Polar Switching and Cybotactic Nematic Ordering in 1,3,4-Thiadiazole-Based Short-Core Hockey Stick-Shaped Fluorescent Liquid Crystals.

We report here the synthesis and thermotropic properties of novel short-core hockey stick-shaped liquid crystalline molecules based on the 1,3,4-thiadiazole core. Polar switching behavior is observed in the cybotactic nematic and smectic mesophases for the bent-core thiadiazole derivatives. The presence of the lateral methoxy moiety in the outer phenyl ring of the four-ring molecules facilitates the formation of spontaneous ordering in the nematic phase observed via X-ray diffraction measurements. Anomalous temperature dependence of spontaneous polarization on cooling is explained by the possible antiferroelectric packing of the molecules that require higher electric field for switching. The compounds exhibited a strong absorption band at ∼356 nm and a blue emission band at ∼445 nm with a good quantum yield of φf ∼0.39. The mega Stokes shift is observed and depends on the nature of the solvent.

Introduction

Nematic phase is the most technologically important and least ordered mesophase and the backbone of the multibillion-dollar display industry which serves for the upliftment of the status of human living during the last two decades. Modulation in this nematic phase, like the formation of twist-bend heliconical structure or macroscopic biaxiality, is the thrust area of contemporary liquid crystal (LC) research due to their promising potentiality for use in ultrafast switching devices. The available theory that correlates the reduced symmetry in biaxial nematics (NB) or spontaneous chiral symmetry breaking in twist-bend nematics (NTB) and modulated molecular structures with relevant features of molecular interactions is not yet completely resolved. It has been well predicted that the introduction of bent curvature in molecular architecture can lead to stabilize these modulations via formation of biaxial ordering[1] or by inducing twist-bend helicity in the nematic phase.[2] Hence, a good number of bent-core molecules have been synthesized that exhibited nematic ordering having a rich variety of complexity of molecular structures regarding the bending unit (bent angle and polarity), linking moieties between two aromatic rings (direction and polarity), substitutions at the outer and/or core unit (position, size, and polarity), terminal chains (nature and length), and so forth.[3] Unfortunately, till today, there is no unambiguous claim of biaxial nematogens that can revolutionize the modern LC-based switching device technology. The biaxiality in the nematic ordering arise due to the formation of macroscopic-sized clusters by the intermolecular coupling and preorganization of the constituent molecules[4] and is known as “Holy-Grail of LC science”.[5]

The incorporation of heterocyclic rings such as 1,3-oxazole, 1,3,4-oxadiazole, 1,2,4-oxadiazole, triazole, thiazole, 1,3,4-thiadiazole, and so forth, instead of usual 1,3-disubstituted phenyl or naphthalene moiety, as a core unit in bent-core LCs produces new multifunctional materials due to the presence of the heteroatoms which provides a reduced symmetry, strong lateral and/or longitudinal dipole, and donor–acceptor interaction within the molecule that change polarity, polarizability, and geometry of the molecule which in turn affects the self-assembly process of the mesophase, transition temperature, electronic behavior, dielectric biaxiality, and other mesomorphic properties. Of these heterocyclic bent-core units, the use of the 2,5-disubstituted 1,3,4-thiadiazole unit in bent-core compounds exhibiting the nematic phase is rare and limited.[6] The majority of the 2,5-disubstituted 1,3,4-thiadiazole-based mesogens reported in the literature are either rod-shaped or rod bent-shaped molecules, exhibiting conventional nematic and smectic phases at the higher temperature.[7] Very recently, we have reported new four-ring hockey-stick LCs of 1,3,4-oxadiazole[8] and 1,3,4-thiadiazole bent-core units.[9] It is observed that most of the 1,3,4- oxadiazole and 1,3,4-thiadiazole-derived compounds exhibited the nematic phase. It is interesting to note that the terminal chain of the 1,3,4-thiadiazole-based hockey stick-shaped molecule drastically influences the mesophase behavior. The lower chain length exhibited the nematic phase, whereas the higher chain length displayed the SmA phase. Further, the isotropic temperature of the 1,3,4-thiadiazole-based molecules are quite high (∼300–350 °C). The high isotropic temperature of the 2,5-disubstituted-1,3,4-thiadiazole compounds which restricts the characterization of the phase and limits the application of the materials in display devices. The transition temperature and the phase behavior of the heterocyclic-based bent-core mesogens are sensitive to the structural modification, in particular, lateral substitution or functionalization of the molecule.[10]

Therefore, we have designed and synthesized a new series of hockey stick-shaped molecules containing the 2,5-diphenyl-1,3,4-thiadiazole derivative, possessing a lateral methoxy moiety in the terminal phenyl ring at the long arm of the molecule in order to understand the phase behavior. The compounds are shown to exhibit an ordered pattern in the nematic phase observed via X-ray diffraction (XRD) measurements. Most importantly, polar switching is observed in the lower part of the nematic and smectic phase in these thiadiazole-based compounds. The blue light emission band at ∼445 nm with a good quantum yield of φf ∼0.39 was also observed in these materials.

