Phosphorescent columnar hybrid materials containing polyionic inorganic nanoclusters

The ternary polyionic inorganic compound Cs 2 Mo 6 Br 14 and 18-crown-6 ethers bearing two o -terphenyl units have been combined to design phosphorescent columnar liquid crystalline hybrid materials. The obtained host–guest complexes are very stable even at high temperatures. Depending on their surrounding atmosphere, these hybrids switch reversibly from a high-to-low luminescence state and show a very stable emission intensity up to 140 8 C.

The ternary polyionic inorganic compound Cs 2 Mo 6 Br 14 and 18-crown-6 ethers bearing two o-terphenyl units have been combined to design phosphorescent columnar liquid crystalline hybrid materials.The obtained host-guest complexes are very stable even at high temperatures.Depending on their surrounding atmosphere, these hybrids switch reversibly from a high-to-low luminescence state and show a very stable emission intensity up to 140 8C.
Columnar liquid crystals (CLCs) have attracted much interest in materials chemistry since 1977 when Chandrasekhar discovered them during his studies of discotic benzene hexaalkanoates. 1 These materials self-organise spontaneously usually into columns, show self-healing properties and have been shown to be useful in the design of many devices including photovoltaic solar cells, field effect transistors, one dimensional conductors and light emitting diodes. 2 It was also found that non-disk-shaped molecules can form columnar mesophases.Recently, they were also used to organise small amounts of bulky inorganic components such as gold nanoparticles, 3 gold nanorods, 4 or quantum dots over a wide range. 5Addition of new functionalities to CLCs, e.g., phosphorescence, appears then, to be particularly appealing in terms of application prospects but still remains very challenging in terms of LC phase or emission stability.We present herein the first hybrid CLC containing up to 34.4 wt% of polyionic, bulky, transition metal cluster (TMC) salts.TMCs are aggregates of metal atoms linked by metal-metal bonds. 6In particular, octahedral TMC based compounds of the general formula A n M 6 X i 8 X a 6 where A = alkali cation, M = Mo or Re, X i = halogen or chalcogen, X a = halogen or organic ligands are known for their phosphorescence with evidenced dual emission for Cs 2 Mo 6 Br 14 . 7We recently developed three strategies to integrate these isotropic emitters in smectic or nematic and cholesteric LCs. 8,9However, up to now, a TMC containing CLC remains unknown.To design such self-assembling materials, we present herein a supramolecular approach in which the nanocluster alkali counter cations are complexed within the cavity of 18-crown-6 ether derivatives CE n that were previously used by some of us to design CLC (Scheme 1). 10 Indeed, although the LC properties of CE n ligands and their respective complexes with monocharged species have been studied, 11 we report for the first time the use and the influence of bulky polyionic inorganic species on the mesomorphic properties of CE n as well as the surprising influence of the supramolecular organization on the luminescence properties of hybrids.
The dibenzo [18]crown-6 derivatives CE n with two lateral tetraalkyloxy o-terphenyl units, as well as the Cs 2 Mo 6 Br 14 cluster This journal is © The Royal Society of Chemistry 2016 ternary salt, were prepared according to the reported procedure with conforming analytical data. 10,12his TMC salt was chosen for its high thermal stability, a prerequisite for developing new classes of clusters containing thermotropic hybrid materials.Complexes were obtained by mixing 1 equivalent of Cs 2 Mo 6 Br 14 dissolved in acetone with 4 equivalents of CE n dissolved in dichloromethane.The reaction mixture was then stirred at 50 1C for 14 hours.After evaporation of solvents, the residue was taken up by dichloromethane to remove, by filtration through a microfilter, any traces of unreacted TMC, and heated under a high vacuum to yield the respective dry complexes (2CE n : Cs) 2 Mo 6 Br 14 = CL n .Their purity was verified by elemental analysis (ESI). 1 H NMR experiments performed on free CE n and their respective complexes CL n did not reveal many changes due to the Cs + complexation within the crown ether cavity (ESI, † Fig. S1 and S2).Only a slight upfield shift of signals corresponding to the methylene groups of the crown ring from 4.26 ppm to 3.96 ppm was observed upon complexation.This shift could be due to (1) a small decrease in the dihedral angle of the OCH 2 CH 2 O fragment upon ion complexation to accommodate the bulky Cs + cation and (2) an electric field caused by the presence of the salt.It is noteworthy that the complexation of smaller ions such as K + within the macrocyclic cavity induced the opposite effect. 11As previously observed, 9 it is mandatory to maintain a Cs : CE n ratio of 1 : 2 in order to maintain a full homogeneity of the cluster salt/CE n complex.
