Successful preparation of new liquid crystalline 3 (or 4)-n-alkanoyloxy benzoic acids, were designed and fully characterized via experimental and theoretical approaches. Elucidation of their molecular structures were carried out by elemental analyses, NMR and FT-IR, spectroscopy. Thermal and mesomorphic properties of all symmetrical dimers having -symmetrical alkanoyloxy chain, were analyzed by differential scanning calorimetry (DSC) and their mesophases identified by polarized optical microscopy (POM). Results revealed that, the smectic mesophase covered all designed symmetrical dimers with thermal stability depending on the length of terminal alkanoyl moiety. Furthermore, the results of the Density Functional Theory (DFT) calculations supported the observed experimental data for the mesomorphic behaviour. The para-derivatives (In) of the alkanoyloxy benzoic acids were predicted to be stable with greater hydrogen bonding interactions than the meta (IIn) counterparts. The computed reactivity parameters revealed that the acids reactivity was influenced by the position of ester substituent. The absorbance spectra of both para and meso derivatives were noticed to be blue shifted with the increase of alky side, however, the energy band gap of meso-derivatives was found to be slightly higher than para-derivates. The synthesized materials exhibit broad photoluminescence spectrum which was noted to be red shifted with the increase of side chain length. The fluorescence lifetime was found to be increases with the increase of alkyl side chain length, and meso-derivatives have slightly longer lifetime as compared to the para-derivatives.
Article
First Mesomorphic and DFT Characterizations for supramolecular assemblies of 3 (or 4)-n-alkanoyloxy benzoic acids and their optical applications
https://doi.org/10.21203/rs.3.rs-2227706/v1
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published 23 Aug, 2023
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3 (or 4)-n-alkanoyloxy benzoic acids
symmetrical supramolecular-H bonding
smectic phase
characterizations
mesophase stability
DFT
optical properties
Liquid crystals (LCs) can play an important role in a variety of applications, including displays, solar cells, sensors, and modulators, because to their promising properties [1] LCs are also appropriate for applications in biology, food production, medicine, and oil recovery. As a result, research into the development and design of liquid crystalline systems has increased recently [2–5]. Supramolecular interactions resulting from H-bonded systems have received a lot of attention in recent years [6–11]. Non-covalent interactions and functional molecule geometries are potential applications for liquid crystal (LC) materials. The shape of LC molecules is determined by their molecular architecture [12–14], and hence plays an important role in the formation of mesophase. The supramolecular complex of LCs (SMLCs), or the association of molecules, is one of the most essential interactions in biological and chemical processes. Mesomorphic stability is mostly determined by the polarity and/or polarizability of the molecule's core mesogenes. Calamitic intermolecular H-bonding interactions are at the core of many of the studied symmetrical and non-symmetrical H-bonded mesogens [15–20]. Another study looked at the production of angular complexes by the establishment of intermolecular H-bonds [21]. Several mesogenic derivatives, as a central linking group, have been reported in the formation of LC dimes [22–26].
H-bonded interaction has been recognized in the production and stabilization of LC mesophases in recent years [27], and its involvement in mesophase self-assembly has also been studied [28, 29]. Thus, in thermotropic LC systems, intermolecular H-bond interactions have shown promising potential [30, 31]. Furthermore, aromatic carboxylic acid dimerization is the first example of LC production [32]. The geometry of the resulting SMLCs has a significant impact on the behavior of LC material. Pyridyl and carboxylic components also serve as proton-acceptor and proton-donor moieties, respectively, in several SMLCs [33, 34].
Gray et al. [35], who researched the thermal characterizations of 4-n-alkoxybenzoic acids, presented the first examples of supramolecular hydrogen bonded liquid crystal (SMHBLC) systems. The production of calamitic symmetric mixes of interacting benzoic acid molecules forming a supramolecular core via hydrogen bonding interactions was attributed to the mesomorphic features of alkoxy-substituted benzoic acids [35, 36]. This approach has recently been expanded to the development of novel supramolecular complex such asnew twist-bent nematogens (bent SMHBLCs) [37]. The production of supramolecular mesogens via hydrogen bonding interactions is generally more efficient than covalent bonding due to lengthen the mesogenic part, and a new method for incorporating its functionality into the molecular skeleton in a controlled and effective manner has been developed [38].
