DFT study on some polythiophenes containing benzo[d]thiazole and benzo[d]oxazole: structure and band gap

ABSTRACT The content of this paper focuses/shed light on the effects of X (X = S in P1 and X = O in P2) in C11H7NSX and R (R = H in P3, R = OCH3 in P4, and R = Cl in P5) in C18H9ON2S2-R on structural features and band gaps of the polythiophenes containing benzo[d]thiazole and benzo[d]oxazole by the Density Function Theory (DFT) method/calculation. The structural features including the electronic structure lattice constant (a), shape, total energy (Etot) per cell, and link length (r), are measured via band gap (Eg) prediction with the package of country density (PDOS) and total country density (DOS) of material studio software. The results obtained showed that the link angle and the link length between atoms were not changed significantly while the Etot was decreased from Etot = – 1904 eV (in P1) to Etot = – 2548 eV (in P2) when replacing O with S; and the Etot of P3 was decreased from Etot = – 3348 eV (in P3) when replacing OCH3, Cl on H of P3 corresponding to Etot = – 3575 eV (P4), – 4264 eV (P5). Similarly, when replacing O in P1 with – S to form P2, the Eg of P1 was dropped from Eg = 0.621 eV to Eg = 0.239 eV for P2. The Eg of P3, P4, and P5 is Eg = 0.006 eV, 0.064 eV, and 0.0645 eV, respectively. When a benzo[d]thiazole was added in P1 (changing into P3), the Eg was extremely strongly decreased, nearly 100 times (from Eg = 0.621 eV to Eg = 0.006 eV). The obtained results serve as a basis for future experimental work and used to fabricate smart electronic device.


Introduction
In recent years, polythiophene-containing heterocycles have many advanced applications based on their high environmental sustainability, structural flexibility, optical stability, and electrochemical characteristics [1][2][3][4][5][6][7][8][9][10][11][12]. They were reported as potential functional materials, such as organic field-effect transistors [13,14], organic light-emitting diodes [15,16], organic photovoltaic cells [17], and other optoelectronic devices [18,19]. Moreover, they also have many applications in pharmacology as water-soluble sensing agents for the recognition of DNA, proteins, and metal ions [20][21][22], thermochromism, photochromism, and biochemist [23][24][25]. Among those, benzothiazole-based polythiophenes have attracted much attention thanks to their wide range of biological activities [26,27]. A novel conducting poly[3-(benzothiazole-2-yl)] thiophene polymerized by electrochemical and chemical synthesis has been studied for its optical absorption and photoluminescence characteristics [28][29][30]. Some technologically advanced methods were applied for the synthesis of 3-(benzothiazole-2-yl)thiophene from the reaction of thiophene-3-carbaldehyde with o-amino thiophenol in refluxing ethanol [31] or under microwave radiation without solvent and catalyst [32]. However, very few studies based on the molecular orbital calculations have been performed for oligothiophenes containing heteroaromatic side chains. Basing on theoretical prediction, Radhakrishnan S. et al. using suggested the structure optical properties relationship of oligothiophenes having four thiophene units [27,33]. In general, the periodic calculations of geometrical stability and electrical properties of the polythiophenes are often unattended with their published experimental data while these theoretical properties are important bases for deeply understanding the nature and application ability in the energy industry and electrically conducting materials. Therefore, structural analysis based on the theoretical calculation of polythiophene derivatives is very necessary. Among theoretical quantum mechanical methods, Density Functional Theory (DFT) method is well-known as an effective method for evaluation of the transition temperature, electronic properties, and structural characteristics of the π-conjugated polythiophene derivatives [34][35][36][37][38][39][40][41][42]. However, only a few studies on the electronic structures of polymers using the DFT method have been done to control the band gap for determining the alternation between conductors and insulators in solar cells, diodes, or transistors. A popular pathway to synthesize new polythiophene derivatives is substitution in polythiophenes, for example, replacement of S atom with Se or Te atoms [43] or replacement of H atoms with CH 3 , NH 2 , NO 2 , or Cl [44]. The DFT method has been applied for the assessment of structural and electronic characteristics of 4 H-cyclopenta [2,1-b,3;4-b′] dithiophene S-oxide derivatives including X (X: O, S, S = O, BH 2 , SiH 2 ) as a bridge [45]. The other five-membered ring molecules and ionization energies (IEs) and the heats of formation of thiophene were calculated and reached a high precision level of ab initio predictions [46]. Recently, the team members also studied the factors affecting the structural, mechanical, and magnetic properties of metal Fe [47,48], Al [49][50][51], Ni [52][53][54], Ag [55], alloys AlNi [56], NiCu [57,58], FeNi [59,60], AgAu [61], NiAu [62], and replace the H derivatives of poly C 13 H 8 OS-H with metal atoms Br, Cu, Kr, Ge, As, Fe [63] showed that the E g band gap decreased, leading to an increase in the conductivity. The obtained results will contribute to research to find materials new for application in the industrial age. In addition, the electronic and optical characteristics of 4 H-cyclopenta [2,1-b:3,4-b′]bithiophene derivatives combining with a variety of functional groups including carbon atoms and heteroatoms in the 4-position were predicted using DFT calculations [64]. Along with that, the energy band gap (E g ) of C 13 H 8 OS was decreased to E g = 1.621 eV when doping with Br, while the energy band gaps of C 13 H 8 OS was increased to E g = 1.646, 1.697, 2.04, and 1.920 eV, respectively, when doping with H, OH, OC 2 H 5 , or OCH 3 groups. The obtained results proved that the substituents had a remarkable effect on the link length as well as band gap of polythiophene derivatives, and molecular shape. More recently, our research group studied some novel polythiophenes containing benzo[d]thiazole that presented a catalyst-and solvent-free microwave-assisted synthesis of mono thiophene [8]. Their structure and properties were determined by FT-IR, 1 H-NMR, 13 C-NMR spectra, single-crystal X-ray diffraction, and the TGA method [32]. We have also mentioned the chemical polymerization of the monomers using anhydrous FeCl 3 as an oxidant in anhydrous chloroform. However, the structures, phase transition temperature, and electronic property data of these polymers are rather limited. The purpose of this work is to predict the theoretical structure and properties in the ideal state of some newly synthesizing polythiophenes containing benzo[d]thiazole and benzo[d]oxazole using DFT calculation. With the available results, we are looking forward to synthesize these polythiophenes using chemical polymerization to environmental stability, improve their processability, and electrical properties.

