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Luminescent liquid crystalline materials based on palladium(II) imine derivatives containing the 2-phenylpyridine core† Marin Micutz,a Monica Iliş,a Teodora Staicu,a Florea Dumitraşcu,b Iuliana Pasuk,c Yann Molard,d Thierry Roisneld and Viorel Cîrcu*a In this work we report our studies concerning the synthesis and characterisation of a series of imine derivatives that incorporate the 2-phenylpyridine (2-ppy) core. These derivatives were used in the cyclometalating reactions of platinum(II) or palladium(II) in order to prepare several complexes with liquid crystalline properties. Depending on the starting materials used as well as the solvents employed, different metal complexes were obtained, some of them showing both liquid crystalline behaviour and luminescence properties at room temperature. It was found that, even if there are two competing coordination

Received 5th August 2013, Accepted 16th October 2013

sites, the cyclometalation process takes place always at the 2-ppy core with (for Pt) or without (for Pd)

DOI: 10.1039/c3dt52137k

the imine bond cleavage. We successfully showed that it is possible to prepare emissive room temperature liquid crystalline materials based on double cyclopalladated heteroleptic complexes by varying the

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volume fraction of the long flexible alkyl tails on the ancillary benzoylthiourea (BTU) ligands.

Introduction Liquid crystals incorporating metal ions (metallomesogens) with luminescent properties are of great interest for their promising application in electrooptical devices, in particular for display applications based on liquid crystals or organic light-emitting diode (OLED) technology. Emissive properties were reported for a series of purely organic liquid crystals1 or metallomesogens and the latter ones were recently reviewed.2 There are several studies dealing with light-emitting metallomesogens based on palladium(II)3 or platinum(II)4 complexes, most of them showing the metal in a cyclometalating environment.5–8 The explanation might be that one of the best strategies used to promote luminescence in d8 metal complexes is to employ ligands with a very strong ligand field in

a Department of Inorganic Chemistry, University of Bucharest, 23 Dumbrava Rosie st, sector 2, Bucharest 020464, Romania. E-mail: [email protected], [email protected] b Centre for Organic Chemistry “C. D. Nenitzescu”, Romanian Academy, Spl. Independentei 202B, Bucharest 060023, Romania c National Institute of Materials Physics, P.O. Box MG-7, Magurele, 077125, Romania d Sciences Chimiques de Rennes UMR 6226 CNRS Université de Rennes 1, Avenue du Général Leclerc, 35042 Rennes Cedex, France † Electronic supplementary information (ESI) available: Crystallographic data including the cif files, emission and absorption spectra, powder X-ray diffraction data, DSC traces and polarising optical microscopy pictures. CCDC 948666–948668. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt52137k

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order to raise the metal-centred (MC) state, as it is the case for cyclometalating ligands, mostly with 2-arylpyridine or 2-thienylpyridine derivatives.9–11 Bruce et al. reported phosphorescent liquid crystalline complexes of platinum(II) showing a stimulus-dependant emission12 as well as highly luminescent (yields higher than 0.5) Pt(II) containing metallomesogens.13 The recently reported Pt(II) metallomesogens bearing 3(2-pyridyl)pyrazole chelates show quantum yields near 1, when recorded in degassed dichloromethane.4a All these platinum(II) luminescent metallomesogens reported so far show relatively high transition temperatures with mainly columnar and, in few cases smectic phases stable at high temperatures. These photophysical studies were done either in the solid state, liquid state or in films obtained by solidification of the mesophase in order to preserve the specific ordering of the liquid crystalline state. The luminescence properties of d8 systems depend on the presence of a sufficiently high energy gap between the lowest emitting excited state and the upper-lying metal-centered excited states when high-field ligands are employed. By comparison with Pt(II) complexes, it has been proved in several cases that the use of cyclometalated ligands is not enough to promote room temperature luminescence properties in Pd(II) complexes. For this reason, luminescent Pd(II) complexes at room temperature are very rare when compared to their Pt(II) analogues, in particular because of the presence of low-lying metal-centred excited states which deactivate the potentially luminescent metal-to-ligand charge transfer

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(MLCT) and ligand-centered (LC) levels through thermally activated processes.14 In most cases, luminescent Pd(II) and Pt(II) complexes contain heterocyclic ligands, usually with one or two pyridine rings, while imine ligands were completely ignored from this point of view in spite of their wide spread in the design of Pd(II) or Pt(II)15 metallomesogens. Several examples of luminescent, either cycloplatinated16 or platinum(II) complexes with N,N-donor diimine ligands are known.17 The double cyclopalladation reaction with various substrates has been intensively studied18 while its application in the design of liquid crystalline materials was less explored.19 In this paper we describe the first example of a doubly cyclopalladated columnar Pd(II) metallomesogen based on an unsymmetrical imine ligand containing the 2-ppy unit that shows luminescence in the solid state and in solution at room temperature. This system can offer the possibility of developing a whole new range of such complexes due to the different possibilities for tuning the mesogenic properties of such compounds, as well as other related physico-chemical properties.

