The involvement of metal ions within the self-assembly spontaneously occurring in surfactant-based systems gives additional and interesting features. The electronic states of the metal, together with the bonds that can be established with the organic amphiphilic counterpart, are the factors triggering new photophysical properties. Moreover, the availability of stimuli-responsive supramolecular amphiphile assemblies, able to disassemble in a back-process, provides reversible switching particularly useful in novel approaches and applications giving rise to truly smart materials. In particular, small amphiphiles with an inner distribution, within their molecular architecture, of various polar and apolar functional groups, can give a wide variety of interactions and therefore enriched self-assemblies. If it is joined with the opportune presence and localization of noble metals, whose chemical and photophysical properties are undiscussed, then very interesting materials can be obtained. In this minireview, the basic concepts on self-assembly of small amphiphilic molecules with noble metals are shown with particular reference to the photophysical properties aiming at furnishing to the reader a panoramic view of these exciting problematics. In this respect, the following will be shown: (i) the principles of self-assembly of amphiphiles that involve noble metals, (ii) examples of amphiphiles and amphiphile-noble metal systems as representatives of systems with enhanced photophysical properties, and (iii) final comments and perspectives with some examples of modern applications.
Modern technology applications have ever-increasing demand of hybrid nanomaterials based on metal complexes not only because of their small size but also thanks to the presence of exotic properties arising from the combination of molecular properties and novel and/or synergistic emergent ones. Electronic states and optical properties, indeed, are the consequence of the various interactions involved and of the specific self-assembly, which can be sensitive to various external stimuli [
Self-assembly in amphiphiles is particularly interesting: they in fact simultaneously possess both polar and apolar moieties within their molecular architecture [
The presence of coordination bonds with metal atoms further enriches the scenario of self-assembly and the relative applications. In this ambit, noble metals are the best candidates since their chemical and optical properties are peculiar and well studied [
Here, we want to give a modern vision of the topic, introducing the simultaneous consideration of two elements:
A schematic and general vision based of the physics of complex systems. The possibility of a metal to bind molecular functional groups offers new possibilities in their intermolecular assembly. Usually, organic materials and inorganic ones are dealt with different disciplines (organic and inorganic chemistry, respectively) since they have different types of interactions. Despite facing different aspects, chemists are perfectly aware that two independently stable compounds can combine to form a new chemical compound with properties quite different from those of the constituents; this is usually seen as a “reaction” giving a change of the system. This vision is correct, of course, but we want here to present an alternative vision borrowing the concepts from the physics of complex systems: the different types of interactions are the perfect prerequisites for the rising of the so-called To make the concept of amphiphile more actual. Usually, amphiphiles are modelled as simultaneously having a polar and an apolar part within their molecular architecture. These
These new concepts are schematically depicted in Figure
Schematically representation of new amphiphile concepts.
In this ambit, small amphiphiles are versatile molecules since they have a reduced steric hindrance allowing a higher orientational freedom in their self-assembly thanks to their reduced size. In addition, if various polar and various apolar groups are concurrently present in different locations of the molecule, then a complex distribution of polar and apolar interactions is foreseen giving enriched scenario of self-assembly possibilities. At the same time, noble metals have unique photophysical and chemical properties. When the building blocks are amphiphiles based on luminescent metallorganic or coordination complexes having sensitive photophysical properties, advanced dynamic functional systems are obtained with striking possibilities of applications in major fields like biomedical, electrooptics, sensing, etc. [
While there is plenty of works focusing on the aspects of self-assembly, preparations of metal-amphiphile assemblies, and their applications in photophysics, the novel aspect of this contribution is to furnish to the reader a panoramic view of this exciting problematic in the fascinating framework/formalism of physics of complexity, simultaneously offering a concrete and updated picture of the state-of-the-art of the published works.
We wanted to prepare an easy-to-read critical summary interconnecting different disciplines like physics of complex systems (i.e., those systems where the overall properties cannot be described in terms of the properties of their constituents since emerging properties arise due to peculiar interactions, synergistic effects, and collective phenomena), coordination chemistry, and applications, with the final aim to tickle the reader’s imagination and to hopefully stimulate new ideas and research.
