Micelles and Vesicles 59 C. Double Complex Salts 67 D. ID Arrays 74 B. Crystalline Assemblies 90 V. Systems based on metal complexes with d 6 and d 8 electronic configuration forming assemblies such as micelles, vesicles, and gels, as well as crystalline structures, will be illustrated. The question is how much we can rationalize the behavior and predict the structures and their properties on the basis of the design. Introduction Photochemical processes in complex-ordered materials are responsible for life as we know it on our planet's surface.
Self-assembly is a nature-inspired process, in which small molecules spontaneously arrange in an ordered fashion, leading to functional structures displaying characteristics which are not present at the level of individual molecules. The information contained within the structure of the small entities is translated into complex functions through the cooperative interactions between these building blocks. To obtain the desired output, the components must be chosen with great care and organized in space, energy, and time. Further, a variety of different architectures can be set up with few building blocks that can be repeatedly combined in different ways, just as observed in natural photosystems.
The driving forces which hold together these entities are mainly dealing with electrostatic interactions, metal coordination, hydrogen bond, n-n stacking as well as hydrophobic, van der Waals, and dispersion interactions. Nowadays, one of the most striking goals is to provide building blocks that can be rationally mixed and matched to have functional supramolecular architecture and materials with properties which can be modified upon an external stimulus.
By mimicking biological systems, it is possible nowadays to create species able to act as supra molecular sensors, as well as to develop catalysts which undergo allosteric control 8 , or produce by light molecular movements 9. Thus, taking inspiration from nature offers the possibility to deliberately design cooperative synthetic systems, blossoming novel, and fascinating scientific scenarios in both fundamental and applied research.
A more ambitious goal is not to mimic nature but to provide simpler artificial systems which can replace natural ones. In many cases, in nature, the assemblies are constituted of organic chromophores. Much less common is to encounter assemblies of organometallic species or hybrid organic-inorganic arrays.
Combination of different metal complexes, possessing complementary properties, for example, emission colors, absorption, ability to accept and donate energy and charge, can lead to white-light emission, light-harvesting systems, photoinduced energy and charge-transfer CT processes, and photocatalysis. Herein, we will discuss several approaches that have led from molecular entities to supramolecular soft and hard systems. Along our journey, we will elaborate on design features, photophysical properties, structure-activity correlation, and potential applications.
The selected examples are based on noncovalently linked luminescent systems in which the self-assembly process generates new functions. We wish to give to the reader only the photophysical basis of selected d 6 and d 8 complexes to follow better the discussion in the next sections. In brief, in a metal complex, its molecular constitution can be described as a metallic center surrounded by an organic coordination sphere. Ground and excited-state electronic wave functions facilitate the presentation of electron density relocations that can be visualized and interpreted as transitions involving localized molecular orbitals MOs.
However, the limitation of this model should be kept in mind. This is particularly true for the nature of the transitions, which actually occur between electronic states that cannot even be regarded as purely zero-order in nature. Electronic states are then approximated as combinations of zero-order wave functions, yielding representations of mixed nature 23, If contrasted with the ground state, the first two mainly involve changes within the organic portions LC or the inorganic part MC of the molecule.
The latter one CT involves a vectorial redistribution within or between the organic ligands intra- or interligand charge transfer, respectively, ILCT , or more frequently, between the organic part and the metal center. At this point, the electron spin has not been considered so far. Within the orbital approximation, the excited electronic states involve half-filled orbitals occupied by unpaired electrons. However, relativistic effects due to acceleration of electrons in the immediacy of the elevated nuclear charges of heavy atoms originate a perturbation that, once again, leads to mixing of zero-order wave functions.
Representation of electronic states as purely singlet or triplet in nature breaks down and gives way to mixing, by spin-orbit coupling, of electronic wave functions with defined spin. The spin-orbit coupling constant, which determines the magnitude of the perturbation, roughly scales as Z 4 , where Z refers to the nuclear charge.
The properties of photoactive transition metal complexes, such as those described herein, are fundamentally marked by spin-orbit coupling. This radiationless process is followed by fast relaxation into the lowest excited state with preponderant triplet character. Moreover, the emitting triplet state can be of a different origin than the optically excited singlet state, thus leading to even more pronounced shifts.
