Synthesis and mesomorphic properties of iron(II)-containing dendrimeric complexes derivative of 3,4-n-dodecyloxybenzoyl poly(propylene imine)

ABSTRACT Synthesis and characterisation of liquid crystalline (LC) Fe(II) complexes with ‘two-chain’-substituted poly(propylene imine) (PPI) dendrimeric ligand of the first to fifth generations are presented. Compounds were synthesised by complex formation between the metal salt and the corresponding dendrimeric ligands. The purity and structure were proved by different methods. The calculated amount of iron in the complexes was confirmed by the experimental data with a great degree of precision. Iron ions are incorporated into the dendrimer at two sites: at the border and inside of the dendrimeric core. A tetragonal coordination of iron was found. Mesomorphic properties of dendrimer iron(II) complexes were studied, a hexagonal columnar mesophase (Colh) was evaluated by the results of X-ray scattering. Upon excitation at absorption bands, iron dendrimeric complexes exhibit fluorescence properties. GRAPHICAL ABSTRACT


Introduction
Dendrimers are a group of repetitively branched molecules, 1−10 nm in diameter, containing the core molecule and branches terminated with different functional groups. [1,2] With their relatively rigid branched structure and distinctly different chemical environments at the core and within the branches, dendrimer molecules may be used for storage and separation of different functional groups on a nanometre scale, making them excellent building blocks for photonics devices. [3][4][5] Recently, metal-containing dendrimers have attracted much attention in many research groups. It can be easily explained by the fact that dendrimers have a large number of potential coordination sites. This is the reason why these types of materials can be used as catalysts and as components of sensors. [6] In this connection, it will be very interesting to synthesise new organometallic dendrimers or complexes of dendrimers with metals. [7] The metallodendrimers, that is, metal-containing dendrimers, are particularly attractive since in addition to the numerous assets listed above, the metallic residues can be integrated into various parts of the dendritic architecture and according to different modes, increasing furthermore their molecular complexity. [8] In recent years, there has been an increasing interest in the field of liquid crystalline (LC) dendrimers. [9] Mesomorphic properties in dendrimers (phase type and stability) can be induced and controlled by a dedicated molecular design which depends on the chemical nature and structure of both the functional groups and the dendritic matrix. [10] In a pioneering work, Lattermann et al. synthesised poly (propylene imine) (PPI) dendrimers with the non-mesogenic 3,4-bis(decyloxy)benzoate groups, [11] Figure 1.
Induction of a Col h phase was systematically observed from zero generation (G0) to fifth generation (G5). The lyotropic phase behaviour of dendrimers of poly(alkylene imine) with non-mesomorphic peripheral units, that is, decyloxybenzoyl-substituted PPI dendrimers as well as model compounds of their main parts, decyloxybenzoyl-substituted alkylamines has been studied. For the first time, the induction/variation of mesomorphic properties in binary systems of these dendrimers or of related model compounds with suitable organic solvents has been established. [12] A more thorough systematic study on both PPI and polyamidoamine (PAMAM) side-chain LC dendrimers was carried out by Serrano et al. [13] As expected, dendrimers functionalised by anisotropic units bearing one terminal alkoxy chain led to the formation of smectic phases, whilst the same dendritic matrices functionalised with mesogenic units bearing two or three terminal alkoxy chains exhibit solely a Col h phase. [14] In the last 20 years some complexes have been prepared using dendrimers of poly(alkylene imine) as ligands for the complexation of transition metals: Co, Ni, Cu and Zn. [15] Recently, complexes of PPI dendrimers with salts of bivalent copper have been obtained and their structure and mesomorphic properties were investigated. [16,17] Complexation of various loads of copper(II) ions to related LC PPI dendrimer derivatives of the zero and first generations equipped at the periphery by di-3,4-decyloxyphenyl amide groups were reported to show Col h mesophases. [17] Complex analysis by EPR spectroscopy revealed that at low loadings of Cu(II) per dendritic ligand, the ions form monomeric complexes with a planar N 2 O 2 coordination geometry involving both carbonyl oxygen and amido nitrogen atoms. At intermediate and high loads, two metal complexation sites were identified. Namely, a pseudo-tetrahedral N 2 O 2 site involving the same atoms as above and a square pyramidal N 3 O 2 site by the additional bridging with a tertiary amino nitrogen atom from the inner core of the dendrimer, with possible intermolecular interactions. [18] It is known that for PPI dendrimers, nitrogen atoms of the primary amine end groups and tertiary amines possess strong electron-donating properties [19] and that the bis(3aminopropyl)amine functionality acts as a strongly complexing tridentate coordinating site for the metal (III) chloride (MCl 3 , with M = Co, Cr, Fe). [20][21][22][23] It has also been reported for the spherical phenylazomethine dendrimer (DPA), whose imine groups act as strong coordination sites with the ferric ions, that FeCl 3 molecules incorporate into the dendrimer and the imine group complexes FeCl 3 (with equilibrium constant of complexation K = 10 8 [M −1 ]), so that 3Cland one imine of DPA are the ligands of Fe 3+ ion. [24] Based on the literature data mentioned above we can assume that FeCl 2 molecules are coordinated into the substituted LC PPI dendrimers at two sites: into the dendritic core and on dendrimer periphery.
Motivation for this work arises from our desire to find suitable photoactive material for fabricating lightemitting diodes and highly luminescent solids for semiconductor lasers. [25,26] Our aims were to synthesise stable complexes of the ligands PPI dendrimer with iron(II) salts and to investigate the structure and the mesomorphic properties of this compound. As a first step to solve this problem, we will try to identify the location of metal ions in the dendrimer architecture and to find out mesomorphic and fluorescence properties of iron-containing dendrimers.