Results and discussion We have started from a series of imine ligands that contains the 2-phenylpyridine core, 1a–f (see Scheme 1), a widely used mesogenic unit,20,21 in order to take advantage of its ability to promote emission properties when linked to either Pt(II) or Pd(II) ions. First we were interested to prepare the analogous complexes of Pt(II) by using various starting materials, such as K2PtCl4, [Pt(μ-Cl)(η3-C4H7)]2, or [PtCl2(DMSO)2] that are extensively used in the cycloplatination reaction.22 The product of the first cyclometallation step was not purified further (2), but used subsequently in the following step involving the bridge-splitting reaction with a simple BTU derivative. Surprisingly, in all cases, except when using [PtCl2(DMSO)2] as a starting material, the final product shows that the splitting of the imine bond occurred in the first step when the 1 H-NMR of the dinuclear product 2 shows a signal at ∼10 ppm assigned to CHO proton and, only the simple heteroleptic complex (3) with 4-(2-pyridyl)benzaldehyde and BTU ancillary ligands was obtained. Such a cleavage of an imine ligand was previously shown to occur with various metals and starting materials.23 Yellow-orange crystals were isolated from the reaction mixture and studied by single-crystal X-ray diffraction. The crystal structure confirmed firmly that the Pt(II) ion shows a very strong preference for the 2-phenylpyridine coordination site and that the BTU ligand is coordinated in a normal fashion via the O and S atoms to complete the square-planar geometry around the Pt(II) ion24 (Fig. 1). Moreover, these complexes show interesting luminescence properties with quantum yields around 8% in non-degassed dichloromethane solution. If the starting material used in attempted cyclometalation reaction was [PtCl2(DMSO)2], then light-yellow complexes of the general formula trans-[PtL(DMSO)Cl2], where L =

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Dalton Transactions

Fig. 1

Molecular structure of 3.

Schiff base ligand, were obtained in refluxing toluene and their structure was confirmed by single-crystal X-ray diffraction studies. Such a chemical behaviour was reported in few other cases when reaction between [PtCl2(DMSO)2] with various Schiff bases having a pyridyl ring or other various pyridine-containing ligands25 led to monocoordination of the ligand through the pyridinic N atom by replacing only one molecule of DMSO solvent of the initial starting molecule.26 The crystal structure shows that the platinum(II) atom has a square-planar geometry as expected and it is surrounded by two chlorine atoms in trans positions, one sulfur atom of the coordinated solvent and the nitrogen atom from the pyridine ring of the Schiff base (Fig. 2). All the bond lengths and angles at the central Pt atom are in the normal range for such complexes (Pt–Cl 2.3037(13) Å and 2.3016(13) Å respectively, Pt–N 2.078(4) Å and Pt–S 2.2170(13) Å)27 (see ESI, Table 2†). Following these experimental results, we turned our attention to the cyclopalladation reaction by employing simply the palladium acetate as a starting material and, in this way, we were able to isolate in toluene red products of doubly cyclopalladated imine ligands as confirmed by 1H and 13C NMR spectroscopy, as well as by MS. Thus, in the 1H NMR spectra two different methyl signals assigned to two different acetato groups could be seen in the 2.5–1.5 ppm range. As previously described in the literature,19b such a product could have a polymeric structure. Additional support for the nuclearity of the new complexes arises from mass spectrometry. The MS

Fig. 2

Molecular structure of 4b.

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pattern for complex 5b shows m/z values for [M + Na]+ (739.3) and [M + AcO + 2Na]+ (823.2), together with some other higher m/z values fragments and some other recombination fragments. Our aim was to prepare new emissive liquid crystalline materials and, for this reason, the products of the first cyclometalation step were not deeply investigated. According to these observations we proceeded further with these μ-acetatobridged Pd(II) complexes and the bridge cleavage reaction was performed in the presence of either a simple BTU derivative or by employing BTU derivatives containing a long alkoxy mesogenic group in order to establish a correlation structure – mesomorphic behaviour for such doubly cyclopalladated complexes. The synthesis pathway used to prepare the palladium(II) complexes is presented in Scheme 1. The double cyclopalladated complexes were prepared by reacting the acetato-bridged complexes with BTU derivatives in dichloromethane solvent in the presence of potassium carbonate. All of the complexes 9–12 were obtained in good yield as yellow to orange products, which are stable under ambient conditions. The formation of the mixed-ligands double cyclopalladated complexes can be confirmed readily by IR and 1H-NMR spectroscopy when the coordination of the N-benzoylthiourea derivatives in the

Scheme 1

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deprotonated form was confirmed by the disappearance of νNH (∼3300 cm−1) and νCvO (∼1670 cm−1) frequencies (compared to the IR spectra of free ligands) together with a shift of νC–N frequency towards lower wavenumbers. All these information suggest the absence of NH hydrogen located between carbonyl and thiocarbonyl groups of the benzoyl thioureic moiety, information that is further supported by 1H-NMR spectroscopy. Despite the BTU derivative being unsymmetric, the final products (9–12) were found to contain only one of the two possible syn and anti isomers with respect to the positions of sulfur atom of the BTU ligand and nitrogen atom of the metallated imine ligand. For the purpose of elucidating the products of this reaction, N,N-dipropyl-N′-benzoylthiourea ligand was employed in the first stage. Successful isolation of yellow crystals by slow evaporation from a mixture of dichloromethane and ethanol afforded a single crystal X-ray study to be performed and the molecular structure to be unequivocally confirmed. The molecular structure of doubly cyclopalladated complex having eight carbon atoms in the terminal alkyl chain is shown in Fig. 3. The complex has two different Pd(II) ions with a square-planar arrangement, one coordinated in a cyclometalated fashion to the 2-phenylpyridine core and the second, in a similar cyclometalated fashion, coordinated to the imine core, both having completed the coordination

Synthetic routes and chemical structures of the complexes.

