Journal of Materials Science: Materials in Electronics https://doi.org/10.1007/s10854-018-9468-3

Synthesis, characterization, thermal, electrical and electrochemical studies of oligo nitrobenzimidazoles and their p–n diode applications Siddeswaran Anand1 · Athianna Muthusamy2 · Nagarajan Kannapiran3 Received: 11 March 2018 / Accepted: 11 June 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2018

Abstract Three novel nitro oligobenzimidazoles, oligo-2-(6-nitro-1H-benzo[d]imidazol-2-yl)phenol (OBINP2), oligo-3-(6-nitro-1Hbenzo[d]imidazol-2-yl)phenol (OBINP3) and oligo-4-(6-nitro-1H-benzo[d]imidazol-2-yl)phenol (OBINP4) were synthesized by oxidative polycondensation of benzimidazole monomers with NaOCl in aqueous alkaline medium. The structure of the monomers and oligomers were confirmed by FT-IR, UV–Vis, 1H and 13C NMR spectroscopic techniques. The monomer BINP2 and its oligomer are showing dual emission through excited state intramolecular proton transfer process. The band gap values of monomers and oligomers were calculated from both UV–Vis spectroscopic and cyclic voltammetric data. Theoretical band gap values of monomers obtained from DFT were compared with experimentally calculated band gap values. The electrical conductivity of I­ 2 doped and undoped oligomers were measured using two point probe technique and are showing good correlation with the charge densities on imidazole nitrogen obtained from Huckel method. The conductivity of oligomers increases with increase in iodine vapour contact time up to 144 h. The variation of dielectric properties of oligomers has been investigated at different frequency and temperature. Among the oligomers, OBINP3 is having greater thermal stability as evidenced by its high carbine residue of around 65% at 600 °C in thermogravimetric analysis.

1 Introduction Polymeric diodes and photovoltaic devices have been focused by many researchers due to their potential applications in electronics as light emitting diodes, field effect transistors and solar cells [1–4]. The research on polymer diodes has been accelerated to achieve highest energy conversion efficiency [5]. Generally, polymeric diodes have been fabricated with either n or p-type Si wafer as it is advantageous over the other fabricated diodes because of their low dark current and extended efficiency [6]. They are Electronic supplementary material  The online version of this article (https​://doi.org/10.1007/s1085​4-018-9468-3) contains supplementary material, which is available to authorized users. * Athianna Muthusamy [email protected] 1



Department of Chemistry, Muthayammal Engineering College, Namakkal, Tamil Nadu 637408, India

2



PG and Research Department of Chemistry, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore, Tamil Nadu 641020, India

3

Department of Chemistry, United Institute of Technology, Coimbatore, Tamil Nadu 641020, India



fully flexible, easily adaptable to geometrical constrains and are extensively applied in many fields due to their low cost, low temperature fabrication process and compatibility with biochemical sensing [7]. These low cost diodes are finding several applications comprising of image sensing, chemical, biological, automation, remote sensing, image processing and position determination [8]. The most interesting advantage of organic polymer diodes is use of anisotropy of optical absorption results from molecular orientation [9]. The combination of transparent electrodes and polarized light detection that results from optical anisotropy of molecules provides unique image sensing devices that capture images under light illumination. Polyheteroaromatic compound are considered to be one of the promising materials for the fabrication of diodes, as they possess interesting electrical, optical, magnetic, chemical and electrochemical properties. Polyheteroaromatic compounds like, polypyrrole, polypyrene, polyaminoanthraquinone, polynaphthalene, polyquinoline and polyphenols have low hole and high electron mobility and acts as an electron donor when compared to inorganic semiconductor materials [10–16]. Among these, polybenzimidazoles (PBIs) are interesting one as its basic nitrogen site act as strong proton acceptors and facilitate proton conduction when doped with

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acid at higher temperature [17, 18]. PBI fibers have high thermal stability, non-flammability, high chemical resistance and are traditionally used in firefighter’s turnout coats and astronaut space suits [19]. Besides, PBIs are used for various purposes, particularly high temperature applications, fluorescence sensor for halide ions, fiber spinning, hollow filaments optoelectronics and photovoltaic applications [20]. The present work is dealing with the synthesis of oligobenzimidazoles (OBIs) by oxidative polycondensation and study on their optical, thermal, electrical properties with a special focus on the CV with different scan rates and diode applications.