The major triplet character of the long-lived, lowest electronic excited states also renders them prone to quenching by molecular oxygen, which can be used for sensing or photosensitizing purposes. The metallic center is therefore shielded from the environment, and intermolecular interactions mainly perturb the organic ligands and, consequently, the ground and excited electronic states whose energy content they determine.
For the octahedral Ru II complexes, and the other d 6 metal ions, the ct l and 7t L orbitals are completely filled as well as the HOMO x M t 2 g 6 is fully occupied and the ground state configuration is closed shell. The ground state is therefore a singlet, while the excited states are either singlet or triplet.
The polypyridine complexes, accounted as paradigmatic examples of MLCT states, involve 4d-orbitals from which electron density is displaced to the organic ligand. Strong a-donors and n- acceptors lead to a larger d-orbital splitting and, consequently, destabilize MC states, making thermal activation less accessible. This leads to red-shifted emissions and shortened excited-state lifetimes, due to a faster nonradiative deactivation, which can be rationalized in terms of the energy gap law Further, the splitting induced by a given ligand field is more pronounced for the diffuse 5d-orbitals of Os II than for the 4d-orbitals of Ru II , which further increases the gap between the emissive MLCT and the nonradiative MC states.
This is necessary to push up in energy the MLCT states, which otherwise possess a very low-energy content leading to extremely low-emission efficiencies according to the energy gap law. Further, strong field ligands induce a larger splitting of d-orbitals, thus making nonradiative MC states less accessible and favoring radiative processes. Luminescent Ir III complexes are often comprising cyclometalating ligands, and even though they possess the same electronic configuration of the Ru II and Os II , the nature of their excited state is less defined. In fact for many complexes, the lowest emitting excited state is a mixed MLCT and LC, which is reflected in longer excited-state lifetimes and often structured emission.
A remarkable behavior is observed for the excited-state lifetime, as even for larger contribution of the 3 LC states, a short lifetime few microseconds is recorded which is related to the heavy atom effect which induce a strong triplet-singlet mixing. Such properties allow a very fine-tuning of the excited-state energies, and the emission maxima can vary from UV to near IR. In the case of Pt II , however, the d 8 configuration leads to square-planar coordination geometries.
Modulating the intermolecular distance allows, for instance, to tune the absorption and emission wavelengths of the assemblies, as the d-d electronic interaction is a function of distance. Pd II shares structural and chemical features with Pt II within the group 10 of the periodic table, even though its photophysical properties are less appealing.
Nonetheless, Pd complexes are very well known for their catalytic properties, which might be exploited in photoactive supramolecular architectures. In the next section, we will divide the d 6 and d 8 metal complexes and describe the possible modulation of their properties upon aggregation. Photoresponsive Assemblies Based on Noncovalent Interactions Synthetic chemists have applied the concepts of multivalency and cooperativity to supramolecular chemistry to create, for instance, biomimetic receptors able to recognize small molecules 29,30 , polysaccharides 31 , and DNA Most of these systems are based on calixa[n]arenes , cucurbituril CB 36,37 , and cyclodextrine CD see Fig.
It has also been shown that organic chromophores can be used to reversibly assemble and disassemble large structures. For example, Zhang and coworkers 41 reported the light controlled assembly of a system based on a-CD and diazobenzene-containing surfactant. As depicted in Fig. Schematic representation of some host systems used for host-guest assemblies: from left: calixarene, cucurbituril, and p-cyclodextrine. Schematic light-induced assembly and disassembly process.
This is due to the increased ste- ric demand because of the change in conformation of the azobenzene moiety, within the hydrophobic cavity of the a-CD. In such systems, photoinduced energy transfer can occur from the periphery, upon complexation of the iridium units, toward the central ruthenium acceptor, or switched in the other direction, from the ruthenium to the periphery when the osmium moieties are assembled see Fig. The lowest excited state is in fact localized on the osmium center, while the highest luminescent excited state belongs to the iridium complex see Fig.
In these cases, anthracene and osmium derivatives were employed as final energy donor and acceptor, respectively. After self-assembling of the three different photoactive components see Fig. To switch the direction of the energy transfer process, Ir complexes must be replaced with the osmium analogues. Right: emission spectra for the three complexes. Schematic formula of the multicomponent system and its energy levels. Due to the energy cascade process, the electronic energy can be transferred from the excited anthracene to the osmium acceptor green via the ruthenium moiety An interesting self-assembled light-harvesting antenna system based on a ruthenium complex and a Nd III emitter has been reported by Balzani et al.
