Optoelectrical, morphological and mechanical features of nitrophenyl supported poly(1,3,4-oxadiazole)s and their nanocomposites with TiO₂

Abstract

Two nitro phenyl supported poly(1,3,4-oxadiazole)s viz.; poly(pyridine(2-nitrophenyl)-1,3,4-oxadiazole) [PPNO] and poly(2-(o-nitrophenyl)-5-phenyl-1,3,4-oxadiazole) [PNPPO] were initially synthesized by a dehydrocyclization reaction, and then reinforced with TiO₂ nanoparticles. They were characterized in terms of optoelectrical, morphological and mechanical properties in order to examine their suitability to function as active or electron transport layers in polymer light emitting diodes (PLEDs). The nanocomposites have been found to possess charge transfer characteristics from the macromolecular systems (PPNO or PNPPO) to TiO₂ particles, upon photo-excitation. Cyclic voltammetric examinations indicated a reduction in band gap for PPNO and PNPPO in the presence of TiO₂ nanoparticles. The nonlinear optical responses and optical limiting behavior of PPNO, PNPPO and their nanocomposites were also evaluated by the Z-scan technique, using nanosecond Nd:YAG, 532 nm laser radiations. In view of user-friendly device fabrication, flexible films of PPNO and PNPPO were fabricated by blending the composites with 2 wt% of poly(methylmethacrylate). Along with good mechanical properties, PPNO and PNPPO showed high fluorescent quantum yield, presenting them as promising candidates for PLED fabrication. Interestingly, the current–voltage (I–V) characteristics propose their additional application as semiconducting packaging materials for optoelectronic devices.

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1 Introduction

π-Conjugated materials are of great importance in the quickly growing field of organic electronics since they can function as active layers in light emitting diodes, photovoltaic cells, photo detectors, sensors and other devices. The reinforcement of such systems with compatible nanoparticles can lead to the generation of novel composite materials with enhanced optical, electronic, gas barrier, mechanical, and magnetic properties.^1–3 They have been found to be promising candidates for solar cells, light emitting diodes, field effect transistors etc.^4–7 Meng et al. prepared graphene oxide/poly(3-hexylthiophene) [GO/P3HT] composites via a silylation reaction of –OH groups and amidation reaction of carboxylic acid groups on the surface of GO sheets for application in optoelectronic devices.⁸ Lu et al. developed a film based on poly(diphenylacetylene)/aligned multi-walled carbon nanotube (MWCNT) composite on an isotropic poly(dimethylsiloxane) substrate that exhibited anisotropic solvent-driven bending/unbending actuation.⁹ Fan and Xu fabricated carbon nano tube composites with poly aniline, polypyrrole and polythiophene (CN/PANI or PPy or PTh).¹⁰ The conductivity of such doped composites were found to be 4 orders of magnitude compared to that of the respective undoped systems. Fratoddi et al. prepared novel hybrid systems based on gold nanoclusters and Pd containing conjugated polymers derived from a monosubstituted acetylene, 3-dimethylamino-1-propyne (DMPA), for sensing applications.¹¹ Kang et al. performed the electrochemical sensing of glucose by the use of glucose oxidase (GOD)–graphene–chitosan nanocomposites.¹²

The advantage of the direct employment of inorganic, nanosized crystalline particles, into semiconducting polymer systems, lies in the enhancement of mechanical, thermal and optoelectronic features of the base polymers by a relatively easy route.^13,14 Titanium dioxide (TiO₂) is a semiconducting and responsive inorganic filler with a variety of applications in diverse areas such as self cleaning,¹⁵ anticorrosion,¹⁶ coatings,¹⁷ and solar cells.¹⁸ For optoelectronic applications, it can function as an electron acceptor having a compatible band gap with many π-conjugated systems.¹⁹ In addition, the favorable mechanical strength, thermal stability, and high surface area propose TiO₂ as an ideal partner for many polymeric materials.

