Novel electron-deficient phenanthridine based discotic liquid crystals

Journal of Molecular Liquids 272 (2018) 583–589

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Novel electron-deficient phenanthridine based discotic liquid crystals A.R. Yuvaraj a, Anu Renjith a,b, Sandeep Kumar a,⁎ a b

Raman Research Institute, Raman Avenue, Sadashivanagar, Bangalore 560080, India Indian Institute of Science, Bengaluru 560012, India

a r t i c l e

i n f o

Article history: Received 19 June 2018 Received in revised form 23 September 2018 Accepted 25 September 2018 Available online 26 September 2018 Keywords: Discotic liquid crystals Pictet-Spengler Hetero-polycyclic Mesophase Conductivity

a b s t r a c t Nitrogen-containing polycyclic core 5-phenylnaphtho[1,2,3,4-lmn] phenanthridine and its nitro-functionalized positional isomers were synthesized using Pictet-Spengler reaction between hexaalkoxytriphenylene-1-amine and various aryl aldehydes. The hexagonal columnar phase was observed in all the synthesized novel compounds. Mesomorphic characterization was carried out using conventional thermal analysis and X-ray diffraction techniques. These compounds showed room temperature liquid crystal phase and there is no crystalline phase observed even at −20 °C upon cooling from the isotropic temperature. Further, the long range of mesophase was confirmed by polarizing optical microscopy. Subsequently, charge transfer complexes were prepared by mixing the synthesized polycyclic hetero-aromatic compounds with 2,3,6,7,10,11-hexakis(octyloxy)triphenylene. In UV–Vis absorption spectra, a redshift was found for the charge transfer complexes; which confirmed the interaction between donor-acceptor counterparts. The electrical conductivity of the charge transfer complexes and their pure target compounds was measured. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Heterocyclic and polycyclic aromatic hydrocarbons play an important role in the photophysical, electronic and supramolecular properties [1,2]. The stable nitrogen-containing polycyclic aromatic compounds exhibit excellent electronic properties at the molecular level and hence they can be useful in various organic electronic technologies [2]. In the recent era, nitrogen-containing heterocyclic compounds were used as a building block to obtain nitrogen-doped graphene; which are employed in the application of catalysis as well as electronics [3]. Actually, direct incorporation of heteroatoms in a polycyclic aromatic compound is a challenge. There are many synthetic strategies and structureproperty relationships available in the literature to understand the significance of heterocyclic aromatic hydrocarbons [4]. The synthetic methodologies have a profound impact on the design of novel polycyclic hetero-aromatic functional materials with tailor made properties. The tuning of the structure and hence the property of these class of compounds usually helps to improve the efficiency of light-emittingdiodes, photo-voltaic cells and field effect transistors [5,6]. The heterocyclic aromatic supramolecular compounds provide good carrier transport and physical properties [7]. In particular, the electron transport and electron injection properties of the compounds are enhanced by replacement of CH group with nitrogen heteroatoms [7a–f]. Wei et al. [8] reported the synthesis of polycyclic heteroaromatic compounds 1,5,9-triazacoronenes. Coronene is graphitic fragment with a zigzag ⁎ Corresponding author. E-mail address: [email protected] (S. Kumar).

https://doi.org/10.1016/j.molliq.2018.09.120 0167-7322/© 2018 Elsevier B.V. All rights reserved.

periphery [9–11]. Nevertheless, there are only a few synthetic routes available in the literature to prepare nitrogenated polycyclic compounds [12]; such as 1,2-diazacoronene [12a] and 1,2,7,8tetraazacoronene [12b]. The synthetic methods of such compounds require vigorous conditions; Diels-Alder reaction of diethyl azodicarboxylate with perylene and maleic anhydride refluxed at 350 °C. Since, these compounds are disc-shaped planar heteroaromatic systems, our aim is to evaluate the liquid crystal (LC) properties of them. Discotic liquid crystals (DLCs) are composed of planar disc-shaped molecules, which have strong ability to form columnar or nematic phases with long-range order. They exhibit excellent charge carrier mobility because of their well-ordered arrangement at the molecular level. So, most of the DLCs are considered as efficient organic semiconductors and these are promising candidates for the photovoltaic technology [13,14]. The formation of CT complexes is important for the enhancement of stability and mesomorphic range of the LCs. In 1989, Ringsdorf et al. introduced the concept and mechanism of CT in DLCs [15]. They have shown that mesomorphism can be induced in amorphous polymers on doping with electron acceptors. Due to the presence of low molecular weight electron acceptors, the disc-shaped counterparts arranges in a columnar fashion. In this work, our plan is to synthesize some new polycyclic heteroaromatic DLC compounds. We have synthesized a new series of disk-shaped 5-phenylnaphtho[1,2,3,4-lmn] phenanthridine derivatives. Interestingly, these compounds are LC at room temperature and they are not producing crystalline state even at −20 °C. Using these target compounds, the CT complexes are prepared by mixing them with

