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Influence of Thiophenes on Molecular Order, Mesophase and Optical Properties of #-Conjugated Mesogens Rajasekhar Reddy Katha, Elumalai Varathan, Nitin Prakash Lobo, Shanmugam Easwaramoorthi, and Tanneru Narasimhaswamy J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08240 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 9, 2016

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The Journal of Physical Chemistry

Influence of Thiophenes on Molecular Order, Mesophase and Optical Properties of π-Conjugated Mesogens K. Rajasekhar Reddy, † E. Varathan, ‡ Nitin P. Lobo,‡ S. Easwaramoorthi, ‡ and T. Narasimhaswamy*† † Polymer Laboratory and ‡Inorganic & Physical Chemistry CSIR-Central Leather Research Institute, Adyar, Chennai 600 020, India

ABSTRACT Increasing interest on π-conjugated aromatic cores built essentially with thiophene rings is recognized owing to their applications in optoelectronics. In this investigation, an attempt is made to understand the influence of terminal thiophene rings on the molecular order, mesophase and optical properties of mesogens in which phenyl benzoate is part of the core. Accordingly, mono, di and terthiophene units are linked to two phenyl ring core by Suzuki cross coupling reaction. The synthesised thiophene based π-conjugated mesogens exhibit enantiotropic nematic and smectic phases with excellent mesophase range. The tendency for smectic phases and the mesophase range enhanced with increased thiophene rings. The layer ordering in smectic A and smectic C phase is established by powder X-ray diffraction while the orientational order of all the rings of core unit is accomplished by 13C NMR spectroscopy. Thus the

13

C-1H dipolar couplings determined from 2D separated local field NMR

experiments show a very high value for terminal C-H of thiophene ring (~ 9-11 kHz) irrespective of number of thiophenes in the mesogenic core. The Density functional theory and Time dependent density functional theory calculations indicate the intramolecular charge transfer transition between the phenyl-thiophene to phenyl benzoate unit. The solution absorption and fluorescence spectral studies reveal interesting features. The mono thiophene based mesogen is non-fluorescent while those based on bithiophene and terthiophene show intense fluorescence. The well resolved vibronic peaks observed in fluorescence spectra of mesogens are characteristic for oligothiophenes. Further, the fluorescence excitation 1 ACS Paragon Plus Environment


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anisotropy measured by monitoring the vibronic features of the mesogens is found to be similar signifying that the emission originates from the identical electronic energy level. Therefore, the investigation encompassing wide ranging techniques manifest that the insertion of more thiophenes in the mesogenic core favour polymesomorphism and intense emission enabling them for application in polarised emission. INTRODUCTION Thiophene centered thermotropic liquid crystals are increasingly gaining prominence due to their extensive array of applications varying from thin film transistors to organic photovoltaics.1-3 Usually mesogenic core built with all thiophene rings are limited and in majority cases thiophene in combination with other moieties particularly the phenyl rings is common.4,5 The important facet of the formation of core unit in thiophene mesogens is the selection of linking unit. For instance, if the core consists of phenyl as well as thiophene rings, the kind of linking between them decides the final application of the molecules.6,7 If ester or imine is used as linking units then the resultant mesogens are often not suitable for functional applications.8,9 On the other hand, if the thiophene and phenyl are directly connected, owing to the better conjugation, the molecules exhibit interesting absorption or photoluminescence properties.10,11 Further, it is not only the direct link between the thiophene and phenyl rings but also the number of thiophenes present in the core decides the target applications.12 Samulski et al. earlier showed that the replacement of a phenyl ring with a thiophene in the core profoundly influences the molecular topology as well as the mesophase transitions.13,14 Thus the dramatic change in the mesophase transitions and molecular shape in phenyl thiophene versus all phenyl ring mesogens is attributed to the geometry of the respective thiophene and phenyl rings.15,16 Accordingly, core unit consisting of irregular pentagon of thiophene and regular hexagon of phenyl profoundly influence the overall molecular shape of the mesogen.17 We have been exploring the molecular structure and 2 ACS Paragon Plus Environment

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topology of thiophene mesogens in liquid crystalline phases by high resolution solid state 13C NMR spectroscopy.16-20 This technique is particularly suitable for probing the structure at atomistic level since the orientational constraints of thiophene as well as phenyl rings are accessible through

13

C-1H dipolar couplings from 2D separated local field (SLF) NMR

experiments. As a consequence, the derived orientational constraints of the constituent moieties from 2D experiments can be correlated to functional properties of the mesogen. Our earlier work

16-20

unambiguously demonstrated the utility of

13

C NMR for probing the local

geometry of the thiophene in rod-like as well as bent-core mesogens. Inspired by the treasure of information extracted from 13C NMR investigation, in this work, three thiophene mesogens built with phenyl and thiophene direct link are studied by

13

C NMR to extract the

orientational order and the photoluminescent properties to examine their suitability for polarised emission. Thus the mesogens are characterized by hot-stage polarising microscope (HOPM), differential scanning calorimetry (DSC) and variable-temperature X-ray diffraction (VT-XRD) techniques for the mesophase properties and 13C NMR for the molecular order. A detailed investigation of absorption and fluorescence spectral properties of mesogens are carried out in solution to understand the influence of thiophene rings while density functional theory (DFT) as well as time dependent density functional theory (TD DFT) methods are used to gain more insight on the optical properties. EXPERIMENTAL SECTION Materials Thiophene 2-boronic acid (Alfa Aesar, London), 2, 2’-bithiophene-5-boronic acid pinacol ester (TCI, Japan), 2, 2’: 5’, 2’’-terthiophene-5-boronic acid pinacol ester (Sreeni Labs, India),

4-bromo

phenol,

dicyclohexylcarbodiimide,

1-bromododecane,

4-(dimethylamino)

methyl

4-hydroxybenzoate,

pyridine,

N,

N’

tetrakis(triphenylphosphene)

palladium (Aldrich, USA) were purchased and used without further purification.