Usually no single compound achieves all of the desired fundamental characteristics, hence LC materials for device displays are generally hybrids. As a result, the investigation of mixtures of LC components is a major topic [37]. The benzoic acid derivatives are the most commonly used compounds for forming LC materials via H-bonding interactions [38]. The purpose of this research is to examine the mesophase behavior of possible SMC formation via H-bonding interactions (In and IIn) produced between pairs of 3 (or 4)-n-alkanoyloxy benzoic acid derivatives with varying terminal alkanoyloxy chain lengths ranging from 5, 9 to 15 carbons. The influence of the mesogenic cores as a function of the location of the alkanoyloxy chain in the designed complexes will also be investigated. Moreover, the experimental results would be substantiated by the theoretical calculations via DFT approach. Furthermore, the optical and photophysical properties of synthesized LC materials were investigated by recording the absorption spectra, steady state and time resolved spectra to find its applications on nematic display devices.
2.1.Synthesis
To a solution of the corresponding hydroxybenzoic acid (20 mmol) in 40 mL of DCM, Triethylamine TEA (40 mmol), dimethylaminopyridine (DMAP) (2 mmol) and the appropriate acid chloride (20 mmol) were added at 0°C. The reaction mixture was allowed to reach the rt and stirred overnight at this temperature. The volatiles were then removed under reduced pressure, and the resulting oily residue dissolved in aqueous solution of 1N hydrochloric acid and extracted with diethylether (5×20 mL). The combined organic layers were washed with NaHCO3, dried over Na2SO4, filtered and concentrated under reduced pressure. Final purification by recrystallized with ethyl alcohol, afforded the desired esters (Scheme 1).
4-(hexanoyloxy)benzoic acid
1H NMR (850 MHz, DMSO) δ 7.99 (d, J = 8.5 Hz, 2H, Ar-H), 7.25 (d, J = 8.6 Hz, 2H, Ar-H), 2.60 (t, J = 7.4 Hz, 2H, CH2), 1.67–1.63 (m, 2H, CH2), 1.36–1.32 (m, 4H, 2CH2), 0.89 (t, J = 7.1 Hz, 3H, CH3). 13C NMR (214 MHz, DMSO) δ 171.96 (CO), 167.12 (CO), 154.49 (C), 131.60 (CH), 128.75 (C), 122.48 (CH), 33.89 (CH2), 31.04 (CH2), 24.40 (CH2), 22.26 (CH2), 14.26 (CH3).
decanoic 4-(decanoyloxy)benzoic anhydride
1H NMR (850 MHz, DMSO) δ 7.99 (d, J = 8.3 Hz, 2H, Ar-H), 7.23 (d, J = 8.4 Hz, 2H, Ar-H), 2.59 (t, J = 7.4 Hz, 2H, CH2), 2.18 (t, J = 7.4 Hz, 2H, CH2), 1.67–1.62 (m, 2H, CH2), 1.49 (m, 2H, CH2), 1.26 (m, 22H, 11CH2), 0.86 (t, J = 6.9 Hz, 6H, 2CH3).13C NMR (214 MHz, DMSO) δ 174.75 (CO), 171.93 (CO), 167.18 (CO), 154.29, 131.30, 128.46, 122.39, 49.05, 34.14, 33.93, 33.72, 31.75, 29.35, 29.31, 29.23, 29.17, 29.13, 29.11, 29.02, 28.92, 28.84, 24.97, 24.72, 22.56, 14.39.
4-(palmitoyloxy)benzoic acid
1H NMR (850 MHz, DMSO) δ 7.99 (d, J = 8.6 Hz, 2H, Ar-H), 7.24 (d, J = 8.6 Hz, 2H, Ar-H), 2.60 (t, J = 7.4 Hz, 2H, CH2), 1.67–1.62 (m, 2H, CH2), 1.38–1.33 (m, 2H, CH2), 1.31–1.20 (m, 22H, 11CH2), 0.86 (t, J = 7.1 Hz, 3H, CH3). 13C NMR (214 MHz, DMSO) δ 171.89 (CO), 167.17 (CO), 154.25 (C), 131.25 (CH), 128.75 (C), 122.48 (CH), 33.93 (CH2), 31.76 (CH2), 29.52 (CH2), 29.50 (CH2), 29.47 (CH2), 29.47 (CH2), 29.41 (CH2), 29.30 (CH2), 29.17 (CH2), 29.11 (CH2), 28.80 (CH2), 24.71 (CH2), 22.56 (CH2), 14.42 (CH3).