Excellent reports on polymer–titania nanocomposites for various device applications ranging from solar cells to environmental sensing,²⁰ biomedical diagnosis,²¹ and energy storage²² are available in literature. Zhang et al. synthesized a poly(methylmethacrylate) (PMMA)–titania nanohybrid using a chelating ligand as coupling agent.²³ Dinari and Asadi prepared triethoxy silane-terminated polyimide–titania nanocomposites (PI/TiO₂ NCs) by the incorporation of TiO₂ nanoparticles into silane terminated PI. The thermal, mechanical and optical transport properties of the nanocomposites have been found to be significantly improved upon comparison with pure PI.²⁴ Youssef prepared polyaniline/TiO₂ nanocomposites via an in situ emulsion polymerization for conductive packaging applications.²⁵ Gao and co-workers developed a material for supercapacitors based on polypyrrole–titanium dioxide brush like nanocomposites through an electrochemical route.²⁶ The energy and charge transfer mechanisms at the polymer–inorganic filler interface for many composites are still under active investigation and the impact of the interfacial charge transfer on the optoelectronic device performance is not well elucidated so far.

In this work, we report the synthesis and characterization of two macromolecules viz.; poly(pyridine(2-nitrophenyl)-1,3,4-oxadiazole) [PPNO] and poly(2-(o-nitrophenyl)-5-phenyl-1,3,4-oxadiazole) [PNPPO] and their novel composites with TiO₂ for optoelectronic applications. The morphological, optoelectrical, and mechanical features of the base systems have been examined. Special attention has been given to examine the influence of TiO₂ nanoparticles on the emission features of PPNO and PNPPO. The nonlinear optical responses of the base materials and the composites have also been investigated.

2 Experimental

2.1 Materials

All chemicals used in this work were of analytical grade and were procured from Sigma Aldrich Bangalore, India.

2.2 Instrumentation

FT-IR spectra were received from a Perkins-Elmer Fourier-Transform Infrared Spectrometer (FT-IR), after pelletization with potassium bromide. The molecular weights of the polymers were examined at 27 °C by high performance gel permeation chromatography (GPC; Shimadzu, Japan) with an injection volume of 2 μl and a flow rate of 1 ml min⁻¹. The eluent used was DMF. In view of the easiness in product fabrication, the flexible film forming ability of PPNO and PNPPO was confirmed by mixing them with appropriate quantities of poly(methylmethacrylate) in DMF solvent, followed by casting. Powder XRD examination was performed on a Rigaku Miniflex X-ray diffractometer with a CuKα radiation source (λ = 1.5418 Å). A scanning electron microscope (SEM), LEO 1550 VP using silicon tips with an accelerating voltage of 15 000 volt was employed for studying the morphological features of the polymer films and nanocomposites. Atomic force microscopic (AFM) examination was carried out at room temperature using contact mode Park Systems XE-100, South Korea. The UV-Vis spectra were obtained from a Perkin-Elmer 3100 Spectrophotometer. Fluorescence spectra were taken using a Perkin-Elmer LS 45 spectrofluorometer. Each fluorescence spectrum was obtained at an excitation wavelength λ_{abs. max} of the corresponding sample. The Z-scan measurements for nonlinear responses were done using a standard open aperture Z-scan technique developed by Sheik-Bahae and co-workers.²⁷ A Q-switched, frequency doubled Nd:YAG laser with 10 Hz repetition rate for excitation was employed for this purpose. The laser pulses possessed a pulse width of 7 ns at 532 nm wavelength. Cyclic voltammetric (CV) examinations were performed on a CHI600D electrochemical work station with a platinum working electrode and a Pt wire counter electrode at a scan rate of 50 mV s⁻¹ against an Ag/Ag⁺(AgNO₃) reference electrode. The current–voltage characteristics of the samples were measured using an Agilent B2902A precision source measure unit. The tensile strength measurements were carried out using a universal testing machine (Shimadzu Autograph AG-X plus 10 KN, Singapore), as per ASTM D638 standard. The samples in dumb bell shape (gauge length = 2.5 mm and width = 1 mm) were drawn with a rate of 1 mm min⁻¹ at room temperature.

