Spectroscopic Study of Electronically-Doped Polyanilines

Polyanilines namely emeraldine, nigraniline and pernigraniline were prepared by chemical method and were electronically-doped with organic acceptors such as, TCNE, TCNQ, DDQ, Chloranil and KI-I2. The FTIR spectra of 20% and 50% doped polyanilines were obtained and analyzed using small polaron model which connected electrical conductivity with IR absorption. Emeraldine prepared by standard method and pure emeraldine as readily obtained wee used. The absorption profiles as experimentally obtained and as calculated using small-polaron model were compared. This comparison leads to frequency –dependence of the real part of refractive index which shows dispersion shape as desired.

Electronic doping is done rarely. Therefore, in the present study electronically-doped polyanilines using organic electron-acceptors such as TCNE, TCNQ, DDQ, Chloranil and KI-I2 have been prepared and studied using FTIR spectroscopy.
Protoemeraldine(brown), emeraldine (green), nigraniline(blue) and pernigraniline (violet)were mixed with organic accrptors such as TCNE, TCNQ, DDQ ,Chloranil and KI-I2 and grinded in a morter with a pastle till colour changed. Further these fine homogeneous powder were mixed with dry IJTSRD | Jan-Feb 2017 Available Online@www.ijtsrd.com spectrograde KBr powder and again grinded. Palates in the form of circular discs were prepared from these mixtures with a manally operated compressing machine. These discs were placed in a dark chamber of GXFTIR single beam spectrophotometer fabricated by Perkin Elmer Company, USA.

RESULTS AND DISCUSSION
Low -mobility material cannot be understood within the framework of the usual band piture and therefore, small polarons due to strong electron-phonon coupling are found to be responsible for transport properties. The dc hopping conductivity of small polarons in crystals with the resonance integrals only between the nearest-neighbours is given by Where = ( + 1) Is related with occupation of a site, = is the concentration of the charge carriers and = / ħ has dimension of mobility. W is the hopping probability. Under certain approximation, the d.c. conductivity takes the form (29).
For frequency dependence 0f conductivity Where ( ) denotes contribution to dielectric constant (ω) resulting from lattice mechanism. Then σ(ω) is also complex and real and imaginary parts σ ' (ω) and σ ' (ω) are given by And σ h is the d.c. hopping conductivity. For high temperatures, International Journal of Trend in Scientific Research and Development, Volume 1(2), ISSN: 2456-6470 www.ijtsrd.com

