Article pubs.acs.org/Macromolecules
Chirally Homogeneous and Heterogeneous Dendritic Liquid Crystals A. Belaissaoui,* I. M. Saez, S. J. Cowling, and J. W. Goodby Department of Chemistry, University of York, Heslington Road, York YO10 5DD, U.K. S Supporting Information *
ABSTRACT: We have coupled methyl α-D-glucoside (G) and methyl α-D-mannoside (M) as the chiral structural variant moieties in the core and at the branching sites within dendritic scaffolds surrounded by 12 cyanobiphenyl mesogens (CB). Systematic studies of the influence of the chiral moieties on thermal and mesomorphic properties were carried out by positional permutation approach of the pyranose units G and M within the core and branching points. The mesomorphic properties of the chirally homogeneous dendrimers GG4/MM4 and the corresponding chirally heterogeneous dendritic homologues GM4/MG4 were investigated by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Remarkably, the thermal profile and the mesophase structure of the four dendrimers appear to be significantly independent of the nature of the central chiral core. The outer chirality at the periphery significantly dominates the liquid crystalline properties.
■
INTRODUCTION Although remarkable progress has been made in nanoscience, there are still huge challenges in arranging molecular functional moieties into structures where they are organized in a controllable and preprogrammed fashion to perform specific and predefined tasks at macroscopic scale. We have focused on functional liquid crystals as they combine the functional components, relevant to perform chemical, biological, and physical tasks, with the high self-organization, remarkable control, and large variety of mesophase morphologies present in liquid crystals. Dendritic systems1−3 are useful models since their size, shape, constitution, branching, functionality,4 and morphology can be readily controlled at the molecular level. Their defined singular functional and structural features are essential ingredients in the molecular engineering of liquid crystals that can lead to unique materials with fine-tunable functions controlling the mesophase properties. Following our studies on supermolecular5−7 liquid-crystalline chiral tripedes and tetrapedes,8,9 we have developed novel functional liquid crystalline dendritic10−12 analogues that combine mesogenic units with chiral functional moieties in a single molecule. In this article, we describe the architectures of a homologous series of chiral dendrimers13−18 that we have prepared with the aim to investigate the impact of the chirality in dendritic systems and how the chiral information encoded at molecular level is transferred and expressed at various levels of organization, from the molecular through to the macroscopic level into the mesophase. In this study, we have coupled methyl α-D-glucoside (G) and methyl α-D-mannoside (M) as the chiral structural variant moieties in the core and at the branching points within dendritic scaffolds surrounded by 12 cyanobiphenyl mesogens (CB). The two pyranose cores differ only at the C2 position with R configuration for the glucoside and S for the mannoside. We report the influence of the stereogenic © 2013 American Chemical Society
centers on the mesomorphic properties of the chirally homogeneous dendrimers GG4/MM4 (the core and branching points having identical pyranose units) and compare it with the corresponding chirally heterogeneous dendritic homologues GM4/MG4 (the core and branching points having different pyranose units) (Figure 1).
■
EXPERIMENTAL SECTION
The C2 epimers methyl α-D-glucoside (G) and methyl α-D-mannoside (M) were used as starting materials for the synthesis of dendrimers building blocks, carbohydrate core molecules G0 and M0 as well as dendrons G1 and M1, as shown in Scheme 1. Allylation reactions of glucoside G and mannoside M were carried out using allyl bromide and sodium hydride NaH in DMF to yield quantitatively their per-allylated corresponding derivatives Ga and Ma, followed by hydroboration reaction with 9-BBN and oxidation, to afford the tetra-O-(3-hydroxypropyl)pyranose homologues G0 and M0 in excellent yields. The carboxylic acid tripedes G1 and M1, which we have previously reported the synthesis,9 were coupled with the tetraol glucoside and mannoside derivatives G0 and M0 by esterification reaction under DCC/DMAP coupling conditions. The reaction was monitored by GPC until completion to yield the monodisperse dendrimers GG4, MG4, GM4, and MM4.