3 Results and discussion

3.1 Synthesis and characterization

The synthesis strategies towards PPNO and PNPPO are shown in Scheme 1.

Scheme 1 Synthesis strategies for PPNO and PNPPO.

3.1.1 Synthesis of poly(pyridine(2-nitrophenyl)-1,3,4-oxadiazole) [PPNO]. The monomer 2-nitroterephthalodihydrazide was initially prepared by the reaction between 2-nitroterephthalic acid and excess hydrazine monohydrate in methanol medium. PPNO was synthesized through a one step method by the reaction between 2-nitroterephthalo hydrazide and 2,6-pyridine dicarboxilic acid in phosphoric acid medium. Phosphoric acid was employed as a polymerization solvent as well as a dehydrating medium. In a typical procedure, 2-nitroterephthalohydazide (0.349 g, 0.00326 mole) and 2,6-pyridine dicarboxilic acid (0.5 g, 0.00326 mol) were added to phosphoric acid, in a 100 ml, three necked flask. The solution was heated to 90 °C, while stirring, for one hour and then at 150 °C for 12 hours in nitrogen atmosphere. The reddish brown colored viscous gum formed was poured into cold water. It was neutralized with 5% Na₂CO₃ solution, filtered off, washed with water and methanol several times and dried at 80 °C in vacuo. The yield of PPNO, so formed, as a dark grey solid, is 65%. M_n = 48 672 g mol⁻¹, M_w = 55 843 g mol⁻¹, PDI = 1.23.

3.1.2 Synthesis of poly(2-(o-nitrophenyl)-5-phenyl-1,3,4-oxadiazole) [PNPPO]. PNPPO was synthesized by a one step method by the reaction between 2-nitroterephthalic acid and terephthalodihydrazide in phosphoric acid medium. The dehydrocyclisation reaction with the same molar ratio of monomers, as in the case of PPNO, was followed to synthesize PNPPO which appeared as a dark brown powder with 69% yield. M_n = 67 622 g mol⁻¹, M_w = 71 277 g mol⁻¹, PDI = 1.05.

3.2 FT-IR spectroscopic analysis

In the FT-IR spectra, given in Fig. 1(a) and (b) for PPNO and PNPPO, the N–O asymmetric and symmetric bands appear at 1432–1467 cm⁻¹ and 1356–1255 cm⁻¹ regions respectively. The peak at 3085 cm⁻¹ can be attributed to the aromatic C–H stretching for both the polymers. In the FT-IR spectra of the monomers, the broad peak around 3400 cm⁻¹ region has been assigned to intermolecular or intramolecular hydrogen bonding (ESI†). Interestingly, the O–H stretching frequency due to hydrogen bonding disappears in the spectra of polymers. This has been taken as a strong evidence for the formation of the expected macromolecular structures. In the FT-IR spectra of the polymers, the absorption at 1686 cm⁻¹ and 1683 cm⁻¹ have been assigned to conjugated C=C stretching frequency for PPNO and PNPPO, respectively. The strong peaks around 1594 cm⁻¹ for PPNO and around 1536 cm⁻¹ for PNPPO indicate the C=N stretching vibrations. The strong and low intensity peaks at 1077 cm⁻¹ and 990 cm⁻¹ for PPNO and 1112 cm⁻¹ and 938 cm⁻¹ for PNPPO confirm the formation of 1,3,4-oxadiazole units in them.

Fig. 1 FT-IR spectra of: (a) PPNO and (b) PNPPO (c) TiO₂, PPNO/PMMA, PPNO/PMMA/TiO₂ (d) TiO₂, PNPPO/PMMA, PNPPO/PMMA/TiO₂.