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Tetrahydrofuran and ethanol (SD Fine, India) were used as received. N, N-Dimethyl formamide, ethyl acetate, diethyl ether, n-hexane, isopropanol, n-heptane, dichloromethane, potassium hydroxide, potassium carbonate, anhydrous sodium sulphate, and silica gel (100– 200 mesh) were obtained from Merck, India and used without further purification. Instrument Details FT-IR spectra of all of the compounds were recorded by ABB BOMEM MB3000 spectrometer using KBr pellets. 1H and 13C NMR spectra of the mesogens were recorded on Bruker Avance-III 400 MHz, at room temperature using tetramethylsilane as an internal standard in CDCl3. The resonance frequencies of 1H and 13C were 400.23, and 100.64 MHz, respectively. The nature of the mesophase and the temperature of occurrence of different phases were examined using Carl Zeiss Axiocam MRC5 polarizing microscope equipped with a Linkam THMS 600 stage with a TMS 94 temperature controller. The photographs were taken by Imager A2M digital camera. Differential scanning calorimetry traces were recorded by DSC Q200 instrument with a heating rate of 10 °C per minute in nitrogen atmosphere. The samples were subjected to two heating and two cooling cycles, and the data obtained from second cycle was considered for discussion. UV-Visible absorption spectra were recorded using a Schimadzu UV-1800 spectrophotometer and the steady state fluorescence, excitation spectra were measured using Cary Eclipse fluorescence spectrophotometer. Fluorescence excitation anisotropy spectra were measured using manual polarizer in parallel and perpendicular configuration, wherein the emission was monitored at respective maximum wavelengths. Fluorescence lifetimes were measured using Edinburg FLS980 time correlated single photon counting spectrofluorometer. The samples were excited at 375 nm, 70 ps light and the decay profiles are monitored at respective emission maximum wavelengths.


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Powder X-ray measurements Variable temperature Powder X-ray diffraction (VT-XRD) studies were carried out on unoriented samples (Lindemann capillary; diameter of 1 mm; Hampton Research, Aliso Viejo, CA, USA) using a PANalytical instrument (DY 1042- Empyrean) operating with a line focused Ni-ﬁltered Cu K α ( λ= 1.54 Å) beam and a linear detector (PIXcel 3D). The sample temperature was controlled with a precision of 0.1 °C using a Linkam heater and a temperature controller.21 Computational Details The ground state geometries of TPDB, BTPDB and TTPDB were optimized by density functional theory (DFT) with Becke’s three-parameter functional and the Lee-Yang-Parr functional (B3LYP)22,23 with 6-31G* basis set. Based on the gas phase optimized geometry of TPDB, BTPDB and TTPDB, spectral properties in chloroform were calculated by time dependant density functional theory (TD-DFT) method with Polarizable Continuum Model (PCM) at B3LYP/6-31G* level. All the calculations were carried out using Gaussian 09 program package.24 Solid-state NMR experiments Solid-state NMR experiments of samples were recorded on a Bruker Avance III HD 400 WB NMR spectrometer (9.4 T) equipped with Bruker double resonance VTN static probe with 5 mm horizontal solenoid coil. The 1H and 13C resonance frequencies were 400.07 and 100.61 MHz, respectively. The fine powder samples at room temperature were packed in a 4 mm Zirconia rotor with a Zirconia cap and placed inside the 5mm glass tube. TPDB and BTPDB samples were initially heated from room temperature to the isotropic phase and then slowly cooled to the respective mesophases to achieve the sample alignment. For TTPDB since the clearing temperature is above the limit of the static probe, the sample is heated up to 210 ºC and cooled slowly to get the orientation. The 1D static


13

C NMR spectra in liquid


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crystalline phases were recorded by standard cross-polarization (CP) scheme with a contact time τ of 3 ms, number of scans 128, recycle delay of 8 s and 62.5 kHz of radio frequency (RF) field strength on both the 1H and

13

C channels during the period τ. SAMPI-4 pulse

scheme 25 (Figure S1 of Supporting Information) was employed to obtain 2D high resolution SLF NMR spectra under static condition. The utility of SAMPI-4 for structurally different mesogens were described in recent work.16-20 2D SAMPI-4 experimental parameters for TPDB, BTPDB and TTPDB mesogens were as follows: CP contact time τ =3 ms, t1 increments =128 /100, number of scans=32 / 24, 1H 90º pulse duration= 4 µs and recycle delay=16 s/18 s/15 s (to avoid RF heating effect). The data were zero filled in both the t2 and t1 dimensions, yielding a 4096 × 256 real matrix. A shifted sine bell window function was applied to the time domain data and the spectrum was processed in the phase sensitive mode. For both 1D and 2D experiments, SPINAL-6426 heteronuclear decoupling sequence employed during

13

C acquisition with RF strength of 30 kHz. Sample heating was achieved

by running the bearing air through a heating coil and regulated by Bruker BVTB-3500 temperature control unit and the 13C chemical shifts were referenced to adamantane methine resonance at 29.5 ppm (external reference). RESULTS AND DISCUSSION The oligo thiophenes are important class of π-conjugated molecules which not only exhibit interesting

opto-electronic

properties

but

also

serve

as

excellent

models

for

polythiophenes.27,28 Among them, those based on terthiophene attracted much attention as key moiety of the core for π-conjugated mesogens due to their application as organic semiconductors.28-30 Thus terthiophenes end caped with alkyl chains are explored as mesogens which depending upon the length as well as branching of terminal chains, the mesophase transitions show large variation.31-34 In the present work, the thiophenes are placed at one end of the core (Scheme 1) to investigate their influence on mesophase, optical


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and order parameter properties. In contrast to known terthiophene based mesogens in the literature, the molecules investigated in the present work can be further chemically modified to generate new series of mesogens through direct arylation since the thiophene rings are located at one end of the core. The structural integrity of synthesised mesogens is confirmed by FT-IR (Figure S2 of Supporting Information)., 1H NMR and