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Dalton Transactions Table 1

Compound

Transition

T/°C

ΔH/kJ mol−1

1a

Cr–N N–I Cr–N N–I Cr–N N–I Cr–N N–I Cr–N N–I Cr–N N–I Cr–Cr′ Cr′–I I–Cr Cr–I Cr–Colh Colh–I I–Colh Colh-g g-Colh Colh–I

118 142 116 140 114 137 115 134 118 132 118 128 87 96 84 171 87a 156a 152b 24 21 165b

32.3 1.0 36.2 0.9 45.8 1.2 52.2 1.2 57.4 1.7 64 1.8 10.5 49.3 50.0 49.1 61.2 0.2 — — — —

1b 1c 1d

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1e 1f 7 10 11

Fig. 3

Molecular structure of 9.

12 a

environment by the deprotonated mononegative BTU ancillary ligand. All the bond distances as well as the angles at the two Pd atoms are similar to those found in either cyclometalated 2-phenylpyridine Pd(II) complexes with different ancillary ligands or cyclometalated imine Pd(II) complexes with various BTU derivatives. Such complexes have an overall rigid planar conformation, making them very interesting candidates for discotic materials by subsequent appropriate substitution with promesogenic groups. Now, if the simple BTU derivative is replaced with alkoxysubstituted BTU derivatives, yellow-orange solids can be obtained and, according to the number of mesogenic groups per BTU ligand, a different mesomorphic behaviour could be seen. Room-temperature liquid crystals are materials with melting points below 25 °C. In order to decrease the melting point of the mesogenic materials, our strategy was to increase the volume fraction of the long flexible carbon chains around the doubly cyclopalladated rigid core.28 In this respect, we progressively increased the number of alkyl chains onto the BTU ancillary ligands, taking advantage of our previous strategy to attach long alkyl chains either in 4; 3,4 or 3,4,5 positions of the benzoyl fragment of these ligands.29 Liquid crystal properties All the six new Schiff bases derived from the 2-phenylpyridine unit exhibited an enantiotropic nematic phase over a relatively short range, between 10 and 30 °C. This phase was evidenced by polarising optical microscopy observations when a fluid and homogeneous texture, either Schlieren- or marbled-textures, was developed on cooling from the isotropic liquid (Table 1). Additionally, the enthalpy values associated with these transitions fall well within the typical values for a nematic to isotropic transition. The melting and clearing points follow a normal trend with a little variation on passing to the superior analogue, with respect to increasing the number of carbon

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Thermal and liquid crystalline properties

Data taken from the first heating-cooling cycle. measured by POM.

b

Temperatures

atoms in the alkyl chain. The investigation of complexes 10–12 using POM and DSC showed that only complexes 11 and 12 form liquid crystalline phases. On heating, the complex 10 melts straight to the isotropic phase around 170 °C while on cooling the isotropic phase the complex simply starts to solidify around 120 °C without crystallisation. This behaviour shows that the five alkyl chains around the discotic molecular shape of compound 10 are not sufficient enough to reduce the melting point and stabilise a liquid crystalline phase. A reduced stability of liquid crystalline phases for Pd(II) complexes having alkylated 2-phenylpyridine cyclometalated ligands and acetyl-acetonates as ancillary ligands was noticed and this was explained by a slight deviation from a planar geometry of this unit with consequences on the packing of aromatic organometallic cores.20 In contrast, by successive addition of alkyl chains onto the BTU ligands, complexes 11 and 12 display large range liquid crystalline phases. The DSC trace for complex 11 exhibits two transitions on the first heating cycle. The first endothermic transition at 80 °C was assigned as a transition from a crystalline state to a columnar phase (Colh) as identified by XRD studies. This transition was followed by a much weaker one around 160 °C assigned to isotropisation with an enthalpy of 0.2 kJ mol−1. The cooling trace showed no transition corresponding to an isotropic liquid to liquid crystalline phase, this broad transition that starts at around 152 °C being detected only by polarising optical microscopy. Further cooling resulted in no crystallisation process and only a glass transition could be detected at 24 °C (temperature recorded at half inflexion point). The following heating–cooling cycles exhibited no peaks, but the glass transition in the same interval meaning that the complex 11 remains in the mesophase from 24 °C up to 155 °C. The mesophase formed on cooling from the isotropic

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Fig. 5 X-ray diffraction pattern of 11 at 85 °C after cooling from isotropic phase.

Fig. 4 Columnar texture of compound 11 at 90 °C on cooling from the isotropic phase (a); mosaic texture of 12 seen between crossed polarisers at 80 °C on cooling from the isotropic phase (b) (magnification ×200).

liquid of compound 11 displayed a typical texture for columnar mesophases (Fig. 4). The texture of 11 showed large homeotropic areas with columnar axes orientated perpendicular to the glass substrates, meaning that the material has a strong tendency to orient with its optic axis perpendicular to the surface of the untreated glass microscope slides. This texture remains virtually the same after cooling to room temperature when the compound solidifies as a glass. The complex 12 that possesses one additional alkyl chain onto the BTU ligand and nine alkyl chains all together in the molecule was isolated as a mesomorphic paste from the reaction mixture. Its DSC trace shows no transition on the first heating cycle, except a glass transition with the inflexion point at 21 °C. The clearing temperature, 165 °C, was detected by POM. Powder X-ray diffraction studies were performed on cooling the samples from the isotropic liquid to the mesophase. All compounds show diffraction patterns typically observed for mesophases in the temperature range of liquid crystal phase and frozen, glassy mesophases. Additionally, virgin samples of 11 and 12 were subjected to X-ray diffraction studies at room temperature and the results confirmed the crystalline nature of complex 11 (Fig. S3, ESI†) before melting to the liquid crystalline state. In contrast, complex 12 shows a slightly similar pattern with the ones recorded at higher temperatures, thus confirming its liquid crystalline state at room temperature. The 1D-WAXS patterns recorded for complexes 11 and 12 (Fig. 5 and 6) are typical of a disordered hexagonal columnar mesophase and contains a series of several Bragg diffraction peaks in the low angle region with their ratio 1 : 31/2 : 2 : 71/2 : 3… that resembles a hexagonal lattice. The results are collected in ESI (Table 3†).