Results and Discussion

Design and Synthesis

1,3,4-Thiadiazole-derived new hockey stick-shaped short-core molecules possessing the lateral methoxy moiety at the terminal phenyl ring located in the long arm of the molecular framework having an imine linkage have been designed. The molecule is an unsymmetrical bent-core molecule, possessing two arms of different lengths. Of these, one arm contains two phenyl rings and is considered to be the longer arm of the molecule, containing the 4-n-alkyloxy chain of different lengths (4-n-butyloxy or 4-n-octyloxy or 4-n-dodecyloxy or 4-n-octadecyloxy), while the 4-n-butyloxy chain at the other arm of the molecule is considered to be the shorter arm of the molecule, possessing one phenyl ring. The methoxy group is introduced at the terminal phenyl ring of the elongated arm of the molecule. Detailed synthesis of the compounds is represented in Scheme and was carried out via the following procedure as elaborated in the Experimental Section. The intermediate compound, 2-(4′-nitrophenyl)-5-(4″-n-butyloxy)phenyl)-1,3,4-thiadiazole (1), was synthesized by reaction of 4-nitrobenzoic acid-N′-(4′-n-butyloxybenzoyl)hydrazide with Lawesson’s reagent in dry toluene under the nitrogen atmosphere. Further, the nitro group of compound (1) was reduced using stannous chloride to produce 2-(4′-aminophenyl)-5-(4″-butyloxy)phenyl)-1,3,4-thiadiazole (2). 3-Methoxy-4-n-alkyloxy-benzaldehydes (3) were synthesized by Williamson etherification reaction of 3-methoxy-4-hydroxy-benzaldehyde (vanillin) with n-alkyl bromides. Schiff base condensation of 3-methoxy-4-n-alkyloxy-benzaldehydes (3) with 2-(4′-aminophenyl)-5-(4″-butyloxy)phenyl)-1,3,4-thiadiazole (2) was used to obtain hockey stick-shaped molecules containing 1,3,4-thiadiazole (CV-). The high-resolution mass spectrometry (HRMS) and elemental analysis of the synthesized compounds were in consistent with the targeted molecular formula which in turn confirmed the pureness of the compounds (see Supporting Information, Figure S4). The characteristic Fourier transform IR (FT-IR) spectra of the compounds (CV-) are presented in Supporting Information, Figure S1. The FT-IR spectra of the CV- compounds showed the typical peaks of the thiadiazole ring of ∼1070 cm–1 for (=C–S–C=) and 1607 cm–1 for (−C=N−) stretching. The stretching frequency of the imine (−CH=N−) linkage was in the range of 1625–1635 cm–1. The stretching frequency of −C=N– in the thiadiazole ring and the linking −CH=N– moiety overlap with each other making it hard to distinguish the linking imine moiety from the thiadiazole (−C=N−) moiety. The observed singlet peak at the range of δ = 8.61–8.58 ppm corresponds to imine proton (−CH=N−) of the compounds, which approved the formation of the Schiff base moiety. The methoxy proton (−OCH) showed a singlet peak at δ = 3.98 ppm. All other protons in the aromatic region was found to be in the range of δ = 6.52–8.06 ppm (see Supporting Information, Figure S2a–d). 13C NMR spectra were in good agreement with the final molecular structure of the compounds (Supporting Information, Figure S3a–c).

Scheme 1

Reagents and reaction conditions: (i) Lawesson’s reagent, dry toluene, reflux, stir under the nitrogen atmosphere, 6 h; (ii) SnCl2·H2O, ethyl acetate, reflux, 4 h; (iii) CH2Br (n = 4, 8, 12, and 18), K2CO3, KI (catalytic amount), dry acetone, reflux 20 h; and (iv) absolute ethanol, 2–3 drops of glacial CH3COOH, reflux, 4 h.

Mesophase Behavior

Polarizing Optical Microscope and Differential Scanning Calorimetry Study

The phase transition temperatures, associated enthalpy, and entropy of the synthesized new thiadiazole-based hockey stick-shaped molecules CV- ( = 4, 8, 12, and 18) obtained from differential scanning calorimetry (DSC) at a scan rate of 5 °C min–1 in the second heating and cooling scans are summarized in Table . The mesomorphic behavior of the new hockey stick-shaped molecules was investigated under polarizing optical microscopy (POM) with crossed polarizers. All the new compounds CV- ( = 4, 8, 12, and 18) exhibited the nematic phase with underlying smectic C phases. The compound CV-4T on heating melts at 132.4 °C to a focal conic texture which on further heating results in the observation of the schlieren texture at 153.8 °C and finally becomes isotropic liquid at 272.8 °C. The compound CV-4T on slow cooling from the isotropic liquid, droplet texture of the nematic phase appeared at 269.0 °C. The nematic droplets coalesce to from the schlieren texture and immediately transform to the homeotropic texture. On further cooling, the weak birefringent greenish-color homogenous optical texture grows from the background of the homeotropic texture at 225.6 °C named as the NCyb phase. On further cooling, an arc-like texture appeared from the weakly birefringent optical texture and finally transformed into a focal conic texture at 145.8 °C. The optical textures of CV-4T are presented in Supporting Information, Figure S5a. The compound CV-8T on heating melts at 97.9 °C to the focal conic texture which on further heating transforms to the schlieren/droplet texture at 175.9 °C and finally becomes isotropic liquid at 234.4 °C. On slow cooling, the sample from the isotropic liquid, droplet texture of the N phase appeared at 233.4 °C (see Figure a). The droplet texture coalesces to form the schlieren texture with the appearance of the secondary schlieren texture at 232.0 °C (Figure b). The formation of the secondary schlieren texture is attributed to nonsingular domain walls which nucleate during the surface anchoring transition in an uniaxial nematic phase as reported in bent-core 1,3,4-oxadiazole-based molecules.[11] On further cooling, the homeotropic domain appeared in the texture, and finally, the texture becomes the weakly birefringent greenish texture at 190.0 °C (see Figure c). The batonnet-like texture developed from the greenish texture as a distinct transition (Figure d) at 177.0 °C, and finally the batonnets coalesce to form a focal conic-like texture (Figure e) at 170.0 °C. On further cooling, the broken focal conic texture appeared and transformed to schlieren textures at 130.5 °C (Figure f) and finally crystallized below 100.0 °C. The similar phase behavior was observed for the long-chain compounds (CV-12T and CV-18T) (see Supporting Information, Figure S5b,c).

Table 1

Thermal Data of the Compounds (CV-)

	TGA data			DSC dataa
compounds	To/°C	Tmax/°C	% char	phase transitions
CV-4T	377.4	401.0	1.02	Cr 132.4 (21.30, 52.50) SmC 153.8 (0.94, 2.20) N 272.8 (0.35, 0.64) Iso
				Iso 269.2 (0.76, 1.40) N 225.5b NCyb 145.8 (0.19, 0.46) SmC 105.3 (21.96,58.0) Cr
CV-8T	380.7	424.1	2.45	Cr 97.9 (14.87, 40.08) SmC 175.9 (0.14, 0.31) N 234.4 (0.54, 1.06) Iso
				Iso 233.6 (0.59, 1.17) N 232.0 NCyb 177.0b SmC 130.5b SmCx 113.7b Cr
CV-12T	379.2	423.9	1.69	Cr 99.0 (19.44, 52.20) N 236.6 (0.56, 1.10) Iso
				Iso 235.3 (0.59, 1.10) N 233.3 NCyb 142.0b SmC 72.8b Cr
CV-18T	404.4	439.9	7.91	Cr 92.8 (16.10, 44.00) SmC 95.9 (4.11, 11.10) Nx 203.9 (0.13, 0.28) N 209.8 (0.46, 0.95) Iso
				Iso 208.2 (0.89, 1.85) N 205.9 NCyb 200.9 (0.60, 1.27) SmC 64.1 (26.58, 78.8) Cr

Phase transition temperatures (°C) of the compounds CV- ( = 4, 8, 12, and 18) recorded for second heating and cooling cycles at 5 °C min–1 from DSC and confirmed by POM. The enthalpies (ΔH in kJ mol–1) and entropies (ΔS in J mol–1 K–1), respectively, are presented in parentheses.