3-(decanoyloxy)benzoic acid
1H NMR (400 MHz, CDCl3) δ 8.82 (brs, 1H, OH), 7.96 (d, J = 7.4 Hz, 1H, Ar-H), 7.80 (s, 1H, Ar-H), 7.46 (t, J = 7.7 Hz, 1H, Ar-H), 7.29 (d, J = 7.6 Hz, 1H, Ar-H), 3.18 (m, 2H, CH2CO), 2.47–1.36 (m, 14H, 7CH2), 0.90 (s, 3H, CH3).13C NMR (101 MHz, CDCl3) δ 179.91 (CO), 172.31 (CO), 150.52 (C), 132.30 (C), 129.33 (CH), 127.34 (CH), 126.43 (CH), 123.24 (CH), 45.51 (CH2), 34.33 (CH2), 31.86 (CH2), 29.41 (CH2), 29.26 (CH2), 29.10 (CH2), 24.89 (CH2), 22.66 (CH2), 14.09 (CH3).
3-(palmitoyloxy)benzoic acid
1H NMR (400 MHz, DMSO) δ 7.84 (d, J = 7.7 Hz, 1H, Ar-H), 7.64 (s, 1H, Ar-H), 7.56 (t, J = 7.9 Hz, 1H, Ar-H), 7.38 (d, J = 8.1 Hz, 1H, Ar-H), 2.60 (t, J = 7.3 Hz, 2H, CH2CO), 1.74–1.57 (m, 2H, CH2), 1.28 (d, J = 32.5 Hz, 14H, 7CH2), 0.85 (t, J = 6.6 Hz, 3H, CH3). 13C NMR (101 MHz, DMSO) δ 172.07(CO), 168.43 (CO), 150.97 (C), 132.76 (C), 130.34 (CH), 127.10 (CH), 126.80 (CH), 123.01 (CH), 33.87 (CH2), 31.74 (CH2), 29.47 (CH2), 29.45 (CH2), 29.38 (CH2), 29.28 (CH2), 29.15 (CH2), 29.08 (CH2), 28.79 (CH2), 24.69 (CH2), 22.55 (CH2), 14.40 (CH3).
2.2. Computational details
The molecular structures of acid dimers understudied were fully optimized without geometrical constraint via GAUSSIAN 09 program [39]. Following the successful optimization, frequency calculation was conducted to substantiate the convergence nature of the dimers and real values were predicted for all the frequencies. On the other hand, Frontier molecular orbitals and the molecular electrostatic potential (MEP) surfaces were obtained from the supplementary check files (.chk) associated with the optimization. All the computations were accomplished by density functional theory (DFT) using B3LYP method [40, 41] while 6-311G was used as the basis set.
3.1. Chemistry
Scheme 1 provides an illustration of the synthetic pathway leading to the target molecules. Concisely, compounds In and IIn were prepared with a typical acyl chloride-phenol condensation procedure. Quaternary ammonium salt derived from acyl chloride was formed in situ by addition of triethylamine to an anhydrous solution of acid chloride in presence of catalytic amount of dimethylaminopyridine (DMAP). Then the activated acyl group namely, (hexanoyl, decanoyl or hexadecanoyl) were allowed to react with either 3- or 4-hydroxybenzoic acid to afford 3- or 4- alkanoyloxybenzoic acids respectively. The reaction time ranged from 20 to 24 hours at room temperature, and the yields were between 75 and 86% and all the products were purified by recrystallization as solid products,
The target alkanoyloxybenzoic acids were primarily characterized for the presence of the ester functional group and for the terminal alkyl chain in their structures. In the 1H NMR spectrums, chemical shifts for the hydrogenated carbon of the terminal alkyl chain were assigned in the high fields (δ 0.85–3.18 ppm). Chemical shifts for the aromatic protons were observed in the low fields with signals at δ 7.24ཞ7.99 ppm, that corresponded to four protons on the benzene ring. In the 13C NMR spectrums, the chemical shifts (δ) of the saturated carbons of the terminal alkyl chain appeared in high fields at δ 14.09ཞ45.51 while those of unsaturated carbons on benzene ring appeared in low fields at δ 122.48ཞ154.25. Furthermore, the chemical shifts of carbonyl carbon atoms in ester or carboxy functional groups were observed at lower fields at δ 179.91ཞ 167.12 ppm due to the substantial deshielding effects of carbonyl oxygen and the magnetic anisotropy effect of the adjacent aromatic ring. As an exception compound I9 showed a duplicate number for the saturated carbons of the terminal alkyl chain, which indicate that acylation take place for both phenolic and carboxy group affording acid anhydride with high thermal stability.
