Ftir, uv-vis and conductivity studies in a copolymer of indole and carbazole

The copolymer containing different wt% of indole and carbazole were synthesized by following in situ copolymerization method using ammonium persulfate as oxidizer, at room temperature. Crystalline and amorphous phases were noted from XRD patterns of the samples. Structural and optical studies were carried out by FTIR and UV-Vis spectroscopy. Different functional groups in the prepared copolymer have been identified and optical band gaps were determined to vary from 3.83 eV to 3.96 eV and 3.98 eV to 4.13 eV corresponding to direct and indirect transitions. Dc conductivity measured in the temperature range of 300 K to 423 K is observed to be semiconducting type. Two different regions of variation of conductivity with temperature have been observed and the corresponding activation energy was determined. The increase in conductivity and decrease in activation energy with increase of wt% of Cz content in PInCz copolymer is observed. This may be due to decrease of hopping distance of polarons with increase of Cz contents in the copolymer.


Introduction
Recently, researchers have been showing more interest on copolymers because they are helpful to overcome limitations of the monomers.They exhibit high optoelectronic properties, better thermal, electrochemical properties, and environmental stability etc., compared to their constituent monomers.
Copolymer is a molecule made up of two different monomer units.The process of copolymerization takes place by the simultaneous polymerization of two different monomers.During the process, two different structures are incorporated into single polymer chain which in turn changes the properties of the copolymer molecule helpful for many applications.In the present work, our interest is with insertion of carbazole (Cz) as a comonomer into indole monomer (IN).Because, Polycarbazole has two fused benzene ring, good environmental stability, electrical and thermal conductivity, photoluminescence property and high mobility [1][2][3].
There are some reports on copolymers which were synthesized by different polymerization methods.Sipho E. M et al [4] have been prepared poly (aniline-co-pyrrole) copolymer using ferric chloride and ammonium per sulphate as oxidants separately.From XRD scans they have observed amorphous nature of copolymer and concluded that the conductivity of copolymer decreases by 2 orders compared to monomers it is due to unsuccessful complex of PANI and pyrrole.Sezai Sarac et al [5] have measured the conductivity of n-methyl pyrrole and carbazole with four probe method for different carbazole concentration in copolymer.They have found highest conductivity 9×10 -5 S/cm for 1/3 ratio of monomers.Bhavana Guptha et al [6] reported fabrication and characterization of electrochemically synthesized Indole and Carbazole copolymer Schottky diode with combination of Al/copolymer/indium.The conductivity of Indole is very low which is of the order of 10 -5 to 10 -7 Scm - 1 and it was increased by incorporating Indole to thiophene by following the copolymerization process [7].N. S. Wadatkar et al [8] prepared thiophene-indole copolymer by oxidative polymerization method, they have found optical band gap varying from 3.37 eV to 3.71 eV and observed increase of conductivity and decrease of activation energy for different Stoichiometry ratios of thiophene and indole.They concluded that the thiophene-indole copolymer is suitable for potential applications in optoelectronics devices.

SAMPLE PREPARATION
Analytical grade chemicals such as Indole, Acetonitrile, Ammonium persulfate (APS), methonol, Carbazole and deionised water were used for synthesizing copolymers.The copolymer (PInCz) was prepared by following In situ copolymerization method.Indole and Carbazole monomer solutions were prepared by dissolving 2.672 gm (80 wt%) of Indole in 100 ml of acetonitrile and 0.668 gm (20 wt%) in 50 ml of acetonitrile separately and stirred.Carbazole solution was added to Indole solution stirred for few minutes.Solution of APS was prepared by dissolving 8.016 gm in 50 ml of water and added drop wise to combination of monomer solutions over a period of 30 minutes.The mixture of solutions was stirred for full day.The green colored solution at the end.The obtained solution was filtered and washed with deionised water and methonal successfully.The obtained product was the copolymer and it was dried at room temperature and labeled as PInCz-20.The same procedure was followed to prepare the copolymer of different ratios of Indole and Carbazole monomers (70:30, 60:40 and 50:50 wt%) and labeled the products as PInCz-30, PInCz-40 and PInCz-50 respectively.