13

C NMR spectroscopy

(Figures S3-S33 of Supporting Information). The mesophase transitions are evaluated by HOPM, DSC as well as VT-XRD while solid state

13

C NMR is employed for finding the

order parameters. The absorption and emission properties are carried out in solution while the computational methods are performed to gain molecular insight of the optical properties. Mesophase transitions The mesophase assignment of the synthesized molecules is carried out by HOPM. The samples are first heated to isotropic phase and on cooling, the phase assignment is done based on the texture formed. For TPDB, upon cooling the isotropic liquid, birefringent droplets are formed which later coalesced to give marble texture35 signifying the occurrence of nematic phase (Figure 1A). At 101.0°C, a phase transition is noticed where the texture transformed to homeotropic domains with fan texture, indicating the formation of smectic A phase (Figure 1B). On further cooling, the crystallization of the sample is noticed at 83.9 °C. In the case of BTPDB, upon cooling the isotropic phase, the nematic threads35 are noticed at 199.2 °C (Figure 1C). At 143.0 °C, the birefringent nematic texture changes to entirely homeotropic showing transition to smectic A phase (Figure 1D). At 111.8 °C, crystallization of the sample is noticed with a needle like crystals. In the case of TTPDB, on cooling the isotropic phase, birefringent fluid is noticed which on further cooling, transforms to marble texture with isolated threads showing nematic phase (Figure 1E). At 206.0 °C, the transition bars are noticed and on further cooling, the schlieren texture of smectic C35 mesophase is formed (Figure 1F). The crystallization of the sample is noted at 155.06 °C. Thus TPDB and BTPDB 7 ACS Paragon Plus Environment


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show enantiotropic nematic and smectic A phases while TTPDB exhibits enantiotropic nematic and smectic C phases. Thus increase in thiophene rings at the terminal position, favors smectic C phase as the aspect ratio of the molecule witnesses an increase. Table 1 lists the transition temperatures along with mesophase assignment. The polymesomorphism noticed in HOPM for the mesogens is further ascertained by DSC measurements. The DSC traces of the mesogens are shown in Figure 2. For TPDB, on heating cycle, crystal to nematic and nematic to isotropic transitions are seen while on cooling, additional peak for nematic to smectic A transition is noticed. Thus the smectic A phase observed in HOPM on cooling the nematic mesophase is a monotropic transition. For BTPDB and TTPDB, the DSC data (Figure 2) is in agreement with HOPM observation. It is clear from the transition temperatures (Table 1, Table S1 of Supporting Information) that with increase in number of thiophenes in the core an increase in mesophase range is noticed. Accordingly, the highest mesophase range (~107°C) is noted for terthiophene based (TTPDB) mesogen. Further, an increase in thiophenes also favor tendency for smectic mesophases. Also the crystal to mesophase transition showed a rise with increase in thiophene rings. This confirms that the rigidity of the core increase with more thiophenes, which are directly linked without the bridging

groups.

Therefore,

the

appearance

of

enantiotropic

mesophases

with

polymesomorphism suggests that the molecules are endowed with high anisotropic polarizability36,37 mainly due to the presence of sulphur atom in thiophene rings. 38 The existence of smectic mesophases are further confirmed by powder X-ray diffraction investigations at different temperatures (Figures S34 and S35 of Supporting Information). Since BTPDB and TTPDB only show enantiotropic smectic mesophases, the XRD studies are attempted for these mesogens. The XRD scans of BTPDB is shown in Figure 3A where in the small angle region, a sharp and intense reflection is seen at 2θ = 2.54° corresponding to layer spacing (d) 34.74 Å. In the wide angle region, a broad hump at 4.57 Å is noticed. These


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XRD reflections are clear indication of layer ordering characteristic of smectic mesophase.39 The molecular length (L) arrived from DFT optimized structure of BTPDB is 35.3 Å (Figure 4). The d/L across the temperature range 125- 145°C is constant (0.98) indicating the monolayer nature of smectic A phase. Figure 3B shows the XRD profile of TTPDB at 185 °C and similar to BTPDB, a sharp and intense reflection in the small angle region (2θ = 2.52°) and a broad hump in the wide angle region is noted. The molecular length (L) calculated from DFT optimized structure of TTPDB is 39.1 Å (Figure 4). The d/L ratio measured in the range 160-205 °C is found to vary from 0.89 to 0.92. Since the small angle reflection varies with lowering of temperature, the phase is assigned as smectic C in accordance with HOPM measurements. Further, the tilt angle in smectic C phase is found to be 26.5-23.0° in the temperature range 160-205 °C.40 In both the cases i.e. BTPDB and TTPDB, the broad hump noted at wide angle region indicates the absence of in plane order and liquid like nature of molecules with in the layer. Further it also endorses the fluid nature of the smectic mesophase in concurrence with the HOPM and DSC findings. 13

C NMR Investigations The solution

13

C NMR experiments of all the mesogens are carried out in chloroform

solution. The spectral assignment is accomplished by iterating the spectrum by Chem Draw software and further refined by running the 2D experiments in solution. Accordingly, 1H-1H double quantum filtered correlation spectroscopy (DQF COSY), distortionless enhancement by polarisation transfer (DEPT), 1H-13C heteronuclear single quantum correlation (HSQC) and heteronuclear multiple bond correlation (HMBC) experiments are performed to confirm the assignment of all the carbons of the core unit (Figures S4-S33 of Supporting Information). Figure 5 shows the proton decoupled solution BTPDB and TTPDB mesogens. The

13

13

C NMR spectrum of TPDB,

C NMR spectrum of TPDB (Figure 5A) shows 13

lines accounting for equivalent number of carbons of the core unit. Since the core has two