Indicates the transition observed by POM only.

Figure 1

POM textures of CV-8T (a) droplet texture of the nematic phase at 233.4 °C; (b) secondary schlieren texture in the nematic phase (NCyb) at 232.0 °C; (c) growth of the homeotropic domain with the nematic schlieren at 190.0 °C; (d) appearance of batonnet texture at 177.0 °C; (e) focal conic texture of the smectic C phase at 170.0 °C; and (f) schlieren texture of the smectic phase at 130.5 °C.

Further, the thermal behavior of all the new hockey stick-shaped LC compounds was also examined with DSC on both second heating and second cooling at the rate of 5 °C min–1 under the nitrogen atmosphere. Phase transition temperatures obtained from DSC are agreed well with microscopy observations (see Table ). From Table , it is further noted that the thermal stability of the synthesized compounds increases with increase in the chain length (4-n-alkyloxy), and their decomposition temperature is ∼150 °C above the isotropic temperature, indicating high thermal stability of the compounds which is suited for the physical measurement without the decomposition of the sample. The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) thermogram of the mesogenic compounds (CV-) are presented in Supporting Information, Figure S7.

As revealed in the representative DSC thermogram in Figure , the compound CV-8T exhibits the enantiotropic nematic phase over a wide temperature range with four endothermic phase transitions, namely, crystal to smectic C (Cr–SmC), smectic C to NCyb (SmC–NCyb), NCyb to N, and nematic to isotropic (N–Iso). The measurable isotropic to nematic transitions are associated with a low enthalpy value (0.59 kJ mol–1) and low entropy value (1.17 J mol–1 K–1). Further, on careful insights into the nematic transition, two noticeable transitions are observed having vanishingly small enthalpy and were detected during N–NCyb phase transition, indicating that the two phases are closely related in structure (see inset of Figure ). It is noted that the N–NCyb transition temperature window is quite narrow (∼2 °C). Therefore, the N–NCyb transition enthalpy change could not be detected. The lower value of the enthalpy and entropy change during the transition indicated the small change in the ordering of the molecules in the phase. Moreover, hardly any measurable enthalpy change values associated for the NCyb–N transitions and NCyb–SmC transition are found which can be compared to Iso–N transition, indicating a cybotactic cluster formation in these nematic phases, that is, layer fragments already exist in the nematic phase and are clubbed to infinite layers at the phase transition.[12] These N–NCyb transitions were observed in all the other compounds. However, because of the narrow temperature window and the enthalpy change, it is difficult to calculate their associated enthalpy change during the transition. The DSC thermogram of the other compounds is presented in Supporting Information, Figure S6.

Figure 2

DSC thermogram of the compound CV-8T.

XRD Study

In order to understand the characteristics of the observed phase structure exhibited by the compounds, detailed X-ray investigation was carried out via small- and wide-angle XRD (SAXS/WAXS) measurements. The samples were filled in thin capillaries (0.5 mm) and aligned via repeated heating and cooling. All the compounds exhibited nematic and smectic phase structures at higher and lower temperatures, respectively.

As a representative case, the diffractogram of compound CV-8T at 200 °C comprised one small-angle peak with a d-value of 34.27 Å and a broad peak at a wide angle which designate to the liquid-like order of the alkyl chains (see Figure ). Additionally, a small peak was observed in the wide-angle region with a d-value of 3.54 Å which signifies the presence of core–core correlation between the molecules. During cooling the sample, the small-angle peak gradually became sharper. At 120 °C, the observed d-value of 28.40 Å which is lower than the molecular length (L) of ∼34 Å and a d/L ratio of 0.83 signifies the presence of tilt ordering in the smectic layers. Additionally, in the smectic phase, the second-order reflection peak with a d-value of 14.15 Å was observed.

Figure 3

XRD pattern of the nematic and smectic phase of CV-8T at 200 °C (red colored) and 100 °C (black colored), respectively. Inset shows the zoomed area of the mid-angle regime (left), exhibiting the appearance of the (002) peak in the Sm phase, zoomed area.

As a representative case, detailed XRD analysis of the compound CV-8T and CV-4T has been carried out to explain the ordering phenomena in the nematic phase. Both the compounds exhibited the nematic (N) phase at the higher temperature and the smectic (Sm) phase at the lower temperature. In order to understand the behavior of the N and Sm phase, azimuthal and radial plots of the first peak of the small-angle peak have been calculated. The small-angle peak of the N phase is found to be azimuthally bifurcated, and the bifurcation is prominent with decreasing the temperature in the nematic phase. The 2D XRD image of the cybotatic nematic ordering of CV-8T and CV-4T at different temperatures is presented in Figure and Supporting Information, Figures S8 and S9. However, the bifurcated peak transforms to a single peak in the smectic phase. In the nematic phase of compound CV-8T, the azimuthal plot of the small-angle peak displays its bifurcated nature (Figure a). However, the first small-angle peak of the smectic phase is azimuthally single (Figure c). The angular separation of the bifurcated peak is found to increase from ∼60° to ∼75° with decreasing temperature, and the respective average angular full width at half-maximum (fwhm) (average of the bifurcated peak) is found to decrease linearly from ∼51° to ∼26° in the nematic phase and suddenly decreases to ∼14° in the smectic phase (Figure a). The observation of the bifurcated peak confirms the two different preferential orientations in the nematic phase which is due self-arrangement of the molecules in the nematic phase and could be attributed to the cybotactic nature of the nematic phase. Further, the radial plot exhibits single peak in the nematic as well as in the smectic phase (Figure b,d). The measured d-spacing and correlation length are shown in Figure b. The d-spacing decreases slowly from ∼35 to 34 Å in the nematic phase and suddenly to 30 Å in the smectic phase with the decreasing temperature. However, the correlation length increases slowly in the nematic phase (from ∼40 to ∼55 Å) and then increases suddenly to 140 Å in the smectic phase. On the other hand, in the nematic phase of compound CV-4T, the azimuthal plot of the small-angle peak also displays its bifurcated nature (Figure a), and the first small-angle peak of the smectic phase is also azimuthally single (Figure c). However, the angular separation of the bifurcated peak is found to increase from ∼61° to ∼76° and then decrease to ∼74° with decreasing the temperature, and the respective average angular fwhm (average of the bifurcated peak) is found to decrease from ∼62° to ∼44° and then increase to ∼48° in the nematic phase and then suddenly decrease to ∼37° in the smectic phase (Figure a). Further, the radial plot exhibits a single peak in the nematic and in smectic phase (Figure b,d). The measured d-spacing and correlation length are shown in the Figure b. The d-spacing decreases slowly from ∼30 to 27 Å in the nematic mesophase and then to 25 Å in the smectic phase with decreasing temperature. However, the correlation length in the nematic phase increases slowly (from ∼31 to ∼50 Å) and then increases suddenly to 124 Å in the smectic phase.