However, the organic chromophores are able to efficiently transfer electronic energy to the [Ru bpy 2 CN 2 ] complex, which is coordinated through the cyano groups to the Nd. The excited Ru complex can then emit or sensitize the line-like emission of the Nd ion, as observed by recording the IR emission spectrum. Besides hydrophobic and coordinative interactions, hydrogen bonds and electrostatic interactions have been used to assemble luminescent metal complexes. Upon light excitation, a photoinduced energy transfer process from the excited ruthenium moiety to the osmium unit is observed.
We have shown that hydrogen bonds can be successfully employed to decorate the periphery of dendrimers with emitting Re I complexes Further, electron transfer could be investigated in a system based on the same interaction, in which the electron acceptor, methyl viologen, was bound to the Red complex, acting as an excited-state electron donor Schematic formula of the dendrimer and of the Red guest which can complex via hydrogen bonds, at its periphery.
Micelles and Vesicles An interesting approach to have multiple components non- covalent assembled, based on identical or different metal complexes, keeping a well-defined structure, is to organize them in micelles and vesicles. Such self-assembly, in certain solvents, must be promoted by an amphiphilic nature of the metal complex which can be achieved by tailoring the ligands coordinated to the metal ion.
A fine-tuning of the ligands could also allow obtaining the correct geometrical shape to be able to discriminate between micelles or double layer structures such as vesicles or eventually linear arrangements such as fibers. In a general design, the metal complex represents the head group of the surfactant and often constitutes the polar part, as most of the metallosurfactants investigated contain charged complexes.
However, the rationalization of the assembly is not that straightforward. In fact, secondary intermolecular interactions tc — tc, electrostatic, hydrogen bonds, etc. Even though metallosurfactants are scarce compared to their nonmetallic organic counterparts, there is an increasing interest due of their multiple applications in fields such as probes in emulsion 53 , formation of monolayers 52, , thin film 60 CRISTIAN A. However, much effort has been devoted for shedding light onto the fundamental understanding of the interior structure and dynamics of soft self-assembling materials, as micelles, vesicles, and microemulsion 53, Several papers reported on the spatial determination of photoactive molecule in organized assemblies , and Pallavicini and coworkers reviewed on the use of luminescence as probe in self-assembly of multicomponent fluorescent sensors Most of these systems are designed for catalytic purposes 88 , to act as templating agents for mesoporous materials 89 , and for optoelectronic application see d 8 -based systems.
Moreover, Bruce devoted intensive work to the fundamental understanding of the structure-properties relationship and, in particular, on the role played by the molecular parameters of metal-containing mesogenic molecules, as geometry and volume of the ligands, nature and size of the metal center, on determining aggregation properties and arrangement features of the systems as well as liquid crystalline behavior Verani and coworkers widely investigated stimuli-responsive soft materials with interesting optical and redox behaviors.
Such materials are able to self-assembly in functional ordered structures, as Langmuir-Blodgett films and liquid crystals, and possess potential applications in molecular electronics and magnetic films as well. Further, Bruce et al. Despite most of the investigations on the ruthenium complexes containing micelles deal with the quenching phenomena, there are few papers in which the micelles have been used to enhance the ruthenium emission The complex Ruppz did not show any luminescence in water, although it was a modest emitter in organic solvents as ethanol.
The quenching in such a case is not due to the dioxygen but to the protonation of the pyrazine group in water which turns off the emission. Most of the micellar systems described so far are realized in water. We recently reported on the aggregation features and spectroscopic properties of inverted ruthenium bipyridyl aggregates in low-polarity organic solvents Such systems could help to further shed light on the role played by closely organized metal centers. Indeed, a surfactant that could be able to aggregate into inverted micelles has to match two different key aspects: a small head group and voluminous hydrophobic substituents.
As a result, a molecule with a truncated cone architecture, where the head group represents the narrow extreme of the cone, might be considered. Chemical structure of the investigated metallosurfactants with two left and one middle disubstituted bipyridines. Emission was monitored at and nm, respectively. To compare the properties of the metallosurfactants upon aggregation, the photophysical characteristics of the complexes were investigated in air-equilibrated n-hexane and ethanol conditions. However, contrary to the reference complex, a pronounced and unexpected bathochromic shift was detected in the 'MLCT absorption band in n-hexane when compared to ethanol.