3.2. Mesomorphic behaviours of acid dimers
The two groups of 3- (or 4-)-n-alkanoyloxy benzoic acids (In and IIn) were synthesized, and the possible supramolecular binary complexes that formed from each dimer were thoroughly described. DSC and POM were used to evaluate the resultant mixtures' mesomorphic characteristics. Figure I show the DSC thermograms of I7 and II7 dimers as representative examples. Figure 2 depicts typical textures of the mesophases under POM. Mesomorphic transition temperatures and their related enthalpy for each supramolecular H-bonded acid dimer complex are collected in Table 1. To investigate the impact of terminal alkanoyloxy chain length on the mesomorphic behavior of both prepared dimeric groups, phase transition temperatures, as driven by DSC measurements, are graphically displayed in Fig. 3a,b as a function of the alkanoyloxy chain length (n), which varies among 3, 7 and 13 carbons. Table 1 and Fig. 3a,b show that the melting transitions for all prepared dimers shift irregularly as the length of the alkanoyloxy chain increases, as usual in LCs systems [17, 18]. Furthermore, depending on the terminal lengths of chain n and their positions, all complexes were shown to exhibit monomorphic phase in either an enantiotropic or monotropic approach. For complexes I3, I7, and I13, respectively, the designed supramolecular H-bonding complexes (In) show mesophases with thermal stabilities of 78.5, 101.2, and 106.9 oC. Additionally, as n increases, the observed smectic C (SmC) phase becomes significantly more stable. The formed SmC for I3 symmetrical dimer appears monotropically and has the least thermal stability in group In (78.5 oC). The mesophase range and stability of the complex I7's SmC phase are around 3.8 and 101.2 oC, respectively. The highest thermal stability and smectogenic range, which are about 106 and 51.0 oC, respectively, are displayed by the longest dimeric complex I13. These findings are consistent with earlier publications [36], according to which the mesophase range was shown to be wider the greater the difference between the flexible wings of the mixed components. All dimeric compounds in the second series IIn (Fig. 3b) show monotropic characteristics. Additionally, for II3, II7, and II13, respectively, their SmC stabilities increase in order to 21.6, 42.3, and 45.2 oC. The dimer II13 in the series IIn has the maximum thermal stability, while the supramolecular complex in the complex II3 exhibits the lowest smectogenic stability. The produced dimers have a preferred molecular configuration, according to all mesomorphic behavior studies.
| System | TCr−SmC | ∆HCr−SmC | TSmC-I | ∆HSmC−I | ∆S/R |
|---|---|---|---|---|---|
| I3 | 104.6 | 49.65 | 78.5* | 3.4 | 1.16 |
| I7 | 97.4 | 40.21 | 101.2 | 2.3 | 0.74 |
| I13 | 55.9 | 41.72 | 106.9 | 2.5 | 0.79 |
| II3 | 76.7 | 39.66 | 21.6* | 1.3 | 0.53 |
| II7 | 69.9 | 45.12 | 42.3* | 1.9 | 0.72 |
| II13 | 53.3 | 43.99 | 45.2* | 2.2 | 0.83 |
Cr−SmC= crystal to smectic C phase transition;
SmC−I = smectic C to isotropic liquid phase transition;
*monotropic phase appear on cooling only
For all dimers (In and IIn), the transition normalized entropy changes (∆S/R) were calculated, and the results were listed in Table 1. According to group In's findings, the entropy changes (∆S/R) decreased as the alkanoyloxy chain length increased from n = 3 to 7 carbons before suddenly increasing at n = 13. The varied stability of the created mesophases may be responsible for this inconsistent relationship. Entropy changes (∆S/R) for group IIn results in Table 1 demonstrated an increment values with the increase of the alkanoyloxy chain length. As a result of the establishment of a broad range of SmC phase, the degree of lateral associations between molecules is increasing along with the overall length of the molecule. Additionally, the weak conjugative interactions between the mesogenic cores of the examined dimeric complexes are to responsible for the relatively low amounts of entropy changes reported.
