency. However, conventional TADF materials have been produced from small organic molecules, and device creation has been generally achieved using chemical evaporation. The advantage of the TADF material with dendrimers is devices can be created using the coating method. The production of TADF devices in the solution process is expected with the use of dendrimers [21,22,23,24,25,26].

The integration of photoluminescent units into DPAs has also been reported [27, 28]. The luminescence originates from bismuth complexes assembled in the DPAs. UV–vis absorption spectra recorded during the assembly of BiCl3 on DPAG4 show increased absorption caused by the bismuth complex of azomethines, confirming the occurrence of a complexation reaction (Fig. 2a). In addition, a four-step shift of the isosbestic point was observed during this spectral change (Fig. 2b). The equivalents of the bismuth salts showing isosbestic points are consistent with the number of imines in each generation of DPAG4. This phenomenon has also been observed in the assembly of other metal salts, as reported previously. Based on this result, BiCl3 was assembled stepwise from the inner layer of DPAG4. In the case of BiCl3, photoluminescence accompanied complexation, unlike other metals (Fig. 2c). With increasing amounts of assembled BiCl3, the emission intensity increased while the quantum yield decreased, which was considered due to the relatively weak coordination strength of imines at the outer parts of DPAG4 [29]. Two components were observed in the lifetime of 60BiCl3–DPAG4, indicating the contribution of phosphorescence.

Fig. 2
figure 2

Integration of photoluminescent units into DPAG4. a Stepwise assembly of BiCl3 in DPAG4. b Shift of the isosbestic point for each step. c Photoluminescence spectra of the bismuth-assembled DPAG4. The inset shows photographs captured under ambient light or upon irradiation. d Solid-state emission of the BiCl3-assembled DPAG4. e On/off optical switching using complexation

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The bismuth-assembled dendrimer exhibited solid-state emission and switching behavior (Fig. 2d and e). These properties are superior to those of common phosphors and a model mononuclear complex with N-(diphenylmethylene)aniline because of concentration quenching or quenching in the solid state. In contrast, solid-state luminescence was observed for the bismuth-assembled dendrimer due to the shell effect of the dendrimer. Thus, the dendrimer skeleton contained a high concentration of photoluminous elements without significant quenching. Optical switching of a DPA phosphor was reported via reversibility of the complexation. Acid–base or redox reactions led to reversible switching of the absorption and photoluminescence of bismuth-assembled DPAG4.

Shape-recognition ability of DPAs

Dendrimers are widely used as sensors for various chemical substances [30, 31] and as biosensors [32]. In particular, the introduction of redox-active groups on the dendrimer skeleton is a useful approach. Astruc et al. investigated and developed dendrimers with ferrocene units and used them to effectively detect anions [33,34,35].

In the case of DPAs, further functions are expected. As mentioned above, photoluminescence resulting from the assembly of bismuth salts has strong potential for use in advanced sensors. However, the shape-recognition ability of DPA sensors has attracted greater attention. This feature is based on the semirigid side chain of the DPAs. The chain skeleton of the DPAs is more rigid than that of PAMAMs. Therefore, the shell effect is not as strong as that of PAMAMs; however, the nanosized spaces in DPAs provide shape selectivity for approaching molecules. For example, selective coordination of Y-shaped molecules was reported for Zntpp-DPAG4 (Fig. 1), which was possible because of the unique three-dimensional structure of the cavity (Fig. 3) [13]. Therefore, substrate-selective catalytic reactions are expected to result from the combination of this selectivity with a metal-cluster catalyst synthesized in DPAs.

Fig. 3
figure 3

A space-filling model of Zntpp-DPAG4 and a cross-sectional image with a Y-shaped guest molecule. The guest molecule coordinates to the porphyrin core through interactions with the dendrons. Reprinted with permission from ref. [13]. Copyright 2013, Nature Publishing Group

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Finely controlled metal assembly in DPAs