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Fig. 6 X-ray diffraction pattern for complex 12 recorded at 100 °C on cooling from isotropic liquid.

The X-ray patterns for both Pd(II) compounds show the intensity of the d11 signal extremely weak, but detectable, compared to the d10 and d30 signals (for 11) or d10 and d20 signals (for 12). This rather unusual diffraction pattern observed for both Pd(II) complexes was reported in several other cases showing a columnar hexagonal mesophases and it was attributed to an incomplete powder averaging resulting in unreliable relative intensities of the reflections in the X-ray diffractogram.30 Along with these reflections, a broad peak around 4.6 Å corresponding to the molten state of the chains and a less intense diffraction around 3.6 Å corresponding to shortrange π–π stacking interactions (Fig. 5) were also observed in the X-ray pattern of complex 11. The two-dimensional hexagonal parameter (d10) was evaluated as 36.2 Å corresponding to a distance of 41.8 Å between neighbouring columns. Additionally, a relative sharp signal around 7.7 Å was found for 11 and this was assigned to Pd–Pd interactions as the closest Pd–Pd distance deduced from the solid-state structure of 9 was 7.5 Å. In the X-ray pattern of 12, in the wide angle region, besides the diffuse scattering at 4.7 Å, due to the liquid-like state of the terminal aliphatic chains, an additional broad scattering

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Fig. 7 Proposed model for the molecular organisation of the building blocks in the hexagonal lattice of complex 11 (left) and 12 (right).

of weak intensity at ∼6.5 Å was found (Fig. 6). This corresponds to about twice the stacking distance of the aromatic cores in the columnar phase and points to the formation of an alternating molecular arrangement along the columns (Fig. 7), in order to allow a better space filling of the discs, similar to the observations made for other palladium(II) compounds with 2-phenylpyridine or imine cores.20 Interestingly, the hexagonal lattice parameters for the two complexes are significantly different (41.8 Å for 11 and 38 Å for 12, respectively), with a smaller value for the complexes having a greater number of alkoxy chains at the periphery implying a different organisation within the mesophase. In order to better understand this difference in the mesophase structure, we estimated the number of molecules Z forming a columnar hexagonal unit, using the following equation:31 Z ¼ 31=2 N A a 2 hρM 1 21 where ρ = density of the liquid crystalline phase (estimated to be ∼1 g cm−3), NA = Avogadro’s constant, a = columnar lattice parameter, h = height of a single columnar unit and M = molecular mass. In the case of complex 11 this value was found to be between one and two molecules in the slice of the column, while in the case of complex 12 this value amounts around 1. To propose a packing model for these two complexes, extensive molecular modelling (Hyperchem) were employed based on the X-ray molecular structure of complex 9 (Fig. 8). In this way, for the Colh phase of compound 11, two molecules instead of only one as for complex 12, could be put together in the crosssection of the columns in order to have a lattice parameter of 41.8 Å. Complex 11 with seven decyloxy tails showed a regular intracolumnar distance along the columns, as revealed by the reflection corresponding to h0 ≈ 3.6 Å. To accommodate two molecules per disc unit it is necessary for complex 11 to adopt approximately a half disc shape resulting from the folding of aniline unit around the NH group of the BTU ligand along the long axis of the molecule defined by the 2-phenylpyridine imine ligand in order to facilitate the efficient space filling and to adopt approximately a hemidisc shape. In fact, this bent orientation of the flexible aniline fragment of the BTU ligands is often seen in the X-ray solid state structure of several Ph-NH(CO)-(CS)NH-R BTU molecules and even for palladium complexes with such derivatives and is due to the formation of hydrogen bonding between NH proton and CvS groups.32 This means that, depending on the number of terminal alkoxy

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Fig. 8 Top-view of the columnar hexagonal packing of complex 11 (a) and complex 12 (b).

chains, two different supramolecular organisations within the columns are expected for the two complexes 11 and 12. Photophysical properties The photophysical properties of Pd(II) complexes have been investigated and the results are summarised in Table 2. Their UV-VIS absorption spectra in CH2Cl2 (Fig. 9) feature two highly intense absorption bands at around λmax 290 (ε > 5.2 × 104 mol−1 dm3 cm−1) and 330 nm (ε > 6.6 × 104 mol−1 dm3 cm−1) that are assigned to 1LC(ππ*) ligand centred transitions of the cyclometalated and BTU ligands, based on similarities with the absorptions of the free ligands. The first band

Table 2 Absorption and emission data for the Pd(II) complexes in dichloromethane solution (concentration 5 × 10−4 M) and in solid state

Compound 10 11 12

Absorption, λmax/nm (ε × 10−3/M−1 cm−1) 254(sh, 107.2), 290(115.6), 324(127.2), 381(sh, 58.9), 405(sh, 42.2), 450(15.9), 247(sh, 80.7), 284(52.5), 338(66.9), 407(sh, 22.1), 457(8.2) 251(sh, 73.7), 285(sh, 54.8), 331(66.9), 403(sh, 27.2), 452(10.2)

Emission, λem/nm (λexc/nm; Φ/%) Solutiona

Solid state

410(285/ 0.28)

518, 667, 727, 817(sh)

365(283/ 0.55)

546, 676, 736, 819(sh)

360(283/ 0.39)

556, 683, 744, 823(sh)

a

Quantum yields were determined with respect to [Ru(bpy)3]2+ in water.