3.3. DFT theoretical studies
3.3.1. Relative stability
The stability of compounds could be influenced by the position of substituent on the basis of ortho-para directory. The result presented in Table 2, affirms the para-alkanoyl derivatives (In) of the acid dimer to be more stable than the corresponding meta-alkanoyl counterpart (IIn) by 3 kcal/mol. The orientation of ester substituent in the acid makes it an electron donating group, which in principle, would be more active at para position than the meta and as such giving the In series greater stability over that of IIn via resonance [42, 43]. In addition, steric hindrance is minimal at para position, and this could have aided the stability of In dimers over the IIn counterparts [43]. Another important factor that could have enhanced stability of In derivatives is the interatomic hydrogen bonding interaction between the acids in the dimer. The slightly shorter OH—OC interatomic distance recorded for the para derivatives (Table 2) suggests greater hydrogen bonding interaction which in return aided their stability over the meta-alkanoyl counterparts. Furthermore, the greater deviation of OH—O bond angle of IIn dimers from the linearity (180o) as highlighted in Table 2 attests to their weaker hydrogen bonding interaction compared to the corresponding In series [44, 45]. The theoretical revelations about the greater stabilities of the In series over the IIn counterparts are in consonant with the observations from the thermal stabilities of the mesophases revealed by the phase transition experiment.
| Compound | Relative energy (kcal/mol) | OH—OC Inter atomic distance (Å) | OH—O Bond angle (o) | |||||
|---|---|---|---|---|---|---|---|---|
| n | In | IIn | In | IIn | In | IIn | ||
| 3 | 0 | 2.82 | 1.607 | 1.616 | 176.3 | 175.4 | ||
| 7 | 0 | 2.79 | 1.609 | 1.617 | 176.2 | 175.3 | ||
| 13 | 0 | 2.71 | 1.610 | 1.617 | 176.2 | 175.3 | ||
3.3.2. Reactivity parameters
The HOMO-LUMO energy gap is an essential parameter in accessing the reactivity of chemical compounds. Other indicators for chemical reactivity are the ionization potential (I.P) and electron affinity (EA) [42, 43]. To evaluate the positional effect of the ester substituent together with the increasing size of its alkyl part, on the reactivity of acid dimers resulting from the hydrogen bonding interaction, optimization calculation was carried out while the computed reactivity parameters are listed in the Table 3. The result reveals that the HOMO and LUMO energy parameters are being influenced by the position of ester substituent. Both HOMO and LUMO energy levels of the para-alkanoyl derivatives (In series) were predicted to be lower than that of the corresponding meta-alkanoyl counterparts (IIn series). In the end, lower resultant HOMO-LUMO energy gaps (∆E) were obtained for the IIn members. The lower energy gap suggests that the IIn would be more reactive but less stable than the In counterparts [43] and this is consistent with the stability earlier predicted from the relative energy. On the other hand, the reactivity parameters seem to be less sensitive to the size of terminal alkyl as the similar values were calculated for the respective HOMO and LUMO energy levels in each series of the acid dimer. On the part of polarity of acid dimer, it could be seen that it is being influenced by the position of ester substituent and the length of terminal alkyl as revealed by the calculated dipole moment and isotropic polarizability highlighted in Table 3. The higher value obtained for the dipole moment of the In series attests to their greater polarity over the IIn counterparts and this is further corroborated by the higher isotropic polarizability computed for them [2]. The observed decrease in the dipole moment for both In and IIn with the increasing length of the dimer terminal alkanoyl, is consistent with the reports for the hydrogen bonded dimers having the magnitude of their dipole moment decreasing with the increase in size of the system [2, 46, 47]. On the part of Frontier molecular orbitals of which the representative models are shown in Figs. 4 and 5, both dimer of In and IIn showed some similarities in the orbitals distribution at the HOMO level, as they have their electron clouds being steadily distributed over only carbon atoms of the phenyl ring. Also, similar appreciable electron clouds distribution upon the carbon and oxygen atoms of C = O and the alkoxy oxygen of the carboxylic moiety was predicted