3.1. Production of the Metal Clusters
Long-chain cyano(alkyl-) or (alkoxy)biphenyls formed in the liquid crystalline and crystal solid states dimeric supramolecular structures due to strong dipole–dipole interactions and the anti-ferroelectric arrangement of the molecules by the head-to-tail principle. The characteristic size of such structures was 2.57 nm for the molecular dimer 5CB molecules. The introduction of metal atoms (silver or copper) in a cyanobiphenyl matrix (CB) were been made by the cryogenic co-condensation of metal atomic vapor, and the molecular beam of the mesogenic compounds (5CB, 8CB, and 5OCB) led to the formation of bi-ligand complexes structures in which a between-metal atom was embedded in the molecular dimer structure between the CN-group of one CB molecule and the phenyl ring of the other CB molecule arranged by the head-to-tail principle (
Figure 1). The distribution of the electron density in the complex structure in the case of two border electronic configurations that were obtained by the DFT/B3LYP modelling of the system is shown in
Figure S2.
The appearance of new bands in the region of the triple CN bond stretching vibrations at 2030 and 2080 (2130) cm−1 was recorded in the IR-Fourier spectra of the Ag-cyanobiphenyl (5CB, 8CB, and 5OCB) samples due to the formation of biligand π-complexes of the metals in the zero oxidation state with cyanobiphenyls. The existence of two different bands for CN oscillations indicated the nonequivalence of the two CN groups in the structure of the biligand complex. In the IR-Fourier spectra of the samples, another new band was also recorded at 650–660 cm−1, this one related to the oscillations of the metal–ligand bonds in the π-complex. The Ag-CB complexes were stable at a temperature of 77 K, but they were completely decomposed when the temperature rose to 150–200 K.
For the ESR spectroscopy studies at low temperatures and high vacuum conditions, we used a cryostat of a special design, the scheme of which is shown in
Figure S1. The cryostat allowed for the simultaneous co-condensation of three substances: metal (Ag, Cu), cyanobiphenyl (5CB, 8CB, and 5OCB) and the third reactive component (CCl
4) in the molecular (atomic) beam mode.
The ESR spectra of double co-condensates Ag with 5CB and 8CB that were obtained and registered at a temperature of 77 K contained three groups of signals (A, B and C) (
Figure 2). Doublet signals (A, B) in the region of higher and lower magnetic fields belonged to the biligand π-complexes Ag(CB)
2 [
22]. The formation of the doublet was caused by the difference in the magnetic resonance parameters of the Ag
107 and Ag
109 isotopes that were contained in the natural silver in an approximately equivalent ratio. The values of the magnetic resonance parameters of these signals that were obtained by the computer processing of experimental spectra are typical for the π-complexes of silver atoms. In these complexes, the main contribution to the formation of the molecular orbital on which the unpaired electron is located was introduced by the 5s-orbital of the silver atom. A comparison of the values of the isotropic hyperfine interaction constants of silver atoms that are part of biligand complexes with values that are typical for undisturbed silver atoms [
26] allowed us to estimate the spin density of the s-electron of the silver of the Ag/5CB system as σ
M = 0.89. This value is typical for the π-complexes of silver atoms and shows the degree of partial electron density donation from the silver atom to the system of the π*-orbitals of ligand molecules.
There was also a central asymmetric singlet ESR signal (C) with a g-factor close to the value of 2.0032—the g-factor of the free electron—present spectra of these systems. This signal can be attributed to the spin resonance of the conduction electrons of nanoscale silver clusters [
22]. By using the method of ESR spectroscopy, it is possible to trace the kinetics of the degradation of biligand complexes and the formation of nanoscale silver clusters when the temperature increases.
