Photosensitive bent-core liquid crystals based on methyl substituted 3-hydroxybenzoic acid †

Photosensitive liquid crystals are of contemporary interest not only from the scienti ﬁ c point of view but also for various applications. Herein we report the ﬁ rst photosensitive materials based on laterally substituted 3-hydroxybenzoic acid. The molecular self-assembly of these new materials was characterized using di ﬀ erent physical techniques including di ﬀ erential scanning calorimetry (DSC), X-ray di ﬀ raction (XRD) measurements, electro-optical investigations and dielectric spectroscopy. We show that the number and position of azo units with respect to the lateral substitution on the molecular core signi ﬁ cantly in ﬂ uence mesomorphic properties of the materials. Based on the position of the substituent, either non-polar or polar smectic C phases are formed. The optimum combination of both the structural elements results in an achiral material which shows a broad polymorphism and exhibits a stable dark-conglomerate crystalline phase with macroscopically chiral domains at room temperature. The structure of this phase di ﬀ ers from the previously described ﬂ uid sponge-like distorted smectic phases as well as from helical nano-ﬁ lament phases, thus, adding new information on the possible variations in the nanostructures of this kind of phase. Moreover, the photosensitivity of these materials has been studied using 1 H NMR spectroscopy.


Introduction
Since the discovery of polar order and macroscopic phase chirality of achiral bent molecules, 1,2 bent-core liquid crystals (BCLCs) have become prominent materials of interest for materials science and supramolecular chemistry.Their importance lies not only in understanding the fundamentals of selfassembly in so matter, [3][4][5] but also in practical applications in non-linear optics, 6 sensors 7 and fast switching electro-optical devices. 8Nowadays, the main focus is put on advanced materials, whose properties can be tuned by different external stimuli, in particular, light.Although the rst photosensitive BCLCs were prepared as early as in 1929, 9 their application potential in areas such as optical storage, 10 holographic media 11 and photo-alignment of LC matrixes 12 was only recognized much later.Therefore, many different azo group-containing BCLCs have recently been synthesised and their properties studied, as shown in a recent review. 13e of the most successful motifs in the search for light tuneable BCLCs represents resorcinol central core, which is extended with azo group-containing elongating side arms.5][16] The real breakthrough in the area of photosensitive BCLCs has arrived with the introduction of a lateral substituent on the resorcinol central core.1][22] Replacing one azo group in the molecular structure with an ester unit provided materials whose mesomorphic behaviour depended on the orientation of the ester unit as well as on the position of the halogen substituent with respect to the remaining azo group. 23Apart from the polar electron-withdrawing substituents, also a weak electron donating methyl group has been introduced on the resorcinol central core to form materials with stable DC phases. 24nlike the resorcinol derivatives mentioned above, compounds based on 3-hydroxybenzoic acid have been studied much less.The majority of the reported materials possessed one azo group in the molecular structure and exhibited columnar B 1 , lamellar B 2 or a modulated smectic B 7 -type mesophases. 14,25,26elated materials with two azo groups have been found to be non-mesogenic. 15Despite the fact that BCLCs based on 3hydroxybenzoic acid substituted in position four with all-ester side arms provided thermally more stable mesophases than the corresponding resorcinol-based analogues, 27 no material based on laterally substituted 3-hydroxybenzoic acid has been reported yet.Additionally, the intrinsic non-symmetry of 3hydroxybenzoic acid central core determines different effect of the lateral substituent on the central core.Recently, it has been shown that the presence of a larger substituent in position six of the central 3-hydroxybenzoic acid unit reduces the exibility of derived BCLCs leading to crystalline substances. 28On the other hand, the presence of a larger substituent and the reduced conformational exibility has recently been recognized as an important feature for BCLCs forming DC phases. 13,21,23,24,29n this contribution, we report the effect of the lateral substituent and its location on the mesomorphic properties of the new class of photosensitive BCLCs possessing one or two azo groups in their structure.We have also varied the position of the substituent with respect to the azobenzene wing, which results in a material exhibiting rich polymorphism.

