Tuning the solid-state emission of liquid crystalline nitro-cyanostilbene by halogen bonding

Introduction

Supramolecular chemistry has proven to be an efficient approach for the development of novel smart materials, since it relies on non-covalent interactions, which allow for dynamic responses to external stimuli [1]. In addition, the self-assembly of the complementary molecular entities provides an easy access to functional systems and enables recyclability and self-healing properties of the materials [2]. With respect to the formation of supramolecular liquid crystals, especially hydrogen bonding and halogen bonding have gained considerable attention [3-7]. In 2004, Bruce and co-workers reported the first example of a halogen-bonded liquid crystal based on pentafluoroiodobenzene and 4-alkoxystilbazole [5]. Ever since, several other groups employed halogen bonding for the formation of liquid crystalline materials [8,9]. For instance, Palacio et al. used (E)-1-(4-(octyloxy)phenyl)-2-(2,3,5,6-tetrafluoro-4-iodophenyl)diazene as a photo-switchable halogen bond donor and investigated the light-induced phase transition of the complexes with 4-alkoxystilbazoles [10]. Recently, Li and co-workers reported on a series of halogen-bonded assemblies to induce chirality in nematic liquid crystalline hosts and studied the light-induced manipulation of the photonic properties of these materials [11] In 2019, our group investigated the role of fluorine substitution of the aromatic halogen bond donor on the liquid crystallinity and the photo-response of halogen-bonded liquid crystals [12]. However, all reported halogen-bonded liquid crystals rely on the halogen-bond-acceptor capability of pyridyl units and so far, no study on the fluorescence behaviour of halogen-bonded liquid crystals has been reported.

In 2014, Tothadi and Desiraju reported on a new supramolecular synthon based on the non-covalent interaction between 1,4-dinitrobenzene and iodobenzene [13]. Their structural analysis of a series of ternary cocrystals revealed that the nitro group is a suitable halogen bond acceptor, which interacts with polarised iodobenzene components in three different geometries – symmetrical, unsymmetrical or sidewise. Many other crystal structures support these findings and suggest the suitability of this synthon for the construction of supramolecular entities [14-17]. However, no examples for supramolecular materials employing this complementary interaction have been reported so far. In the present study we report the first halogen-bonded liquid crystal based on the complementary binding of nitro-cyanostilbene and tetrafluoroiodobenzene derivatives. Therefore, tetrafluoroiodostilbene (F₄St) and a series of fluoroiodoazobenzenes (F₄Az, F₃Az, F₂Az, F₂^’Az) were employed as halogen bond donors and combined with nitro-cyanostilbene (NO₂-C_n) as fluorescent halogen bond acceptor (see Figure 1) to form halogen-bonded liquid crystals. The series of fluoroiodoazobenzenes with varying fluorination degree at the iodobenzene moiety was used to investigate the impact of halogen bonding on the properties of the assemblies. Since cyanostilbene molecules are known to show aggregation-induced emission (AIE) behaviour the photophysical properties of the resulting assemblies were investigated via variable-temperature fluorescence spectroscopy [18].

Results and Discussion

The halogen-bonded assemblies were obtained by mixing the acceptor components NO₂-C_n with the donor entities F₄St or F₄Az in a 1:1 molar ratio in CH₂Cl₂. The slow evaporation of the solvent and subsequent drying in vacuo yielded the desired assemblies.

The mesomorphic behaviour of the assemblies was investigated by polarised optical microscopy (POM) and differential scanning calorimetry (DSC). It should be noted that the individual building blocks F₄St and F₄Az as well as NO₂-C₈ and NO₂-C₉ do not exhibit mesomorphic behaviour. In contrast, mesophases were observed for components employing NO₂-C_n with longer alkyl chains. NO₂-C₁₀ showed focal-conic textures at 94 °C during cooling (see Figure 2a) and NO₂-C₁₁ showed an enantiotropic smectic behaviour (see Supporting Information File 1, Figures S11 and S15).