The term amphiphile is quite general, since it refers to any kind of molecule simultaneously possessing both a polar and an apolar part within its molecular architecture. Even the presence of a very small polar group, like –OH, in a much wider apolar structure, like in cholesterol, can confer amphiphilic properties. Indeed, cholesterol tunes membrane fluidity within the phospholipidic bilayer of cell membranes due to its amphiphilic properties.
Although there are many works covering all the specific aspects involved in the self-assembly of such molecules, a critical and multidisciplinary treatment, giving sight from the top of all the aspects, can be of benefit for researches approaching this topic. At the same time, specific references will be also shown as sources of further details.
It is obvious that amphiphilicity triggers the simultaneous presence of different types of intermolecular (noncovalent) interactions: H-bonds, van der Waals interactions,
However, even the presence of a reduced number of weak interactions of the order of the
In this ambit, comprehension and detailed treatment of the main forces acting in nanostructures (hydrogen bonding, hydrophobic effects, screened electrostatic interaction, steric repulsion, and van der Waals forces) are necessary [
The metal can be bonded by opportune polar functional groups of the amphiphiles which are able to form coordination bonds (like hydroxyl, amino, carboxylic, etc. groups). However, there are two basic phenomena at the bottom of the building up of amphiphile-metal assemblies:
The inclusion of metal-containing species (salts, coordination complexes, nanoparticles, etc.) within the compartmentalizing domain formed by the self-assembly of amphiphiles The direct interaction between the metal atoms and the amphiphilic molecules
Let us see these two contributions separately.
Here, it must be preliminarily shown the distribution of polar and apolar sites in amphiphile-based complex systems. The morphologies of the assembled structures can be various and depend on several parameters: composition, surfactant concentration, and type of amphiphiles as well as temperature and pressure. Figure
Some structures formed by the aggregation of amphiphiles. They can be direct (upper) or reversed (lower) depending on the polarity of the solvent.
Such structures can be found in two-component (amphiphile/solvent system) but also in three-component systems (polar/amphiphile/apolar) whose aggregation pattern can be described by triangular phase diagrams.
Generally speaking, the surfactant concentration is of utmost importance: at very low concentration, the entropic driving force makes molecules randomly dispersed, whereas at higher amphiphile concentration, their interactions gives an enthalpic contribution and make them assemble. Anisotropy of structures, bicontinuity of microphases, and liquid crystals are always the consequence of the balance of intermolecular interactions and steric interactions [
Nonionic amphiphiles [
In this scenario, there are different possibilities to find sites for metal binding, which usually are bound to the polar groups. So, the domains formed as a consequence of the polar/apolar nanosegregation can act as local, compartmentalizing nanoreactors, in few words exerting templating functions.
However, they must not be considered as rigid parts nor closed substructures: the weakness of the interactions involved makes them evanescent and characterized by a wide variety of dynamical processes (e.g., conformational change of monomers, lateral diffusion of monomers within the aggregate, and aggregate breaking/reforming/scission/shape fluctuation), each one with a characteristic time scale. These aspects are clearly discussed and commented on in an easy-to-read minireview by Calandra et al. [
The shape of such aggregated structures can be controlled considering the so-called
Changing molecular shape gives different
Typical structures obtained by the various critical packing factors (Cpp).
Cpp | Structure |
---|---|
<1/3 | Spherical |
1/3–1/2 | Cylindrical |
1/2–1 | Vesicles (spherical or ellipsoidal) |
1 | Lamellar |
>1 | Inverse micelles |
This gives the researcher the opportunity to prepare, by organic synthesis methods, ad hoc molecules having desired morphologies; synthetic or natural phospholipids which are generally made up of one hydrophilic head and two hydrophobic tails [
The inclusion of metal-based compounds may influence the phase diagram of the surfactants in water in either cooperative or destructive ways, depending on their nature. For example, transition metal complexes with an overall hydrophobic nature will be hosted in the hydrophobic core of the micelles, while water soluble complexes will be part of the corona shell or solvated in the water media. Accurate photophysical investigations in the case of luminescent complexes may represent a key tool to investigate the exact location of the complexes [
Direct amphiphile polar head-metal atom interaction can be quite strong, thus giving a marked contribution to the overall self-assembly. Usually, Lewis bases donate electron density to the metal centre which behaves as Lewis acid. According to the covalent bond classification proposed by Green and Parkin [ L-type ligands (neutral two-electron donors like in metal-ligand with dative interaction) X-type ligands (where one electron comes from the metal and the other from the ligand) Z-type ligands (if the metal is an electron density donor towards the ligand)
It is interesting to note that it can be regarded as a categorization based on the type of interactions rather than being a classification of the type of ligands. The type and strength of such interactions can depend on several factors: electrostatics and orbital overlap [
Complexes of transition metals form strong bonds with hard Lewis basic ligands like those having N- and O- donor atoms [
Energies of some representative bonds.