Synthesis
The synthesis of the new BCLCs under investigations is shown in Schemes 1 and 2. The protected central units 1-4 were prepared from commercially available starting materials in accord with previously reported methods. 27,28The elongating side arms were synthesised by known synthetic procedures. 18,30,31 N,N 0 -dicyclohexylcarbodiimide (DCC)-mediated esterication of protected acids 1 and 2 with corresponding phenols 5 and 6 in the presence of N,N-dimethylaminopyridine (DMAP) as catalyst gave rise to protected derivatives 7 and 8, respectively (Scheme 1).Based on the character of the silyl (PG ¼ (CH 3 ) 3 -CSi(CH 3 ) 2 ) and benzyl (PG ¼ C 6 H 5 CH 2 ) protecting groups (PG), deprotection was achieved by tetrabutylammonium uoride trihydrate (TBAF$3H 2 O) and transfer-hydrogenation with ammonium formate in the presence of palladium on charcoal, respectively.In the nal step, the isolated hydroxy esters 9 and 10 were acylated with acid chlorides 11 and 12 of the second lengthening arm to yield the target materials of the series I.
Similarly as above, the protected central cores 3 and 4 were esteried in a DCC-mediated reaction catalyzed with DMAP to yield protected intermediates 13 and 14.Subsequent deprotection performed with respect to the used protecting groups provided hydroxy esters 15 and 16, acylation of which afforded the target materials of the series II (Scheme 2).The analytical data and the synthetic details for the intermediates as well as for the nal BCLCs are summarized in the ESI † le.

Experimental methods and set-up
The phase transition temperatures and corresponding enthalpies were determined by differential scanning calorimetry (DSC), namely Pyris Diamond Perkin-Elmer 7 calorimeter was utilized.The samples of about 2-5 mg were sealed into aluminium pans, which were put into the calorimeter chamber lled with nitrogen.
The temperature and enthalpy values were calibrated on the extrapolated enthalpy values of the melting points for water, indium and zinc.All calorimetric measurements were performed on cooling/heating runs at a rate of 5 K min À1 .
The textures were observed under the polarizing optical microscope Nikon Eclipse E600.The cells for texture observation and electro-optical studies were made from glasses with ITO transparent electrodes (25 mm 2 ), separated by mylar sheets dening the cell thickness.No glass-surface treatment was utilized.The cells (usually 3 mm thick) were lled with studied compounds in the isotropic phase by the capillary action.Another sample can be prepared by spreading the droplet in the isotropic phase on the glass surface (one-free-surface sample).The Linkam LTS E350 heating/cooling stage with TMS 93 temperature programmer was used for the temperature control.The stabilization of the temperature was within AE0.1 K.The electro-optical switching characteristics were studied with the triangular-wave method using a home-made set-up in 6 mm polyimide coated ITO cells, EHC Japan.Voltage of 100 V pp at a frequency 10 Hz and resistor 5 kU were utilized.
Dielectric properties were studied using Schlumberger 1260 impedance analyser.Measurements were performed on cooling and the temperature was stabilized during the frequency sweeps in the range of 10 Hz to 1 MHz.The frequency dependent complex permittivity 3* ¼ 3 0 À i3 00 , namely the real, 3 0 , and the imaginary, 3 00 , parts were detected in a broad temperature range on cooling from the isotropic phase.
X-ray diffraction patterns were recorded with a 2D detector (Vantec 500, Bruker).Ni ltered and pin hole collimated CuK a radiation was used.The exposure time was 30 min and the sample to detector distance was 8.95 cm and 26.7 cm respectively.Uniform orientation was achieved by alignment at the airsample interface on top of a small droplet.The samples were held on a temperature-controlled heating stage.
Photochemical properties were studied using Agilent 400 MR DDR2 spectrometer.The selected compounds were dissolved in deuteriochloroform and irradiated at 365 nm with a common laboratory UV lamp for 8 h.The samples were placed into an NMR instrument tempered at 50 C and the spectra were acquired with 700 s steps between measurements, duration of each measurement was 57 s.

Results and discussion
Based on the position of lateral substituent (methyl group) on the central core, the materials were divided into two series: 4-substituted compounds are denoted as series I and 6substituted compounds are denoted as series II (see Schemes 1 and 2).The mesomorphic behaviour of the studied materials was determined from thermal behaviour obtained by DSC, texture observation under the polarizing optical microscope and electro-optical behaviour under the applied electric eld (Table 1).To support the phase identication, X-ray measurements were performed for a selected example.

