DFT study of polythiophene energy band gap and substitution effects - Document

Polythiophene ( PTh ) and its derivatives are polymer-based materials with a [ pi ] -conjugation model. PTh is a useful photoelectric material and can be used in organic semiconductor device devices, such as PLED, OLED, and solar cells. Their properties are based on molecular structure ; the derivatives contain unlike substitutes in the 3 and 5 positions, such as electron-donating or electron-withdrawing groups. All molecular geometries were optimized at B3LYP/6-31G ( five hundred, phosphorus ) horizontal surface of hypothesis. The department of energy break ( [ E.sub.gap ] ) between the HOMO and LUMO levels is related to the [ pi ] -conjugation in the PTh polymer backbone. In this study, the DFT calculations were performed for the nonsubstituted and 3,5-substituted variants to investigate the stability geometries and electrical properties. The theoretical calculations show that the substitute forms are static, have low [ E.sub.gap ], and are in good agreement with the experimental observations. Full text :
1. initiation Conjugated polymers are of considerable interest due to their electronic properties and their likely technical applications [ 1 ]. They have many advantages compared to inorganic semiconductors, such as easy processing and tunable ocular gaps. Their electronic properties are determined by the delocalize [ pi ] -electrons along their carbon paper spine [ 2 ]. Polythiophene is the most crucial conjugated polymer utilized in a across-the-board spectrum of applications such as conducting polymers, light-emitting diodes, battlefield effect transistors, and fictile solar cells, due to its excellent ocular and electric properties american samoa well as especial thermal and chemical constancy. early on syntheses of poly ( 3-alkythiophene ) involve chemical oxidation or electrochemical polymerization in the pursuit of soluble and processable polythiophenes [ 3-5 ]. however, these processes suffered from the major problem that the structure of the polymer is reasonably changeable and undefined [ 6,7 ]. As 3-alkylthiophene is an asymmetrical molecule, there are three potential orientations available when the two thiophene rings are coupled between the 2 and 5 positions. The first of these is the 2-5 ‘ or head-to-tail copulate ( HT ). The second is 2-2 ‘ or tete-a-tete copulate ( HH ), and the third is 5-5 ‘ or tail-to-tail copulate ( TT ) ( see Figure 1 ).

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Polythiophene ( PTh ) and its derivatives are used in several applications such as displays, electromagnetic shielding, and molecular electronics [ 8 ] and, due to them being thermally stable at ambient temperatures, have been used in new ocular devices such as surface light-emitting diodes ( SLED ) and light-emitting diodes ( LED ) [ 9 ]. One of the main goals in the field of electrically conducting polymers is to develop a complete understand of the relationship between the chemical structure of the polymer and its electronic and conduction properties. once such an insight is achieved, the desire electronic properties could be obtained by specific synthesis following molecular design. The conduction properties of an undoped polymer, in terms of the ring theory of solids, are known to be related to its electronic properties, such as the ionization likely ( IP ), electronic affinity ( EA ), and band col ( [ E.sub.gap ] ) [ 10 ]. The key agent that determines the intrinsic properties of the polythiophene variants is their band structures, peculiarly the positions of the conduction and valence bands and the gap between them. therefore, the [ private detective ] -electrons in conducting polymers play a major function in determining their electrical conduction and ring structure [ 11 ]. The department of energy between the valence and conduction ring of a polymer is related to the lowest energy of its monomer units and to the bandwidth resulting from the overlap between the monomer orbitals [ 12 ]. A band gap is defined as the dispute between the highest occupied molecular orbital ( HOMO ) and the lowest unoccupied molecular orbital ( LUMO ) energy levels in the polymer [ 13 ] : [ E.sub.gap ] = [ E.sub.LUMO ] – [ E.sub.HOMO ] ( electron