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Fig. 9 The UV-VIS spectra of palladium(II) complexes recorded in dichloromethane solution.

around 290 nm appeared like a shoulder in the case of complex 12. Sometimes, these are seen like shoulders probably due to broadening resulted from the superposition of π–π* transitions of the two different coordinated ligands. In addition, several shoulders are also present in their UV-VIS spectra (around 250 nm for all complexes), and a moderately intense band that is absent for the ligands is observed at λmax 450 nm (ε > 0.8 × 104 mol−1 dm3 cm−1), with a shoulder at around 400 nm (ε > 2.2 × 104 mol−1 dm3 cm−1). These less intense low-energy absorptions located at λ > 350 nm could be assigned to a mixture of spin-allowed metal-to-ligand charge transfer (1MLCT) and ligand-centred (1LC) transitions, and this assignment is consistent with previous results based on related cyclometalated Pd(II) compounds.33 The solid-state emission spectra recorded at room temperature for complexes 10–12 are shown in Fig. 10. The emission spectra of all three Pd(II) complexes show two maxima at λmax around 660–685 and 725–745 nm, respectively, with a shoulder around 820 nm when the samples are irradiated with λexc = 480 nm. This orange-red emission has also been visually detected at the optical microscope when the samples were irradiated in the 380–420 nm region (ESI, Fig. S1 and S2†). It

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was found that the emission bands slightly red-shift with increasing number of alkoxy chains in the molecules. A similar trend was seen for other luminescent metallomesogens, including Pd(II) complexes,3c with different number of alkoxy tails.34 Taking into account the previous reports on luminescent Pd(II) complexes, this emission can be attributed to originate from excimeric 3ππ* intraligand excited states as a dσ*–π* MMLCT assignment is possibly ruled out due to the lack of evidence for appreciable Pd–Pd contacts (the shortest Pd–Pd distance in the crystal lattice of complex 9 is 7.5 Å). On the other hand, if the samples are irradiated with λexc = 380 nm, another less intense structured emission band was observed, with λmax around 450 nm and two shoulders on each side located approximately at 410 nm and 470 nm, respectively (ESI, Fig. S10†). These latter high-energy emission bands could be tentatively assigned to intraligand emission (3IL) by the cyclometalated ligand in analogy with several other luminescent 2-phenylpyridine containing Pd(II) complexes.14b,35 The luminescent properties of the Pd(II) complexes were also studied in solution when only one emission band was observed, with its maximum located in the 360–410 nm range assigned to an intraligand transition which are blue-shifted when compared to the solid-state emission spectra, as it is normally expected (ESI, Fig. S9†). However, the quantum yields of all compounds recorded in non-degassed CH2Cl2 estimated with [Ru(bpy)3]Cl2 as a standard (Φ = 2.8% in air-equilibrated water)36 were relatively low, and were found in the range 0.28%–0.55%, very similar to the values reported for other Pd(II) luminescent complexes.14b–d Now, if the emission of the two mesomorphic Pd(II) complexes was recorded in the mesophase, at room temperature, after previous heating to 100 °C followed by rapid cooling to ambient temperature, a slight red-shifted emission was observed for complex 11 while the emission features of 12 were virtually the same (ESI, Fig. S11 and S12†). These latter findings confirmed again the existence of 12 in the liquid crystalline state at room temperature. For 11, a slight red shift of the red emission was observed due to the columnar organisation in the liquid crystalline phase (ESI, Fig. S11†).

Conclusions

Fig. 10 The solid-state emission spectra of complexes 10–12 (λexc = 480 nm).

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It has been shown that by incorporating the 2-phenylpyridine unit into the imine ligands and varying the number of peripheral alkoxy chains on the ancillary ligands we can prepare red-emissive Pd(II)-containing liquid crystalline materials with columnar mesophases stable at room temperature. An implication of different number of alkoxy chains on the organisation within the mesophase was seen and this was assigned to a different way of packing inside the columns. We successfully proved that it is possible to prepare red-emissive room temperature liquid crystalline materials based on double cyclopalladated heteroleptic complexes by varying the volume fraction of the long flexible alkyl tails on the ancillary benzoylthiourea (BTU) ligands.