Figure 4

2D-XRD images of modulation in nematic ordering of CV-8T at (a) 230, (b) 220, (c) 210, (d) 200, and (e) 180 °C.

Figure 5

In the nematic phase of compound CV-8T: (a) Azimuthal plot and (b) radial plot of the small-angle peak at 200 °C. The peak is azimuthally bifurcated, in the smectic phase of compound CV-8T; (c) Azimuthal plot and (d) radial plot of the small-angle peak at 160 °C. [Cyan and green represent the Lorentzian fit to the individual peak, and red displays the fit to the complete profile, and half blue-colored circle represents the data].

Figure 6

In the compound CV-8T: (a) variation of angular separation of the bifurcated peak in degree (blue-colored data) and their average fwhm in degree (red-colored data) with temperature, calculated from the azimuthal plot, (b) variation of d-spacing (blue-colored data) and correlation length (red-colored data) with temperature, calculated from the radial plot.

Figure 7

In the nematic phase compound CV-4T: (a) Azimuthal plot and (b) radial plot of the small-angle peak at 170 °C. The peak is azimuthally bifurcated. In the smectic phase of compound CV-4T: (c) Azimuthal plot and (d) radial plot of the small-angle peak at 140 °C. [Cyan and green represent the Lorentzian fit to the individual peak, and red displays the fit to the complete profile. Half blue-colored circle represents the data].

Figure 8

In the compound CV-4T: (a) variation of angular separation of the bifurcated peak in degree (blue-colored data) and their average fwhm in degree (red-colored data) with temperature, calculated from the azimuthal plot and (b) variation of d-spacing (blue-colored data) and correlation length (red-colored data) with temperature, calculated from the radial plot.

Electric Field Studies

The possibility of polar switching is examined for one representative sample CV-8T. To the best of our knowledge, no study has been reported on a measurement of the spontaneous polarization of thiadiazole-based short bent-core LCs. Under an applied triangular wave voltage, only a feeble and broad current peak (peak A) is observed per half cycle in the isotropic and high-temperature side of the nematic phase up to 195 °C (Figure a). As the temperature is lowered, another small peak appears at the right side of peak A and is designated as peak B. Peak B gradually becomes a prominent single peak on cooling (below 170 °C), while peak A remains almost the same (Figure a–d). Based on the temperature dependence, the current peaks A and B can be interpreted as follows: As peak A is almost temperature-independent and present in all the mesophases including the isotropic phase, it can be designated as an ionic peak. However, peak B is strongly temperature-dependent and completely vanishes far below the isotropic–nematic transition. Thus, the possibility of its ionic origin is ruled out, and it is considered as a polarization current peak. The intensity of current peak B increases on cooling up to 160 °C. As the sample is further cooled, the intensity of the current peak starts decreasing. A similar decrease of polar peak intensity on cooling is observed earlier in oxadiazole[13] and resorcinol[14]-based bent-core molecules in the smectic phase near Sm–Cr transition. At ∼115 °C, SmC–SmCA transition occurs, and a small additional peak can be seen to overlap with peak A and is designated as A′ (Figure e). Spontaneous polarization (PS) is measured by calculating the area under peak B (Figure f). PS varies from ∼76 nC/cm2 at 195 °C to a maximum value of ∼155 nC/cm2 at 160 °C. Below this point, PS decreases continuously as Sm–Cr transition is approached and takes the lowest value of ∼82 nC/cm2. The decreasing trend of polarization on cooling can be attributed to the stronger antiferroelectric packing of the molecules that require higher electric field for switching.[15]

Figure 9

Polarization current response of CV-8T at temperatures (a) 230 °C (N), (b) 190 °C (NCyb), (c) 160 °C (SmCS), (d) 140 °C (SmCS), and (e) 115 °C (SmCA). Peak B is considered to have polar origin. (f) Temperature dependence of polarization PS in all mesophases.

Next, we investigated the optical switching behavior in different mesophases before and after applying the electric field (Figure ) under the polarizing microscope. No abrupt change of texture is observed in the nematic phase after applying field (Figure a,b). However, in the smectic phases, textural changes can be noticed, although the color remains almost the same (Figure c–f). The chiral domains get well defined after applying electric field, but the extinction regions which are parallel to either of the polarizers do not move. As confirmed by XRD, the smectic phases are tilted, and hence the polarization reversals of the molecules are taking place collectively around the long molecular axes,[16] leaving the orientation of the direction unchanged.

Figure 10

Textures of CV-8T as observed in a planar cell of a thickness of 3.2 μm under square-wave voltage (60 VPP, 10 Hz). The left column shows textures without applied voltage, and the right column shows textures under square-wave voltage at (a,b) 185 °C (NCyb), (c,d) 165 °C (SmCS), and (e,f) 120 °C (SmCA).

Dielectric Studies

Compound CV-8T is investigated by dielectric spectroscopy in the temperature range of 250–100 °C. Temperature-dependent complex dielectric permittivity is obtained in the planar cell and dielectric data are fitted with the Havriliak–Negami fitting function.[17]

By fitting the obtained dielectric data in the extended Havriliak–Negami function (detailed notations of the equation are described in Supporting Information, page S19) the characteristic parameters of dielectric study such as dielectric strength, relaxation frequency, and so forth were extracted.where Δε is the dielectric strength and τ is the relaxation time of each individual process k involved in dielectric relaxation, ε0 is the vacuum permittivity (8.854 pF/m), σ0 is the conduction parameter, and ω is the angular frequency. The exponents α and β are empirical fit parameters, which describe symmetric and non-symmetric broadening, respectively, of the relaxation peaks. The first term on the right-hand side of eq describes the motion of free charge carriers in the sample. The exponent s of the angular frequency determines the nonlinearity of the dc conductivity arising from charge accumulation at the interfacial layers. In the case of Ohmic behavior (s = 1), σ0 is the Ohmic conductivity of the smectic material.