In our initial set of assemblies, we combined F₄St with NO₂-C_n with varying alkoxy-chain lengths (n = 8, 9, 10 and 11). The halogen-bonded assemblies exhibited mesogenic behaviour starting with an alkoxy chain length of n = 8. POM investigations revealed nematic mesophases for all complexes (see also Figure 2b) which is in contrast to the behaviour of pristine NO₂-C_n showing a smectic phase for the alkoxy chain lengths of n = 10 or 11. A strong odd–even effect was observed for the NO₂-C_n∙∙∙F₄St assemblies which indicates a significant impact of the alkyl chain length on the packing of the supramolecular entities in the solid state. This effect was also confirmed by the fluorescence behaviour (see paragraph on photophysical properties) and affects mainly the transition from the nematic to crystalline phase. In addition, it was observed that only assemblies with an odd number of carbon atoms in the alkoxy chain on the halogen bond accepting moiety (NO₂-C₉ and NO₂-C₁₁) displayed an enantiotropic phase behaviour. For the NO₂-C_n∙∙∙F₄Az assemblies no significant effect of the alkoxy chain length on the transition temperatures was observed. The temperature ranges of the nematic phases are significantly narrower and liquid crystallinity is induced starting with NO₂-C₉∙∙∙F₄Az (see Figure 3). The reduced performance of the azo series compared to the stilbazole series is in line with previous reports and can be attributed to the repulsion of the free electron pairs of the azo group [7]. Interestingly, enantiotropic phase transitions were observed exclusively for NO₂-C₁₀∙∙∙F₄Az, indicating a different solid-state arrangement of the azo compounds compared to the stilbazole-based assemblies. The mesomorphic properties of all nitro compounds and the assemblies are summarised in Figure 3 and Table 1.

The strength of the halogen bond has a crucial impact on the formation of the liquid crystalline phase as shown, e.g., by Bruce et al. [5,19]. They reported on the thermal properties of halogen-bonded assemblies between stilbazoles and iodo- or bromopentafluorobenzene. While the iodo derivative formed assemblies with liquid crystalline properties, no LC behaviour was observed for the assemblies with bromopentafluorobenzene as it forms a weaker halogen bond. Related systems have been investigated by Yu and co-workers, who studied a series of halogen-bonded liquid crystals based on the combination of azopyridines with molecular iodine or bromine [9]. Interestingly, the broadest mesophase temperature ranges were found for the bromine-based assemblies and not as anticipated, for the iodine system which yields a stronger halogen bond.

In order to prove that the halogen bond plays a crucial role for the induction of liquid crystallinity in our assemblies, we synthesised a series of azo compounds with decreasing fluorination degree at the halogen bond donating iodobenzene [12]. Reducing the number of the fluorine atoms at the halogen bond donating moiety lowers the polarisation of the iodine atom and thus weakens the halogen bond. In a first step, we calculated the interaction energies of the assemblies of the azobenzene halogen bond donors F₄St, F₄Az, F₃Az, F₂Az or F₂’Az with NO₂-C₁. For computational efficiency reasons the terminal alkoxy chains were substituted by methoxy groups. The interaction energies were calculated using the latest theoretical counterpoise correction on the mp2/LanL2DZ with the basis set super position error (BSSE). The computed interaction energies for the assemblies (Figure 4, Table S1 in Supporting Information File 1) decrease in the order F₄St > F₄Az > F₃Az > F₂Az > F₂’Az. This is in accordance with our previous results on pyridine-based assemblies and supports the assumption that a reduction of the fluorination degree at the iodobenzene yields a weaker polarisation of the iodine and thus a weaker halogen bond [12].

To investigate this effect experimentally, a series of azo benzenes with different fluorination patterns was synthesised (F₄Az, F₃Az, F₂Az, F₂’Az) and combined with NO₂-C₁₀ in CH₂Cl₂. Upon removal of the solvent the material was investigated with respect to its mesomorphic behaviour using POM and DSC. In a simplified view, the experimental data is in line with the theoretical data and confirms that the formation of thermally stable halogen bonding induces the liquid crystalline properties of the assemblies. We suppose that the formation of the halogen-bonded assembly expands the mesogenic core and yields a more balanced ratio of rigid and flexible segments, which is crucial for the formation of a mesophase [7,20].

The azo compounds with a weakly polarised iodine atom (F₂Az or F₂’Az) have significantly lower interaction energies and halogen bonding appears not sufficiently strong enough to extend the mesogenic core at elevated temperatures. Thus, no liquid crystalline phase was observed for the NO₂-C₁₀∙∙∙F₂Az and NO₂-C₁₀∙∙∙F₂’Az assemblies (see Figure 5). In contrast, the highly fluorinated azo compounds F₄Az and F₃Az with a stronger polarisation on the iodine atom form sufficiently strong halogen bonds with NO₂-C₁₀ to extend the mesogenic core and induce liquid crystallinity (see Figure 6) [12].

