Nematic-to-isotropic photo-induced phase transition in azobenzene-doped low-molar liquid crystals

Abstract

The photo-induced phase transition generated by several 4,4′-disubstituted azobenzenes in host nematic low-molar mass liquid crystals was quantified by means of the efficiency, ΔT_N–I, using polarized optical microscopy (POM) at variable temperature. Non-covalent interactions established between mesogen and azobenzene were the main determinants of the photo-induced phase transition behaviour. The correct design of the chemical structure of the system components allows controlling the temperature range in which the photo-induced mesophase transition occurs. Therefore, more efficient systems were obtained when azobenzenes with an extended aromatic core were used.

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Introduction

Liquid crystals (LC) have unique physical properties, such as self-organization, fluidity with long-range order, cooperative motion and anisotropy.¹ The alignment of the mesogens can be easily manipulated by applying several physical external fields (optical,^2,3 thermal,^4–6 electromagnetic,⁷etc.). This is the basis of all current LC devices.¹ Liquid crystals doped with organic photochromic molecules such as azobenzenes are of great interest in photonic applications.^2,8–11 Azobenzenes can change their shape under light irradiation, allowing the modulation of liquid crystal properties, including phase transition temperature, molecular alignment, helical twisting power and spontaneous polarization, among others.¹² Light has been proved to be the most efficient stimulus because it is a cheap and clean energy source and can be controlled remotely, rapidly and precisely.¹³

When low-molar liquid crystals doped with azobenzenes are irradiated with UV light, the geometry of the azobenzene molecule changes from linear (trans isomer) to bent (cis isomer), thereby causing a drastic decrease in the order parameter, S. This is reflected in a decrease of the nematic-to-isotropic phase transition temperature of the mixture, T_N–I, which can produce a photo-induced phase transition from the ordered nematic phase to the disordered isotropic phase when the mixture is irradiated at a constant temperature in the vicinity of T_N–I.^14,15 Our goal is to obtain azobenzene-doped liquid-crystalline systems that withstand major distortions of the nematic order in a wide temperature range upon light irradiation, thereby generating switches between the ordered (anisotropic) state and the disordered (isotropic) state that function over a wide range of temperatures.

The azobenzene isomerisation process has been the object of many experimental and theoretical studies since its discovery in 1937.¹⁶ Due to the high applicability of azobenzene-doped liquid-crystalline systems, a thorough study of the molecular interactions that control azomonomer photo-induced order distortion in host liquid-crystalline phases is required. It is essential to understand the interactions between dissimilar compounds, as this enables the physical properties of materials to be tuned. In this paper, we show the influence of the chemical structure of the dye on the assembly process with the mesogen molecules. The intermolecular interactions are the main factor controlling the efficiency of the photo-induced isothermal distortion of the host nematic mesophase order.

Experimental

Materials and general instrumentation

All chemicals were used as supplied without further purification. CH₂Cl₂ was distilled from CaH₂; THF was distilled from sodium/benzophenone and DMF was dried by storing it over activated 4 Å molecular sieves under nitrogen atmosphere. Flash chromatography was carried out over silica gel (SDS, 230–240 mesh). The compounds were characterized by ¹H (400 MHz) and ¹³C NMR (100 MHz) and the spectra were collected using a Varian Mercury Spectrophotometer.

Synthesis and thermal characterization of mesogens M1–M7

We used seven distinct rod-like nematic mesogens as host matrices. M1 and M2 were purchased from Alpha Aesar and M3 from Merck. The mesogens M4–M7 were synthesised according to standard procedures reported elsewhere.¹⁷ The chemical structures of the mesogens M1–M7 are shown in Fig. 1. Their mesomorphic behaviour was analysed by differential scanning calorimetry (DSC). DSC thermograms were recorded using a Mettler-Toledo DSC821 module at a scan rate of 3 °C min⁻¹ under nitrogen flow. The nematic-to-isotropic transition temperatures, T_N–I, for the mesogens and their associated enthalpies, ΔH_N–I, are summarized in Table 1 and are consistent with those found in the literature.¹⁸

Fig. 1 Chemical structures of the nematic mesogens M1–M7.