When heating the samples of the double Ag/CB co-condensates from 77 to 150 °C, the relative integral intensity of the doublet signals A and B gradually decreased, and the intensity of the central singlet signal C simultaneously increased. This indicates the thermal destruction of the biligand π complexes Ag (CB)2. The liberated silver atoms acquired the ability to diffuse relatively freely within a solid-phase matrix, which was an unordered glassy liquid-crystalline phase. Due to diffusion, silver atoms either combined to form new nanoclusters or were incorporated into existing ones. As a result, the integral intensity of the spin resonance signal of the conduction electrons (C) increased. In the temperature range of 150–200 K, the intensity of this signal remained almost constant (taking into account the dependence of the signal intensity on the temperature). Heating samples above 200 K led to the crystallization of a glassy liquid crystal matrix and a sharp increase in molecular mobility, thus resulting in the rapid coalescence of nanoclusters to form aggregates that did not give a signal in the ESR spectra.
The addition of the third active component to the system—CCl
4, in our case—in an equimolar ratio with respect to silver led to an increase in the concentration of the π-complexes of silver atoms with cyanobiphenyl molecules (components A and B in
Figure 2). At the same time, the central signal in the Ag/5CB/CCl
4 triple system acquired a more symmetrical shape compared to that in the Ag/5CB double system (
Figure 3 and
Figure 4). Increasing the temperature in the double system to 170–200 K led to an increase of the intensity of the central component and to the appearance of a highly symmetric signal with a g-factor close to the value of the g-factor of the free electron (
Figure 4f,g,h) in the ESR spectra.
The observed effects can be explained by the fact that silver atoms and small silver clusters have different reactivities in relation to the molecule CCl4. The highly symmetric icosahedral Ag13 clusters did not interact with CCl4, and, at the same time, a set of other asymmetric small silver clusters, which gave an ESR signal in the central region of the spectrum, reacted with this molecule. In addition, small clusters of silver with an even number of atoms (Ag2 and Ag4) do not have unpaired electrons and, therefore, did not give a signal in the ESR spectra. When co-condensing under conditions of high molecular mobility, small silver clusters could react with the CCl4 molecule to release silver atoms. This caused an increase in the concentration of the π-complexes of the silver atoms with cyanobiphenyl molecules in the triple system. Increasing the temperature led to the thermal degradation of the biligand π -complexes of the silver atoms with the dipolar dimer of the CB-molecules and the rising of the diffusion mobility of the silver atoms. The silver atoms themselves interacted with CCl4 molecules, forming the reaction product AgCl and nonstable silver clusters, which could decompose and form silver atoms that aggregated at the next stage in order to form non-reactive, highly symmetric icosahedral Ag13 and Ag55 clusters.
The ESR spectra of the samples of copper-cyanobiphenyl (Cu-5CB) double co-condensates, obtained at a temperature of 77 K, contained two signals. The first was a four-component signal (signal A in
Figure 5) that was caused by the formation of the copper-cyanobiphenyl π-complex (the nuclear spin of copper is 3/2). This signal was characterized by a pronounced anisotropy of magnetic resonance parameters. The value of the perpendicular component of the hyperfine interaction constant (HFI) B ≈ 51 MHz. Additional splitting due to the presence of two isotopes in natural copper (Cu
63 and Cu
65) was not allowed due to the proximity of their nuclear g-factors. A comparison of this value with the value B
0 = 386 MHz, typical for an electron that is located on the 4p-orbital of an undisturbed copper atom [
27], allowed us to determine the value of the spin density of an unpaired electron on the 4p
z orbital of a copper atom in the copper-cyanobiphenyl π-complex; that value was σ
M = 0.13. The process of the jumping an unpaired electron of a copper atom from a 4s to a 4p orbital is known for biligand π-complex copper atoms in the oxidation state of zero with organic ligands [
27].
Another component of the ESR spectrum of the copper-cyanobiphenyl double co-condensate (Cu/5CB) (signal B in
Figure 5) represented a singlet with a g-factor close to the value of 2.0032—the g-factor of the free electron. This was caused by the spin resonance of the conduction electrons of the nanoscale copper clusters. The increase in temperature led to the thermal decomposition of the biligand complexes of the copper atoms and the additional formation of nanoscale copper clusters.