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Experimental section Dichloromethane was distilled over phosphorus pentoxide; other chemicals were used as supplied. C, H, N and S analyses were carried out with a Perkin Elmer instrument. IR spectra were recorded on a Bruker spectrophotometer using KBr pellets. UV-VIS absorption spectra were recorded using a Jasco V-660 spectrophotometer. 1H and 13C NMR spectra were recorded on a Varian Gemini 300 BB spectrometer operating at 300 MHz, using CDCl3 as a solvent. 1H chemical shifts were referenced to the solvent peak position, δ 7.26 ppm. ESI-MS analysis was performed on an Agilent VL mass spectrometer. The phase assignments and corresponding transition temperatures for the palladium(II) complexes were determined by polarising optical light microscopy (POM) using a Nikon 50iPol microscope equipped with a Linkam THMS600 hot stage and TMS94 control processor. Temperatures and enthalpies of transitions were investigated using differential scanning calorimetry (DSC) with a Diamond DSC Perkin Elmer. The materials were studied at scanning rates of 5 and 10 °C min−1 after being encapsulated in aluminium pans. Two or more heating/cooling cycles were performed on each sample. Mesophases were assigned by their optical texture and powder X-ray diffraction studies. The powder X-ray diffraction measurements were made on a D8 Advance diffractometer (Bruker AXS GmbH, Germany), in parallel beam setting, with monochromatized Cu-Kα1 radiation (λ = 1.5406 Å), scintillation detector, and horizontal sample stage. The measurements were performed in symmetric (θ–θ) geometry in the 2θ range from 0.5° to 10° or 30° in steps of 0.02°, with measuring times per step in the 5–40 s range. The temperature control of the samples during measurements was achieved by adapting a home-made heating stage to the sample stage of the diffractometer. X-ray single-crystal data for complexes 3, 4a and 9 were collected with a Bruker-AXS APEXII diffractometer. The structure was solved by direct methods using the SIR97 program,37 and then refined with full-matrix least-square methods based on F 2 (SHELXL-97)38 with the aid of the WINGX39 program. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. H atoms were finally included in their calculated positions. Luminescence spectra were recorded on a Fluorolog-3™ fluorescence spectrometer (FL3-22, Horiba Jobin Yvon) in the solid state and employing a Jasco FP-6300 spectrofluorometer (operating parameters: band width – 5 nm; data pitch – 0.5 nm; scanning speed – 100 nm min−1; spectrum accumulation – 3; path length – 10 mm using Quartz SUPRASIL cells) in dichloromethane solution. Synthesis of imine ligands The imine ligands were prepared by the condensation reaction between the 4-(2-pyridyl)benzaldehyde and corresponding 4-alkoxyanilines, catalyzed by glacial acetic acid, in absolute ethanol, using the same procedure described elsewhere for the preparation of imine ligands.40 Below is presented the preparation of 1a. To a solution of p-hexyloxyaniline (0.193 g,

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1 mmol) in ethanol (10 cm3), 4-(2-pyridyl)benzaldehyde (0.183 g, 1 mmol) followed by a few drops of glacial acetic acid were added. The mixture was heated under reflux for 2 h and then cooled to −25 °C to give the crude product. Crystallisation from hot ethanol gave the analytically pure product as an offwhite crystalline solid. Compound 1b. Yield 76%. Calc. for C26H30N2O: %C: 80.79; %H 7.82; %N 7.25. Found: %C 80.65; %H 7.78; %N 7.18. 1H NMR (CDCl3, 300 MHz): 8.77 (d, br, 1H); 8.58 (s, 1H); 8.15 (d, J = 8.5 Hz, 2H); 8.03 (d, J = 8.2 Hz, 2H); 7.84–7.81 (m, 2H); 7.33–7.27 (m, 3H); 6.97 (d, J = 8.8 Hz, 2H); 4.02 (t, J = 6.6 Hz, 2H); 1.87–1.78 (m, 2H); 1.55–1.30 (m, 10H); 0.85 (m, 3H). 13 C NMR (CDCl3, 75 MHz); 158.2, 157.7, 156.8, 150.0, 144.8, 141.8, 137.1, 137.0, 129.1, 127.3, 122.7, 122.4, 120.9, 115.2, 68.5, 32.0, 29.5, 26.2, 22.8, 14.2. νmax(KBr)/cm−1: 3430br, 2954m, 2922s, 2855s, 1621s, 1581m, 1502s, 1468s, 1435w, 1391w, 1290m, 1251vs, 1191w, 1111w, 1004m, 837s, 775m, 729w, 538w. Compounds 1b–f show the same chemical shifts with corresponding integration for methylene protons. Synthesis of Pt(II) complex 3 To a methanolic solution of [Pt(μ-Cl)(η3-C4H7)]2 (0.057 g, 0.1 mmol in 20 ml) solid 1 (0.25 mmol) was added and the suspension was stirred at room temperature for 48 h. The resulting orange solid was filtered off, washed several times with cold methanol and dried. The 1H-NMR spectroscopy evidenced the CHO signal around 10 ppm. This product was reacted further with an excess amount of sodium salt of N,Ndipropyl-N′-benzoylthiourea (BTUPr2Na) (0.072 g, 0.25 mmol) in dichloromethane to give an orange crystalline solid of 3 in 69% yield. Single-crystals were obtained by cooling a mixture of acetone–methanol at −25 °C. Compound 3. Yield 78%. Calc. for C26H27N3O2PtS: %C: 48.74; %H 4.25; %N 6.56; %S 5.01. Found: %C 48.56; %H 4.37; %N 6.48; %S 4.77. 1H NMR (CDCl3, 300 MHz): 10.03 (s, 1H); 9.37 (d, J = 6.4 Hz, JPt–H = 31 Hz, 1H); 8.20 (d, J = 8.5 Hz, 2H); 8.10–7.92 (m, 2H); 7.87 (m, 1H); 7.63 (s, 2H); 7.56–7.38 (m, 4H); 3.91–3.79 (m, 4H); 1.95 (m, 2H); 1.77 (m, 2H); 1.09 (t, J = 7.4 Hz, 3H); 0.98 (t, J = 7.4 Hz, 3H) 13C NMR (CDCl3, 75 MHz); 193.3, 169.0, 168.4, 164.0, 151.0, 145.3, 138.8, 138.5, 137.6, 136.2, 135.2, 131.3, 129.2, 128.3, 123.9, 123.6, 120.0, 54.6, 53.6, 21.4, 20.9, 11.7, 11.6. νmax(KBr)/cm−1: 3444br, 2963w, 2927w, 2871w, 1735s, 1519s, 1483s, 1420vs, 1361m, 1308w, 1228w, 1199m, 1101w, 1067w, 870w, 829w, 775w, 745w, 711m. Synthesis of Pt(II) complex 4 [PtCl2(DMSO)2] (0.084 g, 0.2 mmol) was added to a solution of 1 (0.2 mmol) in toluene (50 ml) and the resulting solution was heated under reflux for 24 hours. On cooling the solution to −25 °C a yellow crystalline product was formed which was filtered off, washed with cold toluene and dried. Single crystals suitable for X-ray studies were grown by slow evaporation from a dichloromethane solution. Compound 4b. Yield 47%. Calc. for C28H36Cl2N2O2PtS: %C: 46.03; %H 4.97; %N 3.83; %S 4.39. Found: %C 45.85; %H 5.08; %N 3.65; %S 4.18. 1H NMR (CDCl3, 300 MHz): 8.90 (d, br, 1H);