The interior space in dendrimers is often used as a nanoreactor; however, the interior space in DPAs differs substantially from that in common dendrimers. This difference stems from the π-conjugated side chain of the DPAs, creating a gradient of electron potentials in the dendrimer skeleton to coordinate metal salts in a stepwise manner from the inner imines [9, 10, 36]. Because this stepwise complex formation defines the number of accumulated metal atoms, the number of atoms in subnanometer particles synthesized in the DPA reactors can be controlled with single-atom precision. Taking advantage of this feature, researchers have prepared various subnanoparticles with a defined number of atoms, including platinum and titanium oxide atoms [37,38,39,40,41,42,43,44,45,46]. As DPAs, TPM-DPAG4 and PyDPAG4 (Fig. 1) are often utilized as templates. TPM-DPAG4 enabled stepwise assembly of 4, 8, 16 and 32 units according to the imines on the layers. In the case of PyDPAG4, stepwise assembly of 1, 1, 3, 2, 6, and 4 units from the inner sites was reported due to the asymmetric structure (Fig. 4). This control of the number of atoms provides finely controlled synthesis of subnanoparticles. Platinum subnanoparticles with different numbers of constituent atoms were shown to exhibit different catalytic activities. For example, a Pt12 cluster exhibits twice the catalytic activity of a Pt13 cluster [46]. Based on these results, a difference of only one atom substantially affects the physical properties of subnanoparticles. Elements that have the potential to be assembled in DPAs are shown in Fig. 5 [10, 43, 47,48,49]. This figure also shows metals that are adaptable for reducing the capsule method described below. Highly toxic or unstable elements were excluded from the study. They produce a wide variation of subnanoparticles. The stepwise assembly of metals in DPAs is also utilized for multimetallic assembly of up to eight elements on the template. As mentioned above, PyTPMG4 possesses the ability of multistep complexation because its core consists of a pyridine part (Fig. 4). This feature is based on the difference in basicity of each imine site. Therefore, it can be used for the precise blending of multiple types of metal salts. For example, 1, 1, 3, 2, and 6 equivalents of Ga, In, Au, Bi, and Sn salts, respectively, were precisely assembled in PyDPAG4. The assembly order was based on the complexation constant. Metals with stronger coordinating features are positioned at the inner side of DPAs. This feature was revealed by the shift of the isosbestic point during the process of UV–vis titration. Multimetallic subnanoparticles are prepared through the reduction of the assembled multiple metals in the dendrimer (Fig. 6). If multiple types of metal salts are reduced in a solution without the DPA template, only phase-separated metal particles will be obtained, and the homogeneous metal blending shown in Fig. 6b does not occur. High catalytic activities of multimetallic subnanoparticles have also been reported [49,50,51,52].

Fig. 4
figure 4

Atomicity-controlled stepwise assembly of metal units using TPM-DPAG4 or PyDPAG4

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Fig. 5
figure 5

List of elements that are assembled in DPAs (blue) and adaptable for the reducing capsule method using DPAs (green)

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Fig. 6
figure 6

Finely controlled synthesis of multimetallic subnanoparticles. a Stepwise process for multimetallic assembly. b STEM observation and EDS maps for Ga, In, Au, Bi, and Sn in a Ga1In1Au3Bi2Sn6 subnanoparticle. Reprinted with permission from ref. [49]. Copyright 2018, Nature Publishing Group

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Utilizing the precise integration ability, researchers have prepared a reducing capsule for the synthesis of subnanoparticles. This approach is possible because of the precise assembly of BH3 units. DPAG4 with four equivalents of BH3 was prepared as a reducing capsule that contains a specific number of reducing agents in the nanosized space inside the dendrimer [53]. This dendrimer is reduced by multiple electrons, and this reducing capsule has been used to synthesize ultrasmall metal clusters. The procedure is simple: the reducing capsule is added to a metal solution, resulting in the formation of ultrasmall metal clusters with a uniform size. Because the number of reducing electrons is defined, this reducing capsule regulates the number of metal ions that are reduced to form clusters inside the capsule.

The advantage of the reducing capsule method is its versatility. Only the reduction potential of the metal must be considered, and various elements, including Ag, Pt, and Au, have been used [53]. Measurements of the particles revealed that their sizes differ, which is a consequence of the different valences of the added metal cations. In the case of monovalent Ag+ cations, Ag24 particles are theoretically generated. However, when Pt4+ is used in the reducing capsule method, Pt6 particles are expected.