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IJTSRD | Jan-Feb 2017 Available Online@www.ijtsrd.com J is exchange integral and E a is activation energy. Thus, the dependence σ'(ω) he form of a weakly asymmetric gaussian which has peak centered about ħ = 4 and having half-width 4 (32,33). Now the absorption coefficient α is related with σ'(ω) according to The above small -polaron model is applied to optical properties of doped-polyanilines since the FIIR spectra of all electronically-doped polyanilines contain a big (intenses and broad asymmetric gaussian band in the range between 1600 cm -1 and 4000 cm -1 .
The protoemeraldine FIIR spectra of 20% electronically-doped these spectra contain asymmetric Gaussian bands in the range between 2400 cm -1 and 3800 cm -1 .
The theoretical Gaussians were calculated using the equation 5 and C was calculated using = × These gaussians are plotted ( Figure 2).
Since α T (ω) assumes n 1 as a constant but actual α E (ω) experimental curves contains n 1 which depends on frequency.
The asymmetry of α T (ω) is contained in ħ factor only while the asymmetry of α E (ω) also contains the factor 1/n 1 (ω). For protoemeraldine type polyaniline, 4E a / ħ=3200-3300 cm -1 , and hence ≅ 0.1 . However , the half-width gives somewhat lesser value of E a .
Protoemeraldine itself has four gaussians in the infrared range which is consistent with its resonance structure. Two Gaussians seem to be merged to form a large and broad asymmetric Gaussian peak as analyzed above in the spectra of electronicallydoped protoemeraldine. Two Gaussian peaks remain intact around 1590 cm -1 and 900 cm -1 with some distortions. The high energy (Frequency) Gaussian covering all the benzenoid and quinonoid rings only get modified by the charge transfer to the acceptor. Thus in electronically -doped polyaniline intermolecular interactions among large oligoaniline molecules are governed by negatively charged acceptor molecules.
Similar is the case of emeraldine (sm) prepared by standard method. A board asymmetric Gaussian band is the characteristic feature of all of the FTIR spectra of CT complexes of emeraldine The highest frequency Gaussian band associated with all benzenoid and quinonoid rings is modified into an asymmetric Gaussian associated with hopping of polarons among the oligoaniline molecules. Here E a =0.07 ev and E a found from the band-width is E a =0.06 ev. Other two out of three Gaussian bands are found at 1150 cm -1 and 600 cm -1 at lower frequencies and remain intact apart from slight distortion α T (ω) which are calculated and fitted are shown. (fig. 3).
The FTIR spectra of CTCS of emeraldine (pure) (emeraldine obtained from chemical company) are also shown. The calculated asymmetric Gaussian curves are also shown ( fig. 4). Emeraldine availed from a chemical method was more oxidized than emeraldine prepared by standard method. Thus emeraldine was more near nigraniline as obtained from a company while emeraldine was more near protoemeraldine as prepared by standard method. The maximum absorption decreased by electronic doping in the case of pure emeraldine while maximum absorption increased by doping the emeraldine prepared by standard method. Electronic doping has effect of deprotonation. Emeraldine has the highest conductivity or absorption among all the polyaniline.
Nigraniline was also 20% doped with various acceptors and FTIR spectra of CTCS of nigraniline are obtain these spectra also contain asymmetric Gaussian band which can be described within small-polaron model. These asymmetric Gaussian bands are theoretically fitted (fig. 5).
The ratio of α T (w)/ α E (w) again gives frequency dependence of refractive index n 1 (w). there is one more Gaussian band around 1130 cm -1 which is somewhat distorted.
The Gaussian band in CTCS of pernigraniline is again sharp similar to those in protoemeraldine cases. The Gaussian again centered around higher frequency (about 3200 cm -1 ) as compared to the emeraldine is sharp and having band tailing towards low frequency side. This Gaussian is also theoretically fitted ( fig. 5). The taling towards low frequency side can be analyzed as nature of transition between valence and conduction band since pernigraniline and its CTCS are purely semiconductors. The spin-orbit splitting of valence band and consequent three transitions from valence sub bands to conduction band are also evident in all of the CTCS of pernigraniline.
Now this follows discussion of 50% doping of emeraldine (SM) and emeraldine(pure) with organic acceptors. As expected, maximum absorption of asymmetric Gaussian is increased by 10-15% as compared to 20% doping. This means that CTCS with more dopant concentration increases electrical conductivities. The FTIR spectra of CTCS of emeraldine (SM) + DDQ complex is an exception where no gaussian band is found in high frequency range rather a nature of transition obeying ℎ = (ℎ − ) is found ( fig. 7).
This shows macromolecular behavior of emeraldine because it is an allowed indirect transition. The FTIR spectra of emeraldine (pure) doped with organic acceptor. Theoretically fitted asymmetric Gaussian bands are also shown ( fig.8). the Gaussian bands in these cases are very broad spanning the entire infrared range. The two more Gaussian bands of emeraldine at low frequencies are merged strongly overlapping each other. Here E a calculated with absorption maximum at ħ = 4 is less than E a calculated using band-width because band width is found to be very large.

CONCLUSION
The electronically-doped polyanilines contain a broad and intense asymmetric Gaussian in the infrared spectra associated with optical properties in small-polaron model. Thus polarons are generated along the chains of polyanilines due to electronic doping. Electronic doping with organic acceptors leads to an enhancement of electrical conductivity which increases with dopant concentration. For the first time different types of polyanilines namely protoemeraldine, emeraldine, nigraniline and pernigraniline are comparatively studied along with their polymeric CTCS with organic acceptors.