Effect of the material
The structural features and band gap of poly C 11 H 7 NS-O (P1), C 18 H 9 ON 2 S 2 -H (P3) are shown in Figure 2 and Table 1.
The obtained results show that in C 11 H 7 NS-O (P1) molecule, the length of C-H links is between 1.106 Å ÷ 1.108 Å, C-N is 1.459 Å ÷ 1.430 Å, C-S is 1.883 Å ÷ 1.924 Å, C-C is 1.532 Å ÷ 1.545 Å, and the link angle H-C-H is in the range of 106. . Besides, the total energy of the system (E tot ) of the P1 and P3 is E tot = -1904 eV and E tot = -3348 eV, respectively. When adding benzo [d] thiazole to the P1, the link angle and the link length of the P3 between the atoms as well as the benzene ring did not change significantly. This indicated that the structure of P1 did not change when it was added to a benzene ring although benzene ring can lead to a change in the shape, size, and total energy of the E tot system (Figure 2a1, Figure 2a2) corresponding to base cell size with wide-band gap decreased from E g = 0.6210 eV to E g = 0.0060 eV (Table 1). These results are important bases for predicting the structure of polythiophenes containing benzo[d]thiazole and benzo [d] oxazole as well as the change of their E tot and E g .

Effects of doping
The P1 and P2 polymers were chosen as the basic materials, then their composition is changed as follows: O was substituted with S in P1: C 11 H 7 NS-O to get P2: C 11 H 7 NS-S. Similarly, replacing H with OCH 3 in P3: C 18 H 9 ON 2 S 2 -H to get P4: C 18 H 9 ON 2 S 2 -OCH 3 and Cl to get P5: C 18 H 9 ON 2 S 2 -Cl was done. The molecular geometry and band gap of the above samples are displayed in Figure 3 and Table 2. The change in link lengths is due to the electrostatic interaction when the O atom in P1 is exchanged by S (in P2) and the H atom in P3 is exchanged by OCH 3 (in P4) and Cl (in P5). This factor has led to a decrease in the total energy of the system (E tot ) corresponding to the polymers as P1: E tot of E tot = -1904 eV, P 2 : E tot of E tot = -2548 eV, P3: E tot of E tot = -3348 eV, P4: E tot of E tot = -3575 eV, P5: E tot of E tot = -4264 eV. Besides, the band gap (E g ) corresponds to the P1: C 11 H 7 NS-O has E g of E g = 0.6210 eV (Figure 3b1). When replacing -O by -S, the structural shape has the form of P2: C 11 H 7 NS-S (Figure 3a2) and E g of E g = 0.2390 eV (Figure 3b2). Similarly, the P3, C 18 H 9 ON 2 S 2 -H has the E g of E g = 0.0060 eV (Figure 3b3) when replacing -H by -OCH 3 , the P4, C 18 H 9 ON 2 S 2 -OCH 3 has the E g of E g = 0.0640 eV (Figure 3b4), -H equals -Cl in the P5, C 18 H 9 ON 2 S 2 -Cl with E g of E g = 0.0645 eV (Figure 3b5). These results indicated that when replacing O with S, the E g decreased, and replacing the O atom in P1 with S led to the decrease of E tot and E g , and with -H of P3 by -OCH 3 , -Cl, then E g increased, and E tot decreased. When a benzo[d]thiazole was added in P 1 (changing into P 3 ), the E g was extremely strongly decreased, nearly 100 times (from E g = 0.6210 eV to E g = 0.0060 eV). The results obtained are very helpful for these future experimental researches. To confirm, we have studied the electron density in the energy bands. The results of electron density in the energy bands of C 11 H 7 NS with different impurities are shown in Figure 4.