Materials and methods
All solvents, which were used for synthesis, such as benzene, ethanol, tetrahydrofuran (THF), methylene chloride and chloroform are available from Merck. All solvents were kept under inert atmosphere over active 4 Å molecular sieve. Anhydrous iron(II) chloride was commercial available from Fluka and used without further purification. Teflon filters tops on a syringe (200 nm and 450 nm meshes) are from ROTH Rotilabo PTFE. Dendrimers 3,4-bis-(decyloxybenzoyl) PPI derivatives were synthesised by the method described earlier. [11,12] The purity and individuality of the desired product has been checked by different methods. Gel-permeation chromatography (GPC) was performed by means of the following chromatography setup: SDV columns (30 × 80 mm, 5 nm particle size) 10 2 , 10 3 , 10 4 pore size from Polymer Standards Service (PSS). Pump: Spectra Physics P100; UV detector: Waters 440, λ = 254 nm; IR detector: Waters 410. Flow rate: 0.5 ml/min. Eluent: THF with 0.25 wt% tetrabutylamoniumbromid; internal standard 1,2-dichlorbenzene. Mass spectra were obtained by MALDI-TOF-MS method (matrix-assisted laser desorption time-of-flight mass spectrometry) on spectrometer Brucker Reflex TM III; matrix: 7-hydroxycoumarin. UV-Vis spectra were registered in solutions of methylene chloride and THF (0.5 mg/10 ml) by a UV spectrophotometer Hitachi U-3000 UV spectrometer using 1-cm quartz cells. Fluorescence spectra were recorded on a Shimadzu RF-5301 fluorometer, using a quartz cell of 1-cm path length. The concentration of the solution was 0.5 mg of substance /10 ml of solvent. The velocity of the spectrum recorder was 0.1 nm/s. The coefficient of limiting diaphragm was 0.5.
The structure and LC properties were investigated by Fourier transform infrared (FTIR) spectroscopy, polarising optical microscopy (POM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and X-ray diffraction measurements. FTIR spectra of compounds were recorded on a BioRad Digilab FTS-40 device in the region of 4000-350 cm −1 on SiO 2 disk. DSC measurements were carried out on two machines: Perkin-Elmer Diamond DSC and NETZCH DSC 204 F1 device in aluminium capsules; the weight of the sample ≈ 10 mg, the heating rate was 10°C/min in N 2 atmosphere. The phase transition behaviour of Fe(II) complexes was observed by means of a polarising microscope Nikon Diaphot 300 equipped with a hot stage Mettler FP 90. TGA was performing on device TGA/SDTA851 from Mettler Toledo, the heating rate was 10°C/min in N 2 atmosphere. X-ray diffraction measurements were executed by Guinier goniometer Huber 600, monochromator Huber 611 with generator Seifert (CuK α1 , λ = 1.54051 Å) and temperature controller Huber HTC 9634.
Elemental analysis was carried out for the following elements: C, H, N, Cl, Fe in two special laboratories: Mikroanalytisches Labor I. Beetz (Kronach) and Mikroanalytisches Labor E. Pascher (Remagen, Germany).