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phenyl rings, one thiophene ring as well as ester linking unit, 13 lines as expected are noted in the spectrum. Among them, lines appeared at 114.2, 122.2, 126.8 and 132.2 ppm are due to methine carbons of phenyl rings and are more intense than others. The thiophene methine carbons are seen in the range 123.0-128.0 ppm. Table 2 lists the assigned carbons of core unit of TPDB mesogen. Figure 5B shows the solution 13C NMR spectrum of BTPDB while Table 2 lists the chemical shift values. The spectral assignment in this case also accomplished as per the procedure followed for TPDB. For the case of TTPDB, the 13C NMR in solution is run at 40 °C due to insufficient solubility of the sample at 25 °C (Figure 5C). Table 2 lists the core unit carbons with the chemical shift assignment. The important feature of the solution

13

C

NMR data of the mesogens is increase in number of lines in the region 123.0-125.0 ppm owing to increase in thiophene rings from TPDB to TTPDB. In other words, presence of terthiophene at one end of the molecule contributes for more number of methine carbons in the range 123.0-125.0 ppm. The static

13

C NMR experiments of all the mesogens are carried out in liquid crystalline

phase. The TPDB and BTPDB samples are heated to isotropic phase where as TTPDB sample was heated upto ~210 ºC in the magnetic field and the corresponding spectra are recorded in the mesophase upon cooling. The alignment of the molecules is confirmed by noticing change in chemical shift values of core as well as terminal chains. The static

13

C

NMR spectrum of TPDB in neamtic phase at 112 ºC is shown in Figure 6A. The spectrum shows well resolved peaks and the chemical shift assignment of them is carried out by considering the structurally similar fragments of reported mesogens. For instance, the ring I resembles with the 4-hexyloxy benzoic acid for which detailed

13

C NMR in nematic

mesophase is known.41 Similarly for the assignment of ring II carbons also the literature data is used.18 For the identification of thiophene methine carbons, on the other hand, the 2D SLF spectral data discussed in next section is considered. Among the peaks noticed in the


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spectrum (Figure 6A), the intense peaks in the region 138.0-161.0 ppm are due to phenyl ring methine carbons. The more intense peak at 148.1 is assigned to C7, C8 and C11 carbons based the 2D SLF data. The thiophene methine carbons are difficult to identify from 1D spectrum due to their relatively low intensity and also proximity to quaternary carbons of the core unit. For the terminal dodecyloxy chain, the peaks have appeared in the range 11.0-66.0 ppm. Among them, the OCH2 (62.0 ppm) and methyl (11.4 ppm) are easily identifiable while the other methylene carbons showed peaks in the range 19.5 to 26.5 ppm. For BTPDB the static 13

C NMR spectrum in smectc A phase at 136 ºC (Figure 6B) shows more number of peaks

due to the presence of additional thiophene ring. The core unit carbons of the mesogen show peaks in the range 140-228 ppm which are attributed to phenyl ring methine carbons. For the assignment of thiophene methine carbons, similar to TPDB case, 2D SLF data is employed as 1D static spectrum showed many signals with comparable intensities in the range 167.0-228.0 ppm. For the dodecyloxy chain, the carbon chemical shifts are noticed in the range 10.0-60.5 ppm in which OCH2 (60.4 ppm) and methyl (10.7 ppm) are easily assigned, while rest of the methylene carbons are observed in the span of 17.5- 26.0 ppm. Figure 6C shows the static 13C NMR spectrum of TTPDB in smectic C phase at 180 ºC, where in the region 140-227.5 ppm, the core unit carbons showed peaks. Similar to BTPDB, the region 140-227.5 ppm is densely populated owing to the presence of three thiophenes among which the phenyl ring methine carbons are more intense. Hence in this case also, the 2D SLF data is used for the assignment of thiophene methine carbons. For the terminal dodecyloxy chain, the peaks are seen in the range 10.0-60.5 ppm similar to TPDB and BTPDB mesogens. Of them, the OCH2 (60.5 ppm) and methyl (10.2 ppm) are easily distinguishable. The other methylene carbons of the chains are seen in the range 17.0-26.5 ppm. It is clear from the inspection of stacked spectra (Figure 6) that the terminal dodecyloxy chain show more or less similar pattern while for the core unit, an increase in number of peaks is noticed with increase of thiophenes at the terminal



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position. Further, as the thiophenes are directly linked without any bridging groups, superposition of lines is noticed with more number of thiophene rings (TTPDB) in the core unit. This trend is also clearly noticed in solution 13C NMR spectrum where with increase in thiophene rings, the methine carbons showed more lines in the region 123.0-125.0 ppm. The 2D SLF experiments for all the mesogens are performed with a view to assign the thiophene carbons easily and also to find the orientational order parameter of all the rings that are part of the core unit. Figure 7 shows the 2D SLF spectra of mesogens in respective mesophases. The spectra show the contours arising from all the resolvable carbons of the mesogens. In contrast to 1D static spectra, the resolution achieved in 2D SLF is high owing to the well spread

13

C-1H dipolar couplings as well as the chemical shift values. For instance,

for TPDB (Figure 7A), the spectrum shows completely resolved contours in the region 135.0220.0 ppm for core unit carbons. The important feature of the spectrum is clear separation of thiophene methine carbons from phenyl rings in the region 147.0-180.0 ppm. This separation is possible due to the difference in the orientational constraints of thiophene and phenyl rings. For example, the four methine carbons of phenyl rings are seen in the range 138.0-161.0 ppm with typical 13C-1H dipolar couplings of 2.42-2.65 kHz whereas the three thiophene methine carbons are found in the span of 147.0-180.0 ppm with 13C-1H dipolar coupling values of 5.09.0 kHz. So, the large variation between phenyl ring contours versus thiophene contours is clear reflection of orientational difference with respect to long axis. Further, a large variation in dipolar couplings of quaternary carbons of phenyl as well as thiophene carbons is also noticed. In a recent work, we showed that the large variation in dipolar couplings of phenyl and thiophene rings despite their similar aromatic nature is due to variation in the geometry.17,42 The phenyl ring possesses hexagonal geometry while thiophene has irregular pentagon. As a result, the thiophene and phenyl rings show large variation in dipolar couplings even though similar magnitude of variation in alignment induced chemical shift is