3.3 Nanocomposite formation

Initially, TiO₂ was converted into TiO₂ nanoparticles (TiO₂ nanoparticles) using a reported procedure.²⁸ Subsequently, TiO₂ nanoparticles were mixed with appropriate quantities of PPNO or PNPPO under ultrasonication in dimethyl formamide for 30 min. Nanocomposite films based on PPNO and PNPPO were fabricated by slow solvent evaporation on glass substrates.

For independent thin flexible film fabrication, a blending strategy was explored using poly(methylmethacrylate) [PMMA]. A fixed concentration of PMMA (0.2 g/100 ml) in DMF has been found to be ideal to prepare polymer nanocomposites films with an average thickness of 55 μm. A representation of the formation of TiO₂ NPs, PPNO and PNPPO films and their composite films is shown in Scheme 2(a) and (b). The IR spectra of TiO₂ NPs, PPNO/PMMA/TiO₂ and PNPPO/PMMA/TiO₂ are shown in Fig. 1(c) and (d). All the characteristic peaks are found to be retained with considerable intensity both in the blended and composite forms, clearly indicating the absence of any chemical interactions amongst them. In other words, the individual components retain their identity in all the compounded forms.

Scheme 2 (a) Synthesis route of TiO₂ NPs (b) nanocomposite film fabrication.

3.4 XRD analysis

Fig. 2(a) shows the XRD patterns of pristine titania. The peaks at 26°, 41°, 54°, and 63° confirm the crystalline features of pure TiO₂.^29,30 The particle size of TiO₂ has been determined from XRD patterns using Scherrer equation.³¹where, ‘τ’ is the mean size of the ordered domains, ‘k’ is a dimensionless shape factor, ‘λ’ is X-ray wavelength, ‘β’ is broadening of half the maximum intensity (FWHM), and ‘θ’ the Bragg angle. It has been found that the TiO₂ nanoparticles possess an average dimension of 50 nm. The XRD patterns of pure PPNO and PNPPO are given in Fig. 2(b) and (c). The PPNO show peaks at 2θ = 15° and 25°. Similarly, the PNPPO shows at 18° and 28°. The blends of PPNO and PNPPO show a relative amorphous nature due to the homogenization upon blending. Films with PMMA exhibit broad peaks of 2θ at 15° and 17°, respectively, indicating their predominant amorphous nature. The XRD patterns of TiO₂ loaded systems (Fig. 2(b) and (c)) show shifts in the 2θ values compared to PPNO/PMMA and PNPPO/PMMA systems; i.e. they show 2θ around 17° and 29° for PPNO/PMMA/TiO₂ and PNPPO/PMMA/TiO₂ composites, respectively. The sharp peaks observed for TiO₂ NPs alone (Fig. 2(a)) can not been seen for the composites. This shows the proper wetting of TiO₂ NPs by the matrix leading to the formation of homogeneous films.

Fig. 2 XRD patterns of: (a) TiO₂ (b) PPNO, PPNO/PMMA, PPNO/PMMA/TiO₂ (c) PNPPO, PNPPO/PMMA, PNPPO/PMMA/TiO₂.

3.5 Scanning electron microscopy (SEM)

The morphologies of PPNO, PNPPO, their films with PMMA and their composites with TiO₂ NPs are shown in Fig. 3. Fig. 3(a) shows the SEM image of TiO₂ NPs with average dimensions of 50 nm. PPNO and PNPPO show layered structures having smooth surfaces. When blended with PMMA these layered structures vanish and relatively homogeneous films are formed. When TiO₂ NPs are loaded into PPNO/PMMA and PNPPO/PMMA films, they get uniformly distributed as evident from Fig. 3(f) and (g).

Fig. 3 SEM images of: (a) TiO₂ (b) PPNO (c) PPNO/PMMA (d) PPNO/PMMA/TiO₂ (e) PNPPO (f) PNPPO/PMMA (g) PNPPO/PMMA/TiO₂.