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8.59 (s, 1H); 8.17 (d, J = 8.5 Hz, 2H); 8.08 (d, J = 8.2 Hz, 2H); 7.95 (td, J = 7.7 Hz, J = 1.6 Hz, 1H); 7.62 (d, J = 6.7 Hz, 1H); 7.49 (ddd, J = 6.0 Hz, J = 1.6 Hz, 1H); 7.29 (d, J = 8.8 Hz, 2H); 6.96 (d, J = 8.8 Hz, 2H); 4.00 (t, J = 6.6 Hz, 2H); 3.30 (s, 6H); 1.86–1.77 (m, 2H); 1.64–1.25 (m, 10H); 0.91 (m, 3H). 13C NMR (CDCl3, 75 MHz): 161.2, 158.3, 156.9, 152.3, 139.4, 130.4, 129.3, 128.3, 127.2, 124.2, 122.4, 118.6, 115.1, 68.4, 44.0, 31.8, 29.4, 29.3, 29.2, 26.1, 22.7, 14.1. νmax(KBr)/cm−1: 3425br, 2923s, 2859s, 1615s, 1583m, 1505s, 1289m, 1248vs, 1125s. Synthesis of μ-acetato-bridged Pd(II) complexes Palladium(II) acetate (0.5 mmol) was added to a solution of 1 (0.5 mmol) in toluene (25 ml). The resulting mixture was heated under reflux for 24 h when a red-orange solid was formed. The solid was filtered off, washed with cold toluene and dried. Compound 5b. Yield 57%. 1H NMR (CDCl3, 300 MHz): 7.96 (td, J = 7.8 Hz, J = 1.6 Hz, 1H); 7.66 (d, J = 7.9 Hz, 1H); 7.50 (s, 1H); 6.97 (m, 1H); 6.80 (s, 1H); 6.70 (m, 4H); 6.58 (s, 1H); 3.95 (t, J = 6.7 Hz, 2H); 2.34 (s, 3H); 1.83 (m, 5H); 1.61–1.26 (m, 10H); 0.90 (m, 3H). Synthesis of N-benzoylthiourea derivatives (BTU) The N-benzoylthiourea derivatives (6 and 8) as well as their sodium salts used in this work were prepared according to the methods published in the literature29,32 (Scheme 1, ESI†). Compound 7. Yield 64%. Calc. for C46H76N2O4S: %C: 73.36; %H 10.17; %N 3.72; %S 4.26. Found: %C 73.15; %H 10.36; %N 3.55; %S 4.01. 1H NMR (CDCl3, 300 MHz): 12.47 (s, 1H); 9.01 (s, 1H); 7.55 (d, J = 8.8 Hz, 2H); 7.42–7.40 (m, 2H); 6.92 (d, J = 8.7 Hz, 3H); 4.08–4.03 (m, 4H); 3.96 (t, J = 6.6 Hz, 2H); 1.90–1.20 (m, 52H); 0.85 (m, 9H). 13C NMR (75 MHz): 178.8, 166.7, 157.9, 154.7, 149.5, 130.6, 126.9, 123.6, 121.6, 114.7, 112.6, 112.2, 69.8, 69.4, 68.3, 32.1, 31.9, 29.9, 29.7, 29.4, 29.3, 29.2, 26.2, 26.1, 22.8, 14.5. νmax(KBr)/cm−1: 2921vs, 2851s, 1733m, 1664s, 1597s, 1561s, 1537s, 1510vs, 1467m, 1353m, 1277s, 1245m, 1209m, 1072s, 828w, 751w, 657w, 518w. Synthesis of Pd(II) complexes 9–12 Corresponding solid N-benzoylthiourea compound, 6–8, (0.30 mmol) or sodium salt of N,N-dipropyl-N′-benzoylthiourea (BTUPr2Na) for complex 9, was added to a suspension of μ-acetato-bridged palladium complex 5 (0.10 mmol) and K2CO3 in dichloromethane (15 cm3) and the mixture was stirred at room temperature for 24 hours. Evaporation of the solvent gave yellow to orange solids, which were purified by chromatography on silica using dichloromethane as an eluant to yield the final products. They were further crystallized from a mixture of dichloromethane–ethanol (1/1) at −25 °C. Compound 9. Yield 72%. 1H NMR (CDCl3, 300 MHz): 9.10 (d, br, 1H); 8.28 (s, 1H); 8.18 (d, J = 6.6 Hz, 2H); 7.97–7.85 (m, 2H); 7.75 (d, J = 7.4 Hz, 2H); 7.63 (s, 1H); 7.48–7.35 (m, 7H); 7.25–7.17 (m, 3H); 6.99 (d, J = 8.8 Hz, 2H); 4.03 (t, J = 7.1 Hz, 2H); 3.92 (m, 4H); 3.80 (m, 4H); 1.97–1.69 (m, 10H); 1.58–1.27 (m, 10H); 1.13–0.85 (m, 15H). Single-crystals suitable for X-ray

