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Electro-optical properties of the perfect reflector material: poly(3-thiophene boronic acid) semiconducting polymer.


Semiconducting polymers have recently become potential materials for applications in electronic and optical devices. Since then, extensive research efforts have been devoted this area to act the exceptional individual properties of intrinsic polymer semiconductors [1]. In general, polymer semiconductors have a relatively rigid n-conjugated backbone [1, 2] and most of conjugated semiconducting polymers like all crystals are periodic arrays. Their electro-optical properties can be enhanced by pending a proper oxidation ligand [2], this also changes the functionality of polymeric backbone. As an example, boronic acid B[(OH).sub.2] is very active ligand (acts as strong Lewis acid) of the including complex structures [2-4] since electron deficiency character of including boron atom. The recent developments of boronic acids containing materials that have been applied to antiviral, antibacterial and anticancer therapy (see the review paper by Trippier and C. McGuigan [5]) and also for its structure, properties, and biomedical applications of boronic acid derivatives, see the reference 6. Otherwise, boron-containing polymers and other complex structures are monitoring to dynamic covalent functionality [2], for example, in a recent study [7] boronic acid inclosing organic materials were used for the characterization of thin films. Finally, we can say that the extraordinary structural features of organoborane polymers, organoboronic acids and boronates, and their useful compounds may play important roles in several areas from phar maceuticals of material science to develop new drugs and materials [2-7].

On the other hand, thiophene-based organic polymers have recently attracted significant attentions in both academia and industrial applications of electronic and optical technologies [1, 8-20], and their laterally pending boronic acid-containing derivatives have been confirmed that they find usage in a range of technology from biosensors [12] to biomedical applications [5, 6, 10-12, 21], More recently, a theoretical description of the photo induced electron transfer in these systems were given as between the separate donor and acceptor moieties [22], and as a result organoborane polymers were being studied for use in organic light emitting devices, such as optical, electronic, and sensor applications [23, 24].

Recently, a thiophene-backbone boron containing polymer the poly(3-thiophene boronic acid) (PTBA), which is an interesting organic polymer as well as optical semiconductor material, was synthesized by Yakuphanoglu and Senkal [25], and the polymerization of PTBA, electrical conductivity and also optical properties (absorption spectra, reflectance, refractive index and complex dielectric constant) were reported. Then in 2011, PTBA was resynthesized [24] by performing electrochemical experimental techniques as a novel hydrogen peroxide ([H.sub.2][O.sub.2]) biosensor, and also, enzyme-PTBA polymeric biocomposites prepared for high performance biosensing [26].

In this study, two possible phases of PTBA were optimized without any symmetry imposing and then corresponding electronic and optical properties calculated. The geometry optimizations were performed employing a unit cell containing two thiophene-boronic acid repeat units. It is well known that, such a small unit cell immediately imposes a certain constraint to the possible polymer conformations and polymerization chain arrangements available to the simulation, but in a view of previous experimental morphology [24, 25] such conformational degrees of freedom are necessary for realistic studies of the polymeric matter. However, there is typically a complicated relationship between polymer morphology and electronic structure. Then, detailed theoretical study of electronic properties, such as density of states, Mulliken atomic charges and bond populations with bond lengths were calculated. To this end, we calculate optical properties and concentrate on the discussions in connections of electronic and optical properties.