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equimolar amount of 2,3,6,7,10,11-hexakis(octyloxy)-triphenylene. The electrical conductivity of the CT complexes and pure target compounds is measured.

2. Experimental The disk-shaped 5-phenylnaphtho[1,2,3,4-lmn] phenanthridine derivatives 6a–6d are synthesized in five steps (Scheme 1). Firstly, Catechol 1 was alkylated using the calculated amount of 1-bromooctane in presence of a mild base potassium carbonate. Alkylated catechol 2 was stirred with three equivalence of iron(III) chloride for an hour in order to get 2,3,6,7,10,11-hexakis(octyloxy)-triphenylene (HAT8) 3 [14a]. Then, mono-nitro triphenylene derivative 4 was synthesized by nitration using concentrated nitric acid [8]. Further, 4 was reduced using H2/Raney Ni to obtain 5 [8]. The target compounds 6a–6d were synthesized using Pictet-Spengler reaction [8]. In this reaction, hexaalkoxytriphenylene-1-amine 5 reacts with aldehydes such as benzaldehyde, 2-nitrobenzaldehyde, 3-nitrobenzaldehyde and 4nitrobenzaldehyde; which undergoes condensation followed by cyclization. Here, acetic acid is used to create acidic media and to promote the ‘insitu’ cyclization. General procedure for Pictet-Spengler reaction (6a): The intermediate compound 5 (1 g, 0.98 mmol) and benzaldehyde (0.116 g, 1.08 mmol) were refluxed in glacial acetic acid (10 mL) media for 10 h. Then, the reaction mixture was quenched with distilled water and extracted the target compound using dichloromethane. Further, the target compound was purified by column chromatography (silica gel: 100–200 mesh; mobile phase: 20% ethyl acetate/petroleum ether). The obtained dark yellow semisolid liquid crystal compound was dried over high vacuum for 10 min. The same procedure was used to synthesize nitro functionalized positional isomers 6b–d.

