A novel organic semiconductor 4-phenylthiazol-2-yl-(phenylhydrazono) acetonitrile (PTPA) thin films: synthesis, optical and electrical properties

In this study, 4-phenylthiazol-2-yl-(phenylhydrazono) acetonitrile (PTPA) azo dye was synthesized and studied from optical and electrical point of view. The tautomerization phenomenon of the PTPA dye was clarified using one-dimensional (1D) and two-dimensional (2D) nuclear magnetic resonance (1HNMR and 13C NMR), absorbance (UV-Vis), emission, and Fourier transform infrared spectroscopy (FT-IR). X-ray diffraction (XRD) evaluations were indicated that PTPA in powder and thin films crystallizes in a monoclinic system structure with nonstructural characteristics. Spectrophotometric measurements of absorbance A (λ), transmittance T (λ) and reflectance R (λ) at normal incidence light in the wavelength range 200–2500 nm were used to determine the optical band gap, extinction coefficient, k and refractive index, n. Also, non-linear optical parameters such as the third order non-linear susceptibility, χ(3) and nonlinear refractive index, n(2) of PTPA have revealed an awe-inspiring switching behavior, implying the possibility of using PTPA in optical switching systems. Finally, the electrical conductivity of the PTPA was shown to increase with rising temperature, indicating that it is a typical organic semiconductor. Mott’s parameters were determined and discussed at low temperatures. Thus, PTPA is a promising organic semiconductor with broad utility potential in organic electronics such as organic light-emitting diodes (OLEDs).

Organic semiconductors (OSCs) are becoming more widespread due to their enormous advantages over their inorganic counterparts.For example, they are light, easy to make, processed at low temperatures, mechanically flexible, low cost, and abundantly available [1][2][3][4][5] .Additionally, due to their distinguishable qualities, OSCs have lately been used to manufacture numerous high-performance electronic technology products, including solar devices, organic sensors, nonlinear electrical conductors, digital technology, memory devices, and lasers [3][4][5][6][7] .On the other hand, organic semiconductors have some disadvantages, such as limited electrical mobility and the instability of organic thin films during fabrication, storage, and operation due to their reactions with water and oxygen 8 .Therefore, novel OSCs with high chemical and thermal stability are required for future scale-up of the manufacturing process.
Commonly, azo dyes are the chief and most diversified category of synthetic organic chemicals used as colorants for inks, paints, toners, and laser and copier photoreceptors [9][10][11][12][13] .Azo dyes stand out for their simplicity in synthesis, widespread availability, low cost, chemical stability, and wide range of luminescence efficiencies.Moreover, azo dyes have solution processability, a diverse spectrum of absorption bands, fascinating electrooptical characteristics, and the ability to change their spectroscopic and electrical properties [9][10][11][12][13][14][15][16] .These features make azo dyes an encouraging candidate and a promising material with a broad spectrum of essential applications for optoelectronic applications such as solar cells 17 .
Many efforts have been made to synthesize OSCs with high nonlinear optical (NLO) properties for photonics and optoelectronics applications, such as optical limiting and switching 9,10 .Azo materials have a wide range
Preparation of PTPA thin films.The prepared PTPA azo dye thin films were deposited onto a clean quartz substrate by spin coating technique using a chloroformed solution (20 mg/ml).The thickness of the thin films (100 and 190 nm) was estimated using the Alpha-Step IQ profilometer.Methods.NMR spectroscopy measurements.The Bruker AvanceII+ 400 MHz, 9.4 T, Ultrashielded 1 H frequency: 400 MHz spectrometers were used to obtain the 1 H and 13 C NMR spectra, which operated at 400. 13  MHz for 1 H and 100.62 MHz for 13 C. 1 H and 13 C chemical shifts (δ) are given in ppm relative to the solvent (CDCl 3 ) main peaks (δ= 7.26 ( 1 H) and 77.16 ( 13 C)).The manufacturer's software (TOPSPIN 2.1) was used for all 2D tests (COSY, HSQC, and HMBC).All measurements were taken at room temperature.