These results suggest the formation of aggregates in the lower polarity solvent, being most likely reversed micelles or reversed vesicles. The cationic heads of the ruthenium complexes being in the core of the aggregates are organized close to each other feeling a higher polarity environment despite the low polarity of the solvent. The long component was assigned to the monomeric nonaggregated ruthenium amphiphilic molecules, while the short component would arise from the aggregated and closely-packed molecules. The quenching effect is due to the triplet-triplet annihilation, which strongly reduces the luminescence excited- state population of the complexes.
The biexponential decays turned in a monoexponential lifetime Fig. The nature and the structure of such aggregates were also investigated by means of dynamic light scattering and atomic force microscopy. We discussed how the numbers, size, and nature of the akyl chain s can influence the formation of inverted vesicles or micelles which could open new perspective toward the synthesis of nanomaterials and, in particular, of photocatalysts.
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Multi - chromophoric aggregates can also have an important role as light-harvesting units or for the constitution of multicolor emitting materials. Both complexes are charged and contain long chains dialkyl-bipyridine, as the hydrophobic part of the structure. Also, the steady-state and time-resolved emission properties of 1 and 2 were investigated in the presence of a conventional cationic surfactant, as cetyltrimethylammonium bromide CTAB. Both emissions display monoexponential decays.
However, above the CMC, biexponential lifetime profiles and enhanced emission quantum yields are observed for both complexes. Comparison with the excited lifetime of the parental complexes showed that the longer component can be assigned to the aggregated species. Schematic formulas of the metallosurfactants and their self- assembly in mixed aggregates. Upon combination of complexes 1 and 2 in an equimolar 0. No bimolecular quenching process could be detected at the employed concentrations for the reference complexes.
The nonluminescent cationic surfactant seemed to play a noninnocent role. Indeed for concentration of CTAB above the CMC, both 1 and 2 displayed an increase of their emission intensity as well as an elongation of their excited-state lifetimes, suggesting an incorporation of the metallosurfactant in the CTAB-based micelles. In Fig. These experiments clearly showed the dependence of the intramicellar energy transfer process occurring between the two amphiphilies due to the micellization equilibrium of CTAB.
As a result, two emission colors can be obtained and one could even imagine to have more than two emitters or to combine other properties within the same aggregate. These aggregates could therefore be employed as novel electroluminescent materials, as the size of the spherical aggregates is compatible with ink-jet printing and the emitters can be tuned in color and efficiency in a desired way.
Left: Time-resolved intensity decays and fits for 1 at a 0. Reproduced with the permission of the American Chemical Society Work focusing on these compounds was principally prompted by fundamental studies of excited-state electron and energy transfer processes - as well as their potential use as biological sensors , organic light-emitting diodes OLEDs - , and light-emitting electrochemical cells LEECs - Besides soft structures containing luminescent metal complexes, strong effort has been devoted to the development of luminescent porous organometallic frameworks as recently reviewed by Allendorf and coworkers Also, Kitagawa and coworkers reviewed on the synthetic strategies and properties of functional from ID to 3D coordination networks - In these materials, the metal atoms play a fundamental role in the structure formation as well as in the spectroscopic properties of the crystalline materials.
The latter are usually formed from purely organic compounds linked by hydrogen bonds, and the individual building blocks are commonly referred to as tectons. As already mentioned, the supramolecular assemblies can lead to new properties and photoinduced processes which have been studied in great details in solution see previous section.
A chain of ligated metal ions with alternating charges require a judicious choice of building blocks with particular steric and electronic needs, as the planarity of the metal complexes involved and the possibility of x-sta eking as complementary stabilization force. This theme has been recently reviewed by Doerrer; thus no more room will be devoted to it hereafter The double salts are obtained because the small counter-ions yield to easily removable water-soluble salts once the metal complexes are combined.
X-ray crystal structure of a single crystal and its unit cell packing for complex 1. This is the reason for choosing the luminescent ionic complexes possessing complementary colors and high-emission quantum yields. We were able to precipitate the double salts 1 as crystalline materials suitable for single-crystal X-ray analysis, and the structure of one of them shows that they form fascinating 3D porous networks Fig.
As can be easily seen, the pores are formed because of several noncovalent interactions which held together the entire crystalline structure. We noticed that to be able to crystallize the material, a number of characteristics must be incorporated into the building blocks. The large and isolated channels run along the crystallographic c axis and display an irregular shape with a minimum cross-section of 3. Interestingly, upon removal of the solvent from the channels, the crystallinity of the compound decreases, but the porosity is maintained. The process of filling and emptying the pores is perfectly reversible, and we demonstrated that the porous network possesses cavities able to host solvent molecules, or electroactive molecules.