6a: (911 mg, 91%), m.p. 99.5 °C; 1H NMR (500 MHz, CDCl3): 8.42 (s, 1H, Ar), 8.2 (s, 1H, Ar), 8.0 (s, 2H, Ar), 7.7 (d, J = 6.5 Hz, 2H, Ar), 7.4 (d, J = 7.5 Hz, 3H, Ar), 4.4 (t, J = 6.5 Hz and 6 Hz, 2H, OCH2), 4.39 (t, J = 6.5 Hz, 2H, OCH2), 4.33 (q, J = 6.5 Hz, 6H, OCH2 × 3), 3.5 (t, J = 5.5 Hz and 6.5 Hz, 2H, OCH2), 2.2–1.0 (m, 72H, CH2 × 36), 0.9 (t, 18H, CH3 × 6); 13C NMR (125 MHz, CDCl3): 158.5, 151.5, 150.9, 150.0, 149.5, 145.2, 144.4, 144.0, 136.2, 129.3, 127.4, 126.9, 124.5, 124.1, 123.3, 123.2, 117.9, 113.8, 110.5, 108.8, 107.7, 107.0, 77.2, 77.0, 76.7, 75.4, 74.8, 71.3, 69.9, 69.5, 31.9, 31.8, 31.6, 30.5, 29.8, 29.7, 29.6, 29.5, 29.4, 29.3, 29.2, 26.3, 26.2, 25.7, 22.7, 22.6, 14.1, 14.0; Elemental analysis [Found %(Calculated %)]: C 79.77(79.80), H 10.25(10.18), N 1.18(1.27). 6b: (889 mg, 89%). m.p. 121.3 °C; 1H NMR (500 MHz, CDCl3): 8.4 (s, 1H, Ar), 8.27 (s, 1H, Ar), 8.24 (d, J = 8 Hz, 1H, Ar), 8.0 (d, J = 7 Hz, 2H, Ar), 7.7 (t, J = 7 Hz and 7.5 Hz, 1H, Ar), 7.6 (d, J = 7 Hz, 1H, Ar), 7.5 (t, J = 7.5 Hz and 8 Hz, 1H, Ar), 4.4–4.2 (m, 10H, OCH2 × 5), 3.8 (t, J = 6.5 Hz and 8.5 Hz, 1H, Ar), 3.6 (t, J = 6 Hz and 8 Hz, 1H, OCH2), 2.0–1.0 (m, 72H, CH2 × 36), 0.9 (t, 18H, CH3 × 6); 13C NMR (125 MHz, CDCl3): 155.4, 151.5, 150.4, 150.0, 149.5, 148.2, 144.3, 140.7, 136.1, 132.2, 131.9, 127.7, 124.6, 124.1, 123.8, 123.5, 123.3, 121.9, 118.2, 113.8, 110.7, 109.0, 107.7, 75.7, 73.8, 71.1, 69.9, 69.8, 69.5, 31.93, 31.90, 31.87, 31.84, 30.3, 29.8, 29.7, 29.6, 29.5, 29.48, 29.46, 29.35, 29.32, 29.2, 26.3, 26.2, 26.0, 25.7, 22.7, 22.6, 14.1, 14.0; Elemental analysis [Found %(Calculated %)]: C 76.59(76.66), H 9.72(9.69), N 2.41(2.45). 6c: (903 mg, 90%). m.p. 123.5 °C; 1H NMR (500 MHz, CDCl3): 8.6 (s, 1H, Ar), 8.4 (s, 1H, Ar), 8.3 (d, J = 8 Hz, 1H, Ar), 8.2 (s, 1H, Ar), 8.08 (d, J = 7.5 Hz, 1H, Ar), 8.03 (s, 2H, Ar), 7.6 (t, J = 8 Hz, 1H, Ar), 4.4 (t, J = 6 Hz and 6.5 Hz, 2H, OCH2), 4.4–4.3 (m, 8H, OCH2 × 4), 3.6 (t, J = 5 Hz and 5.5 Hz, 2H, OCH2), 2.0–1.0 (m, 72H, CH2 × 36), 0.9 (t, 18H, CH3 × 6); 13 C NMR (125 MHz, CDCl3): 155.8, 151.6, 150.7, 150.2, 149.7, 147.5, 145.7, 144.5, 136.1, 135.7, 127.7, 124.8, 124.4, 124.0, 123.4, 123.2, 122.4, 122.3, 117.3, 113.7, 111.0, 109.3, 107.6, 77.5, 77.2, 77.0, 76.7, 75.6, 74.5, 71.2, 70.0, 69.9, 69.5, 31.9, 31.8, 30.5, 29.8, 29.7, 29.6, 29.5,

OC8H17 OC8H17 OH

OC8H17

(i)

C8H17O

OC8H17

OH 1

2

C8H17O

R N

6

OC8H17 OC8H17

3

OC8H17 OC8H17

C8H17O

C8H17O

(ii)

(iii)

OC8H17 OC8H17 (v)

C8H17O

(iv) NH2

C8H17O

OC8H17 OC8H17

5

C8H17O NO2

C8H17O

OC8H17 OC8H17

O2N

OC8H17 OC8H17

4

OC8H17 OC8H17

NO2 NO2

R= 6a

6b

6c

6d

Scheme 1. Synthesis of positional isomers 5-phenylnaphtho[1,2,3,4-lmn] phenanthridine derivatives. (i) Br-C8H17, K2CO3/KI, Me2CO, 3 h, reflex; (ii) FeCl3, CH2Cl2, 1 h, RT; (iii) Conc. HNO3, CH3NO2/CH2Cl2, 12 min, RT; (iv) H2/Raney Ni, dry THF, 2.4 h, RT; (v) RCHO, AcOH, 10 h, reflux.