2 Experimental 2.1 Materials 4-Nitro-1,2-phenylenediamine, o, m and p-hydroxy benzaldehyde were purchased from Sigma Aldrich. Sodium bisulphite, potassium hydroxide, NaOCl (4%), hydrochloric acid, hydrogen peroxide, sulphuric acid, Ag paste and the solvents used were purchased from Merck chemical company and used as supplied. n-type Si wafer was purchased from Vacutech systems Bangalore, India.

2.2 Synthesis of monomers The benzimidazole BINP2 was synthesized by condensing 4-nitro-1,2-phenylenediamine with o-hydroxybenzaldehyde in DMF solution using sodium bisulphite as a catalyst. The typical route of the synthesis of benzimidazole monomers

Scheme 1  Synthesis of monomers

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Journal of Materials Science: Materials in Electronics

are outlined in Scheme 1 [21]. The brown coloured solid BINP2 formed was filtered, washed with hot water and dichloromethane, and dried in vacuum oven. The other two benzimidazoles, BINP3 and BINP4 were synthesized by adhering the similar procedure. All the benzimidazoles were obtained with good yield (BINP2: 86%, BINP3: 74% and BINP4: 80%). BINP2: 1H NMR (in ppm): 13.23 (s, –NH, –OH), 7.06 (d, Ar–Ha), 7.50 (t, Ar–Hb), 7.17 (t, Ar–Hc), 7.89 (d, Ar–Hd), 8.22 (d, Ar–He), 8.60 (d, Ar–Hg) and 8.27 (s, Ar–Hf). 13C NMR (in ppm): 160.17 (C7), 155.46 (C2), 147.68 (C10), 142.43 (C11), 137.89 (C13), 132.25 (C5), 130.12 (C3), 122.56 (C4), 119.68 (C9), 118.08 (C6), 117.31 (C1), 116.52 (C8), 112.56 (C12). BINP3: 1H NMR (in ppm): 12.85 (s, –NH), 9.79 (s, –OH), 7.09 (d, Ar–Ha), 7.07 (d, Ar–Hb), 7.49 (t, Ar–Hc), 7.88 (d, Ar–Hd), 7.72 (d, Ar–He), 8.53 (d, Ar–Hg) and 8.22 (s, Ar–Hf). 13C NMR (in ppm): 158.05 (C7), 154.75 (C3), 143.42 (C11), 147.89 (C10), 139.82 (C13), 130.44 (C1), 127.88 (C5), 119.26 (C6), 119.03 (C9), 118.32 (C8), 114.76 (C4), 114.25 (C2), 111.33 (C12). BINP4: 1H NMR (in ppm): 12.73 (s, –NH), 10.33 (s, –OH,), 8.13 (d, Ar–Ha & Ha′), 6.97 (d, Ar–Hb & Hb′), 7.76 (d, Ar–Hc), 8.16 (s, Ar–He), 8.43 (d, Ar–Hd). 13C NMR (in ppm): 159.21 (C7), 151.68 (C4), 147.81 (C10), 142.13 (C11) 139.18 (C13), 129.22 (C2 & C6), 117.36 (C9), 116.08 (C3 & C5), 116.04 (C8), 114.39 (C12), 114.29 (C1).

2.3 Synthesis of oligomers The monomer BINP2 was converted in to oligomer, oligo2-(6-nitro-1H-benzo[d]imidazol-2-yl) phenol (OBINP2) by OP using NaOCl as oxidant. 0.009 mol of BINP2 was