for the two acid dimers (In and IIn). However, their HOMOs differ at alkanoate part, as noticeable electron densities were obtained for the carbon and oxygen of the C = O of the alkanoate of In series but, those for the IIn counterparts were electron deficient. In the case of the LUMO, electron clouds distribution significantly covered both carbon atoms and the π system of phenyl ring for the In and IIn dimers. Moreover, electron clouds were predicted to be fairly spread over the carbonyl and alkoxy oxygen atoms of both alkanoate and carboxylic moiety of In members, whereas those for the –COOH group of the IIn derivatives appeared electron deficient. Another computed parameter worth mentioning is the molecular electrostatic potential (MEP) as it normally reveals the extent of electron density for a particular region (Fig. 6). The red cloud over the carbonyl and alkanoyl oxygen atoms of the carboxylic moiety and carbonyl oxygen of the terminal alkanoate of both In and IIn dimers, portrays these regions to be of high electron density but low electrostatic potential. However, the blue cloud over the region of alkanoate carbonyl carbon and its immediate methylene for the two acid dimers signifies high electrostatic potential with low electron density [48–50].
| Compound | EHOMO (eV) | ELUMO (eV) | ∆E (eV) | Dipole moment (Debye) | I.P (eV) | E.A (eV) | Isotropic polarizability (Bohr**3) | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| n | In | IIn | In | IIn | In | IIn | In | IIn | In | IIn | In | IIn | In | IIn | ||||||
| 3 | -7.156 | -7.062 | -1.988 | -1.951 | 5.168 | 5.111 | 2.8528 | 0.2765 | 7.156 | 7.062 | 1.988 | 1.951 | 333.35 | 320.86 | ||||||
| 7 | -7.154 | -7.059 | -1.984 | -1.949 | 5.170 | 5.110 | 2.772 | 0.1916 | 7.154 | 7.059 | 1.984 | 1.949 | 428.66 | 415.17 | ||||||
| 13 | -7.153 | -7.058 | -1.984 | -1.948 | 5.169 | 5.110 | 2.7304 | 0.1702 | 7.153 | 7.058 | 1.984 | 1.948 | 570.26 | 556.15 | ||||||
3.3.3. Energy
Being an extensive property, the magnitude of energy is always dependent on the size of a system. This assertion is justified by the trend of increasing value of zero-point energy, thermal energy, and the thermodynamic variables with the size of the dimers as computed in Table 4. This observation assents to the reports on the energy of systems being directly related to their sizes [48–50]. In addition, the trend of increasing calculated thermal energy with the size of the system is consistent with the increasing thermal stabilities of mesophases for both In and IIn series with the length of alkanoyloxy chain, as revealed by the smectic C to isotropic liquid phase transition temperatures (TSmC-I).
| Compound | ZPE (kcal/mol) | Thermal (kcal/mol) | Enthalpy (kcal/mol) | Gibbs (kcal/mol) | Entropy (cal/mol K) | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| n | In | IIn | In | IIn | In | IIn | In | IIn | In | IIn | ||||
| 3 | 340.662 | 340.644 | 363.158 | 363.112 | 363.750 | 363.704 | 291.528 | 291.246 | 242.235 | 243.025 | ||||
| 7 | 483.987 | 483.955 | 513.287 | 513.231 | 513.879 | 513.824 | 424.927 | 424.314 | 298.347 | 300.217 | ||||
| 13 | 699.002 | 698.913 | 738.489 | 738.404 | 739.081 | 738.996 | 624.885 | 623.905 | 383.015 | 386.019 | ||||
The knowledge of the optical properties of liquid crystalline material is of important to find its applications in optoelectronic devices. For this purpose, we have investigated the absorption and emission spectra of synthesized liquid crystalline materials. The absorption spectra of liquid crystalline p-alkanoyloxy (In) and m- alkanoyloxy (IIn) benzoic acid are illustrated in Fig. 7a. The peak absorbance corresponds to the C-band was noticed around 265 nm for II3 sample [51, 52]. As the alkyl side chain length increases peak slightly blue shifted to 264 nm (for II7) and 265 nm (for II13). The blue shift in absorption spectra originated due to stearic hindrance effect of alky side chain [53]. The absorption spectra of meso samples were found similar to the para samples. The energy band gap of liquid crystalline 4-alkoyloxy benzoic acid and 3-alkanoyloxy benzoic acid were evaluated through Tauc plot shown in Fig. 7b. According to Tauc relation [54, 55] , the intercept on energy axis provides the band gap. The band gap of I3 sample was found to be 4.14 eV and noted to be increased to 4.33 eV for I13 sample. The bandgap of IIn samples were slightly higher than In samples with Eg = 4.19 eV, 4.25 eV and 4.35 eV for II3, II7 and II13, respectively.