Series I
Compound Ia with all-ester linkages exhibited a monotropic SmC s phase.The introduction of an azo group in the elongating side arm connected to the carboxylic unit of the central core (Ib) led to an interesting polymorphism.On cooling, a sequence of a nematic phase, three consecutive SmC phases and a DC phase were observed.We denoted these synclinic smectic C phases as the SmC s1 , SmC s2 and SmC s3 , respectively, on cooling.The DC phase is stable down to room temperature and crystallises on subsequent heating.On DSC thermograph the crystallisation of the DC phase takes place at about 85 C and is present in a form of the opposite peak on the second heating curve (Fig. 1a).On further heating, the crystal phase (Cr) transforms to the SmC at 102 C and at 107 C the phase transition to the isotropic phase takes place.We will describe all observed mesophases later.On changing the positions of the azo and ester linking group in compound Ic, the mesomorphic behaviour completely disappeared.For compound Id with the azo group in both the elongating side arms, a sequence of two monotropic mesophases has been observed on cooling from the isotropic liquid.Namely, a narrow temperature interval of a nematic phase is followed by a smectic C phase (Fig. 2).
Textures and their features under the applied electric eld and at various temperatures were observed using a polarizing optical microscope.For compound Id, a very narrow nematic phase is observed above the tilted smectic phase (Fig. 2a).For all studied compounds from series I, the observed smectic phases reveal broken fan-shaped texture (for illustration see Fig. 2b for Id).The extinction position of all smectic mesophases is inclined from the layer normal by an angle of about 45 degrees, which provides evidence for the synclinic character of SmC phases.For compound Ib, we have observed a sequence of three SmC s mesophases and their planar textures are differing only slightly (see Fig. S1 in ESI †).On the other hand, homeotropic textures are not so uniform and reveal specic features for various SmC s phases of Ib.In Fig. 3a a phase transition from nematic to smectic phase is shown and we can observe "transition bars", which for calamitic LC compounds can accompany N-SmC phase transition. 32Fig. 3b and c show the schlieren texture in the SmC s1 and the SmC s2 phase, respectively.The lines appearing in the SmC s2 phase can be connected with undulations or defects arising in smectic planes.In Fig. 3d the texture in the SmC s3 phase is depicted.
For all compounds of series I, no response to the applied electric eld was obtained in the observed SmC s mesophases, which provided further evidence on their non-polar character.Due to the narrow range of the SmC s3 phase exhibited by Ib, we Scheme 2 Synthesis and designation of compounds of series II.
were not able to determine its response to the applied electric eld.However, the textures of all three consecutive SmC s phases are very similar and thus we assume that also the low temperature SmC s3 phase is of non-polar character.This result has further been supported by the dielectric spectroscopy measurements, see below.
In case of compound Ib, on further cooling from the SmC s3 phase, a highly viscous optically isotropic phase appears (Fig. 4b).Rotating the analyzer by a small angle leads to the appearance of dark and bright domains.Rotating the analyzer in the opposite direction reverses the dark and bright domains (Fig. 4a and c), while rotating the sample itself under crossed polarizers does not lead to any change in the dark texture.The overall distribution of the dark and bright domains is about 1 : 1 in both cases.This indicates that the domains are chiral with opposite handedness, which is a typical feature of chiral phases. 13he dielectric properties of Ib were investigated within the broad temperature interval of all mesophases.Fig. 5 shows the real, 3 0 , and imaginary, 3 00 , part of permittivity with respect to the temperature, T, and frequency.There is no distinct mode, which can be attributed to the polar arrangement.The values of permittivity are rather small, which support the idea of the nonpolar character of the observed SmC s mesophases.We can identify the phase transition temperatures, which are connected with small step-like changes of the real part of permittivity, 3 0 .On the other hand, the imaginary part of permittivity reveals anomalies at the phase transition temperatures.In Fig. 5 these anomalies are marked by arrows.We can speculate that it corresponds to director uctuations, which are more intensive in the vicinity of the phase transition.We can expect that in the The phase transition were taken from the second heating and cooling runs at a rate of 5 K min À1 .Phases are abbreviated: SmC ssynclinic smectic phase; SmC a P Aanticlinic antiferroelectric SmC phase; Isoisotropic liquid; Crcrystalline phase; DCdark conglomerate phase.* -Iso-N enthalpy could not be separated from N-SmC s enthalpy.Temperature is given in C and phase transition enthalpy in kJ mol À1 .observed non-polar SmC s mesophase the bent-core molecules freely rotate along their molecular axis with rather high frequency and this molecular mode contributes mostly at the phase transitions.The values of permittivity are rather small, which support the idea of the non-polar character of the observed SmC s mesophases.On the other hand, we can identify the phase transitions because they are connected with molecular uctuations.We can speculate that the bent-core molecules freely rotate along their molecular axis and this motion contributes mostly at the phase transitions.X-ray scattering measurements were conducted for compound Ib as it shows the interesting phase sequence of nematic and different SmC phases ending with the DC phase.Due to the very narrow range of its existence $1 K, it was not possible to carry out XRD investigations of the nematic phase.However, within all smectic phases, the X-ray intensity exhibited a sharp peak at small scattering angles and a diffuse maximum in the wide angle region (Fig. 6a for Ib in the SmC s1 phase).The layer spacing, d, can be established from the position of the intensity maximum at small scattering angles.The diffuse scattering peak at wide angles corresponds to the average distance between molecules z4.5 Å.For compound Ib the corresponding d value has been found d ¼ 47.8 Å at T ¼ 100 C, which is substantially smaller than the molecular length, l, calculated in ab initio calculations l ¼ 56.3 Å. Comparing X-ray data with calculations, the tilt angle can be estimated to be about 32 degrees.We conrmed a lamellar character of the SmC mesophases observed on cooling from the isotropic phase for other compounds.Due to the monotropic character of the mesophases and their fast crystallisation, it was not possible to nish detailed X-ray measurements.
X-ray investigations in the DC phase of Ib (Fig. 6b) show an intense layer signal with corresponding weak harmonic reections in small and medium angle region.All scatterings form closed rings with uniform intensity distribution, which provides evidence for the disordered meso-structure of the DC phase.The observed layer thickness points to tilted organization within layer with a molecular tilt angle of about 29 .Such a value is typical neither for DC sponge phases nor for helical-nanolament phase (B 4 phase).Furthermore, the broad signal  in the wide angle region could be tted to six maxima in the 2q range between 15 and 26 (Fig. 6b).This pattern clearly excludes uid sponge phases that show only a very diffuse wide angle scattering.However, such reections can not be ascribed to a B 4 phase, which typically shows the reection as sharp separate signals.These results indicate that the DC phase under investigation is similar to the recently described new sub-type of a DC phase exhibited by related 4-methylresorcinol-based materials with one inverted ester. 24The reections in small and medium region appear at very similar positions.The wide angle region shows more diffuse pattern with two more maxima, which could arise from slightly different molecular packing within the studied DC phase.Therefore, inverting the direction of the ester group retains the DC phase but slightly change its ne structure.