The study of film samples of hybrid nanosystems “silver-4-cyano-4-pentylbiphenyl” (Ag-5CB), “silver-4-cyano-4-pentyloxybiphenyl”(Ag-5OCB) and “copper-4-cyano-4-pentylbiphenyl” (Cu-5CB) by transmission electron microscopy showed the formation and stabilization of spherical metal nanoparticles in a glassy-like frozen liquid crystal matrix with a size of 1–2 nm (
Figure 6). These samples were obtained by the co-condensation of the components on a surface that was cooled with liquid nitrogen (temperature 77 K) and subsequent annealing at a temperature of 150–200 K. Metal nanoparticles in film samples can be partially and directly formed during the low-temperature co-condensation and can be additionally formed as a result of the thermal decomposition of the unstable biligand π-complexes of metal atoms M(5CB)
2 [
22]. The metal atoms that are liberated in this process can be localized in the regions of a micro-heterogeneous, disordered glassy liquid crystal matrix that is formed by the alkyl chains of the terminal substituent in 5CB dimer structures. High molecular mobility is maintained in these local micro-regions, even at low temperatures. As a result, the released metal atoms can relatively easily diffuse in the solid-phase matrix and interact with each other, forming nanoparticles that lost the mobility.
The study of selected areas of film samples by electron diffraction has indicated that the resulting metal nanoparticles have a crystal structure with an fcc-cubic lattice that is typical for of silver and copper crystals. The size of silver and copper nanocrystals that are formed in mesogenic CB micro heterogeneous matrices at low temperatures is close to the critical size of metal nanoparticles, at which there is a transition from a crystal-like fcc-cubic structure to quasi-crystalline icosahedral-like structures.
The heating of the samples of the hybrid metal–cyanobiphenyl nanosystems to temperatures above 200 K led to the crystallization of a glassy liquid crystal matrix and was accompanied by a sharp increase in molecular mobility. In this case, the fast coalescence of nanoclusters occurred along with the formation of larger metal aggregates that were not registered by the ESR method. Such behavior of the hybrid metal–cyanobiphenyl system was the result of specificity of intermolecular interactions and, as a consequence, the supramolecular organization of the mesogenic cyanobiphenyls matrix and the alteration of its character with temperature.
In cyanobiphenyl molecules, there is a rigid biphenyl core, as well as terminal substituents in the 4 and 4′ positions. On the one hand, there are flexible, non-polar long-chain alkyl or alkoxyl groups, and on the other hand, there are high-polar cyanogroups. The presence of cyanogroups leads to the high polarity of the molecules of cyanobiphenyl as a whole. Intensive dipole–dipole interactions of cyanobiphenyl molecules lead to antiferroelectric ordering, with the formation of dimers arranged on the principle of “head-to-tail.” In turn, cyanobiphenyl dimers are capable of further self-organization due to staking interactions between biphenyl rings and other types of intermolecular interactions. In this case, the type of self-organization depends on the sample’s prehistory and can change when the temperature changes. The introduction of d-metals (Ag or Cu) into the system, due to the formation of biligand metastable complexes of metal atoms, partially changes the system of intermolecular interactions, breaking the antiferroelectric molecular ordering at long distances.
The complexes were stabilized in the slightly crystallized solid CB matrices up to 200 K. At temperatures close to this value we observed solid–solid phase transition in the CB’s solids and the formation of dimeric crystal structures. At temperatures higher than 200 K, the complexes possessed the effective degradation and aggregation of liberated metal atoms in small (1–2 nm in size) global metal nanoparticles stabilized in the metastable solid state which was formed by the CB dimers and localized in the area of the more mobile alkyl chains of CB’s matrices. When the molecular flows of the metal and the cyanobiphenyl co-condensed on a cold surface at a temperature of 77 K, the atoms and molecules that fell on the surface maintained a high molecular mobility for some time. During this time, some of the metal atoms formed biligand π complexes with the cyanobiphenyl molecules. The other metal atoms could aggregate and form nanoscale clusters. Such a mechanism was evidenced by the data obtained by ESR spectroscopy. Cyanobiphenyl molecules that were not included of the biligand π complexes formed molecular dimers. However, the time at which the atoms and molecules maintained a high molecular mobility was not sufficient for the self-organization of the cyanobiphenyl molecules and their packing into ordered crystalline phases. As a result of this partial self-organization, we obtained a metastable solid-phase matrix that was partially crystallized and formed nematic-ordered (in the case of 5CB and 5OCB) or smectic-ordered (in the case of 8CB) glassy-like phases.