The dielectric spectra of compound CV-8T exhibited typically two distinct relaxations: one at low-frequency region (∼70 Hz to 4 kHz), denoted as peak P1, and the other at high-frequency region (∼100 kHz to 6 MHz), designated as P2 (Figure a). The origin of P2 can be identified as rotation of the molecules along their short axes, while the low-frequency peak P1 can be attributed to collective motion of the bent-core molecules. Such low-frequency collective modes have been observed earlier in oxadiazole-based compounds[13] and four ring bent-core LCs.[18] The dielectric strength of peak P1 (δε1) shows strong temperature dependence, and the transition from the N–NCyb phase and NCyb–SmC phase can be distinctly identified (Figure b). δε1 increases on cooling in the nematic phase but tend to decrease near N–NCyb transition, attaining minima and again rises sharply in the NCyb phase. In the SmC phase, δε1 has the highest value of ∼50 but decreases slightly on cooling. In the SmCA phase, the dielectric strength decreases rapidly owing to strong antiferroelectric ordering among the molecules. The relaxation frequency (fR1) is the highest at high-temperature nematic phase (∼4 MHz) and continuously decreases up to 66 Hz at 100 °C due to increase in viscosity of the medium which impedes the collective motion of the molecules. The dielectric strength of P2 increases, and relaxation frequency decreases monotonously on cooling (Figure c).

Figure 11

Dielectric investigation of CV-8T. (a) Dielectric spectra at different temperatures; temperature dependence of dielectric strength and relaxation frequency of the peak, (b) P1, and (c) P2.

Photophysical Studies

In Dichloromethane

The photophysical characteristics of the hockey stick-shaped molecules were examined via UV–visible absorption and emission spectroscopy in dichloromethane (DCM) solution. The absorption and emission spectra of CV-4T, a representative hockey stick-shaped compound in dilute DCM (c = 1 × 10–5 M), are presented in Figure a. An absorption maximum at 358 nm having high molar extinction coefficient (εmax = 59 500 M–1 cm–1) can be attributed because of the spin-allowed π–π* transition of the π-conjugated aromatic system involving the phenylthiadiazole framework.[9] The optical energy band gap (Eg) of CV-4T as estimated from the onset of the absorption maxima was found to be 3.03 eV. This small band gap of the compounds qualifies as the prospective contender for application in organic light-emitting diodes and organic semiconductors. The compound (CV-4T) in diluted solution displayed an emission band in a region of 375–550 nm with maximum emission intensity in the violet region at 430 nm. The emission peak appears at ∼430 nm with a Stokes shift of about 72 nm (4677 cm–1). This large Stokes shift arises because of the presence of a push–pull organization in a molecule-like two strong electron-donating 4-n-alkoxy moiety and an electron-deficient 1,3,4-thiadiazole moiety.[19] The large Stokes shift value reflects the structural relaxation of the excited molecule and significant changes in molecular conformation upon excitation.[20] The solution absorption spectra of other hockey stick-shaped compounds have almost an identical absorption band λabs at ∼358 nm. Indeed, no considerable difference in absorption properties was observed with the variance of terminal aliphatic chains.

Figure 12

(a) UV–visible absorption spectra and emission spectra of CV-4T in DCM (c = 1 × 10–5 M); the absorption maxima were used as excitation wavelength; (b) UV–visible absorption spectra of CV-4T in different solvent mediums (c = 1 × 10–5 M); (c) normalized emission spectra of CV-4T in different solvent mediums (c = 1 × 10–5 M); (d) emission of the blue light in different solvent polarities; (e) normalized fluorescence spectra of CV-4T in toluene in different concentrations; and (f) concentration-dependent fluorescence spectra of CV-4T.

Solvent Effect

The absorption spectra of CV-4T in different solvent polarities are shown in Figure b. The relevant data from absorption spectroscopy are presented in Table , where the solvents are presented in order of increasing polarity. It was noted that absorption spectra are fairly independent on the solvent polarity, which clearly indicates that the dipole moment of the ground state and corresponding Frank–Condon excited state is similar. Interestingly, emission maxima are considerably red shifted with increasing the solvent dipole moment (see Figure c). This may be explained by the fact that upon excitation with UV-light, thiadiazole compounds were excited to a higher level of vibrational energy of the first excited singlet state, and additional vibrational energy was quickly dissipated into neighboring molecules of solvent, gradually relaxing to the lowest vibrational energy level. The neighboring solvent molecules assist to stabilize and further lower the energy level of the excited state by solvent relaxation around the excited fluorophores. This effect of reduction in the energy separation among the ground and excited states results in a red shift of the fluorescence emission. Upon increasing the solvent dipole moment yields consistently greater reduction in the energy level of the excited state, while reducing the solvent polarity decreases the solvent effect on the excited state energy level. Moreover, solvent relaxation effects in the fluorescence could result in a dramatic consequence on the magnitude of Stokes shifts. With the increase in the solvent dipole moment, the emission of the compound (CV-4T) shifted from the violet (∼423 nm) to blue region (∼452 nm), and the Stokes shift value increases on increasing the solvent polarity (see Figure d and Table ). The compound in the high polar solvent, namely, acetonitrile exhibit a large Stokes shift of 97 nm (6045 cm–1) nm, whereas the other low polar solvents have a Stokes shift value of ∼71–89 nm (∼4768–5618 cm–1). Similar behavior is reported in the donor−π–acceptor stilbene molecule.[21] Therefore, such compounds having a high Stokes shift value in polar solvents are the potential candidate for use in the fluorescent sensors.[22] The large Stokes shift and the solvatochromic effect in the excited state depicts that these molecules exhibit charge separation, namely, an intramolecular charge transfer character in the excited state (ICT state).[23] Furthermore, fluorescence quantum yields (φf) in the solution state of the hockey stick-shaped molecule were determined following the standard procedures, with quinine sulfate in degassed 0.1 M H2SO4 as a reference standard (φ = 0.54).[24] Interestingly, the compound CV-4T exhibits moderately high quantum yield (φf ≈ 0.39) in the tetrahydrofuran solvent as compared to the other solvents (see Table ).