Journal of Materials Science: Materials in Electronics

dissolved in equimolar amount of aqueous KOH at 60 °C and 35 ml of NaOCl was added in drop wise with constant stirring [22]. After refluxing for 24 h the reaction mixture was neutralized with 4N HCl and the oligomer formed was filtered, washed with hot water and methanol to remove the salts and unreacted monomers respectively. The other two oligomers were also synthesized by adopting the similar procedure. The oligomers were obtained with good yield. (OBINP2: 76%, OBINP3: 74% and OBINP4:73%). The polymerization reactions along with abbreviations of oligomers are shown in Scheme 2. OBINP2: 1H NMR (in ppm): 10.35 (Terminal –OH), 13.32 (s, 1H, –NH) and 6.62–9.07 (m, Ar–H). 13C NMR (in ppm): 162.28 (C7), 159.42 (C2), 149.12 (C10), 142.88 (C11), 139.81 (C13), 132.70 (C5), 131.86 (C3), 125.99 (C4) 119.50 (C9), 118.83 (C6) 117.26 (C1), 116.87 (C8), 112.63 (C12), 155.53, 152.74 (C–O–C) 130.64, 123.52, 127.46 (C–C). OBINP3: 1H NMR (in ppm): 9.87 (Terminal –OH), 13.25 (s, –NH) and 6.49–8.62 (m, Ar–H). 13C NMR (in ppm): 162.30 (C7), 157.87 (C3), 148.65 (C10), 142.66 (C11), 139.12 (C13), 130.23 (C1), 129.88 (C5), 118.03 (C6), 117.68 (C9), 116.68 (C8), 114.82 (C4), 111.72 (C12), 156.12, 152.11 (C–O–C), 128.16, 122.54, 113.71, 112.78 (C–C). OBINP4: 1H NMR (in ppm): 10.20 (Terminal –OH), 13.35 (s, –NH) and 6.80–8.75 (m, Ar–H). 13C NMR (in ppm): 160.13 (C7), 159.21 (C4), 149.18 (C10), 133.12 (C11) 139.28 (C13), 128.54 (C2 & C6), 119.84 (C9), 117.63 (C3 & C5), 116.68 (C8), 114.39 (C12), 115.70 (C1), 151.88 (C–O–C), 128.84, 130.15, 122.12 (C–C).

2.4 Diode fabrication The p–n diode was made by coating the p-type oligomer over n-type Si wafer. The heavy metal oxide present on Si wafer

was removed by immersing into 2:1 mixture of ­H2O2 and con.H2SO4 for 10 min and washed with distilled water thoroughly. The native oxide was removed using HF/H2O (1:10) mixture and rinsed with distilled water for 10 min [23]. The oligomer solution was prepared by dissolving it in DMF with constant stirring for 6 h and was coated on a cleaned n-type Si wafer using DELTA spinner with 2000 rpm for 30 s. The dark brown coloured thin film formed was dried at 100 °C for 1 h. The ohmic contact with the film was created by applying silver paste on the rear surface of Si wafer and dried at room temperature.

2.5 Instrumentation The infrared and UV–Vis spectra of monomers and oligomers were recorded with Perkin Elmer FT-IR 8000 and Systronics double beam UV–Vis 2202 spectrophotometer respectively. The 1H and 13C NMR spectra of the compounds in DMSO-d6 were recorded using Bruker AV400 MHZ spectrometer. The molecular weight of oligobenzimidazole was determined by Gel permeation chromatography (GPC) using polystyrene standard and eluted in THF at a flow rate of 1.0 ml/min at 25C on a Waters HPLC model (GPC, water 600 HPLC) fitted with water 2414 Refractive index detector and Styragel HR5E 4E 2/0.5. The fluorescence spectra of monomers and oligomers were recorded on a JobinYvon Horiba Fluoromax-3 spectrofluorometer in DMSO solution. The quantum theoretical calculations using DFT, at B3LYP/6-31(d, p) basis in the Gaussian 09 package were carried out for monomers. Cyclic voltammetry (CV) measurements of monomers and oligomers were done in DMSO using CH instrument 608D electrochemical analyser at a scan rate of 0.2 V/s. Platinum, glassy carbon, Ag/AgCl and tetrabutyl ammonium hexafluoro phosphate were used as

Scheme 2  Synthesis of oligomers

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Journal of Materials Science: Materials in Electronics

counter, working, reference electrode and supporting electrolyte respectively. The electrical conductivity of iodine doped and undoped oligomers were measured with two point probe technique with help of Keithley 2400 electrometer in the range of 1–10 V. The oligomer pellets were exposed with iodine vapour in a desiccator at ambient temperature and pressure. The dielectric studies of oligomers were carried out at different temperatures in the frequency range of 50 Hz to 5 MHz using Hioki 3532-50 LCR meter. TGA measurements were made in HITACHI 7300PC thermal analyser between 30 and 500 °C (in ­N2; rate, 10 °C/min).