In the absence of a guest, we observed not only that the emission of the blue-green emitter is completely quenched by energy transfer but also that the emission of the single crystal is bathochromically shifted with respect to the ones which are supposed to be the smaller band gap species, namely, the cations. Insertion of toluene molecules inside the pores, which most likely increases the distance between the complexes due to the breathing of the crystal, was monitored by confocal microscopy, and a blue shift of the emission accompanied by the visualization of the energy transfer process was detected.
Even more interesting was the possibility to almost selectively quench the cationic iridium complexes by efficient photoinduced electron transfer and to modulate the color of emission of the 70 CRISTIAN A. Further, due to the different oxidation states of the metals employed, multiredox reactions could be envisaged just upon light excitation. Confocal microscope emission spectra of a single crystal of complex 1 before black spectrum and after red spectrum insertion of anthraquinone molecules inside the pores. As can be seen by eyes the emission of the crystal change from orange to green.
Reproduced with the permission of Wiley-VCH Aggregation-Induced Emission Enhancement in Crystals It is well known that aggregation of light-emitting compounds, both organic and organometallic, is usually associated with a strong quenching of emission efficiency. This effect was first recognized by Forster and coworker studying the fluorescence of pyrene Most of the technological applications in which light-emitting molecules are used as emitters in OLED, biological probes and sensors, require a rather high local concentration of such molecules and often the molecules are investigated in solid state.
Usually, in these conditions, a strong aggregation-caused quenching effect takes place, which in turn represents a strong limitation in the real world for using these classes of compounds. In this respect, Tang and coworkers first reported in on an example of aggregation-induced emission AIE , Most of the reported compounds which show this effect are organic molecules , where restriction of intramolecular rotation is generally accounted for being the main cause for AIE , In these cases, several different reasons were considered being involved in the AIE effect, namely metallophilic interaction as in the cases of square-planar Pt II complexes, giving rise to new excited state MMLCT with larger transition dipole moments associated to increasing radiative rate constants, or n-n stacking of the coordinated ligand.
It is worth to note that crystalline compounds sometimes proved to be more efficient emitters than their amorphous counterparts, showing the influence of molecular packing on the solid-state emission , , thus providing optimal conditions to investigate their photophysical properties and the influence of aggregation. The availability of different crystalline phases polymorphs of a luminescent molecule is the best example for studying the relationship between crystal packing and optical properties , In this respect, we recently reported on two stable concomitant solid-state polymorphs yellow and orange of the dinuclear complex [Re 2 p-Cl 2 CO 6 p-4,5- Me 3 Si 2 pyridazine ] Fig.
The compound belongs to the recently reported class of dinuclear, luminescent Red complexes of general formula [Re 2 p-Cl 2 CO 6 p-l,2-diazine ] , , which display intense, broad, and featureless emission in fluid solution from excited state that can be consistently ascribed to a 3 MLCT. During the crystallization process, the concomitant formation of two crystalline phases of the compound was observed. Most likely, the restricted rotation of the Me 3 Si groups in the crystals is responsible for the enhancement of the emission with respect to the solution.
This statement is supported by the fact that similar dinuclear complexes lacking the Me 3 Si group possess such higher emission quantum yields. These findings highlight how packing can strongly perturb the photophysical properties of the molecules even in the absence of particularly short interactions.
Molecular Systems Based on Aggregates of d 8 Metal Complexes The square-planar coordination geometry of d 8 complexes opens many possibilities for the design of supramolecular architectures. Their tendency toward aggregation and stacking is a crucial feature to trigger self-assembling processes. Depending on the balance between stacking tendency and solvent affinity, soft or crystalline materials can be obtained. The substitution symmetry is a further parameter to be considered for the controlled assembly, while the intrinsic properties of the monomeric constituents can be tuned by judicious choice of ligands.
As previously mentioned, the interaction between protruding, doubly occupied dz 2 orbitals critically affects the photophysical properties upon aggregate formation. A particularly interesting feature is represented by the possibility of tuning the distance between the monomeric units and, consequently, the degree of electronic coupling 74 CRISTIAN A. It has been shown that they can form aggregates or even excimers, causing shifts in the emitted wavelengths and affecting the photoluminescence quantum yields , Therefore, controlling the aggregation and being able to predict and design appropriate compounds with the desired properties are an important step to fully exploit their potential for technological purpose.