Table 2

Photophysical Data of CV-4T

compound	medium	absorptiona λmax/nm (ε)b	emissiona λem/nm	Stokes shift νss/cm–1(nm)	FWHM/nm	quantum yield (φf)c	band gap/eVd
CV-4T	TOL	352 (23 700)	423	4786 (71)	65	0.049	3.08
	DCM	358 (59 500)	430	4677 (72)	67	0.093	3.03
	EA	356 (32 400)	437	5207 (81)	63	0.004	3.04
	THF	356 (36 000)	445	5618 (89)	63	0.390	3.04
	ACN	355 (38 900)	452	6045 (97)	66	0.005	3.05

Measured at c = 1 × 10–5 M.

ε = M–1 cm–1; here, TOL = toluene; DCM = dichloromethane; EA = ethyl acetate; THF = tetrahydrofuran; and ACN = acetonitrile.

Fluorescence quantum yield (φf). The standard was quinine sulfate (φ = 0.54 in 0.1 M H2SO4).

Energy gap of highest occupied molecular orbital–lowest unoccupied molecular orbital determined from the onset of the lower energy of the absorption maxima.

Concentration-Dependent Emission Spectra

In the fluorescence spectra of CV-4T at different concentrations in toluene showed a small red shift (∼8 nm) on increase in the concentration of the compound as depicted in Figure e. This red shift is due to the development of the aggregated species with an increase in concentration of the monomer which suggests the formation of the aggregates from monomer concentration and the population of the J-aggregates increases.[25] Similarly, the red shift is also observed in DCM. Similar behavior was not observed in polar solvents such as tetrahydrofuran and acetonitrile. Interestingly, it was observed that in polar solvents, the intensity of the peak decreases with increase in concentration as presented in Figure f. At concentrations larger than 1 × 10–5 M, self-absorption starts to decrease the intensity of the emission peak, but the spectral shape is or else unaffected up to 1 × 10–4 M, signifying that merely intrinsic intramolecular emission happens.[26]

Conclusions

A new series of 1,3,4-thiadiazole based on short-core hockey stick-shaped molecules possessing a lateral methoxy group have been designed and synthesized. The compounds exhibited a long range of the enantiotropic nematic phase with an underlying tilted smectic ordering. The molecules in the nematic phase are arranged in such a fashion to exhibit four-spot pattern 2D images in XRD studies, indicating the presence of cybotactic ordering. Polar switching was observed in the low-temperature nematic region and smectic phases of CV-8T. It turns out to be antiferroelectric organization near crystallization. This is an unusual phenomenon of polar ordering in the nematic and smectic phases of the thiadiazole-based hockey stick-shaped molecule. The compounds showed a strong absorption band at ∼356 nm and a blue emission band at ∼445 nm with a decent quantum yield of ∼0.39 in tetrahydrofuran as compared with other solvents. The mega Stokes shift is observed, and the Stokes shift value increases on increasing the polarity of the solvent.

Experimental Section

Synthesis of 2-(4′-Aminophenyl)-5-(4″-butyloxy)phenyl)-1,3,4-thiadiazole (1)

2-(4′-Nitrophenyl)-5-(4″-butyloxy)phenyl)-1,3,4-thiadiazole (1.0 g, 3.1 mmol) was dissolved in ethyl acetate. To this solution, stannous chloride (2.1 g, 9.3 mmol) was added, and the reaction mixture was allowed to reflux for 4 h. After the completion of the reaction, the mixture was poured into ice-cooled water, and 10% aqueous sodium bicarbonate solution was slowly added to maintain the pH ∼8–9 and extracted with ethyl acetate. A pale yellow solid was obtained after the removal of the solvent from the extracted organic part, which was recrystallized by ethanol to yield off-white solid. Yield: 0.79 g (86%). mp: 125 °C. FT-IR (KBr, cm–1): 3193, 3135, 2958, 2873, 1603, 1571, 1449, 1418, 1303, 1253, 1173. 1H NMR (CDCl3, 400 MHz): δ (in ppm) 1.01 (t, 3H, J = 7.4 Hz, −CH3), 1.48–1.85 (m, 4H, 2 × −CH2), 4.04 (t, 2H, J = 6.4 Hz, −OCH2−), 4.62 (s, 2H, −NH2), 6.74 (d, 2H, J = 8.4 Hz, ArH), 6.99 (d, 2H, J = 8.8 Hz, ArH), 7.80 (d, 2H, J = 8.4 Hz, ArH), 7.92 (d, 2H, J = 8.8 Hz, ArH).

General Synthesis of 3-Methoxy-4-n-alkyloxy-benzaldehyde (3)

To a solution of 4-hydroxy-3-methoxybenzaldehyde (2 g, 13.1 mmol) in dry acetone, potassium carbonate (1.8 g, 13.1 mmol), n-alkyl bromide (13.1 mmol) [n-butyl bromide (1.78 g), n-octyl bromide (2.53 g), n-dodecyl bromide (3.25 g), n-octadecyl bromide (5.30 g)], and a catalytic amount of potassium iodide was added, and the mixture was refluxed for 20 h. The reaction was monitored by thin-layer chromatography. After the completion of the reaction, the crude product was filtered off and chromatographed on silica gel using a mixture of chloroform and hexane (05:95) as the eluant. The pure product was obtained by the removal of the eluant under reduced pressure.

3-Methoxy-4-n-butyloxybenzaldehyde (3a)

Yield: 2.0 g (74%). FT-IR (KBr, cm–1): 2924, 2848, 2767, 2619, 1682, 1595, 1516, 1466, 1429, 1398, 1342, 1272, 1142, 1029. 1H NMR (CDCl3, 400 MHz): δ (in ppm) 0.98 (t, J = 6.0 Hz, 3H, −CH3), 1.47–1.56 (m, 2H, −CH2−), 1.87 (p, J = 7.0 Hz, 2H, −OCH2–CH2−), 3.93 (s, 3H, −OCH), 4.11 (t, J = 6.0 Hz, 2H, −OCH2−), 6.97 (d, J = 8.0 Hz, 1H, ArH), 7.41 (s, 1H, ArH), 7.44 (dd, J = 8.0 Hz, 1H, ArH), 9.85 (s, 1H, −CHO).