The surface smoothness and the dispersion of TiO₂ NPs in the matrix have also been confirmed by atomic force microscopic images of the systems viz.; PPNO/PMMA, PNPPO/PMMA, PPNO/PMMA/TiO₂ and PNPPO/PMMA/TiO₂ as shown in Fig. 4. The AFM topology images show smooth surface features with low root-mean-square (RMS) roughness values ie; 6.04 nm and 8.1 nm for PPNO/PMMA and PNPPO/PMMA blends respectively; and 8.7 nm and 3.1 nm for PPNO/PMMA/TiO₂ and PNPPO/PMMA/TiO₂ nanocomposite respectively. This level of smoothness and uniformity of the surface is highly desirable in the fabrication of active layers for optoelectronic applications.

Fig. 4 AFM images of: (a) PPNO/PMMA (b) PNPPO/PMMA (c) PPNO/PMMA/TiO₂ (d) PNPPO/PMMA/TiO₂.

3.6 Optical properties

The absorption and emission features of PPNO, PNPPO, their blends with PMMA and TiO₂ based composite systems were examined in solution phase with DMF as solvent, by UV-Vis absorption spectroscopy and fluorescence spectroscopy. In addition, the absorption and emission features of PPNO/PMMA, PNPPO/PMMA, and their composites with TiO₂ were studied in the solid phase. The nonlinear optical properties of PPNO, PNPPO and PNPPO/TiO₂ systems have also been studied by open aperture Z-scan measurements.

3.6.1 UV-Vis absorption spectrum. UV-Vis absorption spectra of the different systems under investigation have been depicted as Fig. 5(a)–(d). The UV-Vis absorption spectra of PPNO and PNPPO in solution phase exhibited strong λ_max absorption bands at 506 nm and 517 nm, respectively assignable to the π–π* transition. These λ_max values have an absorption onset at 538 nm and 553 nm corresponding to the optical band gaps of 2.30 eV and 2.24 eV, respectively. The band gap values fall well within the range of that of semiconducting systems.^32,33 When TiO₂ is loaded into either PPNO or PNPPO, the absorption onset gets red-shifted and thus the respective band-gaps get reduced. The blending of PPNO or PNPPO with PMMA does not cause any major changes in the absorption characteristics as evidenced from Fig. 5(a). The TiO₂ loaded PPNO/PMMA and PNPPO/PMMA, in solution phase, possess similar absorption edges and band gaps as those of PPNO/TiO₂ and PNPPO/TiO₂ systems.

Fig. 5 UV-Vis absorption spectra of: (a) PPNO in solution phase (b) PPNO in solid phase (c) PNPPO in solution phase (d) PNPPO in solid phase.

The features of all the systems in the solid phase (as flexible films) are shown in Fig. 5(b) and (d). The absorption peaks have been found to be similar to those in the solution phase. However, there is a reduction in the absorption intensity in the solid phase compared to those in the solution phase. This can be attributed to the strong influence of PMMA macromolecular chains on the absorption features of PPNO or PNPPO chains.