We begin with the search ground state phase and geometry optimization of PTBA, without any symmetry imposing, by the density functional theory calculations. All calculations were performed by CASTEP simulation package [27], The ion-electron interaction is represented by Vanderbilt ultrasoft pseudopotential [28] and exchange-correlation effects were treated within the generalized gradient approximation (GGA) [29]. The lattice parameters, atomic coordinates, electronic and optical properties were calculated out using the level PBEsol [30] of GGA approach. PBEsol is the latest progresses of the Perdew-Burke-Ernzerhof (PBE) [31] exchange correlation functional and it is an explicitly designed to improve the description of equilibrium lattice parameters and other properties of crystal and molecular solids [32], After convergence tests on cut-off energy and kpoint set, the plane wave cut-off energy was chosen 500 eV and k-point separation set to 0.025/[Angstrom], which is corresponding to set of 5 X 11 X 6 and the number of k-point 124 for Brillouin Zone sampling. In the calculations, the optimizations are satisfied by four convergence criteria as follows: (i) the total energies of 5.0 X [10.sup.-6] eV/atom, (ii) the maximum ionic Hellman-Feynman force acting on each atom of 0.01 eV/[Angstrom], (iii) the maximum displacement of 5.0 X [10.sup.-4] [Angstrom], and (iv) the maximum stress acting on each atom of 0.02 GPa. It is note that through the optimizations all atomic coordinates and unit cell parameters were fully relaxed with BFGS algorithm [33] (suggested by Broyden, Fletcher, Goldfarb and Shannon). Density mixing was preferred by the Pulay scheme and in this basic setup FFT grid density precise 72 X 36 X 64 and augmentation density scaling factor is 1.5. Unfortunately, the experimental data for PTBA are more limited [24, 25] and there is no theoretical results for comparison. Then, to validate our calculations we also re-optimized the unit cell with three different simulation approaches: the local density approach (LDA) of Castep package [27], the revised the Perdew-Burke-Ernzerhof (RPBE) [34] level of the generalized gradient approximation (GGA) of [DMol.sup.3] package [35] and atomic based with adjusted basis set Newton-Raphson (ABNR) approach of Forcite package [36], DNP atomic basis sets were used in RPBE approach of [DMol.sup.3], which are comparable to 6-31G** sets [35], LDA optimization was performed with the same convergence criteria of PBEsol calculations (see above), and the RPBE and ABNR calculations were performed with fine criteria of the packages separately.

Then, molecular dynamics (MD) calculations were performed for both observed phases [25, 37], The constant-temperature, constant-pressure ensemble (NPT) were chosen with the temperature range from 300 K to 800 K and the pressure at 0.0 GPa which allows control over both the temperature and pressure. At the last step, we calculate the optical properties of PTBA, related to the complex dielectric function [epsilon](w) = [[epsilon].sub.1] (w) + i[[epsilon].sub.2]{w). The imaginary part, [[epsilon].sub.2](w), is gained from the transitions between the occupied and unoccupied states in view of the selection rules [38], and the real part, [[epsilon].sub.1](w), can be derived from the imaginary part by mean of Kramer-Kronig relationship [39].


As seen from Fig. 1, PTBA structure consists of pending molecular boronic acids [B[(OH).sub.2]] and [pi]-conjugated polymeric backbone thiophene rings [[C.sub.4]HS]. The polymeric rings (backbone) nearly lie at the (0n0) layers. There are two possible one dimensional polymer phases of poly(3-thiophene-A), which are called here symmetric and asymmetric polymeric structures. Where A denotes boronic acid B[(OH).sub.2] and symmetric (asymmetric) means of thiophene-A polymeric nano structure is in the mirror symmetry (asymmetry) of polymeric chain direction (see Fig. la and b). Our DFT result shows that asymmetric phase (the ground state) has lower total energy of -1.07 eV per formula unit than the symmetric phase, which was already observed [25]. Otherwise, despite not to be ground state of poly(3-thiophene-boronic acid) polymer, symmetric chain phase was recently observed in macromolecular alkylthiophene and thioalkylthiophene structures [37].

Then, we decided that asymmetric phase of poly(3-thiophene-boronic acid) polymer is the ground state, and its unit lattice and the bonds of nearest neighbor atoms were displayed on the Fig. 1a. For different views of asymmetric and symmetric phases, see Fig. 1c-e. Hereafter, we will call PTBA as the asymmetric phase. Calculated lattice parameters of the unit cell (two formula unit) of PTBA (2([C.sub.4][H.sub.3]B[O.sub.2]S)}, a = 8.620 [Angstrom], b = 4.356 [Angstrom], c = 7.774 [Angstrom], and the volume of is 255.0 [[Angstrom].sup.3]. The length of asymmetric phases (along the c-axis of unit cell) is about 7.774 [Angstrom]. The optimized unit cell parameters and the shortest boron-carbon and carbon-carbon bod lengths were depicted in Table 1, i.e., all structural parameters have been calculated for the equilibrium geometry for each calculation scheme, respectively. Also, Fig. la and b show the calculated bond lengths for both phases. However, because it is impossible to include so many values in figures we presented only bond lengths for both possible structures using the level PBEsol [30] of GGA approach. However, for the equilibrium geometry, calculated fraction coordinates of atoms of two formula units of PTBA {2([C.sub.4][H.sub.3]B[O.sub.2]S)} were listed in Table 2. As seen from Table 1, the unit cell parameters and bond lengths from all different approximations look very similar and agree well with each other, only the values of [DELTA]V/V with the LDA level of DFT is about 12% larger than that of PBEsol result (of [DMol.sup.3] is about 12% smaller), which is the highest discrepancy in all values, so, these may establish the consistency and accuracy of our calculations.