The normalized photoluminescence (PL) spectra of liquid crystalline alkanoyloxy benzoic acids were recorded by exciting the sample with 320 nm (Fig. 8a,b). The materials exhibit broad emission spectra in the range of 410 nm – 575 nm with peak emission at ⁓460 nm. The emission was noted to be red shifting with increase of side chain length. Moreover, the emission peak of IIn was noted to be slightly red shifted as compared to In, with peak emission at 472 nm for II13. The fluorescence decay spectra shown in Fig. 8b was recorded to investigate the impact of side chain length on life time of excited charge carriers. It was found that the decay spectra well fitted with exponential decay Equation: , where A and B are fitting constants and τ is charge carrier lifetime. The evaluated fluorescence decay parameters are presented in Table 5. The life time was evaluated to be 156 ps, 232 ps and 1.981 ns for the samples I3, I7 and I13, respectively. The life time was found to be increases with the increase of alkyl side chain length. Further, the life time for the samples having functional group at m-position (IIn), noted to be higher than the samples having functional group at o-position (In). Again, the life time for m-positional samples were also noted to be increasing with increase of side chain length with life time of 489 ps, 519 ps, and 855 ps, for the samples II3, II7 and II13, respectively.
| Sample | I3 | I7 | I13 | II3 | II7 | II13 |
|---|---|---|---|---|---|---|
| Eg (eV) | 4.14 | 4.33 | 4.33 | 4.19 | 4.25 | 4.35 |
| τ | 156 | 232 | 1981 | 489 | 519 | 855 |
New six symmetrical 3 (or 4)-n-alkanoyloxy benzoic acid liquid crystals were synthesized and investigated by experimental and theoretical tools. The results revealed that all dimers are monomorphic exhibiting purely SmC phase. The theoretical results fairly agreed with experimental data on the mesomorphic behaviour and further revealed the In series to be more stable but less reactive than the IIn counterparts. Moreover, the predicted reactivity parameters suggested that the position of ester substituent appreciably affected the electronic properties of the benzoic acids understudied. Optical bandgap was found to be increases with the increase of alkanoyl side chain length and meta-derivatives have slightly higher bandgap as compared to the para-derivatives. Moreover, the fluorescence lifetime was found to be increases with the increase of alkanoyl side chain length, and meta-derivatives have slightly longer lifetime as compared to the para-derivatives.
Conflicts of Interest: The authors declare no conflict of interest.
Author contributions:
Formal analysis, H.A.A., S.A.p., F.S.A and M.S.Kh; Funding acquisition, M.J., A.H.E and M.S.Kh.; Methodology, M.A.E., H.A.A.,M.T.K., M.J. and H.A.A.; Project administration, F.S.A. and H.A.A.; Resources, F.S.A., M.A.E., M.T.K., and H.A.A.; Sofware, S.A.P. and M.A.E. ; Writing—original draf, H.A.A., M.S.Kh., H.A.A., M.T.K. and A.H.E.; Writing—review and editing, H.A.A., A.H.E ., M.J., S.A.P., F.S.A., M.S..Kh. and M.T.K. All the authors approved the final version.
Data availability:
The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.
All data generated or analysed during this study are included in this published article and its supplementary information files.
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Scheme 1 is available in supplementary section.
No competing interests reported.
- Scheme1.png
Scheme 1. Synthesis of title groups, In and IIn.
- Supplementarydata.docx