A.R. Yuvaraj et al. / Journal of Molecular Liquids 272 (2018) 583–589

29.4, 29.3, 29.1, 26.3, 26.2, 25.8, 22.7, 22.6, 14.1; Elemental analysis [Found %(Calculated %)]: C 76.72(76.66), H 9.75(9.69), N 2.38(2.45). 6d: (909 mg, 90%). m.p. 118.6 °C; 1H NMR (500 MHz, CDCl3): 8.4 (s, 1H, Ar), 8.3 (d, J = 8 Hz, 2H, Ar), 8.2 (s, 1H, Ar), 8.0 (s, 2H, Ar), 7.9 (d, J = 8 Hz, 2H, Ar), 4.45 (t, J = 5.5 Hz and 6.5 Hz, 2H, OCH2), 4.4–4.3 (m, 8H, OCH2 × 4), 3.6 (t, J = 5 Hz and 6.5 Hz, 2H, OCH2), 2.0–1.1 (m, 72H, CH2 × 36), 0.9 (t, 18H, CH3 × 6); 13C NMR (125 MHz, CDCl3): 156.1, 151.7, 150.71, 150.72, 150.6, 150.2, 149.7, 147.1, 144.49, 144.44, 136.1, 130.2, 124.8, 124.0, 123.4, 123.1, 122.39, 122.33, 117.3, 113.6, 110.8, 109.3, 107.6, 107.0, 76.7, 75.5, 74.6, 71.1, 69.9, 69.5, 31.9, 31.86, 31.83, 31.81, 30.5, 29.8, 29.69, 29.63, 29.5, 29.49, 29.42, 29.39, 29.35, 29.32, 29.1, 26.3, 26.26, 26.23, 25.8, 22.69, 22.66, 22.61, 14.1 14.0; Elemental analysis [Found %(Calculated %)]: C 76.72(76.66), H 9.73(9.69), N 2.39(2.45). 3. Result and discussion 3.1. Liquid crystal characterization Differential scanning calorimeter (DSC) and polarizing optical microscope (POM) were used to evaluate the thermal behaviour of the target compounds 6a–d. These compounds have long-range mesophases, which can be confirmed from DSC graphs (Table 1 and Fig. S1). For the compound 6a, DSC endothermic peak was found at 82.60 °C [ΔH = 0.60 kJ mol−1] in mesophase to isotropic transition. In the isotropic to mesophase transition, the exothermic enantiotropic peak was observed at 73.95 °C [ΔH = 0.81 kJ mol−1]. The compounds 6b–d also showed similar thermal behaviour. All the DSC thermal analytical data are summarized in Table 1. The corresponding microscopic textures were captures in POM analysis and shown in Fig. 1. Clearly, the target compounds 6a–d showed liquid crystals phases at room temperature (22 °C). These compounds did not show crystalline phase even at −20 °C upon cooling, which is confirmed from the DSC analysis (Fig. S1). Hence, these compounds 6a–d are liquid crystals in the wide range and applicable in any field of LC technology. Further, these compounds were watched carefully under the polarizing optical microscope (POM) with variable temperatures at a constant cooling rate of 5 °C min−1. The evaluation of mesomorphism under the POM was supported by the recorded DSC data. The POM images of 6a–d were captured at required temperatures. Interestingly, the microscopic textures were unchanged in wide mesophase range. At room temperature, the microscopic images of 6a–d are given in Fig. 1. These images showed the typical textures hexagonal columnar phase. 3.2. X-ray diffraction studies X-ray diffraction (XRD) technique was employed to understand the supramolecular organisation and mesomorphism of 5-phenylnaphtho [1,2,3,4-lmn] phenanthridine derivatives 6a–d. XRD studies were performed for these compounds in LC phase while cooling from isotropic phase. For 6a–d, the columnar hexagonal phase was confirmed form the XRD spectra and these results were supported the DSC and POM analysis. In the wide-angle X-ray pattern, a diffused scattering halo was found with d-spacing 4.45 A° (2ϴ = 19.96°), 4.25 A° (2ϴ = 20.84°), 4.35 A° (2ϴ = 20.37°) and 4.39 A° (2ϴ = 20.19°) corresponds

Table 1 The phase transition temperatures (peak, °C) with corresponding enthalpy change in square brackets (kJ mol−1) of new 5-phenylnaphtho[1,2,3,4-lmn] phenanthridine derivatives. Target compounds

Heating scan

Cooling scan

6a 6b 6c 6d

Colh 82.60 [0.60] I Colh 106.60 [1.46] I Colh 103.58 [1.06] I Colh 106.08 ∗ [0.61] I

I 73.95 [0.81] Colh I 98.96 [1.52] Colh I 96.98 [0.93] Colh I 101.93 [0.74] Colh

Notations: Colh = columnar hexagonal phase; I = isotropic phase.