The LC properties of hybrids were studied by polarized optical microscopy (POM), differential scanning calorimetry (DSC, ESI, † Fig. S4-S6) and, small and wide angle X-ray scattering (SAXS, WAXS, ESI, † Fig. S7-S14), the results of which are shown in Table 1.All complexes show enantiotropic LC phases extending over a wide temperature range.This behaviour largely contrasts with the native CE n precursor, for which LC phases were observed at lower temperatures and over narrower ranges.However, the increase in the mesophase stability as compared to the parent CE n precursor is similar to that encountered when soft anions such as iodide or bridging anions such as hexafluorophosphate or thiocyanate are used to counterbalance the positive charge of a potassium cation embedded within the crown cavity. 11This unexpected result is of particular interest as host-guest complexes between the small 18-crown-6 cavity and the sterically demanding Cs + cation are usually supposed to be weak.It could eventually be attributed to the polyionic character of the TMC salt and the strong interactions existing between the complexed Cs + and [Mo 6 Br 14 ] 2À dianion which insure a better stability of the supramolecular assemblies upon heating.Surprisingly, the DSC thermogram of CL 8 reveals, upon heating, the formation of two LC phases, whereas upon cooling, only one first order transition corresponding to the isotropic-to-LC phase transition is observed (ESI, † Fig. S5).Fig. 1 shows the optical fan-shaped focal conic textures observed upon cooling for all samples, which are typical indicators of columnar mesophases.Heating CL 8 from 100 1C up to 160 1C did not lead to any noticeable change in the observed textures.Irradiating the samples with UV light illustrates their excellent homogeneity thanks to the well-known red NIR emission of TMC (vide infra).The same experiments performed at 200 1C also reveal a very good homogeneity showing that the interactions between the crown centre and Cs + cations are strong enough to maintain the integrity of the complex even at very high temperatures (ESI, † Fig. S3).
SAXS and WAXS patterns recorded at 127 1C, 119 1C and 131 1C for CL 7 , CL 8 and CL 9 respectively, contained sharp and intense reflections in the small angle region.We first analysed the XRD data, to investigate possible lamello-columnar packing, examples of which were independently reported by Donnio, Ziessel and Marcelis for metallomesogens, 13 donor-acceptortriads, 14 side-chain LC polymers 15 or triazines with pendant triphenylenes. 16We first checked whether sharp Bragg reflections indicating a smectic order were present for complexes CL n .However, neither characteristic layer reflections in the SAXS diffractogram nor the second halo in the wide angle region were visible.Thus, a lamello-columnar phase or a coexisting lamellar and columnar packing could be ruled out.
Hence, the indexation of observed patterns allowed the identification of a rectangular columnar phase with p2mm symmetry  for the three complexes (see Tables S1-S4, ESI, † for a full indexation).All WAXS patterns contained a broad and large signal around 4.4 Å arising from the fluid-like behaviour of the molten alkyl chains and from the stacking of the aromatic units within the columns confirming the liquid crystalline nature of all complexes.For CL 8 , additional measurements were performed at 150 1C to characterize the unexpected second mesophase detected via DSC upon heating.Three signals with reciprocal d-spacings in the ratio 1 : 1/O3 : 1/2 were observed in the SAXS pattern and are in good accordance with the formation of a 2D hexagonal lattice with a p6mm symmetry (Col h ).In this case, the unit cell parameter (a = 35.5 Å) was calculated by looking at the position of the most intense scattering signal assigned to the (10) reflection.A comparison of previously published XRD data for metal-free crown ether CE 11 17 with those for CL n (Table 1) To rationalize these results, we propose that the nanocluster, because of its bulkiness, cannot be accommodated within the c2mm lattice of CE n , resulting in a p2mm lattice with an elongated b axis, so that the cluster fits in the centre of the unit cell and is surrounded by 4 stacks of crown ethers CE 9 with an intracolumnar distance of 4 Å (Fig. 2).This results in a height of 12 Å for the unit cell and the (1 : 4) cluster/CE 9 stoichiometry is maintained.In that case the inorganic moieties volumic fraction is around 9 vol%.For the corresponding Col h phase of CL 8 , the observed a value should allow a hexagonal packing of the discotic crowns CE 8 , while each Mo 6 Br 14 2À unit is encapsulated by three crowns and leads to a volumic fraction of 22 vol% for Cs 2 Mo 6 Br 14 .