FT-IR, UV-visible and fluorescence spectroscopy measurements.FT-IR spectrometer (JASCO (FT/IR-4100) was monitoring the infrared spectra of PTPA dye in the KBr disk.A Shimadzu 2401PC spectrophotometer was used to censor the UV-Visible absorption spectrum in wavelength range 200 to 800 nm using thermostated (T = 298K) 1cm matched quartz cells.The UV-Visible spectrum of 2.56 × 10 -5 M solution of PTPA dye in CHCl 3 (99.0-99.4% BDH AnalaR) was measured.Emission spectra of the same solution for UV-Visible absorption spectra were measured using a JASCO spectrofluorometer FP-8500 at 400 nm excitation wavelength.
Computational calculations.The semiempirical modeling of the PTPA structure (Scheme 1) was attained using PM6 and PM6/CI (configuration interaction) with optimization in the ground (S 0 ) and first excited (S 1 ) states, respectively.The PM6 (COSMO) model in chloroform was used to investigate this dye's tautomerism.The geometric structure of this dye's Z-hydrazone form was created by eliminating the methyl group from a single crystallographic structure with the deposition number (617431).All geometry calculations were performed on a personal computer using a MOPAC 2009-free license obtained from the internet.A further DFT geometric calculation was completed using Gaussian 03 program via B3LYP method.Input files of PTPA molecule were fixed with Gauss View 5.0.9 in gases state.A full optimization was performed up to a higher basis set 6-31G++ (d, p).
Cyclic voltammetry measurements.Cyclic voltammetry (CV) measurements were achieved using a stock solution of 0.0033 g of PTPA in 5 mL of MeOH with a drop of DMF.400 μL of the stock was added to a 20-mL Britton-Robinson buffer (BRB) solution at a pH of 2.5.Versa STAT4 Versa STAT 4 was also utilized to carry out the electrochemical tests, which were linked to a three-electrode system cell with a carbon paste electrode (CPE) of 0.3 mm diameter as the working electrode, Ag/AgCl as the reference electrode, and Pt wire as the counter electrode.
XRD and optical measurements of thin films.Rigaku RINT 2100 diffractometer was used to record XRD patterns of PTPA azo dye in powder and thin films form.The optical measurement films were recorded at normal incidence using spectrophotometer in the wavelength range of 200-2500 nm (JASCO model V-570 UV-Vis-NIR).
Dc electrical measurements of thin films.Dc electrical measurements were carried out in a flat configuration with Au electrodes spaced 5 mm apart (see Scheme S1).The DC electrical conductivity of PTPA films was evaluated by measuring resistance as a function of temperature using a Keithley 2420 instrument throughout a temperature range (300-450 K).The resistivity was calculated as follows 2 : where, R e represents the electrical resistance, w represents the width of the film, d represents the thickness of the film, and L is the length of the film.
(1) ρ = R e  www.nature.com/scientificreports/Analysis of absorbance, emission, and FT-IR Spectra.The realistic optical band gap ( E Opt g ) of the examined PTPA dye is computed from the maximum wavelength (Fig. 4) of the absorption band (λ max = 398.5 nm).However, the predicted energy difference ∆E S1-S0 between the excited and ground states of Z-hydrazone form in PM6 and PM6 COSMO simulations is 3.183 eV and 3.145 eV, respectively.But, the (∆E LUMO-HOMO ) value of the PTPA resulting from DFT gas state optimization is 3.428 eV.The experimental E Opt g agrees with the theoretically calculated (∆E S1-S0 ) energy difference.This allows the absorption band at λ max = 398.5 nm to be gener-  www.nature.com/scientificreports/ated by an intramolecular charge transfer (ICT) π S 0 →π* S 1 transition involving the entire-electronic system [32][33][34] between the phenyl donating moiety and the thiazole acceptor ring, as shown in the HOMO and LUMO orbitals in Fig. 5.The active electronic density shift from the phenyl-donating ring to the acceptor 4-phenylthiazole is seen in this HOMO and LUMO orbital diagram.According to NMR measurements, PTPA azo dye mostly exists in a single tautomeric form (Z-hydrazone) as seen by the strong absorption band at max= 398.5 nm ( see Fig. 4).Although this dye has a wide emission band at λ em = 526 nm and a shoulder at λ em = 478 nm, it have more than one structural form present in the excited state.FT-IR spectrum (see Fig. S6) of PTPA dye shows distinguishable bands at 3410 (N-H), 3100-2840 (C-H Aromatic ), 2213 (C≡N), 1601 (N=C), 1544 (C=N-N-), 1477-1260 (C=C Aromatic ), 1084 (C-N), and 710 cm -1 (C-S).These bands confirm the suggested Z-hydrazone chemical structure of PTPA dye.