We will highlight the most important achievement in this area and try to correlate the chemical structures with the observed photophysical behavior. We will discuss not only ID, 2D and 3D architectures but also crystalline systems. This variable interaction can be employed for sensing purposes, due to the switchable emission of the frameworks ID Arrays Terpyridine, N ' N' N ligands and their N' C ' N and N" N ' C analogues have been successfully coordinated to Pt II , leading to neutral, mono- or doubly charged species, which in some cases display bright luminescence, both in degassed fluid solutions and frozen matrices.
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In particular, it has been shown by Che et al. Che et al. Organometallic molecules and non- covalent intermolecular metal-metal and ligand-ligand interactions were employed to construct submicrometer-sized nonlinear superstructures. Due to the optoelectronic properties of organoplatinum II complexes, novel mesoscopic applications based on the superstructures could be envisaged.
The preliminary studies carried out have shown that the reported metamorphism Fig. Molecular structure of Pt II complexes and their emission in organic light-emitting field effect transistors. Self-assembly of ionic Pt II complexes in aqueous environments. When the nanotubes were suspended in CH 2 C1 2 solutions, all of them disassembled into the corresponding monomers. However, in the coassembly with trans- [Pt PhCN 2 Cl 2 ], the metal coordination could take place both at an inter and intramolecular level Fig.
Interestingly, structure of the nanotubes affects the electrochemical properties that depend on the degree of shielding of the metallic centers. The complexes in solid state display various colors, depending on the substituent on the aryl acetylide ligand Fig. They showed that these organoplatinum II complexes can self-assemble into nearly bidimensional nanostructures displaying near infrared phosphorescence and light-modulated conductivity. Yam et al. Vapor diffusion pt ii Heat-cool Vapor diffusion Vapor diffusion Fig. Molecular structures and supramolecular assemblies obtained under different conditions, as described by Aida et al.
Structure-color correlation table of Pt II complexes left. Transient conductivity measurement of the FET device right. The transient channel current was recorded with a light switching on and off every 5 s in a s period. Selected Pt II complexes left , as described by Yam et al. They showed that the emission of pure films is responsive to both method of preparation and tribological stimulation so that it is possible to switch in a controllable manner between monomer- and excimer-like states.
Bruce et al. The parent ligands exhibit a rich, smectic polymorphism, but when modified with a fused cyclopentene ring, nematic phases dominate. Reaction of the ligands with tetrachloroplatinate II leads to poorly soluble, dimeric complexes that can be cleaved using dimethylsulfoxide; the resulting monomeric complexes are then readily converted to the fS-diketonate complexes.
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All of the complexes are mesomorphic, and the fi-diketonate complexes A nm Fig. Selected structure of Pt II complexes. Left: fast cooled from the LC phase after the texture is fully developed. Right: fast cooled direct from the isotropic phase. Reproduced with the permission of Wiley-'VCH Orthoplatinated rod-like Pt II complexes. Intermolecular organization clearly has a significant effect on emission characteristics. In most cases, the intermolecular interactions are electrostatic, hydrogen bonds and n—n stacking and are induced by heating and cooling of the solution and most recently by ultrasounds Stable hydrogel is formed with terpyridine-type systems upon application of ultrasound to the water solution.
The nature of the gels can be modulated by the addition of metal ions. Reproduced with the permission of the Royal Society of Chemistry Figure 20 shows our findings based on the use of substituted terpyridine ligands which are able to gelate in water even in the presence of metal ions which can coordinate the tridentate ligand.
Emission can be observed from the gel depending on the nature of the metal ions. As supramolecular gels provide fibrous aggregates with long- range order, they are heavily used in biology and medicine as extracellular matrix, drug delivery, and cosmetics, but they could be also of interest in the fields of optoelectronic devices and sensors.
In this context, organometallic gelators can display multiple functionalities and properties which combine ligands or organic fragments and metal ions which further can lead to metal-metal interactions influencing their properties The cohesion between the small molecules assembling into filaments was given by hydrogen bonds, while the balance between the hydrophobic or hydrophilic backbone bearing adamantane or anisole moieties, respectively, determined the gelating ability either in DMF or water.