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not often seen. The quaternary carbons of both phenyl and thiophene rings are noticed in the range 180.0-220.0 ppm. For the terminal dodecyloxy chain, the methylene as well as methyl carbons are well resolved. This is in stark contrast to 1D static spectra where overlap of many methylene carbons is seen. Table 2 list the assigned chemical shifts as well as the

13

C-1H

dipolar couplings of TPDB from the 2D SLF spectral data. For BTPDB (Figure 7B), owing to the presence of two thiophene rings at the terminal position, in the range 140.0-174.0 ppm, additional contours are seen as against TPDB. The contours arising from quaternary carbons in the range 180.0-228.0 ppm are more are less similar to BTPDB. For the terminal dodecyloxy chain, the pattern is similar to TPDB and OCH2 as well as methyl carbon contours are seen distinctly. The methylene carbons are spread in contrast to static 13C NMR spectra due to variation in

13

C-1H dipolar couplings. The assignment of the 2D contours of

BTPDB is accomplished by making use of the assigned data of TPDB mesogen. The identification of additional thiophene methine carbons is achieved considering the location as well as intensity of the contours in the region 165.0-195.0 ppm. In the case of TTPDB (Figure 7C), the core unit carbons showed contours in the range 140.0-228.0 ppm. Since the mesogen has three thiophenes that are directly linked, more contours (140.0-183.0 ppm) are expected as noted in the spectrum. The thiophene methine carbons are noticed in the midst of phenyl ring methine carbons (Figure 7C). In contrast to other two mesogens, overlapping of many quaternary carbons is noted in TTPDB mainly due to five quaternary carbons of terthiophene unit. For the terminal dodecyloxy chain, the contour pattern is similar to other two mesogens. Table 2 summarises the

13

C-1H dipolar couplings of all the mesogens. The

remarkable feature of 2D SLF investigation is observation of high dipolar coupling for terminal methine carbon of thiophene rings. In other words, the

13

C-1H dipolar coupling

values of C13 of TPDB, C17 of BTPDB and C21 of TTPDB are 8.99, 10.00, and 10.95 kHz respectively. This is in contrast to phenyl ring methine carbons which are in the range ~ 2.4-



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3.4 kHz and other methine carbons of thiophene (2.40-5.40 kHz). So the highest values are noticed for terminal CH carbon of thiophene(s). A large variation in the

13

C-1H dipolar

couplings of core unit carbons is mainly due to variation in the orientational constraints of constituent moieties. In order to understand the orientation of phenyl as well as thiophene rings with respect to long axis, the local order parameters of the ring are determined from 13

C-1H dipolar couplings.

Orientational order parameter It is well known that in 2D SLF experiment, except for the isolated C-H pair, the dipolar coupling information needs to be extracted from the experimental dipolar oscillation frequency for carbons that are coupled to more than one proton.43,44 By implementing the established procedure,16-20,41,42,45 for a given ring i.e. phenyl or thiophene ring, the dipolar couplings are related to the main order parameters, namely, Szz and Sxx-Syy by following equation

46,47

DCH =K [Szz (3 cos2θz-1)/2 + (Sxx – Syy) (cos2θx - cos2θy)/2]

(1)

where K = -hγHγC/4π2r3CH, with γH and γC are the gyromagnetic ratios of 1H and

13

C nuclei

respectively and rCH is the inter nuclear vector between them, θx, θy and θz are the angles formed by rCH with the respective coordinate axes of the molecular fragment (phenyl or thiophene ring) under consideration. For 1-4 di substituted phenyl ring with D2 symmetry, the z, x and y axes are defined as respectively the para axis of the ring, an axis perpendicular to the z axis but in the plane of the ring and the axis perpendicular to the plane of the ring. The standard bond distances rCH=1.1 Å for the C–H bond and rCC=1.4 Å for C–C bond are employed during fitting. Also to arrive at best fit, the C-C-H bond angles are required to be varied slightly ~ 120°.48Owing to the irregular pentagonal geometry of the thiophene ring, three order parameters, namely, Szz, (Sxx − Syy), and Sxz, are prerequisite for an arbitrary 14 ACS Paragon Plus Environment

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choice of the axis system.

49,50

The details about the model adopted for fitting the dipolar

coupling data to get the order parameters for thiophene moiety directly linked to the phenyl ring are described in our earlier articles

16,18,20

and in Supporting Information (Figure S36 of

Supporting Information). Table 3 lists the order parameter values of all the rings of the core unit of the mesogens calculated from equation 1 using

13

C-1H dipolar couplings. Since the

main objective of the investigation is to establish the influence of thiophene rings on the order parameter values of TPDB to TTPDB, the main order parameter (Szz) is considered for discussion. For the rings I and II of TPDB, the order parameters are 0.56 and 0.57 whereas for the ring III (thiophene), the value is 0.64. The higher order parameter of thiophene in contrast to phenyl rings is an interesting observation. This could be due to irregular pentagon shape of the thiophene ring which results in deviation from perfect rod-like shape for the overall molecule. It is also observed from earlier work that whenever phenyl rings are attached to thiophene directly, the thiophene order parameter is higher than the phenyl rings due to inherent change in symmetry of the molecule.16,18 For the case of BTPDB, the main order parameter (Szz) of phenyl rings I and II is 0.68 and 0.69 while for the first and second thiophene rings (Ring III and IV), the values are 0.64 and 0.60 respectively. Further, the thiophene order parameter values are lower than the phenyl rings in contrast to TPDB. It can be visualized from the molecular structure (Figure 4B) that the second thiophene ring is mono substituted while the first thiophene is linked at 2, 5-positions (di substitution). As a result, a bent is expected at second thiophene ring which results in variation in order parameter values of phenyl and thiophene rings.17,20 In the case of TTPDB, the main order parameter (Szz) of phenyl rings is found to be 0.64, 0.65 and for thiophene rings, the values are 0.83 (ring III), 0.78 (ring IV) and 0.62 (ring V) respectively (Table 3). In this case, compared to phenyl ring II, the first thiophene has highest order parameter and follows gradual fall for rings IV and V. In other words, among the thiophene rings, the order parameter show decreasing trend from