Table 1 Nematic-to-isotropic phase transition temperatures, T_N–I, and enthalpies, ΔH_N–I, of mesogens M1–M7 and melting points, T_m, of azobenzenes 1–5 analyzed by DSC. All values were determined by cooling the samples from their isotropic state

Mesogen	T N–I/°C	ΔHN–I/J g^−1	Azobenzene	T m/°C
M1	35	2.7	1	163
M2	68	1.7	2	89
M3	72	3.4	3	97
M4	48	2.0	4	127
M5	50	2.4	5	157
M6	70	2.5
M7	89	3.9

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Synthesis of azoderivatives 1–5

4-Hydroxy-4′-methoxyazobenzene was synthesized from p-methoxyaniline by coupling the respective diazonium salt with phenol in basic media at 0–5 °C.¹⁹ Methylation of 4-hydroxy-4′-methoxyazobenzene using CH₃I in THF at reflux provided 1. Compound 2 was attained by means of Mitsunobu’s reaction between 4-hydroxy-4′-methoxyazobenzene and 5-hexen-1-ol using PPh₃ and DIPAD in THF at room temperature.²⁰ 4,4′-Dihydroxyazobenzene was obtained by cleaving the methyl ether of 4-hydroxy-4′-methoxyazobenzene with BBr₃ in dry CH₂Cl₂ at room temperature.²¹ Williamson’s dialkylation of 4,4′-dihydroxyazobenzene with 6-bromo-1-hexene using NaH as a base in dry DMF at reflux provided compound 3. Compound 5 was synthesized by Steglich’s DCC coupling between 4,4′-dihydroxyazobenzene and 4-(5-hexenyloxy)benzoic acid in CH₂Cl₂ at room temperature.²²4 was attained by the reaction of 4,4′-dihydroxyazobenzene, first with 1 equivalent of 4-methoxybenzoic acid and further with 1 equivalent of 4-(5-hexenyloxy)benzoic acid, using the same reaction conditions as for compound 5. Compounds 1–3 and 5 were obtained with global yields between 85–99%. Compound 4 was attained with a global yield of 17%. The chemical structures of all azoderivatives are depicted in Fig. 2. The melting temperatures to the isotropic state, T_m, of all synthesized azobenzenes were determined by means of DSC and are summarized in Table 1.

Fig. 2 Chemical structures of the azoderivatives 1–5.

Polarized optical microscopy (POM)

Polarized optical microscopy was performed using a Nikon Eclipse polarizing microscope equipped with a Linkam THMS 600 hot stage and a Linkam CI 93 programmable temperature controller at a scan rate of 3 °C min⁻¹. We studied several series of LC/azobenzene mixtures containing azobenzene molar fractions within the range 0.01 to 0.1. Samples were prepared by weighing the exact mass of the appropriate mesogen and azocompound. The resulting mixtures were melted to the isotropic state, thereby generating homogeneous solid solutions of the azobenzene in the liquid crystal matrix. To assure a constant composition, the mixtures were homogenized by adding several drops of CH₂Cl₂. They were then stirred magnetically in the liquid state. Finally, the solvent was slowly evaporated and the mixtures were dried for 24 h. The homogeneity of the samples was checked by local probe POM and DSC experiments.

We analyzed the photo-induced nematic-to-isotropic phase transitions by means of POM at variable temperature. At the first stage, the mixtures were placed between two circular cover glasses and then heated to the isotropic state. The nematic-to-isotropic phase transition temperatures with the trans-azobenzene present in the mixture, T_N–I(trans), were detected by cooling the sample from the isotropic phase with the light source power adjusted to 3 V. Trans–cis photo-isomerisation was performed in situ in the microscope which is equipped with an episcopic illuminator that incorporates a 12 V–100 W tungsten halogen lamp containing light in the UV region. The samples were irradiated with the light source power adjusted to 12 V for 5 min isothermally in their isotropic state at a temperature slightly higher than T_N–I(trans). The increase in the light voltage generates a shift towards the UV region (320–390 nm), thereby causing the trans–cis photo-isomerisation of the azocompound. Furthermore, the nematic-to-isotropic phase transition temperatures with the cis isomer present in the mixture, T_N–I(cis), were determined by cooling the samples from the isotropic state under irradiation with light at 12 V during the cooling process.