Increasing the temperature led to a partial reorganization of the CB matrix’s molecular packing, leading to a rapid increase in molecular mobility. As a result, part of the biligand π-complexes of the metals with cyanobiphenyl molecules was irreversibly decomposed. Finally, they disappeared at a temperature of about 150 K. The released metal atoms combined with their aggregation and formed spherical metal nanoparticles with a size of 1–2 nm. The aggregation of metal atoms was possible due to their relatively high diffusion over local microblasts of the solid–liquid crystal matrix that was formed by a system of mobile alkyl or alkoxyl substituents of cyanobiphenyl molecules. The average diameter of the resulting nanoparticles (1–2 nm) was probably determined by the size of these local areas with a high molecular mobility. The metal nanoparticles were stabilized in a the solid-phase matrix. As a result, the system remained stable in the temperature range of 150–200 K, which was confirmed by ESR spectroscopy data.
The heating of the samples that were obtained by the joint co-condensation of the molecular streams of the metal and cyanobiphenyl above 200 K led to the crystallization of a glassy-like solid–liquid crystal matrix. During the phase transformation, there was a sharp increase in translational molecular mobility. This resulted in the coalescence of small metal nanoparticles into larger aggregates that did not produce signals in the ESR spectra.
The rapid heating of co-condensates to room temperature with a phase transition from crystalline to nematic (in the case of 5CB and 5OCB) or smectic (in the case of 8CB) led to the formation of anisometric nanoparticles. For example, in the Ag-5CB system, the growth of highly anisotropic metal nanorods occurred in the crystal matrix in the temperature range of 273–293 K, which was close to the temperature of the crystal–nematic phase transition. The length of these particles (l, nm) was more than 200 nm, and the width (d, nm) was about 15 nm. Thus, the anisometric ratio of particles (l/d) exceeded 10 (
Figure 7). The mechanism of such a quasi-one-dimensional growth of metal nanorods is associated with maintaining a high degree of ordering and the simultaneously increasing molecular mobility in the pre-transition region.
The study of the samples containing nanorods by X-ray diffraction demonstrated the crystalline nature of these formations. The texture of the resulting nanorods occurred due to directed growth that was parallel to the crystallographic plane (100) and perpendicular to the plane (111). The reason for this phenomenon may be the fact that the plane (111) of the silver crystal structure was characterized by a hexagonal dense packing, while the plane (100) was characterized by a less dense square packing. In this regard, the intensity of the interactions of the terminal cyano (CN) ligand groups with metal atoms located on the plane (111) was higher than those of the metal atoms located on the plane (100). Due to this circumstance, the fusion of smaller nanocrystals with the formation of larger ones occurred mainly along the plane (100). This resulted in the formation of highly anisometric metal nanorods. After heating the samples to temperatures (or close to these temperatures) of the phase transitions to the liquid crystal states of CB-nematic or smectic types, the rapid aggregation of small nanoparticles was observed with the formation of anisometric nanoparticles and their highly-ordered assemblies and nanostructures, with the morphology controlled by the dynamic and structural properties of soft matrix templates of CB.
3.2. Self-Assembling of Metal Nanoparticles in Nematic and Smectic Phases of Mesogenic Cyanoalkylcbiphenyls
Samples that were obtained by the co-condensation of Ag or Cu molecular beams with mesogenic cyanobiphenyls and subsequently controlled annealing were nanohybrid materials that included metal nanoparticles that were stabilized in a matrix of mesogenic ligands, and the morphology of the nanoparticles was largely determined by the processes of the self-ordering of the liquid crystal matrix. This was confirmed by the analysis of the set of data that was obtained by transmission electron microscopy (TEM), the electron diffraction of the selected area (SED), and X-ray diffraction. It was shown that depending on the phase state of the Ag/5CB and Ag/8CB hybrid nanosystems, it was possible to form metal nanoparticles of different sizes and morphologies. For example, the rapid heating of samples of the metal–mesogenic co-condensate with a low metal content to a temperature of 313 K, followed by a phase transition to the isotropic phase, led to the formation of spherical metal particles with a characteristic average diameter of 15 nm. The data that were obtained by the method of IR-Fourier spectroscopy indicated the formation of a stabilizing monolayer of alkyl cyanobiphenyl molecules on the surface of the metal nanoparticles due to donor–acceptor interactions of the terminal CN groups of the ligand with surface metal atoms. Increasing the content of the metal component and conducting the self-assembly processes of nanoparticles in the orientationally-ordered nematic phase led to the formation of anisometric metal nanoparticles with an anisometric factor of l/d equal to 3,5 and their highly-ordered ensembles. Atomic force microscopy data, showing the formation of linear aggregates and regularly-ordered ensembles of anisometric nanoparticles in the nematic phase of 5CB at 300 K, are presented in
Figure 8. A bimodal histogram of the size distribution of the metal nanoparticles for this case is also shown in
Figure 8.