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study were obtained by slow evaporation of a solution of 9 in dichloromethane–ethanol 1/1. Compound 10. Yield 65%. Calc. for C86H114N6O7Pd2S2: %C: 63.73; %H 7.09; %N 5.18; %S 3.96. Found: %C 64.02; %H 7.01; N 5.48; %S 3.71. 1H NMR (CDCl3, 300 MHz): 8.99 (d, br, 1H); 8.17–8.07 (m, 3H); 7.97–7.84 (m, 4H); 7.71 (d, br, 2H); 7.48–7.42 (m, 6H); 7.35–7.27 (m, 2H); 6.99–6.90 (m, 7H); 6.72 (d, J = 9 Hz, 2H); 4.05–3.95 (m, 6H); 3.90 (d, J = 5.8 Hz, 1H); 3.85 (d, J = 5.8 Hz, 1H); 1.89–1.67 (m, 6H); 1.65–1.25 (m, 52H); 1.96 (m, 21H). 13C NMR (CDCl3, 75 MHz): 162.6, 158.8, 147.7, 124.6, 114.7, 114.5, 114.1, 113.6, 110.2, 68.8, 68.5, 39.5, 32.0, 30.7, 29.6, 29.5, 29.4, 29.2, 26.2, 24.0, 23.2, 22.8, 14.3, 11.3. νmax(KBr)/cm−1: 2923m, 2854m, 1604m, 1525s, 1507vs, 1473s, 1435s, 1404s, 1362m, 1297m, 1248s, 1220m, 1115m, 1097w, 1027w, 909w, 828m, 764w, 670w, 517w. Compound 11. Yield 56%. Calc. for C118H178N6O9Pd2S2: %C: 67.43; %H 8.54; %N 4.00; %S 3.05. Found: %C 67.25; %H 8.67; %N 3.85; %S 2.80. 1H NMR (CDCl3, 300 MHz): 9.00 (d, br, 1H); 8.16 (s, 1H); 7.96–7.67 (m, 6H); 7.50–7.27 (m, 10H); 6.96–6.86 (m, 6H); 6.67 (d, J = 8.6 Hz, 1H); 4.05–3.94 (m, 12H); 3.68 (t, br, 2H); 1.92–1.20 (m, 118H); 0.89 (m, 21H). 13C NMR (CDCl3, 75 MHz): 174.2, 164.3, 158.8, 152.6, 152.5, 149.0, 148.3, 148.2, 146.7, 141.5, 138.6, 134.5, 132.1, 130.1, 124.8, 124.5, 122.6, 115.1, 114.7, 114.5, 112.3, 111.8, 110.2, 69.2, 69.1, 68.6, 68.5, 32.1, 32.0, 29.9, 29.8, 29.7, 29.6, 29.5, 29.4, 29.3, 26.3, 26.2, 22.9, 14.3. νmax(KBr)/cm−1: 2921vs, 2851s, 1601w, 1540s, 1500vs, 1473s, 1437s, 1405s, 1302s, 1269s, 1244s, 1160m, 1131m, 826w, 757w, 720w, 652w, 517w. Compound 12. Yield 63%. Calc. for C142H226N6O11Pd2S2: %C: 69.04; %H 9.22; %N 3.40; %S 2.60. Found: %C 68.75; %H 9.47; %N 3.18; %S 2.29. 1H NMR (CDCl3, 300 MHz): 8.99 (d, br, 1H); 8.13 (s, 1H); 7.96–7.83 (m, 3H); 7.47–7.29 (m, 10H); 7.05 (s, 2H); 6.93–6.82 (m, 6H); 4.04–3.85 (m, 14H); 3.65 (t, br, 4H); 1.9–1.62 (m, 18H); 1.55–1.20 (m, 140H); 0.95 (m, 27H). 13 C NMR (CDCl3, 75 MHz): 164.0, 158.8, 155.6, 152.6, 152.5, 149.3, 142.5, 125.1, 124.4, 114.5, 114.4, 110.2, 108.5, 73.6, 73.5, 69.1, 68.9, 68.4, 61.9, 32.1, 32.0, 29.9, 29.8, 29.7, 29.6, 29.5, 26.4, 26.3, 22.8, 13.4, 13.2. νmax(KBr)/cm−1: 2922vs, 2852s, 1603m, 1525s, 1509vs, 1467m, 1437s, 1405m, 1360s, 1307m, 1243s, 1154m, 1113s, 1037m, 829w, 760w, 721w, 582w, 518w.

Acknowledgements This work was supported by a grant of the Romanian Authority for Scientific Research, CNCS-UEFISCDI, project number PN-II-ID-PCE-2011-3-0384.

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