Synthesis, Photo-sensitive and Electrochemical Properties of Rod-like Aromatic Aldehyde with Azo Bridge

Abstract Eight new rod-like liquid crystal molecules composed by a long rigid core of three six-member rings (cyclohexane ring or benzene ring), azo, ester and terminal aldehyde groups have been prepared. These rod-like liquid crystalline molecules were designed to construct new structures to further study photo-isomerization in their mesophases. All the compounds have been characterized based on their basic spectral data, differential scanning calorimeter (DSC) and hot stage polarizing optical microscope (HS-POM). The result showed that all the molecules, even those with the shortest terminal methyl group, have liquid crystalline properties. Their mesophases are nematic within the temperature ranges from 85 to 145°C. They exhibit photo-sensitivities not only in methanol solutions but also in a mesophase when exposed to UV light. The highest occupied orbital (HOMO) and the lowest unoccupied orbital (LUMO) and the differences between the frontier molecular orbitals (Eg) of these compounds were determined by cyclic voltammetry. The effect of even-odd carbon number of the terminal straight alkyl chain on the UV spectral data and the Eg were observed. The difference between the Eg of these compounds are in excellent agreement with the difference of their maximum absorption wavelength in UV spectra. Graphical Abstract


Introduction
Modifying molecules to make them respond to external stimuli, for instance magnetism, heat and light, is an important way to obtain new materials. To study the interrelationship between structures of molecules and their properties is an essential task of material chemistry [1]. Some aromatic azo compounds are often widely used as main raw materials in industry, for example as organic dyes [2,3] food additives [4], radical reaction initiators [5] and therapeutic agents [6]. In recent years a great interest has been placed on molecules which have two or more forms, the interconversion of which can be induced by external stimulus [7][8][9]. Azo compounds are well known for their trans-cis isomerization. Azo functionalized materials are of special interest due to their photosensitive nature, which can be exploited for optical and optoelectronic devices [10]. It is well known that azobenzene lightinduced isomerization [11][12][13][14] leads to formation of new functional azo compounds [15,16]. The growing number of azo compounds have been used in photo switching, information storage, holography, etc. [17,18]. Some research has even been focused on the effect of UV-light on solid state of substances [19].
Generally speaking, methyl group as a terminal chain is too short to form a mesophase in rod-like molecules therefore little attempt has been done so far to include methyl as a terminal chain in the rod-like liquid crystalline molecules. Additionally, some publications have reported the effect of different polar end groups on the mesomorphic property [20]. Our previous publications have described the design and synthesis of several series of liquid crystals with terminal alkyls and a polar end group of CHO [21], confirming that molecules with an end group of CHO were easier to form a mesophase. These molecules have enough thermostability and exhibit a wider temperature range of mesophase than molecules containing other common functional groups, for example alkyl, alkoxy or cyano group. Moreover, an aldehyde can be converted into an aromatic acid [22], a Schiff base [23] or other functional groups [24]. Liquid crystal of formylazobenzene can even be used as a stationary phase in separation of special mixtures [25]. Based on these ideas, we have synthesized eight rod-like liquid crystalline molecules bonded with shorter terminal alkyls (5 ≥ n ≥ 1), aldehyde, and azo bridge bonding. The effect of the terminal cyclohexyl or phenyl ring on the liquid crystalline property and photosensitivity of the compounds was also observed. Furthermore, we wanted to know whether rod-like molecules with the shortest methyl chain would exhibit a mesophase and if there was a relationship between their absorption wavelengths in the UV spectra, their frontier molecular orbitals and their molecular structures.