Since all bonds are covalent in the unit cell, the polymeric PTBA structure could be called covalent polymer structure. As seen from Fig. 1a, the more covalent bonds are the shortest C-C bonds, which have the lengths between [d.sub.C-C] =1-389 [Angstrom] with the highest Mulliken population M = 1.20 and [d.sub.C-C] = 1.406 [Angstrom] with the Mulliken population M = 1.17 (in the thiophene rings) and also the chain bods which connect the thiophene sings [d.sub.C-C] =1.444 [Angstrom] with M = 1.19. As expected, these C-C bands are shorter than two B-C bonds, [d.sub.B-C] = 1.557 [Angstrom] with M = 0.82 and [d.sub.B.C] = 1.559 [Angstrom] with M = 0.86, respectively, as in the boron doped graphene structures ([d.sub.B-C] = 1.458 [Angstrom]) for pristine graphene ([d.sub.C-C] = 1.420 [Angstrom]) [40]. In addition, electron localization function (ELF), the Mulliken atomic charges and atomic bond populations of the ground state phase are supported the discussion about polymer bond structure, see Fig. 2a and b. Our results show that while hydrogen atoms and boron atoms of the boronic acid denote electrons and then become the total charges of (+0.46) - (+0.50) [absolute value of e], and (+0.92)-(+0.95) [absolute value of e], respectively, oxygen atoms gained electrons of (-0.81) - (-0.84) [absolute value of e]. For the backbone thiophene rings, hydrogen atoms and sulfur atoms denote the electrons to the carbon atoms, the total charges of (+0.26)-(+0.28) [absolute value of e], and (+0.37)-(+0.44) [absolute value of e], respectively, and carbon atoms gained electrons of (-0.17)-(-0.29) [absolute value of e]. In totally, occurring charge transfer from the boronic acid (positively charged) to the thiophene ring (negatively charged) can now change the valence state of both. Calculated total distribution of the Mulliken charges of these units are +0.23 [+ or -] 0.01 and -0.23 [+ or -] 0.01 lei for the first coupled form, respectively, and for the second coupled +0.26 [+ or -]0.01 [absolute value of e] and -0.24 [+ or -]0.01 Id, respectively. Finally it can be said that, in PTBA structure there is a polar interaction between two coupled molecular units, i.e., the small molecule of positively charged boronic acid and negatively charged polymerized thiophene backbone, which may be called anionic polymerization (see Fig. 2b) as in the type of vinyl polymerization [41], This polarization may also be called as anisotropic ionic polarization [41], as well as anisotropic dipolar polarization, resulting in a permanent dipole in a boronic acid-thiophene backbone units (see Fig. 2b). As in the electron and hole states of the mesoscopic semiconductors of III-IV compounds [42], the charge centers of PTBA structure confined in two directions to a region of the unit cell, and also diminish of total charges of dipolar units indicate that there is a charge stability.

It was previously shown that the backbone shape and derealization of charge centers affect transport properties in the semiconducting polymers [18, 19], and hybridization between electronic orbitals of neighboring polymer backbones and also pending molecules is expected to be weak. Therefore, the location of these charge centers and geometric topologies are effected by the perturbative relatively weak Van der Waals forces between nearest polymeric layers and ionic polarizations. Because of these characteristic effects, the bond lengths and charge distributions of individual atoms of pending boronic acid and backbone of polymeric thiophene rings becoming different from each other (for bond lengths see Fig. la and b).