The obtained results show that the electronic density of P1: C 11 H 7 NS-O, P3: C 18 H 9 ON 2 S 2 -H with the energy bands (E) of E = -20 eV, -15 eV, -10 eV, -7.5 eV, -5 eV, 0.00 eV, 5 eV, or 7.5 eV has the corresponding electronic density P1: 0.00%, 4.55%, 2.69%, 18.76%, 23.66%, 3.96%, 13.80%, 0.00%; P3: 0.00%, 10.98%, 11.57%, 17.29%, 36.84%, 9.71%, 14.45%, 0.00%. When doping the functional groups -S, -OCH 3 , or -Cl into P1 and P3, we obtained P2, P4, and P5, which showed significant changes in electron density. For example, in the E = -20 eV energy range, the electron density has increased from 0.00% to 2.55% then back to 0.00%; in the energy range of E = -15 eV, the electron density decreases and then increases and vice versa from 4.55% to 4.41%, up 10.98%, 11.82%, down to 6.26%; in the energy range of E = -10 eV, the electron density increases and then decreases from 2.69% to 7.34%, 11.57%, 14.58%, down to 12.05%; at the energy range of -7.25 eV, the electron density change to reach the extreme value in the valence area, tends to change from 18.76% to 14.57% to 17.29%, 22.13%, to 17.21%; in the E = −5 eV energy range, the density of electrocytes change from 23.66% to 22.27% to 36.84%, 39.75%, 36.15%; in the range with E = 0.00 eV the electronic density increased and then decreased from 3.96% to 5.80%, 9.71%, 14.94%, 4.83%;  percentage in the energy range of E = -5 eV (Figure 4). Through the obtained calculations, we confirm that this is still a semiconductor material and conductivity was increased because doping the -S functional group into P1 leads to band gap E g decrease and the conductivity increase. Similarly, doping the functional groups -OCH 3 , or -Cl into P3 leads to an increase in the band gap of E g . Besides, connecting P 1 and P 3 through the benzene bridge results in a huge decrease of E g nearly 100 times, while E tot decreased nearly 2 times. The  results are shown in the first Brillouin region (corresponding to zero) correspond to an increase in electron density from 3.96% to 5.80%, 9.71%, 14.94%, and then to 4.83% that increase in electrical mobility. The electrical mobility of P 1 decreased when it was doped with -S, and increased when -OCH 3 or -Cl was added to P3. It confirmed that the impurities affected the network structure and electronic structure of P 1 and P 3 . The cause of this phenomenon is due to the influence of the electronic structure of the functional groups on the band gap E g and the total energy of the system E tot . The obtained results are very useful for future experimental results used to fabricate smart electronic device.

Conclusions
This paper summarizes the structural calculations and gives a detailed comparison of the lattice structure, and electronic structure of C 11 H 7 NS-X and C 18 H 9 ON 2 S 2 -R in case of X and R was replaced with other elements or groups. The change in link length, link angle, total energy, and band gap of doped polythiophenes is convincing evidence to confirm the influence of the impurities, the benzene ring on the band gap, and electronic structure features of polymers.