The use of higher homology 4-cyano-4′-octylbiphenyl (8CB) smectic liquid crystal 8CB as a soft template matrix allowed for the formation of metal nanoparticles in a layered, molecularly-organized system. In this case, the formation of quasi-fractal flat metal aggregates intercalated between smectic layers of 8CB (
Figure 9) was revealed. This was due to the higher interaction energy between the cyanobiphenyl molecules compared to the interaction energy between the cyanobiphenyl molecules and the metal atoms. As a result, the microphase separation of metal atoms and smectic liquid crystal molecules occurred. Silver atoms displaced into the interlayer space that were formed by a system of mobile alkyl substituents could relatively easily diffuse within this space and interact with each other. In this case, the nature of the diffusion processes with the participation of metal atoms was quasi-two-dimensional, which, in the final case, led to the formation of quasi-two-dimensional metal aggregates.
Thus, the processes of the self-organization of the liquid crystal matrix and the associated processes of the aggregation of metal atoms led to the formation of hybrid nanosystems, including metal nanoparticles of different sizes and morphologies that were supramolecularly organized into highly-ordered nanostructures. In particular, these could be anisometric nanoparticles and rod-shaped aggregates that stabilized in nematic mesophases, as well as flat two-dimensional metal aggregates that formed in the layered structured smectic phases of long-chain cyanoalkylbiphenyls. At low metal content, in the orientationally-ordered nematic phase, globular metal nanoparticles of smaller size were formed and stabilized. Increasing the concentration of metals contributed to the formation of anisometric rod-shaped metal nanoparticles and their aggregates. The diagram of the structural-phase states of the Ag-5CB hybrid nanosystem, indicating the conditions for the formation of metal nanoparticles with a certain morphology, is shown in
Figure 10. This diagram is a summary of the results that were obtained by thermal analysis, poly-thermal polarization spectroscopy, electron diffraction of the sample selected areas (SAED) and atomic force microscopy.
An analysis of the data shown in
Figure 10 allowed us to explain the dependence of the size and morphology of metal nanoparticles that were stabilized by the mesogenic CB ligands of the phase state of the mesogenic matrix and the metal concentration. The shape of the metal nanoparticles was the result of competition between three factors—the general tendency to minimize the surface energy of the particle, the energy of intermolecular interactions of liquid crystal molecules with surface metal atoms, and the elastic energy of the liquid crystal. The rapid heating of Ag/5CB co-condensate samples at speeds exceeding 100 K/min to the isotropic phase existence temperatures (T ˃ 318 K) (region I in
Figure 10) led to the formation and stabilization of globular nanoparticles with d = (15 ± 10) nm in size. The formation of anisometric metal nanoparticles and their orientationally-ordered assemblies occurred in the nematic phase, as seen in region III in
Figure 10. In the two-phase region of the coexistence of the isotropic and nematic phases (region II in
Figure 10) it was possible to form both isotropic globular particles and anisometric rods.
The maintaining of the samples at 273–283 K and the increasing of the metal content in the sample from 1 up to 10 w/w% (region IV in
Figure 10) led to preferential growth of rod-like metal nanoparticles with l/d ratios higher than 10. Small metal nanoparticles of 1–2 nm in size were formed in the solid phase (region V in
Figure 10) at 150–200 K.