3 Results and discussion 3.1 Solubility The solubility of monomers and oligomers was tested in a series of solvents by using 0.1 mg of the sample with 1 ml solvent. They are soluble in methanol, THF, DMSO, DMF, DMAC and insoluble in acetonitrile, n-hexane, acetone, ethyl acetate, ­CHCl3 and ­CCl4. The lowering of solubility of oligomers from monomers is due to the polyconjugated structure with broad chain dispersity of oligomers [24]. The number average molecular weight (Mn), weight average molecular weight (Mw) and polydispersity index (PDI) of OBINP2, OBINP3 and OBINP4 were calculated from polystyrene standard calibration curve. The Mn, Mw and PDI values are found to be 3950, 4621 g/mol and 1.16 for OBINP2, 3942, 4545 g/mol and 1.15 for OBINP3, 3742, 4145 g/mol and 1.10 for OBINP4 respectively. The PDI values show that the oligomers are mono dispersed [25].

3.2 Spectral characterization The FT-IR spectra of monomers and oligomers are shown in SF. 1 and the spectral data are given in Table 1. The structure of the monomers were confirmed with the appearance of –C=N– and –NH– bands with simultaneous disappearance of –NH2 and –CHO bands of the reactants. The monomers and oligomers have shown the characteristic –NH– imidazole bands between 3332 and 3182 cm−1 and Table 1  FT-IR spectral data of monomers and oligomers

Compound

BINP2 BINP3 BINP4 OBINP2 OBINP3 OBINP4

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the characteristic bands of –C=N– stretching are appearing between 1604 and 1686 cm−1. The monomer BNIP2 has ortho –OH group to imidazole ring, forming both inter and intramolecular hydrogen bonding and is appearing as a broad band at 3414 cm−1. Whereas, meta (BINP3) and para (BINP4) –OH containing monomers are forming only intermolecular hydrogen and their –OH stretching vibrations appear as sharp band at 3380 and 3346 cm−1 respectively [26]. The bands appearing in the range of 1221–1248 cm−1 are owing to the –C–O stretching vibrations of monomers. In the spectra of OBIs, the bands appearing in the range of 3414–3428 cm−1 are attributed to the terminal –OH groups. The bands appearing in the range of 1150–1180 cm−1 are due to the C–O–C stretching vibrations of oligomers [27]. Besides, the band at 841 cm−1 in oligomers are due to the C–C stretching vibrations of the phenylene units (C–C linkage) [28]. These spectral results confirm that the polycondensation has taken place through both C–C and C–O–C couplings. The aromatic –C=C– stretching vibrations of monomers and oligomers are appearing around 1490 cm−1. The bands around 3080 cm−1 are attributed to aromatic –CH stretching vibrations of monomers and oligomers. The bands in the range of 1344–1473 cm−1 are attributed to –NO2 symmetry stretching of monomers and oligomers. 1 3.3  H and 13C NMR spectral Analysis

The 1H and 13C NMR spectra of BINP2 and OBINP2 were recorded in DMSO-d6 and are shown in Figs. 1 and 2. 1H and 13C NMR spectral values of monomers and oligomers are given in synthetic part. The chemical shift values of phenolic and –NH– protons of BINP3 and BINP4 are 9.79, 10.33 and 12.73, 12.85 ppm respectively. But in the case of BINP2, these protons are overlapped and appearing as a single broad hump at 13.23 ppm. In BINP2, the phenolic proton at ortho position is strongly deshielded through strong intramolecular hydrogen bonding with imine nitrogen and thereby it is resonating in the down field region than BINP3 and BINP4 containing –OH groups at meta and para positions respectively. Further the –NH– proton is also deshielded to certain extent due to intramolecular hydrogen

Wave number (­ cm−1) –OH

–C=N–

–NH

–C=C–

C–O–C

C–O

Ar–CH

–NO2

3414 3380 3346 3414 3408 3428

1604 1651 1686 1611 1631 1618

3298 3189 3210 3237 3332 3182

1474 1515 1454 1494 1481 1494

– – – 1153 1150 1180

1242 1228 1262 1235 1228 1221

3059 3059 3066 3073 3080 3073

1330 1324 1330 1334 1337 1330

Journal of Materials Science: Materials in Electronics

Fig. 2  13C NMR spectra of BINP2 and OBINP2

Fig. 1  1H NMR spectra of BINP2 and OBINP2

bonding with phenolic group and resonates in the down field region when compared with monomers (BINP3 and BINP4) in which –NH– is not involved in intramolecular hydrogen bonding [26]. The aromatic protons of monomers are resonating in the range of 6.97–8.60 ppm. The –OH and –NH peaks of monomer BINP2 are overlapped due to inter and intramolecular hydrogen bonding, but in the oligomer OBINP2 they are appearing in their own chemical shift value as most of the –OH groups are involved in OP and are not available to form hydrogen bonding –NH moiety. The –NH protons of oligomers are appearing as broad peak in the range of 13.25–13.35 ppm. In oligomers, the imidazole –NH proton is appearing in the down field region when compared to monomer, due to non covalent interaction in the oligomer chain [29]. The terminal –OH groups of oligomers are not expected to be in compatible orientation to form hydrogen bonding with imine and thereby