■
RESULTS AND DISCUSSION The mesomorphic properties of the dendrimers were investigated by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). When viewed through cross polarizers, upon cooling from the isotropic state to 90 °C, GG4 displayed a planar texture showing left-handed helicity, identifying clearly the mesophase as a chiral nematic. At 89.7 °C, a transition to a smectic A mesophase was observed where Received: December 24, 2012 Revised: January 22, 2013 Published: February 6, 2013 1268
dx.doi.org/10.1021/ma3026332 | Macromolecules 2013, 46, 1268−1273
Macromolecules
Article
Figure 1. Structural architectures of chiral liquid crystalline dendrimers GG4, GM4, MG4, and MM4.
to isotropic liquid transition at ∼60 °C. The DSC confirms the POM observations that the transition into the chiral nematic phase is broad for GM4 and much sharper in the case of MM4. The heating DSC curves of GG4/MG4 (outer G) display three endothermic transitions: a glass to a smectic A, followed by two overlapping broad shoulders allocated to smectic A-chiral nematic−isotropic transitions. Comparison of the thermal properties of the dendrimers shows glassification and clearing temperatures depression from GG4/MG4 (outer G) to GM4/ MM4 (outer M). Furthermore, MM4, with mannoside as core and branching units, displays the lowest glass transition (43.1 °C), and conversely the highest glass transition temperature is observed when all connecting units are glucosides GG4 (53.3 °C). The G stabilizing/M destabilizing effect is significantly more pronounced when comparing the clearing temperatures, as the isotropic onsets are reduced from around 90 °C for GG4/MG4 (outer G) to 59/56 °C for MM4/GM4 (outer M), respectively. This is in agreement with our earlier findings on
the phase was characterized by the typical focal conic and homeotropic domains. Similarly, MG4 displayed a smectic A phase with a poorly defined focal conic texture. This texture did not improve with annealing over a prolonged time scale. Slow cooling of the GM4 sample from isotropic liquid revealed the formation of ill-defined birefringent domains, which developed slowly. In contrast, MM4 developed rapidly a thermally reversible grainy texture, without specific characteristics or defects, which did not evolve further. GM4/MM4 showed interference with light indicating left-handed helicity, allowing for the unequivocal assignment of the mesophase of both dendrimers as chiral nematic. The DSC thermograms of the four dendrimers are shown in Figure 2, and the transition temperatures are listed in Table 1. All the materials were in the glassy state at room temperature. On heating, the DSC of the compounds MM4/GM4, which both have mannoside M at the branching sites (outer M), exhibit two thermal events: a glass to a chiral nematic transition around 45 °C and the chiral nematic 1269
dx.doi.org/10.1021/ma3026332 | Macromolecules 2013, 46, 1268−1273
Macromolecules
Article
Scheme 1. Synthetic Route to Chirally Homogeneous GG4/MM4 and Heterogeneous GM4/MG4 Dendritic Mesogens
Figure 2. DSC thermograms of chirally homogeneous GG4/MM4 and heterogeneous GM4/MG4 dendritic mesogens.
