ionic liquid crystals derived from 4-hydroxypyridine

0 downloads 0 Views 551KB Size Report
All bis(pyridinium) salts, except the triflate salt with shorter .... salts produced solid products contaminated with few amount of silver particles that could not be ...
Columnar bis(pyridinium) ionic liquid crystals derived from 4-hydroxypyridine: synthesis, mesomorphism and emission properties Amalia Panăa, Monica Ilişa, Teodora Staicub, Iuliana Pasukc, Viorel Cîrcua* a

Dept. of Inorganic Chemistry, University of Bucharest, 23 Dumbrava Rosie st, sector 2, Bucharest 020464, Romania, e-mail: [email protected], [email protected] b Dept. of Physical Chemistry, University of Bucharest, 4-12 Regina Elisabeta Blvd, sector 3, Bucharest 030018, Romania c National Institute of Materials Physics, P.O. Box MG-7, Magurele, 077125, Romania Abstract A series of flexibly linked bis(pyridinium) salts with various counterions (Br-, PF6-, BF4- and OTf-) was designed and prepared starting from corresponding N-alkylated 4-pyridones precursors with mesogenic 3,4,5tris(alkyloxy)benzyl moieties (alkyl = dodecyl or tetradecyl). These salts were investigated for their liquid crystalline properties by a combination of differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and temperature dependent powder X-ray diffraction (XRD). Their thermal stability was checked by thermogravimetric analysis (TG). All bis(pyridinium) salts, except the triflate salt with shorter terminal carbon chain, display an enantiotropic liquid crystalline behavior with a hexagonal columnar (Colh) phase assigned on the basis of its characteristic texture and XRD studies. It was found that these luminescent bis(pyridinium) salts show weak emission in dichloromethane solutions at room temperature and a pronounced red-shifted emission in solid state. The emission properties of these bis(pyridinium) salts do not depend significantly on the nature of counterion employed.

Introduction Ionic liquid crystals (ILC) are extensively studied nowadays due to their unique properties resulting from the combination of liquid crystal (LC) and ionic liquid (IL) properties. In the past years, several reviews covering this topic were published.[1,2] Different factors are responsible for governing the nature of ILC phases, such as the molecular shapes, location of ionic groups and their size, molecular interactions and microphase segregation. For instance, there is a strong tendency to stabilize lamellar phases due to electrostatic interactions and ion-ion stacking in ILC. Depending on the combination of these factors, the LC behavior ranges from typical calamitic behavior to discotic. On the other hand, the liquid crystalline dimers are well known to exhibit interesting mesomorphic behavior that can be, in many cases, different from the single mesogens.[3,4] Importantly, it has been shown that the thermal stability of the geminal dicationic ionic liquids with a liquid/stability range of over 300 °C are greater than those of most traditional monocationic ionic liquids, and, more recently it has been proven the same trend for the bis(cationic) ILC. For instance, Bruce et al.[5] reported a series of symmetric flexibly linked bis(imidazolium) ionic liquid crystals containing the 3,4,5-tris(alkyloxy) benzyl moiety that showed considerably higher stability range of the columnar phase when compare to their monomeric counterparts ILC. Furthermore, among the different ILC, the imidazolium- or pyridinium derivatives with weakly coordinating anions, such as tetrafluoroborate (BF4−) and hexafluorophosphate (PF6−), are well-known for their high thermal and electrochemical stabilities.[6, 7] Generally, the pyridinium-based ILC have been known since long time, displaying very similar properties with the related imidazolium based ILC.[8] While many bis(pyridinium) salts with flexible spacers have been used as ionic liquids with various purposes

(surfactants,[9-14] catalytic, [15, 16] lubricants,[17] biological [18-22]), to the best of our knowledge there are no reports dealing with flexibly linked dimeric bis(pyridinium) ionic liquid crystals. Gallardo et al.[23] reported a series of bis(pyridinium) ionic liquids with 1,3,4-oxadiazole containing linker which on heating decompose without showing any LC properties. Additionally, it is of interest to note here that many dicationic and tetracationic ILC based on the 4,4’-bipyridinium rigid core (viologen-based ILC) have been investigated so far, displaying mesomorphic properties typically of calamitic and discotic materials.[24-28] By comparison, so far, clearly the liquid crystal behavior of the dimeric imidazolium dicationic ionic liquids has been more often reported, including both calamitic and columnar behavior of these materials,[29-35] while little information is available for the bis(pyridinium) salts. The LC materials displaying columnar liquid crystal phases have attracted considerable interest due to their interesting applications in electronic or optoelectronic devices based on their ability to display improved anisotropic charge transportation.[36-38] From this point of view, there are only few examples of pyridinium based ILCs that display columnar phases. One representative example has a classical hexaalkyloxytriphenylene moiety connected via a flexible alkyl spacer to the pyridinium ring.[39] Recently, we reported a series of columnar pyridinium ILC derived from 4-hydroxypyridine and bearing the 3,4,5-tris(alkyloxy) benzyl moiety and various counterions.[40] In this context, it is worth mentioning here that the 4-hydroxypyridine can be easily functionalized with different mesogenic groups in two steps. The first step involves either N- or Oalkylation,[41-46] leading to either 4-pyridones or O-substituted pyridines respectively while in the second step the second alkylation is achieved. Lin et al.[47] reported a series of ILCs based on both O- and N- alkylated pyridinium salts with simple alkyl groups, derived from the 4-hydroxypyridine motif, that display a SmA phase depending on the alkyl chain length and the counterion employed. Therefore, we were interested in extending this study to the LC investigation of the corresponding flexibly linked bis(pyridinium) salts derived from 4-hydroxypyridine containing different alkoxy chains as terminal groups and various counterions such as bromide (Br-), hexafluorophosphate (PF6-), tetrafluoroborate (BF4-), and triflate (OTf-). Their emission properties were studied both in solution and solid state.