Computational methods

Semi-empirical AM1 calculations were performed on a PC with the GaussView 3.0/Gaussian 03 Revision B.04 software.²³

Results and discussion

trans-4,4′-Disubstituted azobenzenes present rod-like shapes while their cis isomers are bent. Accordingly, given the tendency of trans isomers to align along the director direction small amounts of this isomer can be introduced in nematic mesophases without causing their destruction. Upon irradiation of the resulting guest/host mixtures with UV light, trans–cis photo-isomerisation occurs, and disorganization of the nematic mesogens close to the azobenzene molecules is induced. This effect is extended to all nematic mesophase (domino effect), thereby generating instability in this phase.²⁴ As a result, the nematic-to-isotropic transition temperature, T_N–I, decreases and the system reaches the disordered isotropic state isothermally (Fig. 3). Fig. 4 illustrates a microphotograph series of the photo-induced nematic-to-isotropic phase transition of azocompound 1 dissolved in M2 under UV irradiation at 70 °C, a temperature slightly lower than the corresponding T_N–I(trans) (75 °C).

Fig. 3 Isothermal photo-induced phase transition caused by the irradiation with UV light of an azobenzene-doped nematic liquid crystalline system at an azobenzene molar fraction, x_i, at a temperature between the range ΔT_N–I.

Fig. 4 Solid solution of azobenzene 1 with monomer M2 observed under crossed polarizers. Destruction of the nematic mesophase by irradiation with UV light at a constant temperature T = 70 °C between T_N–I(cis) (69 °C) and T_N–I(trans) (75 °C) (up). Regeneration of the nematic mesophase when the thermal cis–trans relaxation of the azobenzene occurs in the dark (down).

The fundamental factors that are related to molecular structure and could influence the molecular assembly between host–guest molecules are the nature or extension of the rigid core and the length or polarity of the lateral substitution of the photoactive molecule. For this reason, we examined azobenzenes 1–5 as guest components (Fig. 2) in order to determine the effect of their chemical structure on the photo-induced nematic-to-isotropic phase transition of several nematogenic compounds. Three different series of nematic rod-like mesogens (Fig. 1) were chosen for the present study. Cyanobiphenyls (M1 and M2) and cyanobicyclohexyls (M3) were selected as representative mesogens with high dielectric anisotropy that are constantly required for technical applications. Phenyl benzoates (M4–M7) were chosen to analyze the effect of the chemical nature of the mesogen rigid core on photo-induced phase transition behaviour. In the last group of mesogens, the effect of the lateral substitution was also considered. The selected molecules are representative of the most widely studied nematogens for different applications. All the mesogens studied present their T_N–I values at moderate temperatures, between 35 and 89°C.

We determined T_N–I(trans) and T_N–I(cis) for each sample using POM by cooling samples from their isotropic state under irradiation at 3 and at 12 V, respectively. Reduced nematic-to-isotropic phase transition temperatures, T_N–I*(x), for the two isomers, were calculated for each azobenzene composition, x, by means of eqn (1):

where T_N–I(x) and T^LC_N–I correspond to the nematic-to-isotropic transition temperature for a mixture of an azobenzene molar fraction, x, and for the corresponding pure mesogen, respectively. T_N–I*(x) provides information about the stabilization or destabilization of the nematic mesophase with respect to the pure mesogen when an azobenzene is dissolved in it. In this sense, values higher than the unit mean that the liquid crystal phase is stabilized by the introduction of the azo-dye.

Reduced nematic-to-isotropic phase transition temperatures for all the mixtures studied are collected in Table 2. For azoderivatives 1–3, with two aromatic rings in their structure, trans isomers did not affect the stability of the host nematic mesophase greatly. Low reduced nematic-to-isotropic phase transition temperatures values, T_N–I*, from 0.80 to 1.23 were obtained for an azobenzene molar fraction of 0.1. Taking into account all the mesogens studied, the stabilization or destabilization of the nematic phase caused by the introduction of the trans-azobenzene in the mixture did not exceed 23% (trans-1 in mesogen M1). 1–3cis isomers caused moderate destabilization of the liquid-crystalline phase or left it almost unaltered, thereby giving T_N–I* values between 0.74 and 1.06. Azobenzenes 4 and 5, with a more extended aromatic core, produced very high stabilization of the nematic mesophase, independently of the isomerism presented by the chromophore. T_N–I* values for 4 and 5trans isomers were between the range 1.28 to 3.11. Thus, the T_N–I of the pure mesogen increased from 28 to 211%, respectively. For example, the T_N–I for M1 changed from 35 to 109 °C when azobenzene trans-5 was introduced. For all the nematogens studied, 4 and 5cis isomers also stabilized the nematic mesophase. However, this stabilization was always lower than that produced for the corresponding trans isomer, as a result of its bent geometry.