Synthesis of iron-containing complexes from first to fifth generation dendrimers
This article reports the investigation of five LC dendrimeric Fe II complexes (first to fifth generations) derived from decyloxybenzoate-substituted PPI dendrimers (nK2.10), that is, 3,4-bis-(decyloxybenzoyl) PPI by direct complexation with anhydrous iron(II) chloride. The general scheme of the synthesis of iron complexes with dendrimers of first to fifth generations as organic ligands (L) is given below for the third generation, Figure 2. The complexes were obtained with respect to the number of potential coordination places in dendrimeric core of ligands using five times excess of dry FeCl 2 . The complexation reaction was carried out in THF solution under argon atmosphere. For each reaction, an iron chloride solution was freshly prepared by dissolving anhydrous FeCl 2 in THF and stirring it for 8 hours. A small amount of formed black precipitate was filtered off by means of a 200 nm mesh PTFE filter. The ligands were dissolved in THF and stirred for 6 hours under inert atmosphere and mixed with FeCl 2 solution.
After the synthesis, the volume of THF solution was reduced to 1-2 ml and the product was precipitated in ice-cold dry ethanol. Some amount of free FeCl 2 was removed by cooled ethanol, and the residue was dissolved in benzene and then filtered through Teflon mesh filters (0.200 µ). The desired product was isolated by freeze drying from benzene solution. The complex obtained was in the form of yellow-ochre amorphous powder. The characterisation of the synthesised iron complexes is given in the Supporting Information.
The purity of investigated complexes was checked by UV spectroscopy and GPC. [27] Individuality and stability was proved by FTIR and mass spectrometry. [27] 3.2 FTIR spectra of the iron(II) complexes Through the complexation of iron, the bands of the coordinating moieties in the ligand are expected to be affected, giving evidence of the iron location in the dendrimer. The iron-complexed samples were prepared as films from CH 2 Cl 2 solution onto doubled polished silicon plates. It was not possible to disperse the complex in KBr because of displacement of the bromide by the nitrate with evolution of Br 2 as indicated by the violet colour of the pellets after preparation.
As an overview, the FTIR spectra of the second generation complex and ligand are presented in Figure 3. The available moieties for iron(II) complexation in the benzoyl-substituted PPI dendrimers are the tertiary amines in the inner core of the PPI and the secondary amide groups in the interphase between the polar PPI core and the apolar alkoxy shell. Amines are strong nucleophiles, in principle favourably disposed towards coordinating iron. The amide moiety exhibited two possible electron-donating sites: nitrogen and oxygen. appears. Band at 1640 cm −1 corresponds to those C=O which are coordinated to iron and the band at 1632 cm −1 of decreased intensity, corresponds to the fraction of carbonyl groups which are free or not O-coordinated. The coordination of iron to oxygen should be furthermore evidenced by a ν(O-Fe) band in the far-infrared range of spectra.