XRD studies for PTPA thin films.The XRD pattern of PTPA in powder form is shown in Fig. 6.The existence of numerous diffraction peaks of varying strengths in the pattern indicates that the PTPA powder is polycrystalline.The computer programs CHECKCELL and CRYSFIRE are used to determine the Miller indices unit and cell properties of PTPA 35,36 The monoclinic phase structure of PTPA powder is P 21/c with a (412) preferred orientation.According to structural data, a= 21 Å, b= 22 Å, c= 14 Å, α= 90.00°, β= 91.00°, and γ= 90.00° are the lattice parameters.A similar phase structure appeared in BTPA azo dye X-ray crystallographic data 16 .Figure 7 depicts the XRD pattern of the PTPA film on the quartz substrate.Scherrer's equation 37 is used to estimate the average crystallite size, D as 20.36 nm for a film thickness of 100 nm and 34.56 nm for a film thickness of 190 nm.Nanometric measurements of crystallite sizes show that PTPA films have a distinguishing nanostructure.Figure 9 depicts the ( α) of the as-deposited PTPA thin films (100 and 190 nm) with a value in the region of 10 5 cm -1 in the wavelength (λ) range of 200-2500 nm.The redshifts in absorbance coefficient are caused by an increase in the crystallinity of PTPA thin films as the film thickness increases.It is common to use Tauc's equation in many organic materials to investigate the kind of optical transitions and the magnitude of the energy gap at the basic absorption edge using the following expression 40-42 : where, S is a parameter that is affected by the transition probability.It is discovered that the best fit is achieved when S =2.As a result, the kind of electronic transition is an indirect permitted transition (see Fig. 10 1.In accordance with Table 1, the E Opt g of PTPA thin films decrease as the film thickness increases.This decrease in E Opt g is caused by improved crystallinity and an increase in the crystallite size of the PTPA thin films (XRD findings).The HOMO/LUMO calculation was essential for modifying device architectures to achieve the proper energy levels between emitting and conveying materials, thereby averting interface emission and other device issues.Additionally, this is advantageous for enhancing device performance.HOMO and LUMO energy levels of PTPA were determined using CV (see Fig. 11).According to the equations below 43 : where, E ox and E red are beginnings of oxidation and reduction potentials which have values 1.158 and − 1.001 V of PTPA, respectively.HOMO-LUMO energy gap is calculated to be 2.16 eV.The energy band gap calculated from CV is extremely similar to that inferred from absorption onset, demonstrating the reliability of electrochemical examination of LUMO and HOMO energy levels.The distribution of T (λ) and R (λ) spectra for PTPA film in the wavelength range (200-2500 nm) for two distinct thicknesses (100 and 190 nm) are shown in Fig. 12.The absorption edge of PTPA appears in the absorption area (λ >500 nm) (2 edge "valleys" regions).However, all PTPA thin films become almost transparent in the non-absorbing region (λ > 500 nm) owing to a very little amount of lost energy due to scattering.Linear optical constants, including the refractive index, n and the extinction coefficient, k can be calculated from α (λ) and R (λ) as shown below 44,45 :    13a).The dispersion curve of n decreases smoothly with wavelength (normal dispersion) at wavelengths greater than 830 nm (see Fig. 13b).Using the Wemple and DiDomenico (single oscillator) method, n dispersion parameters are able to be calculated at this site with the help of the following equation 44,46 : where, E o denotes oscillator energy and E d denotes dispersion energy.The values of E d and E o can be simply calculated from the slope and intercept of (n 2 − 1) −1 versus (hυ) 2 (see Fig. 14).Table 1 shows the derived E d and E o values.The values of E d fall as the thickness of the films rises, whereas the values of E o grow.Extrapolating the linear component of the dispersion relationship towards h ν = 0 eV yields the infinite wavelength dielectric constant ε ∞ .The formula n o = √ ε ∞ can also be used to compute the static refractive index n o .Table 1 sum- marizes the values of ε ∞ and n o for PTPA thin films.By examining the, n data in the transparent zone and using the following relation the dielectric constant at high frequency ε L and the ratio of carrier concentration to effec- tive mass N m * can be calculated 40,44,47 : where, e represents the electron charge, the values of ε L and N m * which are computed from the intercept and the slope of n 2 versus 2 as shown in Fig. 15.Values of ε L and N m * are tabulated in Table 1.It is evident that the value of ε L > ε ∞ is due to the contribution of free charge carriers to the polarisation process that occurs inside the material when light shines it 40 .Furthermore, as thickness grow, the rising values of E d ,E o , ε ∞, n o , ε L and N m * may be attributed to the decrease of lattice defects and lattice vibration.