Bond | Energy (kJ/mol) |
---|---|
Cu-N bond in [CuCl2(NH3)2] | 90 |
Zn-O in [Zn4O(O2CC6H4CO2)3] | 180 |
H-bond | 10-100 |
C-C covalent bond | 350 |
This clearly indicates that the metal-ligand bond is quite strong. It is so strong that it can be considered as a driving force in soft matter self-assembly. The strength of a bond, indeed, is directly related to the energy that the system gains during its formation: the stronger the bond, the higher the amount of energy involved and, consequently, the stronger the tendency of the system to follow that specific reaction/change pathway. We can also suggest the following relation between bond valences (
The bonds with metallic species can be exploited for surface stabilization/functionalization in nanoparticle capping (see later), and their further ordering can give the so-called “metamaterials” with emerging/collective magnetic, optical, and electronic behaviours, of precious use in nanoelectronic and nanophotonic [
Having explored the aspects involved in amphiphile-metal organization, we now start showing the state-of-the-art of the amphiphile-noble metal complexes where the photophysical properties of the self-assembled structures are investigated.
Once we have shed light on the basic principles for amphiphile-metal bonding and assembly, we must now face the problem of how such assembly interacts to form the final structure. The idea of self-assembled structures of amphiphiles working as templates for metal hosting must somehow be completed since amphiphile self-assembly can be, in turn, dependent on the presence of the metal and vice versa. On the other hand, the amphiphile-metal interactions become in competition with the huge number of other interactions which, even if of lower intensity, can be prominent due to their high number of sites. This situation is typical in soft matter [
The hydrophobicity/hydrophilicity balance turns out to be not the unique driving force, since Isodesmic assembly, where the change in the free energy for the addition or subtraction of a molecule from an assembly is independent of the size of the assembly Cooperative assembly, where more than one molecule is involved in creating or destroying an assembly, so a specific number of molecules are necessary to form an assembly. In such a process, interactions between the molecules play a necessary role More complicated processes of assembly (quasi-isodesmic assembly, activation and growth assembly, nucleation and growth assembly, etc.), behaving as a combination of simple processes
For a review of such mechanisms, see Reference [
The mechanism of aggregation is hard to foresee due to the complexity of the systems. Generally speaking, upon gradual increase of hydrophilicity of the amphiphile backbone, a change from cooperative to isodesmic is observed. It is believed that cooperative self-assemblies occur only in specific hydrophobic/hydrophilic patterns in a delicate balance of all the other interactions like metal-metal,
In Figure
Scheme of the interactions involved in amphiphile-metal self-assembled structures.
This can be taken as a first hint that amphiphile self-assembly and the direct amphiphile-metal interactions are, as a matter of fact, interconnected. In the following paragraph, we will show some applications in support of this view.
The self-assembled structures of amphiphiles can also host molecule clusters [
Self-assembled nanostructures can also present interesting morphological [
The bonds between the surfactants and metal ions constitute interactions used in water purification field. Solvent extraction processes can be used for separation of rare earth elements in hydrometallurgy [
Usually, derivatives of phosphoric acid are used as follows: besides the commonly used bis(4-ethylcyclohexyl) phosphoric acid [
Luminescent micelles are attractive for imaging biological tissues; thus, in recent decades, efforts have been devoted for their obtainment, mainly by the encapsulation of emitters within the micelle cores resulting in luminescent micellar coassemblies [
Metal-based surfactants may directly form micellar structures in water, with unique supramolecular structures and sensitive properties easily tunable by external agents. The advantages over their organic counterparts are resulting from their enhanced photostability, long luminescent lifetimes that remove background interferences, and large Stokes shifts [
Finally, we would like to point out that there can be interesting uses also in catalysis [
The peculiar self-assembly of amphiphiles and their tendency to bind metal surely make these substances an interesting class of materials to be tailored for specific applications in catalysis.