585

to 6a–d respectively (Figs. 2 and S2). These halo peaks indicated the liquid like order of the molten alkyl chains (nC8H17) commonly present in each target molecules. For 6b and 6d, two reflections were observed in the small angle region with the d-spacing ratio 1:(1/ √ 3), which corresponds to the indices 100 and 110. These indices are the typical values for the hexagonal lattice. Thus, 5-phenylnaphtho[1,2,3,4-lmn] phenanthridine derivatives are arranged in the hexagonal pattern to construct the mesophase. A short peak at wide angle region was expected in Figs. 2 and S2; to calculate the d-spacing of core-core intermolecular interactions. Unfortunately, this peak is not found in the required range probably due to the disordered nature of the columnar phase. The inter-columnar spaces are 24.26 A°, 23.38 A°, 24.11 A° and 23.96 A° for the target compounds 6a–d respectively. The XRD was recorded with the function of temperature and (1/ √ 3) peak was clearly observed at 55 °C. Somehow, this peak was not visible at room temperature and hence, XRD is described particularly at 55 °C. The representative diffraction spectrum of 6b is given in Fig. S6. 3.3. Absorption and fluorescence measurements The compounds 6a–d showed three absorption maxima at 264 nm, 340 nm and 392 nm (Table 2 and Fig. S3). Fluorescence measurements were performed using a thin layer (thickness is 5 μm) of each compound. The weak fluorescence property was observed in these compounds. The fluorescence peaks are written in Table 2. The compound 6a has a weaker fluorescence than 6c and 6d, which may be attributed to the presence of strong electron withdrawing auxochrome nitro group at the meta and para positions for 6c and 6d respectively. But, 6b does not show fluorescence property in spite of having a nitro-group at ortho position. 3.4. Study of charge transfer complex The reconfiguration of molecular alignment and order of nanoscale materials have significant interest in both applied and fundamental research. Beyond organic synthesis, preparing the mixture of donoracceptor pairs show peculiar characteristics especially in LC research. Formation of the charge transfer (CT) complex is an excellent tool in order to fine-tune electronic and optical characteristics of aromatic LC compounds. The obtained CT complex materials provide potential in a wide range of technological applications. The synthesized N-hetero-polycyclic compounds 6a-d are taken as electron acceptors; whereas 2,3,6,7,10,11-hexakis(octyloxy)triphenylene (HAT8) is used as electron donors. Various composition or proportions of 6a–d:HAT8 were prepared using ultra-sonication and analysed by UV–Vis spectroscopy. Fig. 3 shows the absorption spectra for the pure HAT8 and 6b as well as, composition 1:1, 1:2 and 2:1 (HAT8:6b). Interestingly, peak of 1:1 composition at 383 nm has shifted to 489.3 nm. This additional peak was not observed in other composition. It means, the strong CT interaction only found in 1:1 ratio. The mesomorphic investigation showed enhanced order and LC properties for exact equimolar concentration compared to other concentrations. Therefore, 1:1 ratio of HAT8:6a, HAT8:6b, HAT8:6c and HAT8:6d were prepared and characterized. As seen in Fig. 3, there is a redshift found in the UV–Vis absorption spectra: 384 nm → 489 nm, 383 nm → 489 nm, 394 nm → 500 nm and 409 nm → 489 nm; after the formation of CT complex (Figs. 3 and S4). This observation gives the evidence for intermolecular interactions of donor-acceptor pairs in the mixture. The process of CT complex formation is further confirmed using XRD. The additional peaks found in this analysis show the improved molecular order of the equimolar mixture as compared with the pure compounds 6a–d (Fig. S4). The detailed interpretation of X-ray diffraction analysis is given in Table 3. The inter-columnar distance is unaltered after the formation of CT complex. The core-core separation was not found in case of the

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Fig. 1. The polarizing optical microscope images of the hexagonal columnar mesophase of the target compounds 6a–d were captured at room temperature on cooling from the isotropic phase. All the optical texture viewed through cross-polariser at 200× magnification.