3.6.2 Fluorescence spectrum. As shown in Fig. 6, both PPNO and PNPPO show their bright luminescent properties in the greenish-yellow region of the electromagnetic spectrum with the respective emission maximum of 551 nm and 556 nm. They are being accompanied by a shoulder peak at 580 nm and 590 nm for PPNO and PNPPO respectively. It has been reported that the presence of shoulder peak in the fluorescence spectrum is an indication of excimer formation³⁴ due to the linear structure of the macromolecular chains. The quantum yields calculated using quinine sulphate as a standard for PPNO and PNPPO in solution phase are 21% and 19%, respectively. As seen in Fig. 6(a) and (b), the emission maxima of PPNO/TiO₂ and PNPPO/TiO₂ composite systems (for 2 wt% loading of TiO₂) have been found to be red shifted to 563 nm and 560 nm respectively, compared to the base systems. Fig. 6(c) and (d) show the influences of the wt% of TiO₂ on the matrix in the emission features. There is a regular reduction in the fluorescence intensity with the increase in the amount of TiO₂ particles. It is also observed that the shoulder peaks found for PPNO and PNPPO alone gradually vanishes when the loading of TiO₂ NPs is increased. For 8 wt% TiO₂ loaded systems only a minor shoulder peak has been found. These observations can be correlated with the possible charge transfer phenomenon between the matrix and nanoparticles. The reduction in the intensity of fluorescence with TiO₂ loading in PPNO/PNPPO can also be attributed to concentration quenching. As it occurs, the probability for excimer formation considerably decreases which is being manifested as the vanishing of the shoulder peak. A representation regarding the charge transfer is given in Scheme 3. The charge transfer in the current systems occurs through particle–particle junctions. The transfer is facilitated by electron hoping when TiO₂ nanoparticles are connected with one another and with the donor systems [PPNO/PNPPO].³⁵ As the photogenerated excitons, derived from the π-conjugated systems, are dissociated the probability for recombination should be reduced for effective electron transfer from donor to acceptor.³⁶ The extensive π-electron delocalization in PPNO/PNPPO offers the probability for the reduction in the recombination of excitons and thus support the charge transfer process. Another interesting possibility, in the present systems, is the electronic coupling of the p-orbitals of the conjugated carbon and conduction band states of TiO₂. It has been reported that the linear optical properties of coupled systems (of two semiconductors) may have an enhancement in charge separation and extension in the energy range of photoexcitation compared to a single system.^37–39

Fig. 6 Fluorescence spectra of: (a) PPNO (b) PNPPO, (c) PPNO with different wt% of TiO₂ (d) PNPPO with different wt% of TiO₂.

Scheme 3 Proposed mechanism for charge transfer phenomenon: (a) charge transfer from the LUMO of PPNO/PNPPO to the conduction band of TiO₂ NPs (b) conjugated polymer nanocomposites (CPNCs) with reduced band gap (c) fluorescence quenching.

Fig. 6(a) and (b) show that the blending of PPNO or PNPPO with PMMA causes a slight blue shift. This is due to the intermolecular interaction between PPNO and PNPPO with PMMA which reduces the π–π interaction among the π-conjugated macromolecular chains. However, these systems also get red-shifted when they are loaded with TiO₂.

3.6.3 Nonlinear optical properties. To confirm the utility of the two macromolecular systems in nonlinear optical devices their NLO responses have been investigated. Typically, open aperture Z-scan measurements have been performed to evaluate the nonlinear optical responses of PNPPO and its composites in solution phase at a high intensity laser radiation to examine their possibility for their application in frequency mixing, optical switching, optical communication and optical limiting applications. The hyperpolarizability concerned with the response of electronic changes in macromolecules to an oscillating applied electric field of a laser beam is responsible for their NLO properties.⁴⁰

In general, the nonlinear absorption mechanism of nano second pulses for π-conjugated polymers can be explained by a five-level model⁴¹ as shown in Fig. 1(a) (ESI†). The five-level energy diagram illustrates the transition between ground state (S₀) and singlet (S₁, S₂) and triplet (T₁, T₂) states. Upon excitation of the material with light energy, population in the first excited state (S₁) happens. When the molecule is irradiated with high intensity radiation, excitation (S₁) into high energy state also occurs. A typical Z-scan set up is also schematically depicted in Fig. 1(b) (ESI†).

Fig. 7 represents the open aperture (OA) Z-scan traces of PPNO and PNPPO in DMF solvent. The solids curves are the theoretical fits for the experimental data. At the inset of the figures, optical limiting curves of both the samples, extracted from the Z-scan data, are shown. Here, the normalized sample transmittance is plotted against the input laser fluence. It is seen that the curves deviate from the conventional Beer's law of linear dependence of output intensity on input intensity under strong irradiance and the solution becomes more opaque, leading to optical power limiting, the nonlinear process.