the protons are not deshielded and appearing in the range of 9.87–10.35 ppm. The aromatic protons of oligomers are resonating in the down field region (6.49–9.07 ppm) than the monomer, confirming that the polycondensation lead to C–C coupling involves the removal of ortho and para hydrogen to phenolic group [30]. The terminal –OH bearing carbon of oligomers are resonating at 159.42, 157.87 and 159.21 ppm for OBINP2, OBINP3 and OBINP4 respectively. The chemical shift values of carbons ortho and para to –OH groups are slightly differing from monomers as they are involved in C–C coupling. These carbons are resonating at 132.70 (C5) for OBINP2, 118.03 (C6) for OBINP3 and 117.63 (C3 & C5) for OBINP4. This confirms that the phenolic –OH, ortho and para carbons have taken part in the OP reaction via C–O–C and C–C coupling [30]. Besides, 13C NMR spectra of oligomers are showing some additional peaks around 155.53, 125.32, and 114.62 ppm indicating the change in chemical environment of aromatic carbon atoms. The peaks in the range of 156.12–151.88 ppm and 130.64–112.78 ppm

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Journal of Materials Science: Materials in Electronics

corresponding to C–O–C and C–C respectively are confirming the two types of couplings [31].

3.4 Optical properties UV–Vis spectra of monomers and oligomers were recorded in DMSO solution and are shown in SF. 2. The aromatic π → π* and imidazole n → π* transitions of monomers are appearing at 210, 260 and 326 nm respectively. The absorption bands corresponding to azole (–NH–) chromophore of monomers and oligomers are appearing around 320 and 330 nm respectively. The π → π* and n → π* transitions of imidazole and aromatic moiety of oligomers are appearing around 210 and 260 nm respectively. Oligomers are showing absorption tail in the region 400–500 nm, due to the increases in conjugation length. The coexistence of both long and short effective conjugation systems generally lead to large absorption band [32]. The optical band gap values (Eg) of monomers and oligomers were calculated from Eg = 1242/λonset [33]. The value of λonset can be obtained from the intersection of two tangents on the absorption edges. The band gap values of monomers are greater than the oligomers, due to the decrease in energy between HOMO and LUMO [34]. The optical band gap of monomers and oligomers were calculated and are given in Table 2. The fluorescence spectra of monomers and oligomers were recorded with 100 mg/L DMSO solution (Fig. 3) and the data are given in Table 2. The monomer BINP2 is showing dual emission, whereas BINP3 and BINP4 are showing only single emission. This can be explained using the phenomena of excited state intramolecular proton transfer process (ESIPT). The monomer BINP2 forms two rotamers (I & II) through intramolecular hydrogen bonding of phenolic group with imine and imidazole nitrogen. The –OH group of rotamer I Table 2  Optical data of monomers and oligomers

Compound

e f

λ Exd

λEme

IEmf

Δλg

212, 268, 342 210, 269, 339 211, 260, 331 211, 266, 326 206, 257, 322 204, 287, 330

413 423 417 428 436 444

3.00 2.93 2.97 2.90 2.84 2.79

340 339 340 340 330 330

360, 469 362, 481 434 461 392 414

438 248 413 467 150 520

129 142 94 121 62 84

 Intersection of two tangents  Excitation

 Emission maximum

 Maximum emission intensity

g

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 Stoke’s shift

Emission Eg (eV)

 Band gap

d

(3)

Eg = ELUMO − EHOMO

λonset

 Absorption maximum

c

The cyclic voltammetric analysis of monomers and their oligomers were recorded in DMSO with the scan rate of 20 mV/s in the potential range of 2 to − 2V. The cyclic voltammograms are shown Fig. 4 and their electrochemical data are given in Table 3. The HOMO, LUMO energies and band gap values are calculated by using following equations [39]. ( ) EHOMO = − 4.39 + Eox (1) ( ) ELUMO = − 4.39 + Ered (2)