Iron(II) complexes absorption bands in the UV/ Vis range
The PPI dendrimeric ligands exhibit absorption maxima in the ultraviolet region at 254-258 nm and 287-289 nm in THF solutions. They are nearly the same for each generation. No significant solvatochromic effect is observed in solvents of similar polarity such as CH 2 Cl 2 or benzene. The band at 258 nm corresponds to electronic transitions between the π-π* orbitals of the aromatic benzene rings. Its position is in good agreement with the absorption estimated by the Scott rules for the present substitution. [28] The band at ca. 289 nm can be ascribed to n-π* transitions of the amide carbonyl group. [29] As an example, the absorption spectrum of the second generation ligand is presented together with those of the complex of the same generation 2-K2.10-(FeCl 2 ) 6 on Figure 4(a). The yellowish iron(II) complexes of the PPI dendrimers are expected to exhibit absorption bands close to the ultraviolet, according to the complementarity principle. Spectra of the iron(II) complexes in THF solution exhibit absorption spectra similar to those of the ligand, slightly shifted to larger wavelengths, Figure 4 (b). The bands appear now at 255 ± 3 nm and 287 ± 4 nm depending on the generation. Absorption band in the visible range is not observed even for concentrated solutions. However, for FeCl 2 dissolved in THF, a broad band or shoulder (~85 nm width at half maximum) at 332 nm is observed, cf. Figure 4(a). This band appears between 310 and 425 nm, where the broadening or shoulder is observed in the spectra of the complexes in THF. Since free iron(II) chloride is not present, the observed broadening or shoulder correspond to new transitions induced by the presence of chloride coordinated to iron.
The absorptions are normalised to the absorption of the π-π* band, since the number of benzene rings does not change through complexation and no significant change in the absorption of this band is therefore expected. The n-π* band of the ligand (at 287 nm) corresponding to the carbonyl moiety, becomes stronger and shifts to lower wavelengths with increasing iron loadings (n), indicating that the number of amide moieties involved in the coordination grows with the iron content. Absorption in this spectral region is reported for copper(II) amido complexes with a N 2 O 2 coordination. [30] An enhancement of the slope of the base line in the region 310-425 nm with increasing iron loading is observed in the absorption spectra of different loaded complexes of the second generation, Figure 4(a).
The absorption spectra of the iron complexes are red shifted compared to those of the ligands. The relative intensity of the absorption band of the amide (at ca. 287-289 nm) grows with increasing iron loading, suggesting that the amide moiety is involved in the iron coordination. A new broad band (or shoulder) corresponding to nitrate coordinated to iron is observed at ca. 310-425 nm in the spectra of the iron (II) complexes.

Mesomorphic properties of the complexes
Investigation by polarising microscopy, DSC and X-ray scattering allowed us to establish that all synthesised complexes are mesomorphic. The DSC data not give effective information about phase behaviour of dendrimeric Fe(II) complexes, it is possible to obtain T g data only, Table 1 temperature is quite widespread fact to polymers and dendrimers [31,32] 6 (2), 83°C for 3-K2.10-(FeCl 2 ) 14.6 (3), 95°C for 4-K2.10-(FeCl 2 ) 21 (4) and 90°C for 5-K2.10-(FeCl 2 ) 106.5 (5). Upon heating in the range of the mesophase, the complexes reveal a non-geometric texture, Figure 5. Figure 5(a) shows transition from «crystal to mesophase» of 2-K2.10 (FeCl 2 ) 6 in the first heating cycle at Т = 78°С. Figure 5(b) shows the texture of 2-K2.10 (FeCl 2 ) 6 in the heating cycle at Т = 116.7°С. This type of texture is characteristic of dendrimer complexes of all five generations. Series of photographs of 2-K2.10 (FeCl 2 ) 6 showing the difference between the Colh phase and the crystalline phase was included into Supporting Information. The complexes demonstrate phase transitions of the «solid to mesophase» and «mesophase to Iso» types with clearing temperatures by POM observation. The phase transition behaviour of samples in the second heating process seems to be «glassy(Col h ) to Col h »-«Col h to Isotrope», with no crystalline phase. The temperatures of phase transitions of iron-containing dendrimers are shown in Table 1.
Mesomorphic texture in cooling cycle obtained by POM indicates that mesomorphic ordering is remained at room temperature after cooling. It is confirmed also by the data of Table 2. Probably it can be explained by forming frozen mesophase with ordering appropriate to Col h mesophase. Texture of mesophase is remained at reverse transition from isotropic to mesomorphic state. Transition of 'mesophase-solid' is not observed.
To establish the type of mesophase X-ray measurements were performed at different temperatures ( Figure 6 and Table 2). The calculations of X-ray results demonstrate hexagonal columnar packing of the molecules in the mesophase.
Hexagonal columnar mesophase (Col h ) is familiar in discotic LC. It is formed depending always on the strong interaction between core-core, and the column being titled as Col h . Therefore, based on the XRD results, we proposed a possible packed model of complexes in mesophase as schematically illustrated in Figure 7.