Photo responsive properties in solution
The synthesis of all target compounds is outlined in Scheme 1 and their structural data are shown above. The spectral values are in accordance with the assigned structure.
It is known that the azo group can isomerize from the more stable trans isomer to the cis one under exposure to UV light. The solutions used in the experiments were kept in the dark for two days so that the compounds were exclusively in the trans form. The UV spectra were recorded over the same time interval until the photostationary states of trans-cis isomerization. The changes of optical absorbances of all these compounds in a methanol solution, recorded before and after irradiation with UV light, are summarized in Table 1; the maximum times of photoisomerization were detected at the photostationary states of every compound. The maximum photoisomerization time of each target molecule has been achieved within 70 min for illumination intensities of 105 μW cm -2 ( Table 1). The spectrum variations are the evidence of the trans-cis isomerization of the azo chromophores. The data depicted in Table 1 show that the compounds with the same terminal rings have the same number and the similar position of the peaks in the UV spectra before and after irradiation. Fig. 1 shows the absorption spectral change of compound 3a (5 × 10 -5 mol L -1 in methanol, quartz cell, length 1.0 cm) after irradiation with 365 nm UV-light. Every peak in the UV spectra can be related to the molecular structure. The small shoulder at ~430 nm is assigned to a n-π* transition and the maximum of the absorbance in the range of 290−380 nm corresponds to a strong π-π* electronic transition of the azo-moiety, while the peak at 230 nm is caused by the aromatic ring system.
The two isosbestic points, located at nearby 288 and 390 nm in Fig. 1, confirm the existence of only two compounds − the cis and trans isomers [26]. After irradiation with UV light the first peaks of 3a, 3b, 3c (compounds with a terminal cyclohexane ring) are unchanged, while both peaks of 3d, 3e, 3f, 3g and 3h (compounds with a terminal benzene ring) change their positions. According to the data presented in Table 1 the terminal alkyl chains promote two effects on the λ max of the last five compounds. The first is a shift of the absorption bands, more pronounced in the azo band than in the aromatic ring. Under exposure to UV light, the second peak of the first three compounds listed in Table 1 exhibits a small hypsochromic shift in the range of 4−8 nm, while the first peak of the benzene absorption band stays unaltered. After radiation with UV light, the first peak of the last five compounds exhibits a small bathochromic shift between 3 and 4 nm, while the change of the second peak is related to the structure of the alkyl chain. In compounds 3d, 3f and 3h, which have an odd number of carbon atoms in the alkyl chain, the second peak exhibits a slight hypsochromic shift in the range of 7−8 nm, while in 3e and 3g, compounds with an even carbon number alkyl chain, a small bathochromic shift between 3 and 4 nm is observed. This means that the number of carbon atoms of the terminal alkyl chain affects the position of a π-π* electronic transition of the azo-moiety in trans-cis isomerization. The second effect, before or after irradiation, is weaker for compounds 3d, 3f, 3h (with an alkyl chain of odd number) than for 3e, 3g (with an alkyl chain of even number). The above tendency was not observed for the first three compounds with terminal cyclohexane rings, indicating that the terminal alkyls have an effect only on the electronic transition of compounds with terminal benzene ring. This electronic effect can be transmitted through a conjugated system between benzene ring and other moieties so that affects the position of the π-π* transitions of azo-moiety. This even-odd carbon number effect of the alkyl chain in micromolecules on the UV spectra has not been reported until now. The original absorbance of 3a is A 1 = 0.79527 at 334 nm (Fig. 1A). Subsequent irradiation with sunlight for 70 min causes the absorbance at 334 nm to increase back to A 2 = 0.78397 (Fig. 1B). It is seen that A 2 /A 1 is 99%, i.e., the azo compounds isomers do recover in 99%.