λmax (nm)

a

b

3.5 Electrochemical properties

Absorption a

BINP2 OBINP2 BINP3 OBINP3 BINP4 OBINP4

undergo ESIPT to imine nitrogen and producing a photo tautomer (shown in Scheme 3). Rotamers (I & II) are giving an emission at lower wavelength and the photo tautomer formed through ESIPT emits at higher wavelength. The oligomer OBINP2 is also showing dual emission at 362, 481 nm as the terminal –OH groups are involved in ESIPT [35]. The emission of oligomers are slightly red shifted compared to that of their monomers, this may be attributed to the interchain interaction in oligomers and also increased number of imidazole units [36]. The oligomers OBINP3 (437 nm) and OBINP4 (414 nm) have exhibited only single emission. The fluorescence spectra of oligomers are slightly red shifted and are having higher Stoke’s shift values when compared to that of their monomers due to the extended conjugation of oligomers via C–C and C–O–C coupling [37]. The Stoke’s shift values of monomer BINP2 is greater than that of BINP3 and BINP4 since they are involved in ESIPT [38]. The greater Stoke’s shift value provides very low back ground signals and allows usage of these materials for the fabrication of fluorescence sensor.

b

c

Journal of Materials Science: Materials in Electronics

Fig. 3  Fluorescence emission spectra of monomers and oligomers

Scheme 3  Interconversion of the rotamers and phototautomer

The electrochemical energy gaps (Eg), HOMO and LUMO energy levels of monomers and oligomers were calculated from the oxidation and reduction onset values. The CV of oligomers have shown each two oxidation and reduction peaks. The two oxidation peaks in the anodic region at 0.747 and 1.51 V are corresponding to the oxidation of imidazole –NH– and free –OH groups to form the polaron structure [40]. The two reduction peaks in the cathodic region at − 0.741 and − 1.225 V

are corresponding to the reduction of keto groups in to hydroxyl and neutralization of the oxidized states of the oligomers respectively. The band gap value of OBNIP3 (2.68 eV) is less than the other two oligomers due to the electronic transition between the HOMO and LUMO energy levels and grater electron conducting nature of OBINP3. The electrochemical band gap values are greater than the optical band gap due to the wide distribution of size in oligomers [41].

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Fig. 4  Cyclic voltammograms of the monomers (a) and oligomers (b)

Table 3  Band gap values of monomers and oligomers

Compound

BINP2 OBINP2 BINP3 OBINP3 BINP4 OBINP4

UV–Vis

CV a

DFT

λonset

Eg (ev)

HOMO

LUMO

Eg (ev)

413 423 417 428 436 444

3.00 2.93 2.97 2.90 2.84 2.79

6.22 6.28 6.17 6.25 6.16 6.22

3.14 3.51 3.07 3.56 3.26 3.51

3.08 2.76 3.09 2.68 2.89 2.71

b

HOMO

LUMO

Eg (ev)c

5.81 – 6.13 – 5.85 –

2.61 – 2.46 – 2.35 –

3.2 – 3.5 – 3.6 –

HOMO highest occupied molecular orbital, LUMO lowest unoccupied molecular orbital

a

 Optical band gap

b c

 Electrochemical band gap

 Theoretical band gap

The electrochemical band gap values of oligomers calculated from cyclic voltammetry are in the order: OBINP3 > OBINP4 > OBINP2. These value are differing from their optical band gap values (OBINP4 > OBINP2 > OBINP3). This variation may be due to the difference in calculation method [42]. Further the theoretical band gap values obtained from DFT calculations (SF. 3) using Gaussian 09 package in vacuum phase are greater than the optical and electrochemical band gap values.

3.6 Conductivity measurements The electrical conductivity of OBIs was measured using two probes connected with Keithley 2400 electrometer (1–10 V) by exposing the oligomer pellet with ­I2 vapour in a desiccator at room temperature. The conductivity was measured for every 24 h for a period of 168 h. Among the two types of nitrogen atoms of OBIs, the I­ 2 gets doped with more basic imidazole nitrogen (–NH) and not with imine nitrogen (–C=N). The p-type oxidative dopant ­I2 forms