Fig. 7 Nonlinear responses of PNPPO, TiO₂ and PNPPO/TiO₂ NCs.

To evaluate the influence of TiO₂ nanoparticles on the nonlinear optical responses of the synthesized π-conjugated macromolecular systems, the NLO responses of TiO₂, PNPPO and PNPPO/TiO₂ as model systems have been studied in solution phase (Fig. 7). A significant enhancement in the NLO response and its third-order nonlinear susceptibility has been observed (Table 1). A considerable reduction in the optical limiting threshold value of the composites has also been observed compared to the unfilled systems.

Table 1 Nonlinear optical responses of PNPPO

Description	Refractive index (n)	Nonlinear absorption coefficient (β) 10^−10	Linear transmission	χ ^(3) (esu)
PNPPO	1.425	1.86	0.73	6.14 × 10^−12
TiO2	1.415	1.82	0.79	5.87 × 10^−12
PNPPO/TiO2	1.425	4.89	0.53	1.05 × 10^−11

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Considering the case of two-photon absorption (TPA), the third-order nonlinear susceptibility can be expressed as a complex quantity of real χ⁽³⁾ and imaginary χ⁽³⁾ parts.⁴²

χ⁽³⁾ = Re(χ⁽³⁾) + Im(χ⁽³⁾) (2)

The imaginary part, Im(χ⁽³⁾), is related to the TPA coefficient, β, through

where, ‘n₀’ is the refractive index, ‘ε₀’ the permittivity, ‘c’ the velocity of the light and ‘ω’ the optical frequency.

The real part is related to γ, the second hyperpolarizability through:

Re(χ⁽³⁾) = 2n₀²ε₀cγ (4)

The modulus of third-order nonlinear susceptibility can thus be calculated as:

The TPA coefficient, β, is related to the linear absorption coefficient, α, as:

α = α₀ + βI (6)

In the case of open aperture Z-scan, the transmittance, T(z) can be determined by using eqn (7).⁴³

where, q_0(z,r,t) = βI(t)L_eff and L_eff = (1 − e^−αl)/α the effective thickness with the linear absorption coefficient, α and I is the irradiance at the focus.

The calculated third order nonlinear susceptibility, χ⁽³⁾ and ‘β’, the nonlinear absorption coefficient of PNPPO as a typical case, are listed in Table 1.

The obvious NLO responses and third-order nonlinear susceptibility of TiO₂ loaded PNPPO are due to its ability to display strong reverse saturable absorption (RSA) resulting from the strong multiphoton absorption or excited state absorption. This observation clearly points towards the potential use of TiO₂ loaded PNPPO composites for use in photonic devices and optical switches, and for optical power limiting applications.

3.7 Electrochemical studies

3.7.1 Cyclic voltammetry. The redox behavior of PPNO, PNPPO and their nanocomposites has been investigated from cyclic voltammetric experiments, using DMF solvent, tetrabutylammonium perchlorate (TBAPC) (0.1 M) as an electrolyte, Pt as the working electrode and Ag/Ag⁺ (AgNO₃) as the reference electrode at a scan rate of 50 mV s⁻¹. Fig. 8(a) and (b) depict the voltammogram for both p-doping and n-doping process. The HOMO and LUMO energy levels were estimated from the onset oxidation and reduction potentials using the eqn (8a) and (b).⁴⁴

E_HOMO = −[E^OX_(onset) + 4.8 eV] (8a)

E_LUMO = −[E^Red_(onset) + 4.8 eV] (8b)

Fig. 8 Cyclic voltammogram of: (a) PPNO and PPNO/TiO₂ (b) PNPPO and PNPPO/TiO₂.