1270
dx.doi.org/10.1021/ma3026332 | Macromolecules 2013, 46, 1268−1273
Macromolecules
Article
packing frustration when bearing short spacer units C6 (M5). However, incorporating longer flexible arms C11 (M10) allows a high degree of structural relaxation, thus inducing a higher phase order smectic A and an enhanced thermal stability of the mesophase. The most noteworthy observation to emerge from the DSC data is the grossly identical thermal profile of the dendrimers bearing the same peripheral pyranose units. Higher clearing temperatures and phase order (smectic A) are displayed by the dendrimers GG4/MG4 having glucoside G at the periphery. Similar to tetrapedes, the general trend of the stabilizing effect associated with the outer glucoside G, due to its inherent structural relaxation, is evident. Conversely, MM4/GM4 show low clearing and glass melt transition temperatures, lower phase order (chiral nematic), and broadened phase transitions, expressing packing frustration, which is attributed to the structural heterogeneous configuration of mannoside M branching units. When comparing dendrimer GG4 and tetrapede G10, both display smectic A mesophase and have almost identical clearing temperature (3 °C difference). This can be ascribed to the structural analogies between GG4 and G10 and their relaxed structures, as both systems possess highly flexible scaffolds and glucoside branching units, which are conformationally mobile and configurationally homogeneous, to find the optimum configuration, allowing the maximum space filling efficiency and consequently enhancing their mesophase thermal stability. In contrast, MM4 displays a lower ordered phase (chiral nematic) and a lower clearing temperature (by 23 °C) than tetrapede M10 (smectic A), which is attributed to the local perturbations stemming from M dendrons. Closer examination of the DSC data shows slightly broader shoulders recorded for the heterogeneous dendrimers GM4/ MG4 in comparison to their corresponding homogeneous dendritic homologues MM4/GG4, respectively. This can be assigned to the mismatch destabilization arising from having different units at the central core and branching points. This effect is demonstrated by kick-line analogy,19 suggesting that the transmission of the local perturbations, resulting from having different conformational and configurational effects, occurs even at long distance. Interestingly, the DSC curves of the dendrimers GG4/MG4 (outer G) show two overlapping broad shoulders attributed to smectic A-chiral nematic and chiral nematic−isotropic phase transitions, revealing the slow gradual conformational changes from the highly ordered smectic A phase going to chiral nematic and isotropic liquid. This can be largely ascribed to the high conformational flexibility of the dendritic scaffold, comprised of mobile pyranoses and alkyl chains, allowing conformational fluctuations and an access to a large number of low-energy conformers; hence, the sequential phase transitions occur gradually. This explains the general trend of the four dendritic analogues displaying broad transitions (Figure 2) and in accord with our earlier results on tetrapedes and tripedes. Thus, the first conclusion that can be drawn is that the mesophase structure and the thermal profile of the four chiral dendritic homologues are dominated overwhelmingly by the chirality at the branchings (outer chirality). When glucosides are at the periphery (GG4/MG4), the dendrimers are found to exhibit smectic A mesophase in addition to a very narrow range temperature chiral nematic phase, whereas having mannoside at the branchings (MM4/GM4) induces chiral nematic phase behavior. Parallels can be drawn again with glucoside and
Table 1. Phase Transition Temperatures (°C) and Corresponding Enthalpy Values (J mol−1) from DSC Heating and Cooling Curves for Chiral Dendrimers GG4, GM4, MG4, and MM4 GG4
MG4
G 53.3 SmA 89.7 (3.56) N*a 91.3 (5.53) I I 92.7 (−4.50) N*a 91.9 (−4.54) SmA 51.6 G G 49.1 SmA (N*)b 90.5 (8.29) I b
I 92.6 (−7.68) (N*) SmA 50.2 G a b
GM4
G 47.2 N* 55.7 (1.92) I I 50.4 G
MM4
G 43.1 N* 59.0 (5.06) I I 62.1 (−5.06) N* 36.9 G
Partial overlap between two transition shoulders SmA-N*-I. Pronounced overlap between two transition shoulders SmA-N*-I.
thermo-mesomorphic behavior of glucoside and mannosidebased tetrapedes and tripedes homologues.8,9 The tetrapedes incorporating glucoside G or mannoside M as chiral cores were surrounded by four CB mesogenic groups linked by either C6 or C11 alkoxylate chain spacers (Figure 3). The glucoside
Figure 3. Molecular structures of glucoside Gn and mannoside Mn tetrapedes (n = 5, 10) (*C2 position).