Nonlinear optical parameters for PTPA thin films.An empirical equation based on extended Miller's rule and linear refractive index dispersion, can be used to determine the NLO parameters, which include the third-order nonlinear susceptibility χ (3) and. the nonlinear refractive index n (2) (see Eqs. (10) and (11).While the nonlinear absorption coefficient ( β c ) can be estimated using the Sheik-Bahae model (see Eq. ( 12)) 48-51 .Figure 16a and b displays the variations of χ (3) and n (2) as a function of photon energy for as-deposited PTPA at two distinct thicknesses (100 and 190 nm).Table 1 provides the projected values of n (2) and χ (3) (esu) when h ν = 0 eV for the analyses films.The potential of optical switching using PTPA thin films is shown in Fig. 16a  and b, where the films' significant peaks are located at (i) 1.16 and (ii) 2.47 eV.Moreover, Table 1 shows values of χ (3) and n (2) at photon energies of 1.16 and 2.47 eV for two distinct thicknesses (100 and 190 nm) of PTPA thin films.The increases in values of noticeable peaks in Figs.16a and b are due to a drop in E g opt as the film thickness grew.PTPA's nonlinear characteristics are revealed an awe-inspiring switching behavior, implying the possibility of using PTPA in optical switching systems.
Changes in βc as a function of hv for PTPA thin films are shown in Fig. 17.When hv is raised, βc upsurges gradually until it reaches a maximum value (see Table 1) after which it drops back down.A number of organic thin films showed comparable performance of βc with hv including NiTPP/FTO 38 , (4TFEO) 4-ZnPc/quartz 40 , CN-PPV/FTO 41 , MnTPPCl/quartz 52 , and Toluidine Blue/quartz 53 .This finding also illustrates how thickness affects the enhancement of βc.This behavior could be explained by a decrease in E Opt g in the instance of the 190 nm thin films.PTPA nanoparticles is investigated through their temperature-dependent conductivity within the range of temperatures (300-450 K).The produced PTPA thin films' dc electrical conductivity is shown in Fig. 18.It is evident from Fig. 18 that the direct current DC conductivity increases as temperature rises.As evidenced by its DC conductivity, the PTPA is a typical organic semiconductor 2 .Plots in Fig. 18      where N(E f ) is represents the density of localized states, T o is degree of disorder and γ is the Debye frequency (10 13 Hz) 2 .The hopping distance and hopping energy, which are provided by the following equations, may be derived by concurrently solving Mott's Eqs. ( 14), ( 15), ( 16), ( 17), ( Figure 19 depicts a plot of ln(σT 1/2 ) versus T 1/4 for two various thicknesses (100 and 190 nm) from PTPA films.8), (19) and (20) which are presented in Table 2.The hopping distance raises as the film thickness increases, but the hopping energy decreases.Additionally, R d > 1 and W E > kT are necessary for the VRH conduction process 2 (see Table 2).Thus, in the low-temperature region, the VRH model is the domain conduction model.