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thiophene rings III to ring V. It is clear that ring III and ring IV are 2, 5-disubstituted, whereas ring V is mono substituted thiophene. Since the mesogen has five rings in the core (two phenyl and three thiophenes), the center of mass may lay on ring III and the long axis passes through ring III. As a result, both sides of ring III have lower order parameters. However, variation in the order parameter of ring II and ring IV with respect to ring III is due to change in the shape. A comparison of order parameter values of BTPDB and TTPDB mesogens indicate that with increase in thiophene rings in the core, the terminal thiophene experience lower order parameter in contrast to the central thiophene ring. Therefore, the 13C NMR investigation provide order parameter values of all the rings of core unit which enables to understand the influence of thiophene rings on order parameter. These findings suggest that in mesogens with more than one thiophene in the core, the order parameters are governed by the shape i.e. mono versus di substitution as well as the geometry of adjacent rings that are part of the core unit. Influence of thiophenes on photo physical properties The influence of number of thiophene units on the electronic properties of synthesised mesogens are carried out in solution by absorption and fluorescence spectral techniques. The UV-Visible absorption spectra (Figure 8) measured in chloroform solvent for TPDB, BTPDB and TTPDB is found to be 282, 344, and 383 nm respectively. It can be seen from the Figure 8 that the absorption maximum peaks are red shifted along with decrease in molar absorptivity due to the elongation of π-conjugation. The lowest energy transition originates from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO), which has been confirmed by the TD-DFT calculations performed at B3LYP/631G* level of theory (Table 4). Further, FMO distribution in the HOMO reveals the delocalization of π−electron on all the thiophene units and phenylene moiety (Figure S37 in Supporting Information). On the other hand, LUMO is localized on phenyl benzoate, which 16 ACS Paragon Plus Environment

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probably introduces intramolecular charge transfer interactions owing to the donor-acceptor configuration. While, TPDB is found to be non-fluorescent, BTPDB and TTPDB show intense fluorescence. However, it does not show any mirror image relationship with the absorption spectra. Interestingly, the fluorescence spectra are found to be well structured with well resolved vibronic features, a characteristic property generally associated with oligomeric thiophenes. Introduction of phenyl substituent in BTPDB and TTPDB does not alter the spectral pattern in contrast to oligomeric thiophenes.51 For BTPDB, two well-resolved emission peaks at 399, 419 nm and a shoulder at 448 nm are observed which originated from the 0,0, 0,1 and 0,2 transitions respectively. TTPDB, on the other hand, shows red-shifted emission with maximum at 446, 471, and a shoulder at 500 nm. In all the cases 0,1 peak is found to be more intense than the 0,0 peak. Interestingly, the vibronic features in both the compounds are separated by 0.15-0.18 eV, which corresponds to strong coupling of C=C stretching mode with electronic transition. Either the absence or presence of fine structures in UV-Visible absorption and fluorescence spectra suggests that the molecules become rigid, after transition from ground singlet to excited singlet state. Apparently, quinonoid structures with intervening double bonds between the two thiophene units with enhanced planarity are formed after the Franck-Condon excitation. To find out the limiting anisotropy (r0), the polarized steady state excitation spectra (Figure S38 in Supporting Information) are measured in glycerol, which hinders the rotational diffusion of the solute owing to high viscosity. Thus the calculated limiting anisotropy can be correlated to the angle (β) between absorption and emission transition moments as given in the equation 2. 0.4 3 − 1/2

(2)

The excitation anisotropy measured by monitoring the emission at all the vibronic features is found to be same indicating that all the fluorescence emission originates from the same electronic energy level and the molecules are found to obey Kasha’s rule. The theoretical 17 ACS Paragon Plus Environment


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maximum and minimum values for r0 is restricted between 0.4 and −0.2, respectively for parallel (β=0°) and perpendicular ((β=90°) orientation. For BTPDB, the r0 is measured to be 0.3 and remains almost constant over the entire spectral region. On the other hand, its next higher homolog with three thiophene rings (TTPDB) show r0 value of 0.16, which gradually decreases with increase in the wavelength till 300 nm and then raises to 0 and remains constant in lower energy region. The lower anisotropy value around 300 nm suggests that the absorption and emission are not collinear and the transition is probably associated with the higher energy transitions. A comparatively lower r0 value for TTPDB than BTPDB is due to the difference in the transition moment angle between absorption and emission. Further, the fluorescence lifetime using time correlated single photon counting technique by exciting the molecule at 37 m, 75 ps light are measured (Figure S39 in supporting Information). Unfortunately, the lifetime of the sample is shorter than the instrument response function (IRF= 600 ps). Nevertheless, the lifetime of TTPDB was found to be slightly larger than BTPDB, probably due to the increased radiative size of the molecule. Conclusions Three mesogens with a core consisting of 4-alkoxy phenyl benzoate directly linked to either thiophene or bi or terthiophene were synthesised by Suzuki coupling from 4-Bromo phenyl4-alkoxybenzoate and respective thiophene boronic acid or pinacol esters using tetrakis(triphenylphosphene) palladium (0). The HOPM, DSC and XRD studies of synthesised mesogens revealed existence of enantiotropic nematic and smectic mesophases. The smectic A and smectic C phases of bithiophene and terthiophene based mesogens were confirmed by XRD studies by noticing a sharp and intense reflection in the small angle region and a broad hump in the wide angle region. Further, the XRD studies showed that the tilt angle is 23.0-26.5° in smectic C phase of terthiophene based mesogen. The high resolution solid state