Series II
The mesomorphic properties of series II differ from those of series I.This change probably results from the different position of the lateral methyl group at the apex of the bent-core mesogen.In case of compound IIa with all ester linkages, a monotropic SmC a P A phase was identied (see Fig. 7d).Surprisingly, the introduction of the azo group in the elongating side arm connected to the carboxylic unit of the central core (IIb) led to the complete loss of mesomorphic behaviour.When changing the positions of the azo and ester linking groups (IIc), the monotropic SmC a P A phase was restored.Compound IId possessing two azo units in the structure was crystalline only.
Compounds IIa and IIc showed two peaks at half-period under the applied triangular eld (Fig. 7a and b).The switching current documents the antiferroelectric nature of the studied mesophases.Based on this observation, the mesophase for both IIa and IIc was assigned as SmC a P A .Due to relatively fast crystallisation of the materials under the applied electric eld, we were able to record only changes of planar textures of IIa (Fig. 7c-e).The dielectric spectroscopy for compound IIa and IIc has been performed and a distinct mode observed in the SmC a P A phase on cooling from the isotropic phase.3-Dimensional plot of the real and imaginary parts of the permittivity for IIa is illustrated in Fig. 8.The observed mode completely disappears in the isotropic as well as in the crystalline phase, so we can conclude that it is connected with collective uctuations of molecules.Such mode is very oen observed for SmCP phases and is explained by vibrations of tilted molecules in polar ordered systems with interactions between neighbouring layers. 33