The gels were doped with an hemicaged Eu III complex, which was retained in the cavities containing the entrapped solvent, which was monitored by the excited-state lifetime of the lanthanide complex. However, the self-assembly process yielding the hydrogel was nicely probed with the aid of a Ru II complex, which is not luminescent in aqueous environments. The luminescence was turned on upon gelation, due to the fact that the organometallic species intercalated into the hydrophobic pockets of the network, thus allowing a highly sensitive monitoring of the self-assembly. These findings constitute a basis for the use of luminescent probes, able to change their properties upon solidification of the systems, for the monitoring of bond forming and breaking in soft materials.
To make use of the luminescence switching effect upon self-assembly, we developed a Pt II complex that by itself is nonluminescent, but is able to stack into luminescent gelating nanofibers The coordination of an alkyl-pyridine ancillary moiety to the 2,6-bis- tetrazolyl-pyridine-based complex enhanced the solution processability. The Pt II gelator is nonemissive in diluted solution. However, in frozen CH 2 C1 2 matrix at 77 K and in thin films, it displays a bright unstructured luminescence centered at nm and also shows an intense absorption band around nm, a feature that is not present in solution at room temperature.
These results are particularly remarkable considering that Pt II complexes usually show rather low-emission intensity and strongly concentration-dependent emission due to aggregate or excimer formation in the solid state and in thin film. Therefore, the turn on of the bright luminescence upon aggregation can be employed to monitor the assembling process with high sensitivity.
The fibers self-assemble in larger entangled structures but their emission properties remain unchanged. Diffusion of hexane into the colorless, nonemissive solution of the complex in CHCI 3 , affords a self-assembled yellow gel that appears highly luminescent under UV irradiation see Fig. Bottom left and center : SEM: micrographs of the self-assembled fibers and TEM micrograph bottom, right of the self-assembled gel. The soft assembly also displays not only the highest photoluminescence quantum yield and radiative rate constant but also the lowest radiationless deactivation rate, if compared with doped polymethylmetacrylate and neat films.
This points to the high degree of order within the filaments, which favors radiative processes and minimizes nonradiative pathways. Self-assembly of Pt II complexes yielding luminescent liquid crystals ,, and AIE have been already described, but metal complexes forming soft structures with such intense phosphorescence are very rare.
Further, the use of aggregates to build up electroluminescent devices is a new interesting strategy not only to develop novel colors but also to take advantage of the AIE in real applications. Therefore, we have explored the application of the Pt aggregates in solution-processed OLED devices, showing high brightness and good color purity Dotz et al. They described a palladium pincer bis carbene complex constituting an efficient organometallic gelator for a variety of protic and aprotic organic solvents even in concentrations as low as 0.
NMR spectroscopy and X-ray diffraction studies indicated that n stacking of the heteroarene moieties, van der Waals interactions between the alkyl chains, and metal-metal interactions may be responsible for the aggregation. Despite the simplicity of their structures, the carbene complexes constituted low-molecular mass metallogelators Fig. They efficiently gel- ate not only a broad variety of protic and aprotic organic solvents but also different types of ionic liquids such as imidazolium, pyridinium, pyrazolidinium, piperidinium, and ammonium salts at concentrations as low as 0.
The morphologies of the resulting 3D gel networks composed of long and thin fibers were studied by TEM and light microscopy for a selection of organic and ionic liquids, showing that the achiral gelators assemble into helical fibers. Structure of gelating Pd II complexes described by Dotz et al. Selected TEM micrographs of gels obtained from different solvents are shown. Selected dark-field optical micrograph of a gel obtained with liquid crystals is shown.
Shinkai et al. The complexes gelated various organic solvents at very low concentrations. Electron microscopy gave visual images of well-developed fibrous structures characteristic of low-molecular weight organogels. The nanofibers are evidently different depending on the electronic states of the different central metals.
The Pt gel shows unique thermo- and solvatochromism of absorption and emission color in response to a sol-gel transition Fig. Further, the Pt gel possesses an attractive ability to avoid dioxygen quenching of excited triplet states, which has a positive effect on the phosphorescence Fig. Quinolinate metal complexes described by Shinkai et al. Phosphorescence spectra and photographs of sol and gel phases of a gelating Pt II complex are shown, as well as a confocal scanning laser microscopy micrograph of the gel.