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charge transfer complex (CT) with imidazole nitrogen and aromatic π-electron of oligomer [40] and there by conductivity increases with increase in ­I2 vapour exposure time and is shown in Fig. 5. The conductivity first increases steeply with doping time and after a period it tends to level off. On doping with iodine for 168  h the conductivities of the oligomers OBINP3, OBINP4 and OBINP2 are increased about ­104, ­104 and ­103 S/cm order of magnitude respectively, when compared with the undoped oligomer. The increase in conductivity due to ­I2 doping and doping mechanism on oligomers has been previously proposed by Diaz et al. [43]. The maximal conductivity values of oligomers, OBINP3, OBINP4 and OBINP2 are 1.157 × 10−7, 3.01 × 10−7 and 8.00 × 10−8 S/cm respectively. The greater value of conductivity of OBINP3 is due to its high electron density on imidazole nitrogen. The trend in the conductivity of oligomers is compared with the charge density on imidazole nitrogen of monomers (SF. 4) calculated by the Huckel’s method. The order of charge density is BINP3 (0.375) > BINP4

Journal of Materials Science: Materials in Electronics

Fig. 5  Electrical conductivity of iodine doped oligomers versus doping time

(0.363) > BINP2 (0.361) and, the conductivity of their oligomers vary in the same order. The increase in conductivity of oligomers on ­I2 doping suggests that these oligomers are suitable candidates for gas sensor, optoelectronic and photovoltaic applications [44].

3.7 Dielectric properties The frequency and temperature dependent dielectric properties of oligomers in pellet form were studied at different temperature in the frequency range of 50 Hz to 5 MHz using Hioki 3532–50 LCR meter. The variation of dielectric constant and loss are shown in Fig. 6. Dielectric constants of oligomers were calculated using the Eq. (4) [45].

𝜀r =

Cd 𝜀0 A

(4)

where C is the capacitance and d is the pellet thickness, ε0 is the free space permittivity of the pellet and A is the cross sectional area of the pellet. The dielectric constant of oligomers increases with decrease in frequency and reached a constant value after certain frequency. At low frequency the dipoles of oligomers have sufficient time to align with the field earlier to the field direction modifies and recorded high dielectric constant [46]. Whereas, at high frequency the dipoles of oligomers do not have enough time to align with the field earlier to the field direction modifies and thereby dielectric constant is less. The dielectric constant is very at low frequency at 393 K as the intermolecular forces between polymer chains are less and enhanced thermal agitation of polymer chains. At low temperature the polymer chain segmental motions are frozen and thereby dielectric constant decreased. The dielectric constants of oligomers are in the

order OBINP4 (6925) > OBINP3 (1464) > OBINP2 (90) at 393 K (50 Hz). The difference in dielectric constant of oligomers can also be verified with help of difference in dipole moment of monomers calculated theoretically using DFT. The values of dipole moment decrease from para (7.1908D) to meta (5.0583D) and to ortho (4.5110D) and is showing direct correlation with dielectric constant of oligomers. The oligomer, OBINP4 recorded greater dielectric constant can be used to make passive component like resistors, capacitors etc [47]. At low temperature the dielectric constant of oligomers is decreased as the space charge polarization is less. Whereas, dielectric constant of oligomers are increased at high temperature at lower frequency, due to the predominance of space charge polarization [48, 49]. The difference in dielectric loss at different temperatures with increase in frequency is shown in Fig. 6b, d, f. The dielectric loss of oligomers is due to three distinct effects such as, space charge polarization DC resistivity and orientation polarization. The high value of dielectric loss at low frequencies and low at high frequencies are mainly due to DC conduction and space charge polarization [50]. In the high frequency region the low dielectric loss obtained suggested that the oligomers can be used for capacitors, radiation detectors, resistors, thermionic valves and electrooptical device applications [51].

3.8 I–V characteristics I–V characteristics of the diodes OBINP2/n-Si, OBINP3/nSi and OBINP4/n-Si were carried out with Keithley electrometer 6517B voltage/current source. The forward and reverse bias I–V characteristics of the diodes at room temperature are given in Fig. 7a. The I–V characteristics of diodes show good rectifying behaviour with forward to reverse current ratio ~ 2 in the range of − 4 to + 4 V. The turn on voltage of the p–n junction is found to be ~ 0.82 V. The turn on voltage, obtained from the diffusion potential, would correspond to a potential barrier such that carrier has to conquer in order to supply to forward current. OBINP3/n-Si diode shows slightly higher rectifying nature than the other two diodes, due to its low band gap resulting in more efficient electron delocalization through the oligomer backbone. The low turn on voltage of the diodes suggests that there is an easy injection of holes and electrons from the respective electrodes and their transport across the layers to cause increase forward current. The current voltage characteristics of the diodes can be described by thermionic emission theory [52, 53]. ( qv ) I = I0 exp −1 (5) nKT