In addition to the calculations of structural and polarizations properties, the electro-optical properties of PTBA were also calculated. We begin with the calculation of electronic partial density of states (PDOS), which is plotted in Fig. 3. In order to further investigation of electronic structure, detailed partial density of states was calculated at the range of -3.1 eV - +3.1 eV with k-point separation 0.025/[Angstrom] (see inset of Fig. 3). We set the Fermi level to zero in all electronic density of states (DOS). The PDOS shows without states in the range about 1.35 eV, the region from -0.028 eV in valance band to the 1.32 eV in continuum band, i.e., PTBA is an extrinsic (p-type) semiconductor with the electronic band gap, [E.sub.g] = 1.35 eV, which is larger than that of polythiophene ([E.sub.g] = 1.05 eV) and poly(3-hexylythiophene) ([E.sub.g] = 0.99 eV) [18], It is well known that DFT estimates for the electronic band gap is essentially too small (experimental optical band gap value of the PTBA was found to be 1.92 eV [25]). The profile of valance band states can be classified into four ranges: the first range is mainly dominated of core s-electron states of boron and oxygen between -22 eV and -20 eV, and of carbon and sulfur between -19 eV and -17 eV, second is from [sp.sup.2] hybrids electron states of boron, carbon and sulfur atoms between -16 eV and -12 eV, third is also from sp2 hybrids electron states of all constituent atoms (but p-electrons are dominant) between -12.0 eV and -2.5 eV. And finally, the fourth range of DOS from -2.5 eV to the Fermi level, is originated from only p-electrons, the upper valance band states especially of carbon and sulfur atom's in which two sharp peaks at -1.860 eV and -2.039 eV dominated by pelectrons of sulfur atoms. The states in the vicinity of the Fermi energy (from the upper band of about -1.50 eV to the -0.028 eV) are dominated by p-electrons of carbon atoms. On the other hand the continuum bands from the Fermi level to about 3.0 eV comes from only p-electrons and the upper bands up to about 7.0 eV, s-[p.sup.2] and s-[p.sup.3] hybrids orbital electrons. As seen from PDOS profile of the material (see inset of Fig. 3), the states the near band edge from -3.1 eV in occupied valance bands to 3.1 eV in unoccupied conduction bands come from p-electrons, especially dominated by p-electrons of carbon and sulfur atoms, which are mainly responsible from intra bonds of polymeric rings as backbone of the structure.

There is a relation, on the other hand, between electronic DOS and optical parameters of the semiconductor structures and one of them constructs optical properties from dipole transitions between valance and conduction bands. If a semiconducting structure is influenced by photons, the relation is defined for dipole transitions from an initial state [E.sub.i] (into the valance band) to the final state [E.sub.f] (into conduction band) as [43]

[[epsilon].sub.2]([omega])=([h.sup.2][[pi].sup.2][e.sub.2]/[m.sub.2][[omega].sup.2]) [[summation].sub.if] [integral] [absolute value of [M.sub.if]] [delta]([E.sub.f](k) - [E.sub.i](k) - h [omega])dk, (1)

where the integral is over the unit cell, summation over all initial valence band and conduction band states of allowed ones, and [M.sub.if] is the momentum matrix elements for interband transitions. Then, imaginary part of dielectric function can be calculated from the Eq. 1, and also, the real and imaginary pars of the dielectric function are related to each other by the Kramers-Kronig relations [39], for detail see the references 42, 44. It is not here that, electronic band gap is of energy an electron hole pair in a semiconductor, whereas optical band gap is the exciton energy of interband transitions. Excitonic effects [27] are not treated in the present optical formalism, but we know that DFT estimates for the electronic band gap is essentially too small and therefore optical functions of DFT really red-shifted [18]. In order to calculate the optical properties of a material, it is efficient to calculate complex dielectric function, [epsilon] = [[epsilon].sub.1] + i[[epsilon].sub.2], or, complex refractive index N = n + ik, where, n and [[epsilon].sub.1] real parts of, and k and [[member of].sub.2] are imaginary parts of complex refractive index and dielectric function, respectively. There is a simple relations [43] between these quantities as [epsilon] = [N.sup.2], with [[epsilon].sub.1] = [n.sub.2] - [k.sub.2] and [[epsilon].sub.2] = 2 nk.

However, calculated optical properties as dielectric function, refractive index, reflectivity, and absorption coefficient are illustrated in Figs. 4-7. It is note that, as seen from Fig. 1, since polymeric flats are in (0n0) surfaces, in our calculations to the normal incident unpolarized light wave direction of [010] is chosen with the smearing 0.25 eV. For easy discussing on the optical properties, it is appropriate to consider the spectra into two energy range: the low energy region of 0.0 eV to 2.0 eV, which is the near band edge region and upper region of 2.0 eV. Generally, if some peaks have large line width in the spectra, they may come from some bands overlapping and interatomic interactions which split the atomic levels into quasi-continuous bands, then, in order to detail analysis of critical points of calculated optical spectra, such as peak positions and amplitudes, we recalculated the properties with the adjustable tiny smearing parameter as of 0.005 eV. The results depicted in the inset of each figure for the range of 0.0 eV - 3.1 eV (up to about 400 nm, including visible range).