volt ). ( 1 ) So the electric conduction is directly related to the HOMO and LUMO energy of the atom. 2. Methods : Computational Details Full geometry optimizations were performed, under no constraints, in the framework of the concentration running hypothesis ( DFT ) by means of the B3LYP functional [ the Becke3 and the Lee-Yang-Parr loanblend functional ], using the gaussian 09 broadcast [ 14 ]. The 6-31G ( five hundred, phosphorus ) footing set was chosen as a compromise between the quality of the theoretical border on and the high computational price associated with the high count of dimensions to the problem [ 15,16 ]. The HOMO, LUMO, and energy gaps were besides deduced from the stable structure of the nonsubstituted forms. In this wallpaper, the transition energies were calculated at the establish department of state geometries using TD-DFT/B3LYP calculations, and the results were compared with the available experimental data. A newly designed affair, the long-range Coulomb-attenuating method acting ( CAM-B3LYP ), considered long-range interactions by comprising 19 % of HF and 81 % of B88 exchange at short stove and 65 % of HF plus 35 % of B88 at long rate [ 17 ]. furthermore, the CAM-B3LYP method acting has been applied and was found to be sanely adequate to of predicting the excitation energies and the absorption spectrum of the molecules [ 18-21 ]. Therefore, the vertical excitation energy and electronic absorption spectra were simulated using the TD-CAM-B3LYP method acting in this work. The investigate polymers, 8T, 8TCOO, and 8TOC ( HH, HT, and TT ) and 8TCOC ( HH, HT, and TT ), are depicted in Figure 1 .

3. Results and Discussion 3.1. Geometric Properties. For all molecules, geometric parameters were obtained after total optimization by B3LYP/ 6-31G ( five hundred, phosphorus ). To investigate the effect of the substituents on the geometries and electronic properties, the optimize structures of respective substitute oligomers built on thiophene 8TOOC, 8TCOC ( HH, HT, and TT ), and 8TOC ( HH, HT, and TT ) are compared with the unsubstituted one, 8T. On the other hand, to investigate the effects of unlike regioregularities, ( HH, HT and TT ) are compared between them and with the 8T and 8TOOC compounds. The geometric characteristics, given as bury distances ( [ d.sub.i ] ) and dihedral angle ( [ [ theta ] .sub.i ] ), are listed in Tables 1 and 2. The optimize structures ( see Figure 4 ) of compounds ( 8TCOOC and 8TOC, with different regioregularities ) are compared with each other and besides with 8T and 8TOOC. The geometric characteristics are given as bury distances ( [ d.sub.i ] ) and used to investigate the effect of unlike regioregularities ( HT, HH, and TT ). The theoretical calculations show that the torsional angles are evaluated to be about [ approximately adequate to ] 180 [ degrees ] ( a quasiplanar conformity ) for 8T and 8TOC ( HT and TT conformations ), whereas the other compounds show an antigauche slant with an average bend angle between 122 [ degrees ] and 170 [ degrees ] for 8TCOC ( HH and TT ) and the 8TCOC ( HT ) shows the value of tortuosity slant between the values of 8TCOC ( HH and TT ) of about 150 [ degrees ] ( see Tables 1 and 2 and Figures 2 and 3 ). The properties of the oligomer for 8TOC ( HT and TT ) can be explained by the effects of the OC [ H.sub.3 ] donor group compared with the C [ H.sub.2 ] -OC [ H.sub.3 ] group. The electronic doublet of the methoxy group increases the junction, which promotes better two-dimensionality of the system, in contrast to the C [ H.sub.2 ] OC [ H.sub.3 ] group. These results are audited by the intercyclic report of the values of the bindings. This shows that, in going from the 8T compound to 8TOOC and 8TOC ( HH, TT, and HT ) compounds, there is a reduction of the values of the bury bindings ( see Tables 1 and 2 and Figures 2 and 3 ). These results may be explained by a repulsion of the OOC [ H.sub.3 ] ( 8TOOC ) and OC [ H.sub.3 ] ( 8TOC ) groups among them that favors the flatness of these systems. The comparison of the values of the bindings of intercyclic 8T compounds with 8TCOC shows an increase of these values for those systems with antigauche torsion angles and indicates the strong interaction ( attraction ) of the C [ H.sub.2 ] OC [ H.sub.3 ] groups among them. 3.2. electronic Parameters. table 3 lists the theoretical electronic parameters of the studied conjugate compounds. The calculate electronic parameters ( [ E.sub.gap ], LUMO, and HOMO ) of compounds 8T, 8TCOC ( HH, HT, and TT ), 8TOOC, and 8TOC ( HH, HT, and TT ) exhibit the destabilization of the HOMO and LUMO levels in comparison with those of the unsubstituted one ( 8T ) due to electron-donating substitution groups, while, in the case of 8TOC, there is a net stabilization of the HOMO and LUMO levels. however, these compounds have a smaller energy break ( [ E.sub.gap ] ) than the unsubstituted one, which is due to the presence of a methoxy group in the [ beta ] position of the thiophene surround. The band gap of 8TOC is much smaller than that of the other substitute compounds. This may be attributed to the number of electron-donating methoxy english groups and besides to the Coulombic interaction between the sulfur of thiophene and oxygen atoms [ 27, 28 ]. The theoretical band gaps computed for disjunct chains are expected to be about 0.2 eV larger than the values computed in the condense phase 29 ]. When taking into report this difference, the band break values obtained are near to the experimental datum for the octamer. The prize of the octamer ‘s 8T and 8TOC band gap ( 2.22 electron volt and 1.93 electron volt, resp., after correction ) is in accord with that measured experimentally for polythiophene, 2.0-2.3eV [ 30, 31 ], and that for poly ( 4,47-dimethylbithiophene ( 8TOC ( TT ) ) ) was estimated to be 1.6 electron volt [ 23 ]. The octamer seems to be a utilitarian model to obtain a better sympathize of the electronic properties of the polymeric system. It is significant to examine the HOMO and the LUMO energies for these oligomers because the proportional order of the take and virtual orbital provides a reasonable qualitative indication of excitement properties. In general, as shown in figure 5 ( LUMO, HOMO ), the HOMOs of these oligomers possess a [ pi ] -bonding character within a fractional monetary unit and a [ private detective ] -antibonding character between the straight subunits. On the other hand, the LUMOs own a [ principal investigator ] -antibonding fictional character within a fractional monetary unit and a [ pi ] -bonding character between the subunits. The experiment shows that the HOMO and LUMO energies are obtained from an empirical formula based on the attack of the oxidation and reduction peaks measured by the cyclic voltammetry. In theory, the HOMO and LUMO energies can be calculated by the DFT calculation [ 32, 33 ]. however, it is noticeable that solid-state packing effects are not included in the DFT calculations, which tend to affect the HOMO and LUMO energy levels in a thin film compared with an disjunct molecule, as considered in the calculations. even if these calculated energy levels are not accurate, it is possible to use them to obtain the information by comparing alike oligomers or polymers. The calculate electronic parameters ( Gap, HOMO, and LUMO ) of compounds 8T, 8TOOC, 8TOC ( HH, HT, and HT ), and 8TCOC ( HH, HT, and HT ) are illustrated in postpone 3. In the character of compounds 8TOC, 8TOOC, and 8TCOC, it is luminary that there is a taxonomic change of the HOMO and LUMO energies into the spine ; substitution raises or lowers the HOMO/LUMO energies in agreement with their electron donor character. however, these compounds have a smaller department of energy gap ( [ E.sub.gap ] ) than the unsubstituted one ( 8T ), which is due to the presence of different substituents as described previously [ 34, 35 ]. The band gap of 8TOC ( HH, TT, and HT ) is much smaller than that of the other substitute compounds. This possibly attributed to the number of electron-donating methoxy side groups. 