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Journal of Materials Science: Materials in Electronics

Fig. 6  Dielectric constant (a, c, e) and dielectric loss (b, d, f) of oligomers OBINP2, OBINP3 and OBINP4 with temperature

where ­Io is the reverse bias saturation current, q is the charge of electron, V is the applied voltage, n is the ideality factor, K is the Boltzmann constant and T is the temperature. The diode ideality factor and reverse bias saturation current were determined from the slope and intercept of the semilogarithmic forward bias of I–V plot. The n and ɸb values are

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obtained from the Eqs. (6) and (7) by substituting the values from Eq. (5) [54, 55]. ) ( q )( dv n= (6) KT d(lnI) where ­A* the effective Richardson constant.

Journal of Materials Science: Materials in Electronics

Fig. 7  I–V and semi logarithmic plot of oligomeric p–n diode

Table 4  Diode parameters S. no.

Parameter

OBINP2

OBINP3

OBINP4

1 2 3

Current Ideality factor Barrier height

2.44 × 10−8 7.18 0.699

8.51 × 10−10 8.04 0.696

2.54 × 10−8 10.06 0.690

𝜙b =

(

KT q

)

ln

(

A∗ T 2 I0

)

(7)

The absolute information about the mechanism of transport through oligomeric thin film can be obtained from the analysis of forward current voltage. The n, ɸ b and saturation current of the diodes were obtained from the slope of the linear forward current region and their values given in Table 4. From the values of n we deduce that the diodes represent a metal interfacial layer semiconductor configuration rather than an ideal diode. The n values of the diodes are greater than the unity, due to the interface states in the oligomer Si interface. The ɸb values of the diodes are 0.699, 0.696 and 0.690 eV for OBINP2/n-Si, OBINP3/n-Si, and OBINP4/n-Si respectively. The R ­ s is subjective by the presence of border coating between the n-Si wafer and oligomer leads to the non ideal forward bias current–voltage (I–V). In general, the forward bias I–V characteristics are linear in the semi logarithmic (Fig. 7b) scale at low voltages but deviate considerably from linearity owing to series resistance when applied voltage is high. The barrier height becomes smaller as the ideality factors increases. This result may be ascribed to cross nonhomogeneities of the obstacle in the diode [56]. The device shows a photovoltaic behavior and it can be operated as photodiode and OLED.

Fig. 8  TG traces of oligomers

Table 5  TGA values of oligomers Oligomer

OBINP2 OBINP3 OBINP4

Ton

156 181 183

% Weight loss temperature in (°C) 10

30

50

65 167 79

200 322 274

315 550 451

% Char residue 600 °C 57 68 64

3.9 Thermal properties The thermal stability of the oligomers was studied using thermo gravimetric analysis in nitrogen atmosphere and the thermograms are given in Fig.  8. The temperatures

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corresponding to 10, 30 and 50% weight loss and the char residue are given in Table 5. The initial weight loss up to 100 °C is due to the presence of occluded moisture [57]. On comparing initial degradation temperatures, the order thermal stability of oligomer is: OBINP4 > OBINP3 > OBINP2. These results suggest the oligomers have good thermal stability at 600 °C is due to greater conjugation and higher aromatic content in oligomer skeleton. The high carbon residue indicates that the polycondensation has proceeded mainly through C–C coupling than the thermally weaker C–O–C coupling [58].

4 Conclusion The structure of nitrobenzimidazoles and their oligomers synthesized by oxidative polycondensation were confirmed by UV–Vis, FT-IR, NMR spectral analyses. The results indicate that the OP has taken place through C–C and C–O–C couplings. The monomers are having higher band gaps than oligomers due to increase in polyconjugation and it can be used for photovoltaic applications. The monomer BINP2 and its oligomer are showing a dual emission, due to the ESIPT. The oligomers are showing grater Stoke’s shift values than their corresponding monomers and they provide very low back ground signals and allow the usage of these materials for the fabrication of fluorescence sensor. The oligomers are having electro-active nature and the band gap values are greater than the optical band gap due to the wide size distribution of oligomers. The electrical conductivity of oligomer BINP3 has increased about ­104 times greater than the undoped oligomer. The oligomers have char residue of 60% at 600 °C indicating that they are thermally stable due to the presence of long conjugated backbone. The dielectric constant and loss of OBIs are increases with increase in temperature.

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