tetrapedes (G5/G10) displayed higher melting and clearing temperatures than their corresponding mannoside homologues (M5/M10), respectively (Table 2). The significantly higher Table 2. Phase Transition Temperatures (°C) and Corresponding Enthalpy Values (J mol−1) from DSC Heating and Cooling Curves for Mannoside and Glucoside Tetrapedes Gn and Mn (n = 5, 10) G5 G10
G 43.5 N* 101.8 (1.91) I I 112.0 (−1.58) N* 41.9 G G 21.0 SmA 94.2 (11.96) I I 100.9 (−12.74) SmA 25.4 G
M5 M10
G 33.6 N* 74.7 (1.47) I I 84.4 (−1.98) N* 35.2 G G 17.6 SmA 82.0 (9.34) I I 87.2 (−10.23) SmA 15.6 G
thermal stability of the glucosides G5/G10 is largely attributed to their associated structural homogeneity, since all four arms can adopt all-axial or all-equatorial configuration in both lowenergy chair conformers. Conversely, mannosides are structurally heterogeneous (broken symmetry) due to the different orientation of the substituent at the C2 position. Furthermore, while glucoside tetrapedes showed a decrease in the clearing temperature with increasing alkyl chain length (Table 2), mannoside tetrapedes followed the reverse trend, indicating a 1271
dx.doi.org/10.1021/ma3026332 | Macromolecules 2013, 46, 1268−1273
Macromolecules
Article
centro-chiral core and the mesogenic dendritic shell. The outer chirality at the periphery significantly dominates the liquid crystalline properties as a result of the relatively close vicinity of peripheral branching units and CB mesogens, thus allowing an effective transmission of the chiral structural information to the dendritic outer shell. Nevertheless, the individual chiral dendrons do not act freely from the central core in the form of discrete units but behave as part of the dendrimer as a whole.
mannoside tetrapedes bearing short spacers C6 (G5/M5), which display right-handed chiral nematic mesophase. The helical configuration is induced by the chirality twist effect on the packing properties in the mesophase. Conversely, wide temperature range smectic A mesophase was obtained when incorporating longer spacers C11 (G10/M10), regardless of the nature of the chiral core. This demonstrates that the structural information from the core is not transferred to the peripheral CB mesogens when incorporating long flexible arms C11, as a result of the high decoupling between the core and the mesogens. In contrast, the chiral information is more efficiently transmitted through shorter linkages C6 leading to chiral nematic mesophase. Comparatively to tetrapedes G10/M10, the four dendritic homologous possess even longer conformationally mobile spacers, comprising of alkyl chains and pyranoses (G/M), connecting the inner flexible core with the outer dendritic mesogenic shell. Hence, CB mesogens and pyranose central core are even more pronouncedly decoupled, resulting in very diffuse core chirality expression at the periphery. The loss of the central core chiral information can be attributed on one hand to the conformational flexibility of the dendritic skeleton and on the other hand to the “dilution effect”,20,21 resulting from the long linkage core-mesogenic shell. Thus, the phase structure of the four LC dendrimers is primarily dominated by the local chirality at the branching sites. Now that we have established that the phase structure of the dendritic homologues appears to be dictated solely by the chirality of the pyranose branching units within the dendrons, the critical question now arises: “does each dendron act as a discrete block independently from the central core or does each dendrimer behaves as whole unit?” If we consider the possibility that dendrons behave discretely, the direct consequence is that the dendrimers GG4/MG4 (outer G) and the glucoside tetrapede G5, both systems bearing C6 outer spacers and outer pyranose unit G, would have relatively similar phase structure behavior. However, this is not the case, since G5 displays purely chiral nematic while GG4/MG4 exhibit mainly thermally stable and highly ordered smectic A mesophase. This can be explained by the flexibility of the dendritic scaffold and the structural relaxation associated with the peripheral glucoside G (homogeneous configuration), thus maximizing the packing efficiency and consequently inducing a smectic A phase behavior. Conversely, MM4/GM4 exhibit lower ordered mesophase (chiral nematic), which can be attributed to the structural frustration associated with the peripheral M (heterogeneous configuration).
■
ASSOCIATED CONTENT
S Supporting Information *
Experimental procedures, POM, MALDI mass, and NMR data. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work has been financed by EPSRC within the EUROCORES Programme SONS II of the European Science Foundation. We also thank the EPSRC Mass Spectrometry Service at Swansea for provision of services.