Electrochemical analysis
The molecular energy level of the HOMO and LUMO can be exactly calculated by the cyclic voltammetric method. The energy levels and energy gaps of the eight compunds were determined by the three electrodes system (SCE, platiunm wire and glassy carbon serving as the reference, counter and working electrodes respectively) with THF as the solvent (concentration is 4 × 10 -3 ) and tetrabutylammonium perchlorate (t-Bu 4 NClO 4 ) as a supporting elecrolyte. The HOMO, LUMO and E g were calculated using the following equations. E HOMO = -e[E onset OX + 4.4(eV)], E LUMO = -e[E onset red + 4.4(eV)] and E g = -e∆Φ, ∆Φ = Φ p -Φ n [27], where e is  elementary charge, Φ p is the onset oxidation potential and Φ n is the onset reduction potential. The typical voltammogram of 3b is shown in Fig. 2. The HOMO, LUMO and the corresponding band gap E g of every compound are summarized in Table 2.
Three trends in Table 2 can be found as follows. The first trend is that the E g of the first three compounds are all greater than that of the last five compounds; the last five compounds, with the terminal benzene ring, have lower energy gaps than the first three compounds with the terminal cyclohexane ring. The results agree with the common conception that the electronic effect of benzene (as a congurate system) can narrow the difference between the energy levels. On the other hand E g (3b) is greater than E g (3a) or E g (3c), coincided with the fact that the energy of the second peak of 3b is greater than those of 3a and 3c ( Table 1). The third trend, that the E g of 3e being greater than the E g of 3d or 3f and the E g of 3g being greater than the E g of 3f or 3h, indicates that among the last five compounds, the compound with a straight alkyl chain of an even carbon number has lower E g than the compounds with odd carbon number. The effect of carbon number on E g is in an excellent agreement with that of the second maximum absorption wavelength in their UV Spectra ( Table 1). The data from Tables 1 and 2 show that, if the E g of any target compound is lower than of other compounds (Table 2), its second maximum absorption wavelength in UV spectra is longer than others, which means that the energy of the second peak in UV spectra is lower than others. The results indicated that because the second peak of each compound has lower energy than the first one (Table 2), the electronic transition between the frontier molecular orbital (E g ) is associated with the second maximum absorption peak in the UV spectra. The E g and absorption wavelengths can be modified by the terminal alkyl group, which requires further study in order to change the photo-sensitive property of molecules within a certain range.

Liquid crystalline properties and photo responsive properties in mesophase
The transition temperatures, associated with enthalpy values obtained from DSC studies and the ranges of decomposition from TG for the target compounds are summarized in Table 3. DSC thermograms were measured at the rate of 10°C min -1 for the heating process. The melting temperatures of these compounds are observed in the range of 84.6−136.6°C and clearing temperatures are 190.7−270°C. The ranges of their decomposition begin from 205 to 287.2°C and end from 367.5 to 408.6°C, suggesting the thermostability of these compounds. Among them, 3a with the terminal methylcyclohexane ring is the least stable one, while 3d with the terminal ethylbenzene ring is the most stable. The DSC analysis reveals that melting temperatures of compounds 3d−3h decrease with an increase of the length of the terminal alkyl chain. The transition temperature observed from POM is in a good agreement with a result obtained from the DSC analysis.  The mesogenic behaviors were investigated by POM. The POM textures of the typical compounds 3a and 3b are shown in Fig. 3, respectively. These compounds all exhibit the appearance that should belong to the nematic phase of schlieren type. There is no other type of mesophase observed during heating or cooling process.
The temperature ranges of their mesophases are from 85 to 145°C. The temperature ranges of the mesophase of both compounds with the terminal n-propyl are 126.8 and 145.4°C, the widest values in their own series, respectively. In other words, the terminal n-propyl creates the widest mesophase in both series. Although there was little report focused on the rod-like liquid crystal molecules designed with a terminal methyl chain, both 3a and 3d with the methyl exhibit a mesophase with temperature ranges of 85 and 94°C, respectively. The ratios of the molecular length to the width of these two compounds are 4.28 and 4.19 (Table 3), respectively, slightly more than 4 (the lowest value for a molecule to exhibit a mesophase), proving that the methyl, as a shortest alkyl chain, can form a mesophase in the rod-like molecules of an appropriate structures. By comparing compounds with the same alkyl chain, we found out that 3a, 3b and 3c with terminal cyclohexane rings have lower melting points, clearing points and narrower ranges of mesophase than 3d, 3e and 3f, respectively. Among them, 3d has the highest c.p and widest range of the mesophase. On one hand, these azo aromatic aldehydes have wider ranges of a mesophase than other liquid crystals with an aldehyde group [28]. On the other hand, they have wider temperature range of a mesophase than other polymers with an azo group [26]. Theoretically, the cis isomer with a bent shape should have no mesophase observed. If so, when UV light changes the structure of the compound from the trans to the cisisomer, the textures should completely disappear after enough exposure time.
When the sample of 3a, as the first sample, was heated to 152°C at the thermoplate, a texture of schlieren type can be observed (Fig. 4A). Keeping the temperature constant, the texture was exposed to UV light of 365 nm with illumination intensities of 105 μW cm -2 within the time interval of 10 min. The changes of the texture were then observed by POM (Fig. 4B-G). From these pictures we can find that the areas of texture are gradually decreasing as the exposure time increases until they completely disappear, i.e. the azo compound did change its shape in a mesophase. The exposure time of other compounds were also recorded ( Table 1), showing that the change of the molecular shapes is more difficult in a mesophase than in solutions. It is because the molecules in a mesophase still need to overcome larger intermolecular forces than in a solution. From the results shown in Table 1, we also found that the molecules with the terminal benzene ring were so difficult to change their configurations that they needed more time of exposure to UV light.