It appears surprising, that NO₂-C₁₀∙∙∙F₃Az shows a broader nematic mesophase range (ΔT = 12.5 °C) than NO₂-C₁₀∙∙∙F₄Az (ΔT = 4.6 °C). However, the increase in the temperature range is predominantly attributed to the lowering of the crystallisation temperature, which indicates that the strength of the halogen bond is not the only contributor to the mesomorphic behaviour of the halogen-bonded materials. The change in the electronic anisotropy by unsymmetrical substitution with fluorine as present in F₃Az will also have an impact on the dispersion interactions and packing in the solid state and adds to the shift of the crystallisation temperature.

Photophysical studies

Recently, our group has shown that self-assembly provides an efficient way to tune fluorescence behaviour of liquid crystalline materials [21]. Phenolic thioethers showing aggregation-induced emission properties were combined with alkoxystilbazoles to form hydrogen-bonded mesogens. Since NO₂-C_n is known to be fluorescent, we were curious how the formation of the halogen-bonded complexes affects the AIE behaviour of NO₂-C_n. Therefore, we studied the photophysical properties of 1:1 assemblies by UV–vis and fluorescence spectroscopy. The NO₂-C_n building blocks are moderately fluorescent in the solid state and F₄St as well as F₄Az show no significant fluorescence. However, upon formation of the halogen-bonded assemblies the fluorescence of the materials is significantly changed. Since the F₄Az-based assemblies did not show fluorescence behaviour, the following discussion focuses on the assemblies NO₂-C₉∙∙∙F₄St as representative example. The fluorescence of NO₂-C₉ appears yellow-green, while F₄St is non-fluorescent. Upon formation of the halogen-bonded liquid crystal, green fluorescence was observed (see Figure 7).

Similar to the observation made for the liquid crystalline behaviour, also the fluorescence behaviour shows a strong odd–even effect for the alkoxy chain length at the NO₂-C_n unit, which indicates a significant difference in the molecular packing in the solid state. The fluorescence properties noticeably changed from the pristine assemblies directly after removal of the solvent, compared to the materials after one heating/cooling cycle (see Figure 8). In addition, the odd–even effect already described for the liquid crystalline properties also affects the fluorescence behaviour of the supramolecular assemblies in the solid state. Directly after removal of the solvent, all materials were obtained as crystalline powders with weak to moderate fluorescence as observed under UV light by the naked eye (see Figure 8). In contrast, after one heating/cooling cycle the fluorescence of the assemblies based on nitro-cyanostilbenes with an odd number of carbon atoms in the alkoxy chain significantly increased, while for the even-numbered NO₂-C_n no significant change in the fluorescence was observed.

The AIE behaviour of cyanostilbenes was first described by Park and co-workers [22]. This readily accessible class of fluorophores can easily be modified and attached to other functional entities, thus it has become a promising candidate for the design of new materials for optoelectronic applications [23]. Recently, it was successfully employed as a fluorescent moiety in the design of luminescent liquid crystals. In supramolecular liquid crystals, however, it did not find application so far [24].

We also studied the temperature dependence of the fluorescence behaviour. In the solid state at room temperature, NO₂-C₉∙∙∙F₄St shows a high intensity emission at λ = 545 nm, which gradually decreases with increasing temperature. At 70 °C, the fluorescence is almost completely quenched. This temperature correlates with the transition from the crystalline phase to the liquid crystalline phase as observed by POM and DSC. The further heating to 110 °C did not yield significant changes in the fluorescence intensities (see Figure 9). Upon cooling the sample to room temperature, the fluorescence returned, which is in line with the previous reports and characteristic for mesogens with AIE behaviour [25].

Conclusion

In summary, the first example of a halogen-bonded fluorescent liquid crystal is reported, employing the interaction between iodofluorobenzene derivatives and nitro-cyanostilbenes. A systematic investigation of the materials revealed the impact of the halogen bond donor on the liquid crystalline properties of the assemblies. While the stilbene-based halogen bond donor induces the formation of nematic mesophases with broad temperature ranges, the temperature range of the mesophase of the azobenzene-based assemblies is significant narrower. Theoretical calculations and the modular use of halogen bond donors with changing fluorination degree reveal that at least three fluorine atoms are needed for the formation of a thermally stable halogen bond to induce liquid crystalline properties.

In addition, the formation of the halogen-bonded assemblies had an impact on the fluorescence and photophysical properties of the supramolecular mesogens showing the characteristic AIE behaviour. The length of the terminal alkoxy chain at the nitro-cyanostilbene had a significant impact on the fluorescence behaviour, which was attributed to the packing differences of the assemblies with an even or an odd number of carbon atoms in the alkoxy chain. Currently we are preparing a comprehensive study investigating the potential of halogen bonding in fluorescent liquid crystalline materials.