3.3. Photovoltaic Properties. broadly, the most efficient polymer solar cells are based on the bulge heterojunction ( BHJ ) structure of the blend of [ pi ] -conjugated polymer donors and fullerene derivative acceptors [ 7, 36-40 ]. here, we studied the photovoltaic properties of polyalkyl thiophene with [ 6,6 ] -phenyl-C61-butyric acidic methyl ester ( PCBM ), which is the most widely used acceptor in solar cellular telephone devices. The equate structure of the photovoltaic devices is schematically depicted in Figure 6. The remainder in the HOMO energy levels of 8T, 8TCOC, 8TOOC, and the LUMO of PCBM was 0.73 electron volt, 0.16 electron volt, and 0.62 electron volt, respectively, but the value of 8TOC is minus ( table 3 ), suggesting that the photoexcited electron transplant from the study compounds to PCBM may be sufficiently effective to be utilitarian in photovoltaic devices [ 24, 41, 42 ]. In principle, the optimization of polymer-fullerene solar cells is based on polish the electronic properties and interactions of the donor and acceptor components thus as to absorb the maximal measure of light and generate the greatest number of complimentary charges, with minimal accompaniment loss of energy and transport the charges to the electrodes. however, it is necessary to know the ideal electronic characteristics that each component should have for the purpose of the next generation, high-efficiency photovoltaic systems. The two components required in these devices for electronic optimization are a soluble fullerene ( by and large a C60 derivative instrument ) acceptor and a polymeric donor that can be processed in solution. Fullerenes are presently considered to be the ideal acceptors for organic solar cells for several reasons. First, they have an energetically deep-lying LUMO which endows the molecule with a identical high electron affinity relative to the numerous electric potential organic donors [ 25 ] .

It is apparent that the active layer donor-acceptor composite governs all aspects of the mechanism, with the exception of charge collection, which is based on the electronic interface between the active layer complex and the respective electrode. Besides the cardinal mechanistic steps, the outdoors racing circuit electric potential ( [ V.sub.oc ] ) is besides governed by the energetic kinship between the donor and the acceptor ( Figure 6 ) rather than the work functions of the cathode and anode, as would be expected from a simplistic view of these diode devices. specifically, the energy remainder between the HOMO of the donor and the LUMO of the acceptor is found to be very closely correlated with the [ V.sub.oc ] respect [ 43-45 ]. The maximal open racing circuit electric potential ( [ V.sub.oc ] ) of the BHJ solar cell is related to the difference between the highest busy molecular orbital ( HOMO ) of the electron donor and the LUMO of the electron acceptor, taking into bill the energy lost during the photocharge generation [ 46 ]. The theoretical values of open circuit voltage [ V.sub.oc ] have been calculated from the following formulation : [ V.sub.oc ] = [ absolute value of [ E.sub.HOMO.sup. ( Donor ) ] ] – [ absolute measure of [ E.sub.LUMO.sup. ( Acceptor ) ] – 0.3. ( 2 ) furthermore, these compounds have assimilation maximum in the vicinity of 400-540 new mexico ( table 4 ) which is the optimum range of preoccupation ( UV-visible ) for photovoltaic cells. In addition, they have an energy opening of the order of 2.00 electron volt ( table 3 ) for the elevation of the electron from the HOMO to the LUMO orbitals. This characteristic allows us to promote these compounds for the production of solar cells. These values are sufficient for a possible effective electron injection system. therefore, all the analyze molecules can be used as organic solar cell components because the electron injection process from the excite molecule to the conduction band of the acceptor ( PCBM ) and the subsequent positive feedback is possible in photovoltaic cells. 4. Optoelectronic Parameters 4.1. electronic Absorption Spectra. For a better sympathy of the electronic properties of thiophene-based molecules, the excitation energy based on the inaugural and second singlet-singlet electronic transitions has been studied. In late years, prison term dependent concentration functional theory ( TD-DFT ) has emerged as a authentic criterion tool for the theoretical treatment of electronic excitation spectrum and late works demonstrate better accuracy for a wide range of systems [ 47, 48 ]. The TD-DFT/B3LYP/6-31G ( five hundred, p ) has been used on the footing of the optimize geometry. The nature and the energy of the singlet-singlet electronic transitions of all the oligomers in all series under study are reported