These findings consistently indicate that introduction of metal chelates into the gel is one of the most effective strategies toward a variety of photo- and electrochemical nanomaterials. A last example refers to the complexes described by Yam et al. They have synthesized a series of alkynylplatinum II terpyridyl complexes and investigated their electrochemical, photophysical, and luminescence properties. This is further supported by the observation of the complete switching off of the 3 MMLCT emission at nm at T ge i.
The systematic study of these alkynylplatinum II terpyridyl complexes has led to a better understanding of the factors that direct the gel-formation properties. Longer hydrocarbon chains are found to enhance solubility in most common organic solvents hindering the formation of stable metallogels, whereas bulky tert-butyl substituents on the terpyridyl ligand forms less stable metallogels, as reflected by its higher critical gelation concentration and lower sol-gel transition temperature than the unsubstituted terpyridyl analogue.
This demonstrates the subtle interplay of factors influencing the formation and stability of these metallogels and shows that with modifications of the ligands as well as variation of the organic solvents, the electronic absorption, and luminescence properties could readily be tuned. General structure of gelating terpyridine-based Pt II gelators described by Yam et al.
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In a further step, Yam and coworkers constructed helical superstructures employing chiral alkynylplatinum II -terpyridyl complexes, obtaining metallogels that show helical fibrous nanostructures. Crystalline Assemblies Eisenberg et al. Upon exposure to methanol vapors, it shifts color from red to orange, and a displacement to higher energy is observed in the emission maximum with an increase in excited-state lifetime and emission intensity. In both nm Fig. Pt II complex displaying reversible color and luminescence switching after exposure to MeOH vapor, as described by Eisenberg et al.
However, the red form is devoid of solvent within the crystal lattice and contains complexes stacked with an almost linear arrangement of Pt II ions having an average distance of 3. Top left: image of the device. Inset: luminescence image of microwires. Bottom left: schematic diagram of the device used for the detection of light. Che and coworkers went a step further and made use of the vaporesponsive behavior in optoelectronic devices. The characterization of photoresponsive transistors employing the above-mentioned microwires revealed that the microwires constitute ambipolar semiconducting materials Fig.
The photoinduced conducting characteristics suggest that the assembled complex possesses stimuli-responsive features with potential applications in semiconductor devices, such as photodetectors. Interestingly, the exposure to different environments can markedly affect the conductivity. Changes in the crystal structure can be also triggered by mechanical stimuli, as elegantly shown by Shinozaki et al.
The mechanochemical behavior of Pt II complexes bearing cyclometallating tridentate ligands was investigated in terms of solid-state luminescence. A broad emission band, which was not detected for the crystal, was observed in the red region of the electromagnetic spectrum for the powder. The phenomenon was very similar to the excimer formation observed in solution. The excimeric state formation is facilitated by the mechanical grinding, and it is described by the quantum chemical mixing of ground and excited Fig.
Mechanoresponsive Pt II complex described by Shinozaki et al. Such crystalline materials could find applications in mechanical sensing. Conclusions and Open Questions In conclusion, the number of functional aggregates in which new properties are generated by the assembly process is already very large and growing every day. In fact, a clever design of the organometallic species can lead to aggregates with emerging properties and novel functions that can be reversibly and dynamically tuned. This behavior resembles the dynamic nature of living systems able to self-organize and adapt in different environments.
Can we foresee that also with metal complexes we are able to interconvert from a soft structure, for example, micelle to another one, for example, fibers or gels by an external input? How many different metal complexes acting as energy and electron donor or acceptor species could we organize, for instance, to realize an artificial noncovalent light-harvesting system? Could we have crystalline materials in which we could use the metal complex framework to perform photoinduced multiredox reactions inside the cavities?
Many more questions will arise from the development of this fascinating area combining functional chemical structures and intermolecular interactions. We are aware that many scientists all over the world are working on such problems and have answered to some of our questions. And for those who are entering the field, we believe that there are so many exciting systems to create and to study that we will have to write several more book chapters. Balzani, V. Nishiyabu, R. Cordier, P. Hue, I.
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Fluorine an untapped resource in inorganic chemistry. This article is part of the themed collection: Fluorine Chemistry. You have access to this article. Please wait while we load your content Something went wrong. Try again? Cited by. Back to tab navigation Download options Please wait Article type: Highlight.
DOI: Download Citation: Chem. Fluorine an untapped resource in inorganic chemistry C. Varlow, D. Szames, K. Dahl, V.