Materials and Characterization
All initial intermediates used in the synthesis were prepared in our laboratory with purity higher than 99%, and characterized by IR, GC-MS and 1 H NMR. Other reagents were from commercial sources and used without further purification. The purity of the compounds was determined using LC-10A (Shimadzu) instrument with methanol as eluent and flowing rate was 1 mL min -1 . Elemental analyses were conducted using PE-2400 analyzer (Perkin Elmer). UV spectra were recorded on Agilent 8453 Spectrometer (Agilent Technologies). IR (KBr) spectra were recorded on The conventional three electrode system was measured against SCE, platinum wire and glassy carbon serving as the reference, counter and working electrodes respectively.

Synthesis of compound 2
2a was prepared by heating and stirring a mixture of 1a (0.81 g, 5 mmol) and SOCl 2 (0.59 g, 5 mmol) and several drops of DMF in toluene (30 mL) at ambient temperature for 5 h, then the solvent was removed under vacuum. A brown liquid (2a) was obtained and used in the next step without further purification.

Thermal analysis
Thermal transitions and enthalpies were determined by DSC measurements carried out using TA equipment with a MDSC Q100 module using a heating rate 10°C min -1 and a nitrogen flow of 50 mL min -1 . TGA measurements were carried out using TA equipment with a Q500 module using a heating rate of 10°C min -1 . Mesomorphic textures were determined using a LV100POL Polarizing Optical Microscopy (Nikon) with a LTS E350 hot stage (Linkam).

UV-vis Spectroscopic Measurement and photoisomerization
The UV-vis absorption spectra were recorded using an Agilent 8453 Spectrometer. The sample were dissolved in spectroscopy grade methanol and maintained in dark for 2 days before the spectra were recorded. A ZF-7A UV lamp was used for photoisomerization analysis (Shanghai Jiapeng Technology Co. Ltd.). The excitation band used had a maximum at 365 nm. All the optical analyses were performed in solution at room temperature. The photoisomerization in mesophase was also carried out using the same UV-lamp for irradiation.

Calculation of the ratios of the length to width of target molecules
The length and width of the target molecules were calculated by Chem Bio3D Ultra 12.0.2 (Combridgesoft). During the procedure the stereostructure of any molecule was first drawn by the software. The optimum configuration of the molecule was then worked out by minimizing the molecular energy. At last, the farthest distance calculated between any two atoms in the horizontal direction was considered to be the molecular length, while the farthest distance calculated between any two atoms in the vertical direction the width, so that the ratios of the molecular length to the width could be obtained.

Conclusions
We have reported here eight new rod-like azo liquid crystal molecules with two different terminal rings. The molecular structure allows all of them, even these with the terminal methyl, to exhibit a mesophase behavior that belongs to the nematic phase of schlieren type. All these compounds can, in a methanol solution, gradually change their shape from trans to cis isomer under irradiation of UV light (365 nm). Sunlight can recover their original structures by transforming the cis isomer to the trans one with a recovery rate of 99%. An interesting phenomenon, involved the odd-even carbon electronic effect on the compounds with a terminal benzene ring, was observed in the UV spectra of these target compounds. We performed photo-induced studies on mesophase of these target compounds, which has been seldom reported until now. Results showed that under irradiation of UV light, the molecules can also change their shapes in mesophase, which was observed by gradually disappearing of the mesogenic textures under POM. The HOMO, LUMO and E g obtained from the cyclic voltammetric method showed that the electronic transition between the frontier molecular orbital (E g ) is related to the second maximum absorption peak of each target molecule in the UV spectra. The effect of carbon number on E g is in excellent agreement with that on the second maximum absorption wavelength in their UV Spectra. The E g and the absorption wavelengths can be modified by changing the terminal alkyl group of the target molecules; the subject requires further study in order to change the photo-sensitive property of molecules in a certain range.