3-Methoxy-4-n-octyloxybenzaldehyde (3b)

Yield: 2.7 g (74%). FT-IR (KBr, cm–1): 2921, 2849, 2763, 2620, 1680, 1593, 1513, 1466, 1425, 1395, 1349, 1272, 1140, 1036. 1H NMR (CDCl3, 400 MHz): δ (in ppm) 0.89 (t, J = 6.0 Hz, 3H, −CH3), 1.29–1.51 (m, 10H, 5 × −CH2−), 1.89 (p, J = 7.0 Hz, 2H, −OCH2–CH2−), 3.93 (s, 3H, −OCH), 4.10 (t, J = 8.0 Hz, 2H, −OCH2−), 6.97 (d, J = 8.0 Hz, 1H, Ar-H), 7.41 (s, 1H, ArH), 7.44 (dd, J = 8.0 Hz, 1H, ArH), 9.85 (s, 1H, −CHO).

3-Methoxy-4-n-dodecyloxybenzaldehyde (3c)

Yield: 3.4 g (80%). FT-IR (KBr, cm–1): 2925, 2850, 2761, 2625, 1680, 1595, 1514, 1467, 1429, 1396, 1348, 1271, 1145, 1030. 1H NMR (CDCl3, 400 MHz): δ (in ppm) 0.89 (t, J = 8.0 Hz, 3H, −CH3), 1.23–1.57 (m, 18H, 9 × −CH2−), 1.87 (p, J = 8.0 Hz, 2H, −OCH2–CH2−), 3.90 (s, 3H, −OCH), 4.09 (t, J = 8.0 Hz, 2H, −OCH2−), 6.97 (d, J = 8.0 Hz, 1H, ArH), 7.40 (s, 1H, ArH), 7.44 (dd, J = 8.0 Hz, 1H, ArH), 9.85 (s, 1H, −CHO).

3-Methoxy-4-n-octadecyloxybenzaldehyde (3d)

Yield: 4.9 g (88%). FT-IR (KBr, cm–1): 2919, 2848, 2763, 2621, 1682, 1593, 1512, 1465, 1427, 1398, 1346, 1271, 1142, 1029. 1H NMR (CDCl3, 400 MHz): δ (in ppm) 0.87 (t, J = 8.0 Hz, 3H, −CH3), 1.21–1.48 (m, 30H, 15 × −CH2−), 1.88 (p, J = 7.0 Hz, 2H, −OCH2–CH2−), 3.92 (s, 3H, −OCH), 4.09 (t, J = 8.0 Hz, 2H, −OCH2−), 6.96 (d, J = 8.0 Hz, 1H, ArH), 7.40 (s, 1H, ArH), 7.43 (d, J = 8.0 Hz, 1H, ArH), 9.84 (s, 1H, −CHO). 13C NMR (CDCl3, 100 MHz): δ (in ppm) 190.98, 154.17, 149.79, 129.79, 126.89, 111.27, 109.12, 69.18, 56.02, 31.92, 29.70, 29.58, 29.53, 29.37, 28.90, 25.88, 22.70, 14.13.

General Procedure for the Synthesis of Compounds (CV-)

2-(4′-Aminophenyl)-5-(4″-butyloxy)phenyl)-1,3,4-thiadiazole (0.07 g, 0.21 mmol) was dissolved in ethanol. To this solution, the ethanolic solution of 3-methoxy-4-n-alkyloxy-benzaldehyde (0.21 mmol) [3-methoxy-4-n-butyloxy-benzaldehyde (0.04 g), 3-methoxy-4-n-octyloxy-benzaldehyde (0.05 g), 3-methoxy-4-n-dodceyloxy-benzaldehyde (0.07 g), and 3-methoxy-4-n-octadecyloxy-benzaldehyde (0.08 g)] was slowly added. To it, 2–3 drops of glacial acetic acid as the catalyst was added and was refluxed for 4 h. The reaction mixture was allowed to cool at room temperature to obtain a yellow solid product. The product was further recrystallized several times with ethanol to obtained yellow solid as a pure compound.

5-(n-Butyloxy)-2-((4-(5-4-n-butyloxy)-1,3,4-thiadiazole-2-yl)phenylimino)methyl)phenol (CV-4T)

Yield: 0.07 g (65%). Yellow solid. mp 134–136 °C. FT-IR (KBr, cm–1): 3001, 2956, 2941, 2866, 2714, 1625, 1601, 1576, 1539, 1510, 1468, 1442, 1421, 1381, 1300, 1251, 1266, 1168, 1173, 1096, 1067. 1H NMR (CDCl3, 400 MHz): δ (in ppm) 0.98 (m, 6H, 2 × −CH), 1.48–1.55 (m, 4H, 2 × −CH−), 1.78–1.90 (m, 4H, 2 × −OCH2–CH−), 3.98 (s, 3H, −OCH), 4.04 (t, J = 6.4 Hz, 2H, −OCH−), 4.10 (t, J = 6.8 Hz, 2H, −OCH−), 6.95 (d, J = 8.4 Hz, 1H, ArH), 6.99 (d, J = 8.4 Hz, 2H, ArH), 7.29 (d, J = 8.4 Hz, 2H, ArH), 7.32 (dd, J = 1.6 Hz, 1H, ArH), 7.61 (d, J = 1.6 Hz, 1H, ArH), 7.94 (d, J = 8.8 Hz, 2H, ArH), 8.02 (d, J = 8.8 Hz, 2H, ArH), 8.39 (s, 1H, −N=CH−). 13C NMR (CDCl3, 100 MHz): δ (in ppm) 167.67, 167.01, 161.50, 160.79, 154.63, 152.11, 149.80, 129.44, 128.94, 128.84, 127.50, 124.89, 122.67, 121.68, 115.03, 111.64, 109.27, 68.75, 67.96, 56.11, 31.20, 31.08, 19.21, 13.91. HRMS (ESI) m/z: [M + H]+ for C30H34N3O3S calcd, 516.2321; found, 516.2318. Elemental analysis calcd for C30H33N3O3S: C, 69.87; H, 6.45; N, 8.15%. Found: C, 69.90; H, 6.42; N, 8.18%.