As expected, the emission properties of [Mo 6 Br 14 ] 2À are retained within the hybrid materials (see ESI, † Fig. S15). 18e photoluminescence spectra (l exc = 380 nm) show a broad and intense band extending from 550 nm up to 950 nm.The influence of the hybrid nanostructures on the inorganic emitter emission was investigated by temperature-dependent luminescence studies.Samples were heated up to the isotropic state and slowly cooled to reach a nanostructured glassy or crystalline state at 25 1C prior to measurements.The phosphorescence signals were collected at every degree upon heating (see the ESI, † Fig. S16 and S17).As previously reported, 19 if the emission intensity variation upon heating is only due to an increase of non-radiative deactivation processes, it should follow an exponential law of the type: I(T) = I 0 exp(ÀA/T) where A is constant and I 0 the emission intensity at a given temperature.Thus, reporting ln(I/I 300 ) = f (1/T) should give a straight line and this is what is observed for Cs 2 Mo 6 Br 14 (Fig. 3).Surprisingly, increasing the temperature from 25 1C up to 140 1C does not have much effect on the ability of the hybrid materials to emit light as an emission intensity loss of only less than 10% was recorded (the intensity loss at 140 1C is about 77% for Cs 2 Mo 6 Br 14 in its powder form).At higher temperatures, the obtained curves significantly deviate from the straight line with a similar behaviour for CL 7 and CL 9 .The separation of the clusters from each other by isolating triangular arrays of crown ethers might be the reason for the increased luminescence intensity of CL 8 in the high temperature Col h phase as compared to the low temperature Col r phase.Presumably, the crown ethers act as an isolating matrix for luminescent clusters, making these hybrids attractive alternatives with regards to polymers for applications requiring temperatures around 100 1C. 20Thus, the columnar nanostructures have a strong influence on the emissive ability of hybrids despite the isotropic nature of the inorganic emitter.Let us emphasize that such unprecedented high stability of the emission intensity vs. temperature is particularly rare and has never been reported so far for TMC or TMC-containing hybrid materials.
As illustrated by absolute quantum yield measurements performed in an air or N 2 atmosphere, the emission intensity can also  This journal is © The Royal Society of Chemistry 2016 be modulated by modifying the local atmosphere of the material.In all cases, a reversible and instantaneous switch from 0.05 or 0.07 in air up to 0.20, 0.26 or 0.30 in an N 2 atmosphere was measured for CL 7 , CL 8 and CL 9 , respectively (Fig. S18 and Video S1, ESI †).This phenomenon, which shows that these materials might be used as efficient oxygen sensors, is due to the O 2 permeability of CL n and is based on the ability of the excited states of the phosphorescent metal clusters to be quenched by molecular triplet oxygen, thus creating singlet oxygen. 21Although we demonstrated recently the same effect when nanoclusters are introduced into gas-permeable polymer matrices, 22 this is the first time that such behaviour is reported in a liquid crystalline material.
In conclusion, we show herein, that nanometric luminescent transition metal cluster compounds are easily integrated in a CLC hybrid material by using a supramolecular approach.In these materials, Cs + cluster unit counter cations are complexed within the cavity of crown ether derivatives functionalized by o-terphenyl units that are well known to generate columnar liquid crystalline phases.We show that the introduction of cluster salts results in columnar rectangular phases of p2mm symmetry and that their thermal stability is largely enhanced as compared to the parent crown ether CE n .Surprisingly, a second columnar hexagonal phase was observed on heating for CL 8 .Luminescence studies reveal that the red NIR clusters emission stability upon heating is unexpectedly largely enhanced as compared to its precursor namely Cs 2 Mo 6 Br 14 .Moreover, we demonstrate for the first time that modifying the atmosphere surrounding the LC hybrids with an inert gas induces a large, instantaneous and reversible increase of its emission efficiency thanks to the good O 2 permeability of the LC matrix.The columnar mesophases observed for these novel clustomesogens in combination with their luminescence properties make them promising candidates for oxygen sensors, optoelectronic or photovoltaic devices.As a further step towards such applications, the mesophase temperature ranges need to be lowered by molecular design.Work towards this goal is currently in progress.
that the symmetry of the Col r lattice changed upon complexation.While the lattice parameter a remains relatively unaffected, i.e. 66.1 Å (CE 11 ) vs. 66.3Å (CL 9 ) despite the difference of 2 CH 2 units in the side chain, the b values almost doubled from 25.1 Å (CE 11 ) to 44.4 Å (CL 9 ).Halos were in a similar range (4.6 Å vs. 4.0 Å).

Table 1
Thermodynamic and X-ray diffraction data for CL n compounds a In kJ mol À1 K À1 .b In kJ mol À1 .c Observed on cooling from the isotropic state; Cr: crystalline state; Col r : columnar rectangular; Col h : columnar hexagonal; I: isotropic state.