■
REFERENCES
(1) Vögtle, F.; Richardt, G.; Werner, N. Dendrimer Chemistry; WileyVCH: Weinheim, 2009. (2) Astruc, D.; Boisselier, E.; Ornelas, C. Chem. Rev. 2010, 110, 1857−1959. (3) Newkome, G. R.; Shreiner, C. Chem. Rev. 2010, 110, 6338−6442. (4) Vögtle, F.; Gestermann, S.; Hesse, R.; Schwierz, H.; Windisch, B. Prog. Polym. Sci. 2000, 25, 987−1041. (5) Saez, I. M.; Goodby, J. W. Struct. Bonding (Berlin, Ger.) 2008, 128, 1−62. (6) Saez, I. M.; Goodby, J. W. Supramolecular Soft Matter: Applications in Materials and Organic; Nakanishi, T., Ed.; John Wiley & Sons: Hoboken, NJ, 2011; pp 301−321. (7) Deschenaux, R.; Donnio, B.; Guillon, D. New J. Chem. 2007, 31, 1064−1073. (8) Belaissaoui, A.; Saez, I. M.; Cowling, S. J.; Goodby, J. W. Soft Matter 2010, 6, 1958−1963. (9) Belaissaoui, A.; Saez, I. M.; Cowling, S. J.; Zeng, X.; Goodby, J. W. Chem. Eur. J. 2012, 18 (8), 2366−2373. (10) Donnio, B.; Buathong, S.; Bury, I.; Guillon, D. Chem. Soc. Rev. 2007, 36, 1495−1513. (11) Donnio, B.; Guillon, D. Adv. Polym. Sci. 2006, 201, 45−155. (12) Marcos, M.; Martin-Rapun, R.; Omenat, A.; Serrano, J. L. Chem. Soc. Rev. 2007, 36, 1889−1901. (13) Murer, P.; Seebach, D. Angew. Chem., Int. Ed. Engl. 1995, 34 (19), 2116−2119. (14) Murer, P. K.; Lapierre, J.-M.; Greiveldinger, G.; Seebach, D. Helv. Chim. Acta 1997, 80, 1648−1681. (15) Seebach, D.; Rheiner, P. B.; Greiveldinger, G.; Butz, T.; Sellner, H. Top. Curr. Chem. 1998, 197, 125−164. (16) Thomas, W.; Tor, Y. Chirality 1998, 10, 53−59. (17) Romagnoli, B.; Hayes, W. J. Mater. Chem. 2002, 12, 767−799. (18) Gibson, S. E.; Rendell, J. T. Chem. Commun. 2008, 922−941. (19) Lockman, J. W.; Paul, N. M.; Parquette, J. R. Prog. Polym. Sci. 2005, 30, 423−452 and further literature cited therein. (20) Guillena, G.; Kreiter, R.; Coevering, R. V.; Gebbink, R. J. M. K.; Koten, G. V.; Mazon, P.; Chinchilla, R.; Najera, C. Tetrahedron: Asymmetry 2003, 14, 3705−3712.
■
CONCLUSIONS In summary, we have used carbohydrates glucoside G and mannoside M building blocks as core and branching units to impart chirality to four mesogenic dendritic molecules. Systematic studies of the influence of the chiral moieties on thermal and mesomorphic properties were carried out by permutation approach of the carbohydrate units G and M within the inner core and the branching sites. The dendrimers GM4/MM4 (outer M) exhibit solely the chiral nematic mesophase at low temperature, while MG4/GG4 (outer G) display wide range smectic A and thermally instable chiral nematic mesophase. Remarkably, the thermal profile and the mesophase structure of the four dendrimers appear to be significantly independent of the nature of the buried central chiral core, which is attributed to the “dilution effect” and the high conformational flexibility of the linkage between the 1272
dx.doi.org/10.1021/ma3026332 | Macromolecules 2013, 46, 1268−1273
Macromolecules
Article
(21) Forrat, V. J.; Ramon, D. J.; Yus, M. Tetrahedron: Asymmetry 2008, 19, 537−541.
1273
dx.doi.org/10.1021/ma3026332 | Macromolecules 2013, 46, 1268−1273