5-(n-Octyloxy)-2-((4-(5-4-n-butyloxy)-1,3,4-thiadiazole-2-yl)phenylimino)methyl)phenol (CV-8T)

Yield: 0.09 g (71%). Yellow solid. mp 98–100 °C. FT-IR (KBr, cm–1): 2993, 2954, 2922, 2844, 2865, 2718, 1603, 1572, 1555, 1537, 1511, 1468, 1444, 1415, 1386, 1352, 1299, 1270, 1249, 1173, 1150, 1098, 1071. 1H NMR (CDCl3, 400 MHz): δ (in ppm) 0.89 (t, J = 6.2 Hz, 3H, −CH), 1.00 (t, J = 7.4 Hz, 3H, −CH), 1.29–1.55 (m, 12H, 6 × −CH−), 1.77–1.85 (m, 2H, −OCH2–CH−), 1.86–1.91 (m, 2H, −OCH2–CH−), 3.98 (s, 3H, −OCH), 4.04 (t, J = 6.4 Hz, 2H, −OCH−), 4.09 (t, J = 6.8 Hz, 2H, −OCH−), 6.94 (d, J = 8.4 Hz, 1H, ArH), 6.99 (d, J = 8.4 Hz, 2H, ArH), 7.29 (d, J = 8.4 Hz, 2H, ArH), 7.33 (d, J = 8.8 Hz, 1H, ArH), 7.61 (s, 1H, ArH), 7.94 (d, J = 8.8 Hz, 2H, ArH), 8.02 (d, J = 8.0 Hz, 2H, ArH), 8.39 (s, 1H, −N=CH−). 13C NMR (CDCl3, 100 MHz): δ (in ppm) 167.66, 167.01, 161.50, 160.79, 154.64, 152.10, 149.80, 129.44, 128.94, 128.84, 127.51, 124.88, 122.67, 121.68, 115.03, 111.65, 109.26, 69.08, 67.96, 56.10, 31.83, 31.20, 29.37, 29.23, 29.02, 25.94, 22.68, 19.24, 14.14, 13.87. HRMS (ESI) m/z: [M + H]+ for C34H42N3O3S calcd, 572.2947; found, 572.2940. Elemental analysis calcd for C34H41N3O3S: C, 71.42; H, 7.23; N, 7.35%. Found: C, 71.46; H, 7.26; N, 7.29%.

5-(n-Dodecyloxy)-2-((4-(5-4-n-butyloxy)-1,3,4-thiadiazole-2-yl)phenylimino)methyl)phenol (CV-12T)

Yield: 0.13 g (73%). Yellow solid. mp 102–105 °C. FT-IR (KBr, cm–1): 3002, 2952, 2912, 2863, 2845, 2714, 2656, 1635, 1604, 1572, 1514, 1470, 1441, 1418, 1302, 1268, 1253, 1218, 1173, 1148, 1098, 1068. 1H NMR (CDCl3, 400 MHz): δ (in ppm) 0.89 (t, J = 6.4 Hz, 3H, −CH), 1.00 (t, J = 7.2 Hz, 3H, −CH), 1.25–1.55 (m, 20H, 10 × −CH−), 1.77–1.91 (m, 4H, 2 × −OCH2–CH−), 3.98 (s, 3H, −OCH), 4.04 (t, J = 6.4 Hz, 2H, −OCH−), 4.09 (t, J = 7.0 Hz, 2H, −OCH−), 6.94 (d, J = 8.4 Hz, 1H, ArH), 6.99 (d, J = 8.8 Hz, 2H, ArH), 7.29 (d, J = 8.4 Hz, 2H, ArH), 7.32 (dd, J = 6.4 Hz, 1H, ArH), 7.61 (s, 1H, ArH), 7.94 (d, J = 8.8 Hz, 2H, ArH), 8.02 (d, J = 8.4 Hz, 2H, ArH), 8.39 (s, 1H, −N=CH−). 13C NMR (CDCl3, 100 MHz): δ (in ppm) 167.67, 167.02, 161.51, 160.80, 154.64, 152.09, 149.79, 129.44, 129.27, 128.93, 128.84, 127.50, 124.89, 122.67, 121.68, 115.03, 114.94, 114.87, 111.64, 109.25, 69.08, 67.96, 56.10, 31.83, 31.20, 29.38, 29.23, 29.02, 25.95, 22.68, 19.24, 14.14, 13.87. HRMS (ESI) m/z: [M + H]+ for C38H50N3O3S calcd, 628.3573; found, 628.3580. Elemental analysis calcd for C38H49N3O3S: C, 72.69; H, 7.87; N, 6.69%. Found: C, 72.64; H, 7.89; N, 6.71%.

5-(n-Octadecyloxy)-2-((4-(5-4-n-butyloxy)-1,3,4-thiadiazole-2-yl)phenylimino)methyl)phenol (CV-18T)

Yield: 0.10 g (67%). Yellow solid. mp 93–95 °C. FT-IR (KBr, cm–1): 2993, 2954, 2920, 2865, 2852, 1631, 1603, 1576, 1539, 1515, 1467, 1442, 1415, 1387, 1330, 1303, 1276, 1252, 1172, 1068. 1H NMR (CDCl3, 400 MHz): δ (in ppm) 0.88 (t, J = 6.8 Hz, 3H, −CH), 1.00 (t, J = 7.2 Hz, 3H, −CH), 1.26–1.55 (m, 32H, 16 × −CH−), 1.79–1.83 (m, 2H, −OCH2–CH−), 1.87–1.91 (m, 2H, −OCH2–CH−), 3.98 (s, 3H, −OCH), 4.04 (t, J = 6.4 Hz, 2H, −OCH−), 4.09 (t, J = 7.0 Hz, 2H, −OCH−), 6.94 (d, J = 8.4 Hz, 1H, ArH), 6.99 (d, J = 8.8 Hz, 2H, ArH), 7.29 (d, J = 8.4 Hz, 2H, ArH), 7.31 (dd, J = 1.6 Hz, 1H, ArH), 7.61 (s, 1H, ArH), 7.94 (d, J = 8.8 Hz, 2H, ArH), 8.02 (d, J = 8.4 Hz, 2H, ArH), 8.39 (s, 1H, −N=CH−). 13C NMR (CDCl3, 100 MHz): δ (in ppm) 167.67, 167.02, 161.51, 160.80, 154.63, 152.10, 149.79, 129.44, 129.26, 128.93, 128.84, 127.51, 124.89, 122.68, 121.68, 115.03, 114.94, 114.87, 111.64, 109.25, 69.08, 67.96, 56.10, 31.83, 31.20, 29.38, 29.23, 29.21, 29.02, 25.95, 22.68, 19.24, 14.14, 13.86. HRMS (ESI) m/z: [M + H]+ for C44H62N3O3S calcd, 712.4512; found, 712.4520. Elemental analysis calcd for C44H61N3O3S: C, 74.22; H, 8.63; N, 5.90%. Found: C, 74.25; H, 8.61; N, 5.84%.