13

C NMR studies of mesogens provided molecular structure in 18 ACS Paragon Plus Environment

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respective liquid crystalline phases. The 13C-1H dipolar couplings determined from 2D SLF experiments were used for computing the order parameters of phenyl as well as thiophene rings. Interestingly, for mono thiophene mesogen, the order parameter of thiophene ring was found to be lower than phenyl ring II while for bi and terthiophene based mesogens reverse trend was noticed. Thus the variation of thiophene order in these mseogens was generally attributed to kind of substitution on thiophene ring i.e. 2- substitution versus 2,5disubstituiton. The solution absorption and fluorescence spectral studies of mesogens revealed that the mono thiophene based mesogen was non-fluorescent while those based on bithiophene and terthiophene showed intense fluorescence. It was also found that the absorption spectra of mesogens were structure less while the fluorescence spectra were well structured with resolved vibronic bands which advocates that the molecules becomes rigid, after transition from ground singlet to excited singlet state. The excitation anisotropy studies revealed that the fluorescence emission originated from the identical electronic energy level. Further, the DFT and TD DFT studies demonstrated the intramolecular charge transfer between the phenyl-thiophene to phenyl benzoate unit. The overall studies unveiled that with increase in thiophene rings in the terminal position of core unit, tendency for smectic mesophases increased while the fluorescence emission experienced a redshift. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. It contains synthetic details, spectral data of intermediates, SAMPI-4 pulse scheme diagram, 1D and 2D solution NMR spectra, VT-XRD plots, thiophene models for order parameter calculation, fluorescence spectra, HOMO, LUMO contour plots. AUTHOR INFORMATION 19 ACS Paragon Plus Environment


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Corresponding Author *E-mail: [email protected]. Phone: 91-44-24422059 ACKNOWLEDGEMENTS The authors thank Dr. B. Chandrasekaran, Director, CSIR-CLRI and Prof. K. V. Ramanathan, NMR Research Centre, Indian Institute of Science, Bangalore, India for their support and keen interest in this project. We thank for the help extended by Dr. V. Subramanian, Dr. B. V. N. Phani Kumar and Mr. R. Ravikanth Reddy of this Institute for computational studies as well as for 2D solution NMR studies respectively. We are grateful to Ms. K. N. Vasudha, Raman Research Institute, Bangalore, India for the powder X-ray measurements. The partial financial support from STRAIT CSC020-WP2 is duly acknowledged and K. Rajasekhar Reddy thanks the Council of Scientific and Industrial Research (CSIR), New Delhi for the grant of Senior Research Fellowship.


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Table 1: Transition Temperatures and Enthalpy Values of TPDB, BTPDB and TTPDB from Second Cooling Cyclea

Mesogen Code

Transition temperatures (°C) and enthalpy values in parenthesis (△H kcal/mol)

Mesophase range

Mesophase transition Crystallization TPDB BTPDB TTPDB

I-N 131.10 (0.27) I-N 199.21 (0.15) I-N 278.54 (0.25)

N-SmA [99.93] (0.19) N-SmA 144.82 (0.05) N-SmC 206.04 (0.07)

SmA-Cr 83.92 (5.38) SmA-Cr 111.80 (6.77) SmC-Cr 155.06 (9.62)

31.17(N) 16.01 (SmA) 56.19 (N) 31.22 (SmA) 72.50 (N) 50.98 (SmC)

a

[ ] Indicates monotropic transition

Cr: Crystallization, N: Nematic, SmC: Smectic C, SmA: Smectic A, I: Isotropic



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Table 2: 13C NMR Data for the Core Unit of Mesogens in Solution and Mesophasesa

C. N. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

TPDB 112 ºC ( nematic) CS AIS DOF (ppm) (ppm) (kHz) 220.4 56.9 1.29 138.2 24.0 2.47 160.8 28.6 2.41 180.1 58.8 1.27 203.6 38.9 0.58 215.9 65.5 1.33 148.1 25.9 2.65 148.9 22.1 2.42 196.2 64.2 1.30 185.2 41.8 0.93 148.9 25.8 5.31 177.4 49.5 5.09 180.1 55.4 8.99

BTPDB 136 ºC (smectic A) CS AIS DOF (ppm) (ppm) (kHz) 227.6 64.0 1.55 141.0 26.7 2.74 164.2 31.9 2.64 187.8 66.4 1.55 209.8 44.9 0.64 224.6 74.0 1.60 150.6 28.3 3.08 152.7 26.0 2.66 203.6 61.3 1.58 192.1 60.3 1.09 158.2 34.5 4.27 173.0 48.6 2.75 183.6 46.8 0.90 183.6 46.2 0.90 167.1 43.2 2.99 153.5 25.6 2.48 178.9 54.3 10.95

TTPDB 180 ºC (smectic C) Solution Solution Solution CS AIS DOF (ppm) (ppm) (ppm) (ppm) (ppm) (kHz) 163.5 163.6 163.8 227.3 63.5 1.53 114.2 114.3 114.5 140.7 26.2 2.56 132.2 132.3 132.3 163.7 31.4 2.65 121.3 121.4 121.6 186.3 64.7 1.34 164.7 164.9 164.8 214.4 49.6 0.34 150.4 150.6 150.8 227.3 76.5 1.53 122.2 122.3 122.3 151.0 28.7 3.30 126.8 126.7 126.7 152.5 25.8 2.90 132.0 142.3 131.8 189.1 57.3 1.61 143.4 131.8 136.3 190.2 53.9 1.49 123.1 123.7 123.8 159.6 35.8 4.38 127.9 124.4 124.4 159.3 34.9 2.41 124.7 136.8 142.6 206.8 64.2 1.43 137.4 136.4 193.7 57.3 1.19 123.9 123.7 151.0 27.3 4.30 127.9 124.2 168.4 44.2 3.05 124.6 137.2 193.7 56.5 1.19 136.6 193.7 57.1 1.19 124.0 177.7 53.7 4.32 127.9 151.0 23.1 3.30 124.6 182.7 58.1 10.00 a CS: chemical shift; AIS: alignment induced shift; DOF: dipolar oscillation frequencies