As seen from Fig. 4, intensive broad peaks of real and imaginary parts of the dielectric function [epsilon]([omega]) are located at the low energy region of 0.0 eV to 2.0 eV, and more broader and less intensive peaks with shoulders are located at the regions of about 2.0 eV - 5.0 eV and 6.0 eV - 12.0 eV, respectively. In the low energy region, the first intensive real peak of resonance located at about 1.36 eV (amplitude ~ 32), followed by another set of less intensive peaks up to the minimum at about 1.65 eV (with negative amplitude ~ -19.5), otherwise, first imaginary peak at 1.38 eV (amplitude ~ 38) and last one at 1.61 eV(amplitude ~ 32)(see inset of Fig. 4). It is interesting that the imaginary part [[epsilon].sub.2]([omega]) is nearly constant between about 1.61 eV - 2.16 eV with diminishing intensity. Zero [[epsilon].sub.2] means that there is no frictional and dispersive effect on a carrier in the structure, i.e., it is not an excellent dielectric material in this frequencies range. On the other hand, dielectric materials have no many carriers but metallic materials have more carriers, and a negative [[epsilon].sub.1]([omega]) means that the material has enough carriers to attaining a metallic state [45], so we get negative values of [[epsilon].sub.1]([omega]) (see the inset of Fig. 4). As seen from Fig. 4, [[epsilon].sub.1]([omega]) is negative at around 1.47 eV, 1.54 eV, 1.65 eV in the low energy range (infrared region). Lowest negative value of [[epsilon].sub.1]([omega]) is appeared at about 1.65 eV (amplitude ~ -19.5). As a result, although PTBA is a semiconducting polymeric material, which has the electronic band gap of 1.35 eV, interestingly it has metallic character at about infrared photon energy ranges. In this range the real part [[epsilon].sub.1]([omega]) is higher than the imaginary part [[epsilon].sub.2]([omega]) as in experiment [25], Generally, it can be said that calculated complex dielectric spectra are in agreement with the previously reported experimental results by Yakuphanoglu and Senkal (see Fig. of 9 in Ref. 25).

The propagation of an electromagnetic wave on the other hand in coupled two different materials can describe by a complex quantity, the linear refractive index, N = n + ik. For vacuum and transparent materials it is purely real, and contrary for well absorbent materials completely imaginary. As seen from Fig. 5, at very low energy (at about 0.01 eV) the value of real part of refractive index is in the order of 2.36 and imaginary part 0.0. While increasing the energy, real and imaginary parts of the refractive index reach to the maxima and then, both reach to the zero. After ~ 2.0 eV, the amplitudes of shoulders are increasing up to energy of ~5.0 eV and then decreasing. In the low energy range (0.0 eV - 2.0 eV), the first peak of and also neighboring peaks are broadening (see inset of Fig. 5) of both real and imaginary parts of the refractive index. The first and last peaks of real and imaginary parts, in this energy range, are located at about 1.36 eV (amplitude ~5.9) and 1.61 eV (amplitude ~4.0), and 1.39 eV (amplitude -3.8) and 1.63 eV (amplitude ~4.8), respectively. Both parts of refractive index diminish interestingly at around 1.91 eV (~650 nm). It should be pointed out that the locations of real peaks in the low energy range are agreement with the experimental observation of refractive index (see Fig. 7 of Ref. 25).

The other optical constant is optical reflectivity, R, illustrated in Fig. 6 and is given in percent of incoming light, which is a function of the energy of light. It is note here that according to Beer's Equation [45], R is the real quantity: R = [[(n-1).sup.2] + [k.sup.2]]/ [[(n + 1).sup.2] + [k.sup.2]]. Due to transitions between the interbands, the optical reflectivity spectra for PTBA is distinctive and depending on the details of its electronic band structure. Of course, many more allowed transitions are possible. There are two regions of the reflectivity spectra in the ranges from 1.38 eV (far infrared waves) to 17.5 eV (the extreme ultraviolet waves).