Several strategies have been adopted in order to induce supramolecular assembling properties in water for
Schematic illustration of different types of amphiphilic complexes and the resulting hydrophilicity/hydrophobicity nature after functionalization.
Transition metal complexes based on d6 and d8 metal centres may exhibit emission from both ligand and metal centres, having excellent photostabilities, large Stokes shifts, and excellent emission efficiencies due to the high spin-orbit coupling constants induced by the presence of the heavy metal that permits populating the triplet excited states. Indeed, emission may occur from ligand transitions like intraligand (IL) or ligand-to-ligand charge transitions (LLCT), or metal transitions that involve metal-to-ligand charge transfer (MLCT) or ligand-to-metal charge transfer (LMCT) electronic transitions. Moreover, many complexes may have triplet-state phosphorescence also due to metal-metal interactions or ligand-ligand stacking, like metal-to-ligand-ligand charge transfer (MLLCT), metal-metal-to-ligand charge transfer (MMLCT), or ligand-to-metal-metal charge transfer (LMMCT).
Finally, the complexes have long-lived emission lifetimes due to the spin-forbidden nature of the relaxation from the triplet state to the ground state. The photophysical properties of transition metal complexes are considered lately to be a winning strategy for applications in biomedicine as therapeutic and/or diagnostic agents [
However, in the aggregate state, new excited states may be formed, different from those exhibited by the isolated molecule, yielding different decay pathways that may be (i) nonradiative—this phenomenon being defined as aggregation-caused quenching (ACQ) or radiative, the so-called aggregation-induced emission (AIE). These new excited states may be formed in the ground state or in the excited states, respectively, through excimer formation. Moreover, the aggregation may induce AIE by restricting the intramolecular motions (RIM), by steric hindrance or by hindering the access of oxygen, a well-known quencher of phosphorescent probes. Finally, the photophysical properties of transition metals are sensitive to the molecular environment; therefore, they can be used not only for sensing but also for proving the supramolecular assembling in dynamic systems like the examples presented further (
Several research groups obtained important results with Pt(II) complexes, exploiting the flat coordination geometry of the square planar metal centre that makes them more prone to be piled up into supramolecular assemblies. In certain conditions, Pt⋯Pt metallophilic interactions may be formed wherein one side contributes to the self-assembling, while on the other side, the close vicinity of the metallic centres induces the formation of new orbitals due to dz2 interactions leading to metal-metal-to-ligand charge transfer (MMLCT) states. Thus, luminescence switching between different emitting states due to the arrangement into supramolecular “soft” dynamic sheet-like, micellar, vesicular, rod-like, or fibrous structures triggered by external stimuli like concentration, temperature, and presence of nonsolvent may be obtained. As a function of substituents and, respectively, the hydrophobic/hydrophilic unit nature, the possible structures formed by amphiphilic surfactant-type Pt(II) coordination complexes in water are represented in Figure
Schematic representation of the possible supramolecular assemblies of amphiphilic coordination complexes in aqueous media.
The stronger the Pt⋯Pt interaction (shorter distances), the more bathochromically shifted is the MMLCT band, and this property can be used to investigate the formation of supramolecular soft nanostructures in solution, as demonstrated by several groups whose research is presented as follows. The Pt(II) complexes are divided as a function of the ligand type.
Chemical structure of the family of Pt(II) complexes based on (2,6-bis(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine) as tridentate ligand.
By playing with solvent composition and light, three different supramolecular assemblies were successfully entrapped and characterized as two kinetically metastable and a thermodynamically stable form. Flash injection of a dioxane solution of Pt_1 into water yielded a metastable kinetic state in which the molecules form soft vesicles with a hydrodynamic diameter
Further, the modulation of the number, charge, and nature of the hydrophilic group was achieved by grafting on the pyridinic coligand through an amide linkage a gallate unit substituted with several hydrophilic flexible groups (complexes Pt_2a-e in Figure
Further, the pyridine coligand was parasubstituted with an alkyl chain terminated with a charged pyridinium unit acting as the hydrophilic moiety, similar with complex Pt_2e (complexes Pt_3a-e in Figure
It is worth mentioning other analogue amphiphilic neutral Pt(II) complexes based on 2,6-bis(tetrazol-5-yl) pyridine and EG functionalized pyridine coligand used to build up supramolecular polymeric 1D and 2D nanostructures; however, no investigations in aqueous solutions were performed [
A structurally similar class of Pt(II) complexes based on 2,6-bis(1-propanesulfonate-1,2,3-triazol-4-yl)pyridine and alkynyl aromatic substituents was designed and characterized by Li et al. [
Chemical structure of amphiphilic Pt(II) complexes Pt_4a-c based on 2,6-bis(1-propanesulfonate-1,2,3-triazol-4-yl)pyridine and alkynyl aromatic substituents.