The nature of ion complexation
We investigated the influence of the metal ion affects at mesomorphic properties in a series of dendrimer complexesderivatives of PPI as compared to the initial ligands. Mesomorphism of ligands was investigated Remarks: T g ,°Cglass temperature; ΔC p , J/g * K; Iso,°Cisotropic liquid; Col hcolumnar hexagonal mesophase; T dec .,°Ctemperature of decomposition. previously. [11,12] The introduction of Fe ion crucially effects on the temperature area of mesophase existence in dendrimer complexes depending on the generation number. In these complexes with a bivalent iron ion thermostability of the mesophase is, on the whole, little dependent on the generation number and rises sharply as compared to that in the ligand. However, the range of its existence narrows down when moving from the lower generations to higher ones, with the columnar hexagonal packing being preserved.   At the same time, the compounds form a tetragonal structure of the complex, Figure 8. In the given compounds such packing structure results in a particularly strong intermolecular interaction of the plane-plane type. The first generation dendrimer complex reveals it graphically. As the generation number is increased, the change in the form of molecule from plank-like to cylindric results in a considerable narrowing of the mesophase existence, Figure 7. It is likely to be due to the emergence of steric effects reducing the interaction between the chlorine atoms. It is worth noting that the series of dendritic complexes reveal a higher thermal stability of the glass(y) stage as compared to ligands. The given trend is especially evident with the increase of the generation number.
Whilst in the dendrimer ligands the increase in the generation number tends to decrease the temperature of the transition from the glassy state to the mesophase, in metal complexes under the same conditions the thermal stability of the glassy state increases, which is particularly evident in Fe(II) complexes.

Photoactive properties
All investigated compounds demonstrate the fluorescence properties. The emission bands and their intensities are given in Table 3 upon complex excitation at 223, 258 and 290 nm absorption bands. All spectra were recorded in THF solution at room temperature.
The dependence of emission spectra of 2-K2.10-(FeCl 2 ) 6 from the absorption bands is shown in Figure 9.
As can be seen from Table 3, the highest emission intensities are observed for compounds excited at 290.8 nm absorption band and the largest fluorescence intensity manifests the iron dendrimeric complex of the first generation. The explanation of the observed results and the study of mechanism and kinetics of the fluorescence properties will be considered in future.

Conclusions
Five dendrimeric complexes from first to fifth generations have been synthesised by complex formation between metal salt and organic ligand. The purity of the synthesised compounds has been checked by UV spectroscopy and GPC. The presence of iron has been established by mass spectroscopy and elemental analysis. The iron content in the complex was calculated, the results agreed excellently with the experimental data. Iron ions are incorporated into the dendrimer at two sites: at the border and inside of the dendrimeric core. A tetragonal coordination of iron was found. The given complexes have been shown to form a Col h mesophase with transition to glass state on cooling.
The thermal stability of the mesophase in all generations of dendritic/dendrimeric metalcomplexes is higher than in initial mesomorphic dendrimersligands.  Introduction of iron ions (Fe 2+ ) is accompanied by the broadening of the mesophase existence region, which in all the samples is represented by a columnar hexagonal packing. This effect is probably due to the contribution of chlorine atoms to the intermolecular interaction and additional rigidity of the complex structure upon introduction of the coordinated iron ion. Upon excitation at absorption bands iron dendrimeric complexes exhibit fluorescence properties.