Table 2 Reduced nematic-to-isotropic transition temperatures at an azobenzene molar fraction of x = 0.1, T_N–I*(x) = T_N–I(x)/T^LC_N–I, for mixtures of azocompounds 1–5 with mesogens M1–M7

Azobenzene		M1	M2	M3	M4	M5	M6	M7
a Phase separation of the azobenzene from the host mesogen was observed.
	trans	1.23	1.10	0.99	1.08	1.01	1.06	0.96
1	cis	1.06	1.01	0.96	0.95	0.92	1.00	0.92
	trans	1.09	1.05	0.80	1.01	0.97	1.02	0.98
2	cis	0.90	0.95	0.74	0.90	0.90	0.97	0.93
	trans	1.10	1.08	0.88	1.04	1.03	1.04	0.98
3	cis	0.98	0.98	0.80	0.93	0.90	0.99	0.93
	trans	2.86	1.83	—^a	2.17	1.75	1.52	1.28
4	cis	2.42	1.66	—^a	1.90	1.55	1.38	1.18
	trans	3.11	1.88	—^a	2.50	2.31	1.74	1.37
5	cis	2.68	1.69	—^a	2.20	2.01	1.61	1.27

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One of the main problems in such an investigation was to quantify the degree of distortion generated in the host mesophase due to the isomerisation of the photoactive molecules. According to Ikeda, efficiency, ΔT_N–I, is the most important parameter to consider in the study of photo-induced nematic-to-isotropic phase transitions. When LC/azobenzene samples were irradiated at temperatures below T_N–I(cis), no phase transition was observed, even after long irradiation, whereas very effective phase transitions were induced by irradiation at temperatures within the range between T_N–I(cis) and T_N–I(trans).³ Efficiency, ΔT_N–I(x), is defined as the temperature range in which the azobenzene at a determined molar fraction x modifies the host mesophase order, as a result of its trans–cis photo-isomerisation, driving the liquid-crystalline system to the isotropic state isothermally. Therefore, the working range is broader with azocompounds that exhibit greater efficiencies. The efficiency of the mesophase distortion, ΔT_N–I(x), was calculated for each azobenzene molar fraction by means of eqn (2):

ΔT_N–I(x) = T_N–I(trans)(x) −T_N–I(cis)(x) (2)

Efficiency values for mixtures of azocompounds 1–5 at a molar fraction x = 0.1 in all the selected host mesogens are summarized in Table 3. The efficiency values, ΔT_N–I, observed for two-aromatic ring azocompounds 1–3 were moderate; all of them fell between 2.9 and 7.1 °C. Azocompounds 4 and 5 showed very high values; all between 8.6 and 15.5 °C.

Table 3 Efficiencies, ΔT_N–I(x) = T_{N–I (trans)}(x)–T_N–I(cis)(x), at an azobenzene molar fraction of x = 0.1, for mixtures of azocompounds 1–5 with mesogens M1–M7. ΔT_N–I values are reported in Celsius

Azobenzene	M1	M2	M3	M4	M5	M6	M7
1	5.9	6.3	2.9	6.5	4.3	4.1	3.8
2	6.7	7.0	4.5	5.7	3.5	3.6	3.9
3	4.0	6.6	5.6	5.5	6.5	3.3	4.5
4	15.5	11.7	—	12.9	10.1	9.7	8.6
5	15.1	12.7	—	14.5	15.1	8.7	9.3

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The evolution of the reduced nematic-to-isotropic phase transition temperature for mesogen M4 mixtures with azocompounds 3 (two-aromatic rings) and 5 (four-aromatic rings) against the azobenzene molar fraction is represented graphically in Fig. 5. An increase in efficiency was observed with the azobenzene proportion in the guest–host mixtures. All the other azobenzene/LC mixtures studied showed the same behaviour.

Fig. 5 Evolution of the reduced nematic-to-isotropic phase transition temperatures, T_N–I*, for azobenzenes 3 and 5 in M4 against the azobenzene molar fraction, x.