Results and discussion The synthesis of pyridone compounds 2a,b and of bis(pyridinium) salts 3-6a,b was carried out according to Scheme 1, following a similar method previously reported by us [40] for monocationic pyridinium salts derived from 4-hydroxypyridine. The 4-pyridone derivatives 2a,b were reacted with 1,10-dibromodecane to give the quaternized pyridinium bromide double salts 3a,b. This second alkylation step proceeds in relatively low yield, 18% and 48% respectively, and excess of 4-pyridone derivative was required as well as longer reaction time and careful chromatographic separation to remove the unreacted 4-pyridone starting material together with the monosubstituted pyridinium salt. Further, the metathesis reaction of the bromide anion with NH4BF4, NH4PF6 and AgOTf produced the corresponding salts 4-6a,b. All compounds were characterized by elemental analysis (C, H, N), IR, 1H- and 13C-NMR spectroscopy, supporting the proposed structure. As a mention, the use of 1,6dibromohexane instead of 1,10-dibromodecane with the aim to prepare the shorter spacer analogues gave rise to contaminated products and difficult to isolate by column chromatography. Therefore they were not included in present discussion. Moreover, the methathesis reaction performed with AgNO3 in order to prepare the nitrate bis(pyridinium) salts produced solid products contaminated with few amount of silver particles that could not be

removed by usual filtration through Celite or recrystallization and, for these reasons, they were not further included in this work. The IR spectroscopy was used to confirm the exchange of the bromide counterion. Thus, characteristic strong frequencies for the new groups were found in the IR spectra of the corresponding pyridinium salts (1083 cm-1 for BF4-, 842 cm-1 for PF6- and 1258 cm-1 for OTf-, respectively). The formation of the pyridinium salts was also confirmed by 1H- and 13C- NMR spectra. It was found that the anion has a significant influence on the chemical shift of the two protons located in the vicinity of the nitrogen atom. While for the bromide salts 3a, b this signal is located at around 9.19 ppm, upfield shifts were observed for all the other counterions (up to 8.61, 8.63 ppm for 4a,b, X-=PF6-, 8.73, 8.78 ppm for 5a,b, X=BF4- and 8.72 ppm for 6a,b, X=OTf- respectively). These chemical shifts deduced from the 1H NMR spectra of pyridinium salts 3-6a,b were found to follow the order: Br->BF4->PF6->OTf-, a similar trend observed for other different series of simple pyridinium salts or imidazolium salts.[8]

RO

OR

RO N

OH

RO

RO Br

TBABr/NaOH THF

N

O

2a, b

1a, b

RO

OR

OR

C10H20Br2

MeCN

RO Br N

O

(CH2)10

O

3a, b

N

Br OR

XRO

RO

MeOH/H2O

OR

OR

RO X N

O

(CH2)10

O

N

4-6a, b

X OR RO X= PF6- (4), BF4- (5), OTf- (6) OC12H25, a R= OC14H29, b

OR

Scheme 1. Preparation of bis(pyridinium) salts with various counterions.