Conclusions
In conclusion, PTPA was synthesized and characterized using IR, UV-visible, and NMR methods.Using 1D and 2D NMRs, it was possible to demonstrate the presence of PTPA dye in Z-hydrazone form, which was stable due to intramolecular hydrogen bonding in chloroform.Optimized structure and electronic properties (E HOMO ), (E LUMO ), and (E g ) were calculated using DFT and PM6 semiempirical calculations.The spin coating method was utilised to prepare PTPA thin films effectively.XRD patterns for all thicknesses revealed that the PTPA films had a monoclinic phase structure.The crystallite size was estimated inside the nanoscale, demonstrating the nanostructure feature of PTPA thin films.According to Tauc's relationship, indirect E Opt g1 PTPA films decreased from 2.13 to 1.99 eV, also E Opt g2 decreased from 3.24 to 2.92 eV as the film thickness increased.The energy band gap measured from CV (2.16 eV) was highly comparable to that predicted from absorption onset E Opt g1 .The dispersion parameters of the reflective index in a normal dispersion zone were investigated using a single oscillator model.Based on a semi-empirical relationship, χ (3) and n (2) of PTPA thin films were calculated at photon energies ranging from 0.5 to 4 eV.Computed χ (3) and n (2) values were on the order of ~10 −10 and10 −9 esu, respectively at hν → 0 .The DC electrical conductivity of PTPA thin films followed a pattern comparable to that of organic semiconductors.VRH conduction was dominant at low temperatures.Based on these findings, the PTPA seems to be a particularly efficient OSC, which is useful in advanced photonic applications.( 19) Scheme 1. Synthesis of 4-phenylthiazol-2-yl-(phenylhydrazono) acetonitrile (PTPA) azo dye.

Figure 1 .
Figure 1.(a) Azo-Hydrazone tautomerism of thiazole azo dye.(b) *The intra and intermolecular hydrogen bonds of the predominant Z-and E-hydrazone forms of PTPA dye.(*Numbering of carbons and protons will not be continuous and not follow numbering rules, where the thiazolylacetonitrile moiety has numbers from 1' to 9' , on the other hand the substituted phenyl ring has numbers from 1 to 7).

Figure
Figure 13a and b illustrates the distribution of k and n spectra for PTPA film in the wavelength range (200-2500 nm) for two different thicknesses (100 and 190 nm).The reduced light energy loss associated with thin PTPA films with low k values suggests potential use in optoelectronics (see Fig.13a).The dispersion curve of n decreases smoothly with wavelength (normal dispersion) at wavelengths greater than 830 nm (see Fig.13b).Using the Wemple and DiDomenico (single oscillator) method, n dispersion parameters are able to be calculated at this site with the help of the following equation44,46 :

Figure 13 .
Figure 13.Variation of (a) k and (b) n vs. λ for PTPA thin films.

Table 1 .
Estimated optical parameters for PTPA thin films.
illustrate Arrhenius's behavior in the two temperature ranges [high I and low II] 2,54-56 .where,σdcrepresents the dc electrical conductivity, σ o represents the zero kelvin electrical conductivity, k B rep- resents the Boltzmann constant, �E σ represents the activation energy, and T represents the absolute temperature.Slope and intersection of the straight lines in the [high I and low II] parts of the plot can be used to determine the values of (ΔE σI , ΔE σII , σ oI , and σ oII ) for all of the thin films under evaluation, as noted in Table2.The shift in slope, and thus the variation in �E σ indicate a transition from extrinsic to intrinsic conduction.The activation energy in intrinsic semiconductors is less than half the value of the optical energy gap, which is already suitable with the determined ΔE σI values for PTPA thin films.Lower temperature conduction is clarified in terms of hopping through a band of localized states, while higher temperature conduction is explained in terms of the thermal excitation of carriers to the band boundaries 2 .Variable Range Hopping (VRH) conduction mechanism at low temperatures (300-330 K) is based on localized states near to the Fermi level.According to Mott's VRH approach, at low temperatures, lengthy hops between sites are more frequent than the total of all other ancillary hops.VRH mechanism is described by Mott's phrase as following2,56-59 :

Table 2 .
Values of activation energies and Mott parameters for as-deposited PTPA thin films of different thicknesses.