The calculated HOMO–LUMO energy levels, and thus the band gap of all the synthesized novel materials are summarized in Table 2. These energy values obviously correspond to the characteristic features of a typical semiconducting system. A considerable reduction in the energy levels and band gap values of polymer NCs systems has been observed compared to the matrices (PPNO and PNPPO). This reduction in energy levels, and thus the band gap of nanocomposites is due to the well dispersion of TiO₂ nanoparticles in the polymeric systems with effective interfacial interaction. It has been pointed out that the change in band gaps or energy levels (or red shift) in the emission or absorption spectrum may due to the effect of illumination of one semiconductor by the energy levels of another semiconductor.⁴⁵ A mechanism for the redox behavior of 1,3,4-oxadiazole unit is illustrated in Scheme 4.

Table 2 Energy level values of the polymers and their nanocomposites

Description	E HOMO (eV)	E LUMO (eV)	Band gap (eV)
PPNO	−6.26	−3.29	2.96
PPNO/TiO2	−6.32	−3.49	2.83
PNPPO	−6.12	−3.49	2.63
PNPPO/TiO2	−6.09	−3.75	2.34

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Scheme 4 Mechanism for redox behavior of 1,3,4-oxadiazole unit in the cases of filled and unfilled systems.

3.7.2 Current–voltage characteristics. The electrical (semiconducting) behavior of the polymers and the nanocomposites has further been confirmed from the current–voltage (I–V) curves as shown in Fig. 9(a) and (b). It has been observed that both the polymers and their composites show good semiconducting properties in the range of micro ampere current at a low voltage.

Fig. 9 I–V characteristic curves for: (a) PPNO and PPNO/TiO₂ (b) PNPPO and PNPPO/TiO₂.

3.8 Mechanical properties

To confirm the utility for the fabrication of conductive packaging materials, TiO₂ loaded PPNO/PMMA and PNPPO/PMMA systems were subjected to the examination of their mechanical properties. The tensile strength values of the systems under investigation are given in Fig. 10. The results indicate the enhancement of mechanical properties of PPNO/PNPPO with increase in the loading of TiO₂ particles. The strong dependence of the mechanical properties of nanocomposites on the external load transfer between the reinforcing nanofiller phase and the matrix has already been discussed in various reports.^46–48

Fig. 10 Tensile strength of PPNO/PNPPO nanocomposites with loading of TiO₂ NPs.

4 Conclusion

Two novel π-conjugated polymers viz.; poly(pyridine(2-nitrophenyl)-1,3,4-oxadiazole) [PPNO] and poly(2-(o-nitrophenyl)-5-phenyl-1,3,4-oxadiazole) [PNPPO] with good solubility, film forming capability, and impressive optoelectronic properties were synthesized and characterized. Their nanocomposites with TiO₂ were also fabricated and subjected to the investigation for the optoelectrical, morphological, and mechanical properties. The crystalline features of the nanocomposites were confirmed by XRD and the morphology by SEM and AFM techniques. The macromolecules and their nanocomposites showed bright luminescence in the greenish-yellow region of the electromagnetic spectrum. The better luminescence features of the TiO₂ loaded systems compared to PPNO or PNPPO alone has been explained in terms of the charge transfer phenomena from the π-conjugated systems to the conduction band of TiO₂. The reduction in the luminescence intensity at higher TiO₂ loading in the matrix has been attributed to the possible concentration quenching. The nonlinear optical properties of the matrix were found to be enhanced upon TiO₂ loading, as typically attested with the PNPPO/TiO₂ system. The examination of the current–voltage characteristics confirmed the semiconducting features of the nanocomposites derived from TiO₂ and PPNO/PNPPO. The tensile strength of the films derived from PPNO or PNPPO reinforced with TiO₂ has been found to be suitable for the fabrication of optoelectronic appliances. The overall results of the investigation clearly indicate the possible utilization of TiO₂ loaded PPNO/PNPPO nanocomposites as emissive layers in PLEDs or conductive packaging materials.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22131a

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