in table 4. All electronic transitions are [ private detective ] [ right arrow ] [ [ pi ] .sup. * ] type and no localize electronic transitions are exhibited among the calculate singlet-singlet passage. table 4 summarizes the transition energies of concentration spectrum and oscillator military capability. Two interest trends in oscillator persuasiveness can be found in all series of compounds : the oscillator intensity OS of [ S.sub.0 ] [ right arrow ] [ S.sub.1 ] is the overwhelmingly largest of all the series of polymers. According to Table 4, we note an excitation energy of about 2 eV for all compounds studied. In addition, preoccupation maxima were located in the 400-540 new mexico range for all compounds, indicating all molecules have lone one band in the visible region ( [ [ lambda ] .sub.abs ] > 400 new mexico ). The concentration spectrum of these compounds showed a major assimilation band assigned to the [ pi ] [ right arrow ] [ [ principal investigator ] .sup. * ] transition local electron. According to the transition character, most of the compounds show the HOMO [ proper arrow ] LUMO conversion as the first singlet excitement. The maximum ( [ [ lambda ] .sub.max ] ) wavelengths for UV-vis absorption spectrum of all compounds simulated are shown in Figure 7. The calculate [ [ lambda ] .sub.max ] by TD-DFT-B3LY and TDCAM-B3LYP is different by about 99 new mexico = [ [ lambda ] .sub.max ] ( B3LYP ) — [ [ lambda ] .sub.max ] ( CAM-B3LYP ). The prediction of the excitation energies and oscillator strengths by using the CAM-B3LYP is in better agreement with the experimental results [ 49, 50 ] than is found for B3LYP, because the CAM- ( Coulomb-attenuating method- ) B3LYP besides takes report of the long-range corrections for describing the hanker [ principal investigator ] -conjugation [ 51 ]. The results are shown in postpone 4, and we note that the excitement energy values [ DELTA ] E obtained by TD-DFT/CAM-B3LYP/6-31G ( five hundred, p ) flat are in agreement with those obtained by the methods DFT/B3LYP/ 6-31G ( d, phosphorus ) level ; therefore the CAM-B3LYP functional is very utilitarian for predicting the excitation energies and oscillator strengths. The values calculated using the TD-DFT CAM-B3LYP function of our compounds in vacuo for predicting the excitement energies and oscillator strengths are listed in mesa 4. The TD/CAM-B3LYP/6-31G ( vitamin d, phosphorus ) method was besides employed to simulate the ocular property of the compounds .

4.2. effect of Solvent. Solvent effects attract considerable attention because most of the chemical processes occur in the solution phase. The solution effects are included here to ensure that the calculations are compatible with the typical conditions. The effects of chemical environment are discussed far as follows. In order to study the effect of solvents on the ground state molecular geometry, excitation energies and maximal assimilation quantum chemical calculations on the learn molecules were carried out in vacuum, in dimethyl sulfoxide ( DMSO ). The longest wavelength peaks from C-PCM-TD-CAM-B3LYP/6-31G ( d, phosphorus ) results are shifted compared to the corresponding ones using the TD-CAM-B3LYP results. The magnitudes of the apparitional shifts are about 15 nm for 8T, 8TOC ( HT ), and 8TOOC and 18.81, 22.01, 23.68, 8.17, and 10.62 new mexico for 8TCOC ( TT ), 8TOC ( TT ), 8TOC ( HH ), 8TCOC ( HH ), and 8TCOC ( HT ), respectively. The preoccupation maximum of all compounds are promote shifted by the consequence of the dimethyl sulfoxide solvent. This might be related to the gradient electrostatic likely and donor impression of the OC [ H.sub.3 ], C [ H.sub.2 ] OC [ H.sub.3 ], and OOC [ H.sub.3 ] groups. further, there is a version of the absorption maximum translation for the lapp compound ( HT, TT, and HH ) ; this is the solution of the different configurations that one compound can take. In DMSO the calculate results for the absorption maximum of all compounds are located between 568.23 and 404.35 nm ( table 4 ). The flower positions of those molecules in DMSO show a crimson switch when compared to those measured experimentally [ 26 ]. 