Photochemical properties
The presence of the azo units in the investigated BCLCs allows the study of photoisomerisation under UV light irradiation.This phenomenon has been reported before for similar azobenzene containing BCLCs in solutions using UV-Vis spectrometry. 13However, herein we study this process using 1 H NMR spectroscopy.This method allows us to determine the amount of Z-isomer formed upon UV irradiation of the predominant E-isomer of an azo group containing materials.Moreover, this method is suitable for the observation of the thermal relaxation of the formed Z-isomer back to the more stable E-isomer in the dark.It is evident that with the increasing number of azo units present in a molecule, the 1 H NMR spectrum of such a compound gains in complexity.In case of two azo groups, the formation of four different chemical species can be envisioned for a non-symmetrical bent-core compound.Therefore, we decided to study substances with one azo group in the elongating side arms, namely polymorphic Ib and crystalline IIb.
From the acquired 1 H NMR spectra (see Fig. S3 and S4 †), thermal relaxation back to the thermodynamically more stable E-isomer from the Z-isomer formed by UV-irradiation is evident.The relaxation rate constants determined from the slope of the decrease was 1.25 10 À4 s À1 for Ib and 1.08 10 À4 s À1 for IIb (Fig. 9).This result indicates a slightly lower exibility of compound IIb, which may be caused by the steric inuence of the lateral substituent in position 6 on the central core.This feature can be used for the design of the next generation of slow relaxing photosensitive bent-core compounds.Such compounds with very stable Z-isomer can be then used for optical data storage or photo-alignment of LC matrices.

Ab initio calculations
The differences in mesomorphic properties can also be explained on the basis of a preferred molecular conformation.The conformers with minimum energy of Ib showing three consecutive non-polar SmC phases and a DC phase, and IIc exhibiting the SmC a P A phase were calculated using density functional theory method at B3LYP 3-21g level in Gaussian soware.Despite the fact that the calculations were performed   for a single molecule in vacuum, the conformers with minimum energy showed strong preference of Ib towards more linear arrangement, while IIc prefers rather a bent molecular shape (Fig. 10).The favourable molecular shapes correspond well with the formed mesophases; more linear molecules tend to organize in mesophases typical for rod-like mesogens, and bent molecules form mesophases typical of bent-core molecules.

Conclusions
New photosensitive BCLCs based on 4-and 6-methyl substituted 3-hydroxybenzoic acid have been synthesised and studied.We show that the position of the lateral substituent on the central core with respect to the position of the azo group-bearing elongating side arm strongly inuences physical properties of the materials.For 4-methyl substituted materials of series I, generally, non-polar synclinic SmC phases have been identied.In case of Ib, rich polymorphism has been observed on cooling from the isotropic phase, namely, nematic phase followed by three synclinic smectic phases and ending with a dark conglomerate phase (DC phase).None of the SmC s phase revealed switching under the applied electric eld.Non-polar character is further supported by the dielectric spectroscopy data.In details, we have described the disordered meso-structure of the DC phase, which exists below the SmC s phases at room temperature.It was conrmed that this DC phase has a different structure from those of B 4 phases and DC sponge phases, but very similar to the DC phases of the related 4-methylresorcinol derived BCLCs.This indicates that inverting the direction of one ester group in the side arm of the bent-core mesogen changes the ne structure of the formed DC phase, thus, adding a new subtype to the family of these phases.
The synclinic SmC phases observed for series I were replaced for series II derivatives either by anticlinic polar SmC phases (SmC a P A phases) or crystalline phases.The formation of the SmC a P A phase (materials IIa and IIc) is probably due to the preferred bent molecular shape of the materials.The absence of mesomorphic behaviour in the case of IIb and IId can be explained by the limited exibility of the side chain next to the lateral substituent.The negative effect of a larger substituent in the vicinity of the carboxylic unit on the 3-hydroxybenzoic acidbased materials in combination with less exible side chains has recently been described. 26It is reasonable to assume that the introduction of the azo unit further limits the overall exibility of the substance required for efficient molecular packing within a mesophase.
In conclusion, the mesomorphic properties of the presented materials are mainly inuenced by the mutual position of the photosensitive unit and the lateral substituent.Thus, proper molecular design may provide materials exhibiting mesomorphic properties typical of calamitic materials or bent-core liquid crystals.Slower thermal relaxation of UV-induced Z-isomer of azo-containing bent-core materials may be benecial for optical data storage and related applications.