Thermal behavior and mesophase assignment The LC phases exhibited by 4-pyridone intermediates 2a,b and pyridinium salts 3-6a,b were assigned based on the polarized optical microscopy (POM) textures as well as variable-temperature powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC) studies. Their transition temperatures and the mesomorphic properties are collected in Table 1. Both 4-pyridone intermediates display a monotropic behavior with a hexagonal columnar phase (Colh), as previously reported by us for shorter analogue.[40] For example, the DSC trace of 2b shows only one relatively sharp peak during the first heating run and this peak was assigned to a simple crystal to isotropic liquid transition. On cooling from the isotropic liquid, a weak and broad transition was observed around 58oC, corresponding to an isotropic to liquid crystalline phase transition. This phase was found to have a monotropic character for 2b, as previously reported for 2a [40] and, on further cooling, was followed by a crystallization process. Moreover, the second and the third DSC heating cooling cycles for 2b show a cold crystallization transition around 40oC, followed by the same transition temperatures as found in the first heating run, suggesting a good thermal stability of this product and the reproducibility of the liquid crystalline phase (Fig. 1). On the other hand, all bis(cationic) salts, except 6a, display an enantiotropic liquid crystalline behavior. The DSC curves of the bis(pyiridinium) salts, except 4a and 6a, show a strong exothermic peak on the first heating run assigned to melting to LC phase and a second broader peak assigned to the isotropization. The melting temperatures are shifted toward lower values on subsequent heating runs, compare to the values recorded in the first heating run. These melting temperatures are perfectly reproducible on the following heating cooling cycles and these values were reported in Table 1 (Fig. 1c). The first heating run of 4a shows a glass transition around 8oC accompanied by a broad transition to isotropic state around 97oC, without any sign of crystallization on the following heating-cooling cycles. The temperature range of the LC phase is greatly influenced by the terminal chain length (twelve or fourteen) and the nature of counterion employed. Interestingly, the triflate salt with 12 carbon atoms in the terminal chains, 6a, has the lowest melting point, 15oC, below ambient temperature and does not show any mesogenic behavior. Cooling its isotropic phase from room temperature down to negative temperatures resulted in crystallization around 15°C. Thus, it seems that a higher number of carbon atoms (fourteen instead of twelve) were enough to stabilize the LC phase for the 6b analogue with the same triflate counterion and 11oC mesophase range. This behavior resembles the general trend found for ILC, where clearing points show a decreasing tendency with an increase in the size of anions. Generally, the compounds with higher number of carbon atoms have higher melting points and isotropization temperatures with broader mesophase ranges (31oC for 3a and 40oC for 3b). For all bis(pyridinium) salts 3-6a,b, the LC phases were assigned to hexagonal columnar (Colh) on the basis of their characteristic texture, pseudo-focal conic and spherulitic textures, when observed by POM (Fig. 3). Importantly, the 3,4,5-trialkyloxybenzyl bromide precursors were used as alkylation reagent for imidazole or pyridinium units to prepare LC materials able to display columnar phases depending on alkyl chains length employed [5, 28, 38] and the assignment of a Colh seems to be reasonable in this respect. Moreover, it has been shown that for guanidinium-based ILC, the presence of three alkoxy chains on the mesogenic 3,4,5-tris(alkoxy) benzene unit led to columnar mesophases, while the presence of only one or two terminal alkoxy groups resulted in either suppression of LC behaviour or a SmA phase. [48]

But, in order to unequivocally confirm the mesophase assignment, several compounds (3b, 4a and 5a) were selected as representative examples to be subjected to variable temperature powder X-ray diffraction measurements. Typical patterns that are characteristic of a hexagonal packing were obtained. For example, on cooling from the isotropic state in the mesomorphic domain, the diffractograms of 3b and 4a show a strong maximum from the (100) reflection followed by a series of four weaker sharp peaks with a d-spacing ratio of d/√3, d/√4, d/√7 and d/√9 from the (110), (200), (210) and (300) reflections, respectively (Fig. 2).

(b) Heat Flow (Endo up)

Heat Flow (Endo up)

(a) 20

0

0

-20

-20

30

40

50

60

70

80

30

90

40

50

60

70

80

90

o

o

T/ C

T/ C

(c)

3

Heat Flow (Endo up)

20

0

10

20

30

40

50

60

70

80

90

100

110

o

T/ C

Fig. 1. DSC traces: first (a) and second (b) heating-cooling cycles for compound 2b and second heating-cooling cycle for 4b (c). Two important observations can be made by comparing the thermal properties of the dicationic bis(pyridinium) salts reported here with the available data on the related monocationic pyridinium salt with dodecyl chains as terminal groups reported elsewhere. First, the exchange of the Br- counterion with bulkier counterions (PF6-, BF4- and OTf-) did not lead to the suppression of LC properties as it was the case for related monocationic salts and, second, the mesomorphic range are significantly extended confirming the idea of a greater stability for dicationic ILC versus monocationic ILC. It is useful to bring into discussion the comparison of the clearing

temperatures for 3-6b series where all the reported temperatures are true thermodynamically values of mesophase existence. Surprisingly, these values (Table 1) suggest that the order of the stabilization effects on the columnar LC structures is PF6->BF4- >Br->OTf-, somehow different from the general trend imposed by the presence of bulkier anions which may disturb their columnar packing. Such a higher stability of liquid crystalline phase by exchanging with a bulkier counterion has been reported previously for ionic liquid crystals based on guandinium or imidazolium moieties. [38, 49, 50] In the present case, it is likely that the larger anion (BF4- and PF6-) allows the carbon chains to interdigitate resulting in a more efficient packing around the bulkier anions and an extended LC range. This statement was further supported by thermogravimetric (TG) analysis performed on several selected compounds, which showed that the decomposition started at more elevated temperatures (around 170-220oC, depending on the nature of counterion employed) than the case of related monocationic pyridinium salts. Moreover, these dicationic salts showed the same trend of thermal stability in respect to the counterion employed (Br-Br->OTf-. These dicationic salts showed the same trend of thermal stability in respect to the counterion employed (Br-