MEASUREMENTS
XRD patterns of copolymer samples were collected from (RIGAKU ULTIMA MINIFLEX-600) in the 2θ range from 2̊ to 80.The FTIR spectra of the samples were recorded using (SHIMADZUIR-Prestige-21) in the wave number range from 400 cm -1 to 4000 cm -1 and UV-Vis spectra of the samples recorded using (UV Visible1899 spectrometer) in the wavelength range from 200 nm to 900 nm.Hydraulic press was used to pelletize the powder sample.The two probe method was employed to carry out dc measurements of the samples in the temperature range from 300 K to 423 K.A constant voltage of 5 V applied across the pellet.
The electrical resistivity, ρ and Conductivity, σ of the samples has been determined using the following expressions.
where, R = (V/I), A is the cross sectional area and t the thickness of the pellets.The obtained conductivity was observed to be within the accuracy range 2 to 3%.

Results and discussion
3.1 XRD Figure 1.depicts XRD scans of PInCz copolymer and it reveals semi crystalline nature of the copolymer.XRD patterns show sharp peaks at 13.80̊ and 16.59, which are the characteristics peaks of carbazole and therefore confirms the presence of carbazole in copolymer matrix [3].In addition to that, the patterns contains the broad humps between 19̊ and 26̊ indicating amorphous nature of copolymer which further confirms the presence of indole in the copolymer matrix [9].The intensity of the peak present at 16.59̊ increases with increase in the Cz concentration confirms copolymerization of indole and carbazole.Similar nature of patterns can be seen in the reports [8,10].The XRD patterns of Polyindole and Polycarbazole monomer presented in our earlier paper [11,12].

FTIR
Figure 2. shows FTIR spectra of PInCz copolymer.The most characteristic difference is variation in the intensity of the band at 1051 cm -1 , appeared in the copolymer alone signifies the incorporation of ClO4 -anions and confirms interaction between indole and carbazole monomers [6].The band at 741 cm -1 is due to CH bond in the benzene ring of indole [6,10,13].The band at 812 cm -1 attributed to C-H deformation in tri substituted benzene ring [6,14].The band at 3204 cm −1 indicates stretching vibration of N-H bond [11,13].The band at 1110 cm −1 originated from the C-N vibration [13] and at 1227 cm −1 stretching of C=N respectively [6,14].A sharp band around 1325 cm −1 refers to C-H out of the plane bending vibration of aromatic ring [14].The presence of aromatic bands -C2=C3-attributed to 1572 cm −1 [13] and the peak at 1613 cm −1 attributed to bending mode of aromatic alkene [6,13].

Uv-Vis
Figure 3. depicts UV-Vis absorption spectra of PInCz copolymers.In our previous work [11] we found two bands at 265 nm corresponds to π−π* transition of the benzene ring and 305 nm to conjugation and π−π* transitions of benzene ring of polyindole.And for polycarbazole we observed, broadband at 279 nm ascribed to bonding and anti bonding (π-π*) transition of the benzoid ring [12].Similarly, PInCz copolymer shows two polaronic transitions at 260 nm and 285 nm and it is also observed that the peak at 285nm is shifted to 281nm as carbazole content increased.These results confirm the occurrence of copolymerization.The observed bands corresponding to Polyindole are comparable with thiophene-indole [8], Polyindole/Poly(vinyl acetate) [15].shows the Tauc's plots for direct and indirect band gaps of the PInCz copolymer.Tauc's plots for all the samples are not shown in the Figure 4 & 5 to save space.The energy gaps were estimated by drawing tangents to the band edges and they were extrapolated on to the x-axis (hν-axis).The tangent where it intersects hν-axis gives value of energy gap.The direct and indirect band gap values of PInCz copolymer of different Indole and carbazole monomer ratios shows higher values compared to individual Polyindole and Polycarbazole and they tabulated in Table 1.Direct energy gaps of PInCz varies from 3.96 eV to 3.83 eV and where as indirect energy gaps from 4.13 eV to 3.98 eV.The obtained direct and indirect band gaps are found to be decreased with rise of Cz wt%.This confirms copolymerization of Indole and carbazole.The obtained band gap values shows that PInCz copolymer is acceptable for photovoltaic activities and suitable for constructing photovoltaic devices.The band gap values are in comparison with the thiophene-indole [8], polycarbazole-polyindole [6]. PInCz-20 1.0 1.5 2.0 2.5 3.0 3.5 4.0 PInCz-50 6