pure state, but hc = 3.63 A° is obtained for CT complexes. This is the direct evidence of enhancement of columnar hexagonal order in the system, which is shown in Fig. 4. Millar indices obtained for these CT complexes are 100, 110, 200 and 210 correspond to hexagonal columnar phase with the lattice constant a = 23.53. Also, the d-spacing values given in Table 3 confirmed the columnar hexagonal phase of CT complexes with the ratio 1:(1/ √ 3):(1/2):(1/ √ 7). The alkyl chain lengths are unchanged because of the same aliphatic chain nC8H17 used in both the donor and acceptor molecules. Thus, XRD analysis confirms the donor-acceptor interactions and enhancement of hexagonal order of CT complexes.

10

6b

The electrical conductivity of the compound in the LC phase depends mainly on the charge transport through the cores and therefore is largely affected by the electron density of the core compound. Thus factors contributing to the increase/decrease of the electron density can impact the electrical conductivity of the compound. As we know, the increase in conductivity with temperature can be attributed to the lowering of the band gap of the material, thus making the HOMO → LUMO transitions facile [14c]. The target compounds 6a–d are highly conducting near to the mesophase to isotropic transition [14].

6d

d = 20.25 A

log intensity (A.U.)

log intensity (A.U.)

105

3.5. DC conductivity measurements

4

d = 4.25 A d = 11.72 A 103

105

d = 20.75 A

104 d = 4.39 A d = 12.01 A 103

5

10

15

20

25

5

10

15 2

Fig. 2. The intensity pattern of the XRD exhibited by 6b and 6d at 55 °C.

20

25

A.R. Yuvaraj et al. / Journal of Molecular Liquids 272 (2018) 583–589 Table 2 Absorption and fluorescence maxima of 5-phenylnaphtho[1,2,3,4-lmn] phenanthridine derivatives. Compounds

Absorption peaks (nm)

Fluorescence peaks (μm)

6a 6b 6c 6d

262, 341, 384 263, 340, 383 264, 341, 394 267, 339, 409

7.40, 10.10, 23.80 – 7.95, 11.45, 18.20 6.20, 12.20, 13.45

Optical Density (A. U.)

1.0

263 nm 279 nm

HAT8 6b 1:1 (HAT8:6b) 1:2 (HAT8:6b) 2:1 (HAT8:6b)

0.8 0.6 0.4

340 nm 383 nm

0.2

489.3 nm 0.0 250

300

350

400

450

500

550

Wavelength (nm) Fig. 3. The absorption spectra of compounds HAT8, 6b and various compositions. Wavelength shift noticed in the absorption spectra from 393 nm to 489.3 nm after CT complex formation for the equimolar mixture HAT8 + 6b.

To measure DC conductivity, the ITO coated cells containing the compounds were kept at a potential of 1 V and the observed current was measured using chronoamperometry. Since all the pure target compounds exhibited room temperature LC phase, DC conductivities were measured from the isotropic phase to room temperature with the constant cooling rate of 5 °C min−1. The observed conductivity of the compounds DLC 6a–d was close to those reported previously [14]. Since the isotropic temperatures varied for compounds 6a–d, the trend in electrical conductivities was analysed up to a temperature of 80 °C.