The first region is defined from far infrared to the longest red waves, 2.0 eV (~620 nm), which is the most important part of spectra. In this region the material has 100% reflected in the red band from 1.66 eV (~747 nm) to 1.91 eV (~650 nm), and also there are seven well defined peaks with amplitude of around 0.60 from 1.38 eV (~898 nm) to 1.61 eV (~770 nm) (see inset of Fig. 6). This would be an agreement with the experimental observation [25] of increasing strongly with wavelength up to about 800 nm and decreasing with increasing wavelength. It is interesting that there is no reflection around 2.0 eV (~620 nm), from 1.94 eV (~639 nm) to 2.16 eV (~574 nm). In the second region from about 2.0 eV (620 nm) to the far ultraviolet waves, including visible range, there is no individual sharp peak bigger than with 0.40 amplitude (very week), which may be the residual influence of the multiply reflections inside the material, to the end of short length waves, except the peak around 4.70 eV (~264 nm) with the amplitude of 0.39 and large broad peak located at about 12.0 eV with shoulders in the region of 9.0 eV-14.0 eV. Incidentally, we have no experimental and/or theoretical data to support our perfect reflection results at present, and therefore further experimental and theoretical studies are desirable. Finally we would like to emphasize that PTBA is really a perfect reflector polymeric material, and consequently, is very worthy in the sense that it enables one to verify theoretical predictions.

However, according to the Fermi's golden rule [44], the transition rate of the absorption is depending on the coupling between the initial and final states and their individual densities. As seen from Fig. 7, aside from the sharp peaks at located around 1.6 eV (~775 nm) and 4.6 eV (~270 nm), the absorption spectra exhibit a strong and extremely broader peak (having many of shoulders) centered at about 11.0 eV with about very large line width (7 eV) and ~170.000 cm 'amplitude. Including these two characteristic sharp absorption peaks, each region of both peak locations are given in two insets of Fig. 7. The first region detailed in the inset of figure (left upside), in this region, from 1.38 eV (~898 nm) to the absorption edge at about 1.66 eV (~747 nm) there are several individual peaks with the amplitudes of about 62.000 [cm.sup.-1] to 125.000 [cm.sup.-1]. It is note here that, an absorption edge was observed experimentally [25] at about 840 nm for PTBA and also the lowest energy absorption shoulder was observed for poly(3-hexylythiophene) at about 2.0 eV [46], In the second inset (middle downside), there are also individual absorption peaks located at about of 4.50 eV - 4.70 eV with ~140.000 [cm.sup.-1] amlitude. Except very small peak at about 1.70 eV (~729 nm) with about 2.300 [cm.sup.-1] amplitude, which may be of the multiply reflections inside the material, there is no absorption peak from 1.66 eV (~747 nm) up to the peak 2.16 eV (~574 nm) with about 20.000 [cm.sup.-1] amplitude. Calculated DFT electronic band gap is about 1.35 eV, as expected it is lower than the experimental value of optical band gap ([E.sub.g] = 1.92 eV) [25], Regarding the electronic and optical properties, DFT estimates for the electronic band gap is essentially too small, meaning that the onset of spectral functions is really red-shifted.

Finally, it can be said that an agreement of our last peak at about 1.66 eV (about 747 nm, red-shifted) with the recent experiment [25] of an absorption edge observed at about 840 nm (about 1.46 eV).