Early methods (metallodendrimer methods) for synthesizing small metal particles using dendrimer templates involved metal assembly as the first step, and this process controls the sizes of the particles. Therefore, suitable conditions for metal salts precisely assembled in dendrimers must be determined. In contrast, the reducing capsule method is based on controlling the number of reducing electrons, not the number of metal atoms (Fig. 7). Because this method does not require the regulation of the integration process of metals, the types of metals used to produce finely controlled ultrasmall particles can be drastically increased.

Fig. 7
figure 7

Synthesis of subnanoparticles using DPAs. a Metallodendrimer and b reducing capsule methods

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Superatom synthesis using DPAs

As a class of subnanosized particles, superatoms have attracted attention in recent years. A superatom, which is composed of several atoms arranged in a highly symmetric structure, exhibits atom-like properties. Theoretical research on superatoms was published in the 1990s [54], and they have been widely synthesized as gas-phase clusters [55,56,57,58,59]. However, for superatoms to be used as materials, they must be prepared as solids. One approach to the preparation of solid superatoms is soft landing of gas-phase superatoms. However, further research is necessary to prepare sufficient amounts of superatoms without decomposition. Therefore, researchers began developing methods for the solution-phase synthesis of superatoms.

Superatoms have subsequently been synthesized using DPAs with control of the atomicity. The first superatom reported to be prepared using a dendrimer template was Al13. Al13 clusters are among the most widely known examples of superatoms; they are composed of 13 Al atoms arranged in an icosahedral structure [56]. Al13 exhibits halogen-like electronic properties and reactivity (Fig. 8a). These features have been confirmed in gas-phase experiments. Its monovalent anion (Al13−) tends to be more stable than clusters with other numbers of Al atoms. Recently, the solution-phase synthesis of superatoms by preparing 13-atom Al clusters using DPAs was reported [12].

Fig. 8
figure 8

Preparation of a 13-atom aluminum cluster. a Superatomic orbitals of Al13−. b Scheme for the preparation of 13 aluminum atomic superatoms inside PyDPAG4

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PyDPAG4, which is composed of a DPAG4 skeleton with a pyridine site in the core, has been used for the synthesis of Al13 clusters. The change in the UV–vis absorption is due to complex formation, and a shift in the isosbestic point is observed, reflecting the difference in basicity of imines on each layer of PyDPAG4. This spectral behavior, like that of other metals, indicates stepwise assembly from the inner imine sites. In particular, the isosbestic point was clearly different before and after the assembly of 13 equivalents, indicating the successful assembly of 13 molecules of AlCl3 inside PyDPAG4. Therefore, the reduction led to the generation of superatoms consisting of 13 Al atoms (Fig. 8b). The electronic states of the synthesized 13-atom aluminum clusters were characterized using X-ray photoelectron spectroscopy. The binding energy in the spectrum of the 13-atom clusters was shifted to lower energies compared with those in the spectra of aluminum clusters synthesized from precursors with other numbers of aluminum complexes. This shift suggests the formation of anion clusters, and stability mimicking that of halogen anions has been experimentally documented via an oxidation reaction [12].

Conclusions

This article focuses on the development of functionalities of DPA dendrimers. DPAs, which have a semirigid skeleton and the ability to precisely accumulate metal salts, will be employed in a remarkable number of applications as luminescent materials, shape-recognition sensors, and subnanosize precise reactors for atomicity-controlled multimetallic particles and superatoms. As a subnanosized reactor, metals can be hybridized on a subnanometer scale even if metals are phase-separated in the bulk state. Therefore, this approach applies to the fabrication of novel alloy materials and unique catalysts. In addition, superatoms synthesized in a solution phase are expected to become new building blocks for materials. Among other applications of DPAs, accurate alignment on the substrate is an attractive goal. In addition, the formation of the connecting structure of DPAs using a cross-linking molecule has been investigated [60]. Based on the linked dendrimers, controllable alignment of functional subnanoparticles or dendrimer-based materials will be possible, which are expected to serve as high-density devices. Because of these advanced features, DPAs are expected to contribute substantially to various fundamental research fields, including nanoscience and metal clusters, and will also be used in next-generation multifunctional materials.