Scheme 1
Scheme 1 Synthesis and designation of compounds of series I.

Fig. 1 Fig. 2
Fig. 1 DSC thermographs for compounds (a) Ib, (b) Id and (c) IId.The inset in the figure (a) shows the phase transition from SmC S1 to SmC S2 taken on cooling, marked with an arrow.The red upper curves show the second heating run, the lower blue lines show the second cooling, taken at a rate of 5 K min À1 .Fig. 2 Planar texture of compound Id at different temperature observed by polarizing microscope (a) in the N phase at T ¼ 116 C and (b) in the SmC s phase at T ¼ 110 C. The width of the figure corresponds to 200 mm.Orientation of the crossed polarisers (A, P) is marked.

Fig. 3
Fig. 3 Homeotropic texture for compound Ib (a) at the N-SmC s1 phase transition at T ¼ 105 C, (b) in SmC s1 phase at T ¼ 100 C, (c) in the SmC s2 phase at T ¼ 86 C and (d) in the SmC s3 phase at T ¼ 81 C. The width of figures corresponds to 250 mm.

Fig. 4
Fig. 4 Textures of the DC phase of compound Ib at T ¼ 50 C: (a) after rotating the analyzer by 10 from the crossed position with respect to the polarizer in clock-wise direction; (b) under crossed polarizers and (c) in anticlockwise direction, showing dark and bright domains, indicating the presence of areas with opposite chirality sense.

Fig. 5 3 -
Fig.53-Dimensional plots of the real, 3 0 , and imaginary, 3 00 , part of permittivity with respect to the temperature, T, and frequency for Ib.The arrows mark phase transition.

Fig. 6 X
Fig. 6 X-ray intensity versus scattering angle for compound Ib: (a) at T ¼ 100 C in the SmC s1 phase and (b) at T ¼ 70 C in the DC phase.The insets show the corresponding 2D X-ray patterns at the indicated temperature.Fig.7 Switching current response curves recorded by applying a triangular wave voltage in the SmC a P A phase for compounds: (a) IIa at 103 C and (b) IIc 99 C; (c-e) optical textures in the SmC a P A phase for compound IIa at 102 C under an applied DC voltage between crossed polarizers in (c) field induced SmC s P F state at +10 V, (d) in the SmC a P A state at V ¼ 0 after switching off the field and (e) the field induced SmC s P F state at À10 V.

Fig. 7
Fig. 6 X-ray intensity versus scattering angle for compound Ib: (a) at T ¼ 100 C in the SmC s1 phase and (b) at T ¼ 70 C in the DC phase.The insets show the corresponding 2D X-ray patterns at the indicated temperature.Fig.7 Switching current response curves recorded by applying a triangular wave voltage in the SmC a P A phase for compounds: (a) IIa at 103 C and (b) IIc 99 C; (c-e) optical textures in the SmC a P A phase for compound IIa at 102 C under an applied DC voltage between crossed polarizers in (c) field induced SmC s P F state at +10 V, (d) in the SmC a P A state at V ¼ 0 after switching off the field and (e) the field induced SmC s P F state at À10 V.

Fig. 9
Fig.9The time dependent decrease of the concentration of Z-isomer in the solution of Ib (triangles) and IIb (dots) depicted in the logarithmic scale.

Fig. 10
Fig. 10 Conformers with minimum energy of Ib and IIc obtained by ab initio calculations.

Table 1
Mesomorphic properties of materials of series I and II a