We have presented results of a computational study on PTBA organic polymer, using first-principles calculations. Our DFT results indicate that asymmetric phase of PTBA polymer is the ground state and that consist of two coupled molecular units, i.e., a small molecule of positively charged pending boronic acid and negatively charged polymerized thiophene backbone which have anionic polymerization also as in the type of vinyl polymerization. Since all bonds are covalent in the unit cell, the polymeric PTBA structure could be called covalent polymer structure. Otherwise, PTBA is an extrinsic (p-type) semiconductor with the electronic band gap, [E.sub.g]= 1.35 eV, which is larger than that of polythiophene ([E.sub.g] = 1.05 eV) and poly(3-hexylythiophene) ([E.sub.g] = 0.99 eV). The states the near band edges from -3.1 eV in occupied valance bands to 3.1 eV in unoccupied conduction bands come from p-electrons, especially dominated by p-electrons of carbon and sulfur atoms, which are mainly responsible from intra bonds of polymeric rings as backbone of the structure. PTBA is an optic material and also perfect reflector polymer in the red band from 1.66 eV (~747 nm) to 1.91 eV (~650 nm). Seven well defined reflection peaks located from 1.38 eV (~898 nm) to 1.61 eV (~770 nm) (see inset of Fig. 6) would be correspond to the experimental observation [25] of increasing strongly with wavelength up to about 800 nm. Although it was not observed experimentally, the refractive index diminishes at around 1.91 eV (~650 nm) and the real part of dielectric function has negative values, attaining a metallic state interestingly in the low energy range (infrared region). It is well known that regarding the electronic and optical properties, DFT estimates for the electronic band gap is essentially too small; meaning of that onset of optical functions is really red-shifted. The last absorption peak at about 1.66 eV (about 747 nm, red-shifted) may be correspond to an absorption edge observed at about 840 nm (about 1.46 eV) at the recent experiment [25], Finally, the results, presented in this work indicated that it is a very promising multi-functional polymeric material for electronic technology and for also optical devices.


The author thanks Professor B. Inem, from Department of Metallurgy and Material Engineering, Gazi University for carefully reading of the manuscript.


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Mehmet Simsek

Department of Physics, Faculty of Sciences, Gazi University, 06500 Teknikokullar, Ankara, Turkey

Correspondence to: M. Simsek; e-mail: Contract grant sponsor: State of Planning Organization of Turkey; contract grant number: 2011K120290; contract grant sponsor: Gazi University BAP; contract grant number: 05/2010-82.

DOI 10.1002/pen.24297

Published online in Wiley Online Library (

TABLE 1. Optimized structural parameters of PTBA from the different
approaches of Castep, D[Mol.sup.3] and Forcite packages.

Quant/             Castep                    D[Mol.sup.3]   Forcite
Method             GGA/PBEsol    LDA/CA-PZ   GGA/RPBE       Ab/ABNR

a ([Angstrom])       8.616         8.524       8.977          8.554
b ([Angstrom])       4.360         4.205       4.772          4.164
c ([Angstrom])       7.774         7.727       7.886          7.670
[delta] (degree)   113.1         117.0       112.7           74.2
[beta] (degree)     90.0          89.2        89.3           93.8
[gamma] (degree)   106.8         112.1       112.2          104.1
V ([[Angstrom]
  .sup.3])         255.0         224.5       284.9          255.0
  ([Angstrom])       1.557         1.539       1.569          1.571
  ([Angstrom])       1.389         1.377       1.400          1.374

a, b, c, [alpha], [beta], and [gamma] represent the lattice parameters
and V is the volume of unit cell in PI symmetry. [d.sub.B-C] and
[d.sub.C-C] represent the shortest boron-carbon and carbon-carbon bond
lengths, respectively.

TABLE 2. Calculated fraction coordinates (u, v, and h1) of atoms,
hydrogen (H), boron (B), carbon (C), oxygen (O) and sulfur (S)
atoms in two formula units of PTBA {2([C.sub.4][H.sub.3]B[O.sub.2]S)}.

            coordinates of atoms

    u          v           w

H   1.037904    0.003363    0.973847
H   1.548068    0.070118    0.487780
H   1.711983   -0.400060    0.084372
H   0.707859   -0.288881    0.607796
H   1.755245    0.039164   -0.098090
H   0.822234   -0.075478    0.366052
B   1.628787    0.030420    0.127525
B   0.944591   -0.035333    0.612641
C   1.300972    0.172948    0.977738
C   1.140381    0.084059    0.901139
C   1.118306    0.086351    0.721272
C   1.269512    0.178383    0.655796
C   1.318936    0.183345    0.479086
C   1.464383    0.136858    0.412507
C   1.487250    0.142242    0.233219
C   1.352223    0.193448    0.159879
O   1.693994   -0.181430    0.179203
O   0.811851   -0.177938    0.692947
O   1.690129    0.161376   -0.001969
O   0.927321   -0.000030    0.449701
S   1.428690    0.263404    0.820620
S   1.208595    0.240169    0.315076
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Author:Simsek, Mehmet
Publication:Polymer Engineering and Science
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Date:Jun 1, 2016
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