Chemical structures of the family of Pt(II) complexes based on 2,6-bis(-benzimidazol-2
The introduction of an aromatic unit into the alkynyl coligand changes the shape of the supramolecular aggregates in water into 2D structures, dictated by the presence and length of alkyl chains grafted [
The amphiphilicity of the Pt(II) complexes based on
Supramolecular amphiphiles were obtained with Pt(II) complexes based on
In the following, the two Pt(II)(
Chemical structures of dinuclear Pt(II) complexes Pt_9a-l.
In DMSO solution the complexes aggregate into nonemissive plate structures with hydrodynamic diameters of ca. 1000 nm and minor involvement of Pt⋯Pt interactions. The addition of water until 20% induces a change in the morphology of the supramolecular structures from plate to fibres with a reduced dimension to ca. 800 nm. The process is accompanied by a color change from yellow to orange associated to the switching on of Pt⋯Pt and
Amphiphilic Pt(II) complexes based on
Subsequently, a comprehensive study of the morphology and photophysical properties of the supramolecular assemblies formed in water by Pt(II) complexes by a successive functionalization of
Regarding complexes Pt_11d-g the functionalization of the
Chemical structures of the different units forming the supramolecular metallaclips Pt_12a and Pt_12b.
Both complexes formed homogenous solutions in water and organic solvents. Complex Pt_12a exhibited a bright yellow fluorescence in polar solvents and blue luminescence in less polar solvents. In water, complex Pt_12a showed emission in the blue region of the spectra with increased intensity due to the formation of sheet-like structures. Hence, tuning of the shape and emission properties of nanostructures was obtained in MeOH solution with the addition of increasing amount of water. At low water content (10%), 150 nm diameter nanoparticles were observed by SEM investigations that grew in diameter with increasing water content until 30%. Between 40 and 70%, the particles assembled into planes while sheet-like structures were totally formed at water content >70%. The sheet-like structure formation was accompanied by a drastic increase of the emission intensity and a blue-shift of the emission maxima.
Regarding complex Pt_12b, it was found that in water, it formed different shape aggregates as a function of concentration. By increasing the concentration, upon CAC which was found to be around
Although higher coordination number complexes have increased difficulty in ordering into soft supramolecular structures due to the bulky voluminous geometry, by a judicious functionalization remarkable self-assembling abilities were obtained also with octahedral Re(I), Ru(II), or Ir(III) species. The great efforts were compensated by the remarkable photophysical properties of these complexes in a biological relevant environment such as tunable emission color, intense fluorescence and emitting excited states generated by spin forbidden transitions. Importantly, the close-pack of these molecules into supramolecular assemblies may hinder the quenching phenomena due to dioxygen diffusion, a major drawback suffered by these complexes. On the other side, a close-pack of chromophores may lower the quantum yields and shorten the lifetimes due to formation of new excited states that may decay to the ground state via nonradiative paths. The protophysical properties of these systems are thus usually resulting from a combination of different processes.
Hydrophobic dinuclear neutral Re(I) tricarbonyl complexes were functionalized with polar tails (3EG or 4EG) through rigid aromatic or flexible alkyl hydrophilic groups (complexes Re_1a-d in Figure
Neutral and ionic octahedral metal complexes.