The Maier–Saupe theory of the nematic mesophase, which is based on the mean field approximation, can be applied to multicomponent systems containing axially symmetric molecules interacting with an anisotropic intermolecular potential.^25–27 A simplification of these formulae can be used to predict phase diagrams for binary solid solutions.²⁸ The usual assumption is that, in the absence of chemical reactions, the bulk properties add up as a weighted sum of the individual properties (geometric rule), but this may not always be the case.²⁹ The nematic-to-isotropic transition temperature for the different trans-azobenzene/LC mixtures can be estimated theoretically by means of the following expression:

T_N–I(x) = ξ(x_azoT_m + (1 −x_azo)T^LC_N–I) (3)

where T_m corresponds to the melting point of the trans-azoderivative and ξ is a dimensionless parameter. Some deviations between the behaviour predicted by the Maier–Saupe theory (ξ = 1) and the real behaviour can be observed. ξ is the relation between the real value and the theoretical one (ξ = T^r_N–I/T^th_N–I) and gives a qualitative measurement of the degree of interaction between the mesogens and the dye molecules. The ξ calculated values for all the pairs of trans-azocompound/LC are collected in Table 4. ξ exhibits values from 0.82 to 1.03 for dyes 1–3 and from 1.23 to 2.30 for azocompounds 4 and 5. The ξ values obtained clearly show that their T_N–I do not algebraically add up, indicating the establishment of non-covalent interactions between the two species. It is well known that the presence of non-covalent interactions, such as π–π stacking^30,31 or hydrogen bonds³² among others, play a crucial role in determining the mesomorphic behaviour of liquid-crystalline systems.

Table 4 Calculated values for the parameter ξ for the trans isomers of azoderivatives 1–5 in the mesogens M1–M7 at an azobenzene molar fraction of x = 0.1

Azobenzene	M1	M2	M3	M4	M5	M6	M7
1	0.90	0.97	0.97	0.87	0.82	0.94	0.88
2	0.94	1.02	0.95	0.93	0.90	0.99	0.98
3	0.93	1.03	0.97	0.95	0.94	1.00	0.97
4	2.27	1.68	—	1.86	1.51	1.40	1.23
5	2.30	1.66	—	2.04	1.90	1.55	1.27

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The deviation of experimental values from the predicted ones was mainly due to the π–π interaction. The π–π interaction was responsible for the higher stabilization of the nematic mesophase caused by four-aromatic ring azobenzenes than that caused by two-aromatic ring azobenzenes. Azobenzenes that produced greater stabilization of the host nematic mesophases were more efficient. In other words, the most efficient systems, with ΔT_N–I values of up to 15.5 °C, were formed by azocompounds that stabilized the host mesophase up to 211% (T_N–I* = 3.11). Otherwise, binary mixtures which displayed low efficiencies of up to 7.1 °C, produced destabilizations or stabilizations that were all lower than 23% (T_N–I* = 1.23). Accordingly, azobenzenes with four aromatic rings are able to interact more extensively with the nematic host and to establish more π–π interactions with the mesogen molecules, due to their more extended aromatic core. When an azo-dye molecule is isomerised, a greater number of mesogen molecules, affected by the π–π interaction, have to be reorganized. Thus, they are forced to abandon the director direction. This effect produces a marked decrease in the order parameter. Consequently, the mixture T_N–I decreases considerably and high efficiency values are obtained for these systems. For azobenzenes with two aromatic rings in the rigid core, the change produced in the order parameter during the trans–cis photo-isomerisation is not as great and lower efficiencies are registered.

A simplified model based on computational calculations is presented in Fig. 6. The length of the rigid part of the mesogens and azobenzenes was determined by means of semi-empirical AM1 calculations. The calculated length of the rigid core was 11.4 and 12.1 Å for cyanobiphenyl (M1 and M2) and phenyl benzoate based mesogens (M4–M7), respectively. The length of the rigid core of the azobenzenes 4 and 5trans isomers was 24.3 Å, while that of azobenzenes 1–3 was 11.3 Å. These values indicate that four-aromatic rings azobenzenes are able to interact with twice as many nematic host molecules as two-aromatic ring azobenzenes. When trans–cis photo-isomerisation occurred, the length of the azo-dye rigid part in its bent form was reduced to 12.1 and 5.7 Å, respectively. Cis isomers of azoderivatives 4 and 5 could still interact efficiently with the mesogen molecules. Therefore, stabilization of the host mesophase was detected, although lower than that produced by the corresponding trans isomers. In the case of azocompounds 1–3, the length of the rigid part of the dye in its bent form was reduced by half. This distance was too short to establish efficient π–π interactions with the surrounding liquid crystal molecules. The nematic order was not reinforced by the introduction of these short cis-azo-dyes and generally destabilization of the nematic phase was produced.