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Table 3: Orientational Order Parameter for Phenyl and Thiophene rings of TPDB, BTPDB and TTPDB Mesogensa

b

c

B

C a

d

D

A

S

CCH Bond Angles

Mesogen Ring

TPDB BTPDB TTPDB

θb 119.6 119.2 120.3 119.5 120.4 118.3

I II I II I II

Mesogen Ring

β

b

θc 119.8 120.0 120.7 120.8 120.1 119.6

CCH Bond Angles

Szz

Calculated dipolar oscillation frequencies (kHz)

Sxx-Syy

b 0.56 0.57 0.68 0.69 0.64 0.65

0.054 0.058 0.062 0.068 0.060 0.066

Szz

Sxx-Syy

θB θC θD TPDB III 1.0 110.7 41.5 32.3 0.64 0.034 III 6.0 114.6 46.2 0.64 0.063 BTPDB IV 8.5 117.6 48.3 23.3 0.60 0.012 III 8.5 117.1 49.1 0.83 0.060 TTPDB IV 8.0 117.0 48.2 0.78 0.041 V 5.5 114.6 45.4 27.4 0.62 0.051 a Ring I (TPDB/BTPDB/TTPDB): a=C1, b=C2, c=C3, d=C4

2.47 2.64 2.75 3.08 2.57 3.28

c

d

RMSD (kHz)

a

2.42 1.28 1.28 2.43 1.30 1.31 2.63 1.56 1.57 2.67 1.57 1.61 2.66 1.48 1.47 2.89 1.46 1.49 Calculated dipolar oscillation frequencies (kHz) B C D A 5.34 5.05 9.00 0.89 4.28 2.76 0.99 1.04 3.00 2.46 10.96 0.90 4.39 2.40 1.30 1.38 4.34 3.06 1.18 1.19 4.32 3.31 10.00 -

0.01 0.02 0.02 0.02 0.07 0.05 RMSD (kHz) 0.03 0.06 0.01 0.04 0.02 0.01

Ring II (TPDB/BTPDB/TTPDB): a=C6, b=C7, c=C8, d=C9 Ring III (TPDB/BTPDB/TTPDB): A=C10, B=C11, C=C12, D=C13 Ring IV (BTPDB/TTPDB): A=C14, B=C15, C=C16, D=C17 Ring V (TTPDB): A=C18, B=C19, C=C20, D=C21 b

Angle between the local z-axis and the corresponding C-C bond (see Supporting Information)



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Table 4: Summary of the Excited State Electronic Transitions obtained from TD-DFT calculations at the B3LYP/6-31G* level.

S1 S2 S3

Absorption (nm) 315 279 266

Energy (eV) 3.93 4.44 4.65

Oscillator strength (f) 0.6027 0.5308 0.1225

S1 S3 S8

372 285 259

3.33 4.35 4.77

1.2350 0.1275 0.2442

S1 S7 S8

430 277 276

2.88 4.46 4.48

1.6146 0.1307 0.2107

Mesogen States

TPDB

BTPDB

TTPDB

a

Dominant contributiona (%) H→L (98%) H→L+1(92%) H-1→L (44%), H→L +3(33%) H→L (99%) H-1→L (70%) H-1→L+1 (44%), H-3→L (41%) H→L (99%) H-1→L+1(51%), H→L+5(32%) H-1→L+1(42%), H→L+5(32%)

H denotes HOMO and L denotes LUMO


Exp. (nm) 282

343

383

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Figure Captions Scheme 1: Synthetic strategy adopted for Thiophene Mesogens Figure 1: HOPM photographs of Mesogens. TPDB: (A) marble texture of nematic phase (129.3°C) and (B) fan texture of smectic A phase (92.5°C). BTPDB: (C) threaded texture of nematic phase (189.3°C) and (D) homeotropic texture of smectic A phase (142.6°C). TTPDB: (E) threaded texture of nematic phase (275.8 °C) and (F) schlieren texture of smectic C phase (180.2 °C). Figure 2: DSC Heating and Cooling Scans of (A) TPDB, (B) BTPDB and (C) TTPDB. Figure 3: Powder X-ray Diffraction profiles of (A) BTPDB and (B) TTPDB in Smectic A and Smectic C phases at 125 °C and 185 °C respectively with inset enlarged Wide Angle region. Figure 4: Planar (A, C and E) and DFT Energy Optimized (B, D and F) Structures of TPDB, BTPDB and TTPDB. Figure 5: Proton-decoupled Solution TTPDB.

13

C NMR Spectra of (A) TPDB (B) BTPDB and (C)

Figure 6: Static 13C NMR Spectra of (A) TPDB in nematic phase (112°C), (B) BTPDB in smectic A phase (136°C) and (C) TTPDB in smectic C phase (180°C). Figure 7: 2D SAMPI-4 Spectra of (A) TPDB in nematic phase (112°C), (B) BTPDB in smectic A phase (136°C) and (C) TTPDB in smectic C phase (180°C). Contours with red color indicates thiophene methine carbons. Figure 8: UV-Visible absorption and fluorescence spectra of TPDB, BTPDB and TTPDB in chloroform solution.



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Scheme 1


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Figure 1



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Figure 2


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Figure 3

(A)

(B)



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Figure 4


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Figure 5



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Figure 6


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Figure 7



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Figure 8


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Influence of Thiophenes on Molecular Order ... - ACS Publications

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