Conductivity
Conductivity found to be of the order of 10 -4 (Ωm) -1 and increased with increase of temperature.Conductivity is also observed to be increasing with Cz content in the copolymer.The increase of conductivity with temperature is due to increase of mobile ions.The copolymer with 50:50 wt% of Indole and Carbazole monomer ratio showed highest conductivity estimated to be 2.97×10 -4 (Ωm) -1 at 413 K in region I and 2.86×10 -4 (Ωm) -1 at 343 K in region II.Conductivity of the PInCz copolymer found to be higher than the Polyindole and Polycarbazole which are reported in our earlier wrok [11,12] and this may be due to reduction in hopping distance and creation of network for transportation of charge carriers inside copolymer matrix.The order of estimated conductivity values are in comparison with the values reported in aniline-co-pyrrole, thiophene-indole [4,8].
The temperature variation of electrical conductivity is analyzed using Arrhenius expression, σ = σ0 exp (Ea/kBT) Where, σ0 is pre exponential factor, Ea the activation energy and kB the Boltzman constant.Figure 6.
shows the plots of ln(σ) versus (1/T) for PInCz copolymer.To save space, the Arrhenius plots for all samples are not depicted in Figure 6.From the figure, we can observe that there exists at least two regions of variation of conductivity.Implying that there are two different sizes of absorption energy for conduction in region I and region II.Linear lines were fit in both the region to estimate activation energy.The conductivity increased because of the reduction in the scattering rate of polarons inside the copolymer matrix.The activation energy is found to be 5.50 meV for PInCz-40 in region I, and 2.53 meV for PInCz-50 in region II are the lowest values compared to the other concentration, indicates in this concentration electron, needs less amount of energy to overcome the potential barrier compared to other concentrations.The behavior of variation of conductivity with activation energy is in comparison with the report [8].The conductivity and activation energy values of PInCz copolymer of different monomer concentration in both region I and II are tabulated in Table 2.

Conclusions
PInCz copolymer in different compositions has been synthesized by in situ copolymerization technique.The copolymer has been investigated for XRD, FTIR and Uv-Vis and conductivity.The XRD patterns reveals semi crystalline nature of copolymer and different functional groups in copolymer were identified by FTIR.The blue shift in the UV-Vis optical absorption gives information regarding intermolecular interaction between the bands of copolymer.Direct and indirect optical band gaps for allowed transitions were estimated by plotting Mott-Davis-Tauc's plots for the absorption edges and it is found that the band gaps of the PInCz copolymer are higher than the PIn and PCz.The Dc conductivity of the copolymer is increased with Corbazole concentration in copolymer and it is observed that (50:50) wt% of monomer has highest conductivity.Variation of conductivity with temperature indicates semiconducting nature of copolymer.Conductivity enhanced, activation energy reduced, and it is due to reduction of hopping distance of the charge carries in the copolymer matrix.

Figure 3 .
Figure 3. Optical Absorbance versus wavelength for PInCz copolymers of different compositionThe direct and indirect optical energy gaps of the PInCz copolymer have been estimated using Mott-Davis-Tauc's, relation for linear absorption coefficient (α)[8],

Figure. 5 .
Figure.5.Tauc's plots of (αhν) 2 versus hν for the determination of indirect band gap for PInCz of different composition.

Table 1 .
Optical Band gap energy (direct and indirect) values for PIn, PCz, PInCz copolymers of different ratios

Table 2 .
DC conductivity and activation energy in region I and II for PInCz copolymers of different ratio Figure6.Plots of ln(σ) versus(1/T) in region I and II for PInCz copolymers of different ratios. 7