The self-assembly of alkoxy-bridged dinuclear Re(I) (complexes Re_2a-c in Figure
Amphiphilic ionic complexes Ru_1 and Ir_1 (Figure
A reverse molecular design was further employed: the molecular amphiphiles were formed by a neutral emissive Ir(III) complex with two cyclometallating phenylpyridine ligands and a picolinate ancillary ligand substituted with long alkyl chains ending with charged solubilizing sulfate groups (complexes Ir_2a-b in Figure
Some examples of luminescent metal complexes that do not have well-defined polar/apolar molecular parts but self-assembly through similar mechanisms and forces into supramolecular structures in aqueous media were quite recently reported. This included an Au(I) linear complex formed with two hydrophilic monodentate ligands and some Pt(II) and Ir(III) complexes that self-assembled into chromonic or chromonic-type lyotropic liquid crystalline states.
The linearly coordinated Au_1 complex (Figure
Linear amphiphilic Au(I) complex based on peptide unit and trisulfonated-triphenylphosphane ligand.
In this ambit, Lu et al. reported luminescent planar-shape cationic Pt(II) complexes having solubilizing chloride or sulfate anions, able to self-assemble in water into ordered polyelectrolitic microfibres (complexes Pt_13a-d, Pt_14a-b, and Pt_15 in Figure
Planar-shape cationic Pt(II) and Rh(I) complexes.
Complex Pt_13a was furthermore covalently interconnected through a flexible oligo(oxyethylene) chain of different length obtaining, thus, dicationic species Pt_16a-d·X (Figure
Planar shape Rh(I) coordination complexes based on 2,6-xylylisocyanide ligands bearing solubilizing counterions (complexes Rh_1a-c in Figure
Octahedral cationic Ir(III) complexes.
A detailed structural analysis by WAXS, SAXS, and SANS experiments performed for Ir_3a complex in both anisotropic gel and isotropic phases showed that in water the complexes self-assembled into double string polyelectrolytic supramolecular columns surrounded by solvated counterions. These aggregated species existed also at low concentration (1%
The gelling and appearance of positional long-range order specific to liquid crystalline systems resulted only from the interactions between increasing number of strands, with preserving their shape, similar to the assembly of chromonic polyelectrolyte columns. Due to structural differences between the typical chromonic planar molecules and the Ir(III) complexes with rather bowl-like geometry, the self-assembly was tentatively explained by a difference in the charge distribution in the coordination shell forming areas with rather hydrophobic character next to areas with rather hydrophilic character that led to aggregation in water.
Accurate photophysical investigations carried out on isotropic water solutions containing the polyelectrolytic strings without positional order and viscous mesophases at room temperature by varying temperature sustained the structural investigations and offered more information about the system behaviour, considering a similar aggregation for all complexes Ir_3a-c and Ir_4. Importantly, a significant improvement of the emission efficiencies and blue-shift of the emission maxima was observed in water with respect to the solvated complexes in diluted methanol. Due to aggregation, the enhancement of rigidity and lack of strong interchromophoric contacts experienced by the Ir(III)
A substantial increase of the number of Ir(III) molecules self-associating to form strands for complexes Ir_3a and Ir_3b, with complete (Ir_3a and Ir_3c) or quasicomplete disappearance (Ir_3b) of the monomeric species was found in the mesophases (2.5%
By changing the ancillary ligand from
Supramolecular self-assembly is a key approach for the design and development of nanostructured systems and has become a fundamental method for the formation of advanced nanomaterials. This strategy focuses on different types of interacting (and communicating) building blocks to perform preprogrammed advanced functions [
The self-assembly processes may become particularly intriguing if amphiphilic molecules are involved. In addition to the wide scenario of possible interactions between amphiphiles (polar and apolar interactions, steric hindrance, H-bonds,
In a similar way, the possibility to control the complexity in the nanoworld would make real the building up of “artificial” systems that could improve human life by realizing novel complex molecular materials/systems/devices amplifying the range of their capabilities in every desired field. We really hope, with this contribution, to have made the reader curious to this exciting aspect, encouraging, at the same time, future research to focus on this aspect. A gradual development is advisable, passing from the exploitation of the individual/specific properties of the compounds we have presented, to the
The data used to support the findings of this study are included within the article.
The authors declare that there is no conflict of interest regarding the publication of this paper.
E. I. S. and C. C. acknowledge the support of the Romanian Academy, Project 4.1. The authors also acknowledge the support from the Romanian Academy and from the CNR-RA bilateral project 2020–2022 (prot. n. 0088276 from 09/12/2019).