4.3. electronic emission Spectra. TD-DFT//B3LYP/6-31G ( five hundred, p ) has been used for optimize structures and CAM-B3LYP/631G ( vitamin d, phosphorus ) was used to calculate the excite department of state and to simulate the emission spectrum of the considered polymers and the first singlet excited states are listed in table 5. like to the absorption spectrum, [ S.sub.1 ] [ correct arrow ] [ S.sub.0 ] fluorescence peaks in emission spectrum ( Figure 6 ) have the greatest oscillator strengths in all molecules and entirely arise from HOMO [ right arrow ] LUMO ( [ pi ] [ right arrow ] [ [ pi ] .sup. * ] electronic transition ). Polymers with aromatic or heterocyclic units by and large absorb light with wavelengths in the range from 300 to 700 nm due to [ principal investigator ] [ right arrow ] [ [ protease inhibitor ] .sup. * ] transitions. The excited states of their chromophores ( excitons ) release energy radiatively adenine well as nonradiatively on returning to the ground state [ 26 ]. The radiative disintegrate of excitons to the flat coat state can emit visible light. These excitons are besides formed when a bias electric potential is applied to an emissive polymer sandwiched between an anode and a cathode [ 52 ]. The theoretical studies of our molecules can be used to derive the Stokes shifts, the dispute between the position of the maximum band of the preoccupation and emission spectrum in wavelength, which is about 80 to 93 nm for the structures 8T, 8TOC ( HH, TT, and HT ), 8TCOC ( TT ), and 8TOOC, but that for the 8TCOC ( HH ) and 8TCOC ( TT ) is 149 nanometer. Among them, the compound 8TCOC has a relatively larger Stokes transfer, which indicates that electron transitions break the hard conjugate effect, leading to noteworthy changes in the social organization and reorganization energy, as discussed above. 5. Conclusions In this survey, we have used the DFT/B3LYP method acting to investigate the theoretical analysis of the geometries and electronic properties of some type-in-derivatives structure of polythiophene. The modification of chemical structures can greatly modify and improve the electronic and ocular properties of pristine studied materials. The electronic properties of the conjugate materials based on thiophene compounds have been computed using the 6-31G ( vitamin d, phosphorus ) footing set at a concentration functional B3LYP level, in order to guide the synthesis of novel materials with specific electronic properties. In summary, the conclusions are as follows. ( one ) The UV-vis assimilation properties have been obtained by using TD/CAM-B3LyP and TD-B3LYP calculations. The receive concentration maximums are in the rate of 350-550 new mexico. ( two ) The HOMO floor, LUMO tied, and band gap of the analyze compounds were well controlled by the acceptor strength. The count band gap ( [ E.sub.gap ] ) of the studied molecules was in the range of 1.91-3.02 electron volt. ( three ) The account values of [ V.sub.oc ] of the study molecules range from 0.16 electron volt to 0.91 eV/PCBM ; these values are sufficient for a possible effective electron injection .

The theoretical results suggest that both the donor strength and the stable geometry contribute significantly to the electronic properties of the conjugate polymers. last, the procedures of theoretical calculations can be employed to predict the electronic properties of the other compounds and far to design novel materials for organic solar cells. hypertext transfer protocol : //dx.doi.org/10.1155/2015/296386 conflict of Interests The authors declare that there is no conflict of interests regarding the publication of this newspaper. Acknowledgments This work has been supported by the Volubilis Program ( no. MA/11/248 ) and the convention CNRST/CNRS ( Project Chimie 1009 ). The authors are grateful to the “ Association Marocaine des Chimistes Theoriciens ” ( AMCT ) for its pertinent avail. Guillermo Salgado-Moran thankfully acknowledges Si. Mohamed Bouzzine for his invitation to participate in this work. Daniel Glossman-Mitnik is a research worker of CIMAV and CONACYT and acknowledges both institutions for partial digest. References [ 1 ] H.-J. Wang, C.-P. Chen, and R.-J. Jeng, “ Polythiophenes comprising conjugate pendants for polymer solar cells : a inspection, ” Materials, vol. 7, no. 4, pp. 2411-2439, 2014 .

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