Fig. 6 Calculated distances for cis and trans isomers rigid part for two-aromatic rings azobenzenes and four-aromatic rings azobenzenes.

Another fact which indicates that π–π interactions play a crucial role in this model is the study of host/guest mixtures with mesogen M3, which does not have any aromatic rings in its chemical structure. For this mesogen, no π–π interactions could be established between the azo-dye and the host liquid crystal. When M3 was mixed with two-aromatic ring azobenzenes (1–3), destabilization in the mesophase occurred independently of the dye isomerism, thereby drastically decreasing the T_N–I value of the mixtures (Table 2). A decrease of about 20% in T_N–I with respect to the pure mesogen was found for azocompound 2. In this way, lower efficiencies were detected due to the absence of π–π interactions between the azobenzene and the mesogen molecules. However, the increase of the length of the alkyl chains of the lateral substituents in the azo-dye produces an increase in efficiency. This can be understood by considering the establishment of van der Waals interactions between the two species, which are more important when azocompound 3, presenting larger alkyl chains, is dissolved in M3. This mixture gives an efficiency of 5.6 °C in comparison with the 2.9 °C that is obtained for azocompound 1 mixed in the same mesogen (Table 3). For the four-aromatic ring azocompounds (4 and 5), phase separation of the azobenzene from the mesogen was observed due to their poor solubility in mesogen M3, caused by the absence of π–π interactions. Thus, no quantification could be carried out.

On the basis of the results presented in Table 3, it can be concluded that cyanobiphenyl liquid crystal hosts produce more efficient systems than those formed by phenylbenzoates liquid crystal hosts. This observation is of great importance for materials design when thinking in technical applications. Three different factors have to be highlighted when considering the different structural features which can influence the efficiency of the photo-induced nematic-to-isotropic phase transitions. In first place, as has been discussed above, π–π interactions established between guest and host seem to be the main factor. In this way, the efficiencies registered in aryl containing host liquid crystals (M1–M2 and M4–M7) are greater than those obtained for dicyclohexyl based mesogens (M3). A second factor is the dipolar moment of the host mesogen. A comparison between mixtures of M1 and M2, which are both cyanobiphenyl based nematic liquid crystals where the only difference is the alkyl or alkoxy chain of the position 4′ of the biphenyl ring, respectively, shows that efficiencies are higher in the latter, which is the most polar, when short azobenzenes (1–3) are introduced. The opposite behaviour was observed for the longest azoderivatives 4–5, where higher efficiencies are obtained for the most non-polar mesogen M1. Consequently, the polarity of the mesogen plays an important role in the photo-induced phase transition behaviour and must be related to the manner in which the host and guest molecules interact. Otherwise, without further experimental work, we are not able to predict the effect of the polarity on the efficiency. Finally, another important factor to take into account are the intermolecular van der Waals interactions, which are more relevant for dicyclohexyl based mesogens, such as M3, where no intermolecular π–π interactions can be established. In this type of mesogen, the introduction of longer alkyl chains into the azo-dye produces an increase in the efficiency of the photo-induced phase transition.

Conclusion

Here we studied the photo-induced phase transition behaviour of several 4,4′-disubstituted azobenzenes in three types of host nematic liquid crystals: cyanobiphenyls, dicyclohexanes and phenyl benzoates. Our analyses indicate that the non-covalent π–π interactions established between both azobenzene and mesogens influence photo-induced phase transition temperatures.

Azobenzenes which present more extended aromatic cores produce more efficient systems, due to the greater change in the order parameter that takes place during the trans–cis photo-isomerisation process in comparison with systems with shorter π-cores. Azoderivatives with four aromatic rings stabilize the nematic mesophase in the two azobenzene geometries, due to the high possibility of interaction with the nematogenic molecules viaπ–π stacking.

We conclude that correct molecular design of the azobenzene chemical structure as well as the choice of an appropriate host mesogen is of fundamental importance for obtaining efficient photo-induced nematic order distortions under isothermal conditions.

Acknowledgements

Financial support from the European project: “Functional Liquid-Crystalline Elastomers” (FULCE-HPRN-CT-2002-00169) and from the Ministerio de Educación y Ciencia (CTQ-2006-15611-C02-02) is gratefully acknowledged. The authors thank Prof. Dr Heino Finkelmann for his continuous support and helpful discussions. J. Garcia-Amorós is grateful for the award of a doctoral grant from the Universitat de Barcelona.

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