587

It is quite evident from Fig. 5a that the position of the nitro group has the significant effect on the electrical conductivity of the molecules. The lowering of electrical conductivity was observed for the nitro functionalized 6b–d, as compared with 6a. Actually, the bulky NO2 group disturbs the order of the arrangement within the LC system. Usually, nitro groups are slightly distorted from the molecular plane. So, the decrease in conductivity of the nitro functionalized compounds 6b–d may be attributed to the poor arrangement of DLC units with respect to 6a. It is clear from Scheme: 1 that in addition to the slightly distorted out of the plane arrangement of nitro group in 6b–d, intramolecular steric hindrance within the nC8H17 and PhNO2 also exists in the ortho isomer. The steric hindrance can alter the dihedral angle which can significantly influence the extended conjugation as reported earlier by Han et al. [16]. The steric hindrance and associated distortion in planarity diminish the electron transport within 6b system, which is been reflected in the least electrical conductivity compared to 6a, 6c and 6d. The donor-acceptor pairs in the mixtures showed interesting results. The electrical conductivity of target compounds 6a–d are lower than HAT8 (Fig. 5a) [14b]; it clearly shows the electron deficiency in 6a–d. So, these compounds act as electron acceptors in the CT complexes. The presence of nitro functional group at various positions is very interesting to discuss in this context, as strong electron withdrawing nitro groups can literally vary the electron density of positional isomers 6b– d. Hence, the electrical conductivity of CT complexes composed by HAT8 and N-hetero polycyclic compounds 6a–d was measured. The composite HAT8 + 6a showed the highest conductivity than the CT complexes of nitro positional isomers; similar to the earlier measurements of pure compounds 6a–d. It shows the formation of a strong CT complex and that donor-acceptor interaction is maximum as compared with the other nitro functionalized positional isomers, which are exhibiting similar DC conductivities. Interestingly, low DC conductivity was recorded for HAT8 + 6d mixture than other mixtures, indicating poor charge-transfer characteristics. This is reflected by the minimal redshift in the UV spectra compared to the other isomers. 4. Conclusion Polycyclic 5-phenylnaphtho[1,2,3,4-lmn] phenanthridine and its nitro functionalized positional isomers were synthesized using PictetSpengler reaction. These discotic compounds consist of six peripheral aliphatic alkyl chain nC8H17 in order to get the mesomorphic property. Long-range hexagonal columnar phase was recorded from −20 °C to about 120 °C and no crystalline phase was observed on cooling from isotropic phase. The hexagonal columnar phase obtained from these target compounds were confirmed by XRD measurements. The CT complexes

Table 3 X-ray diffraction analysis of HAT8 with 6a–d composites at the constant temperature 55 °C (the tabulated values are taken from Fig. S5). Target compound

2ϴ degree

d-Spacing (A°): observed (calculated)

Millar indices

Phase (lattice constant)

Alkyl chain length (A°)

Core-core separation (A°)

Inter-columnar space (A°)

HAT8

4.35 7.56 8.66 4.22 7.35 8.50 4.33 7.50 8.66 11.13 4.22 7.27 8.53 11.21 4.25 7.37 8.50 11.21

20.26 11.67 (11.69) 10.19 (10.13) 20.89 12.01 (12.06) 10.38 (1044) 20.38 11.13 (11.76) 10.19 (10.19) 7.93 (7.70) 20.88 12.14 (12.06) 10.35 (10.44) 7.88 (7.89) 20.76 11.97 (11.98) 10.38 (10.38) 7.88 (7.84)

100 110 200 100 110 200 100 110 200 210 100 110 200 210 100 110 200 210

Hexagonal (23.39)

4.40

3.63

23.39

Hexagonal (24.12)

4.42

3.62

24.12

Hexagonal (23.53)

4.42

3.64

23.53

Hexagonal (24.12)

4.41

3.64

24.12

Hexagonal (23.97)

4.41

3.63

23.97

HAT8 + 6a

HAT8 + 6b

HAT8 + 6c

HAT8 + 6d

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A.R. Yuvaraj et al. / Journal of Molecular Liquids 272 (2018) 583–589

Fig. 4. Improved order of DLC system facilitated by CT complex.

0.4

0.35

HAT8+6a HAT8+6b HAT8+6c HAT8+6d

0.30 0.25

0.3

κ m(S/m)

κ m (S/m)

(a)

6a 6b 6c 6d

0.5

0.2

(b)

0.20 0.15 0.10

0.1

0.05

0.0

0.00 30

40

50

60

70

80

30

o

T ( C)

40

50 60 o T ( C)

70

80

Fig. 5. The DC conductivity values of (a) pure target compounds 6a–d; (b) CT complexes; with the function of temperature.

were prepared using the synthesized N-heterocyclic compounds with HAT8. The redshift observed from the UV–Vis spectra were confirmed the formation of CT complexes. The DC conductivity studies were carried out for the pure compounds 6a–d as well as CT complexes. Formation of CT complexes showed variation in the electrical conductivity as compared with pure N-heterocycles 6a–d.

[2]

Acknowledgement We thanks to Ms. K. N. Vasudha, Dr. H. T. Srinivas, Dr. D. Vijayaraghavan and Mr. K. M. Yatheendran from SCM group - Raman Research Institute, for their technical support. We are thankful to Prof. V. Lakshminarayan for his timely suggestions in the evaluation of DC conductivity. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2018.09.120.

[3]

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