2,1,3-Benzothiadiazole Small Donor Molecules: A DFT Study, Synthesis, and Optoelectronic Properties

We herein report the design and synthesis of small-donor molecules, 2,1,3-benzothiadiazole derivatives (2a–d), by Stille or Suzuki reaction. The synthesized compounds were characterized by spectroscopic and electrochemical methods. The compounds 2a–d absorb the light in a wide range (the UV-green/yellow light (2c)) and emit from green to red/near IR light (2c). Furthermore, these compounds show a narrow energy gap (1.75–2.38 eV), and high Ea values increasing for polymers, which prove their electron-donating nature and semiconductor properties. The measurements were enhanced by theoretical modeling.


Introduction
Organic semiconductors are currently used in many fields of science, e.g., organic light-emitting diode (OLED), solar cells, transistors, molecular imaging, and sensors [1][2][3][4][5][6][7]. Due to their conductive and optical properties, the possibility of easy modification, flexibility, and low production costs, they displace their inorganic counterparts. A recent breakthrough in the performance of organic semiconductor devices has been achieved by developing non-fullerene acceptors (NFAs) that can overcome the drawbacks of fullerenes and have the advantages of high absorption rates, readily tunable optical and electronic properties, and increased solubility [8,9]. Using several variants of the semiconductor structural systems, the properties of the entire system are adjusted, e.g., donor-acceptordonor (D-A-D) molecules [10,11]. An example is the use of small molecules, which are currently being intensively studied due to the shortage of n-type conjugated polymer (n-CP) materials as an acceptor in the active layer and a p-type conjugated polymer (p-CP) as a donor, especially in organic solar cells. Currently, n-CP focuses primarily on perylene diimide, naphthalene diimide, pyridine derivatives, and p-CP containing thiophene and dithienosilole [12][13][14][15][16][17]. However, many of them suffer from some significant weaknesses, such as a poor absorption coefficient and excessively strong crystallinity and stacking, which lead to a limited photocurrent and poor separation in the active layers [18][19][20][21]. To overcome the unfavorable properties of some conjugated polymers (CPs), small molecules are being researched, considering their advantages of easy purification, a narrow bandgap, strong absorption, higher electron mobility, well-defined chemical structures, and good photovoltaic performance reproducibility without batch-to-batch variation [22][23][24][25][26][27][28][29].

Synthesis
The synthetic routes and chemical structures of the 2a-d molecules are depicted in Scheme 1. The target small molecules 2a-c were obtained through a Stille coupling reaction, and compound 2d was obtained through a Suzuki coupling reaction. Selenophene and 3,4-ethylenedioxythiophene were monostannylated to 2-(tributylstannyl)selenophene or 2-(tributylstannyl)-3,4-ethylenedioxythiophene in 99% yield with SnMe3Cl in THF at −80 °C. All the synthesized compounds were further purified by using a column chromatography technique. The molecular structures of these target compounds and their intermediates were established by using different spectroscopic tools. Figure 1 shows the optimized structures and frontier molecular orbitals of 2a-d. The HOMO energy level was found to spread throughout the molecules, whereas LUMO was localized on the benzothiadiazole moiety. The HOMO and LUMO energy levels of 2a-d are stated in Table 1. The observation, however, should be treated with caution, since the orbital energies are basis set dependent. However, these values are reasonably close to both calculated and experimental ionization energies ( Table 1). The efficient small molecule donors should have the HOMO energy levels around −5.2 eV to ensure acceptable Scheme 1. Synthesis routes of the benzothiadiazole derivatives (2a-d).

Synthesis
The synthetic routes and chemical structures of the 2a-d molecules are depicted in Scheme 1. The target small molecules 2a-c were obtained through a Stille coupling reaction, and compound 2d was obtained through a Suzuki coupling reaction. Selenophene and 3,4-ethylenedioxythiophene were monostannylated to 2-(tributylstannyl)selenophene or 2-(tributylstannyl)-3,4-ethylenedioxythiophene in 99% yield with SnMe 3 Cl in THF at −80 • C. All the synthesized compounds were further purified by using a column chromatography technique. The molecular structures of these target compounds and their intermediates were established by using different spectroscopic tools. Figure 1 shows the optimized structures and frontier molecular orbitals of 2a-d. The HOMO energy level was found to spread throughout the molecules, whereas LUMO was localized on the benzothiadiazole moiety. The HOMO and LUMO energy levels of 2a-d are stated in Table 1. The observation, however, should be treated with caution, since the orbital energies are basis set dependent. However, these values are reasonably close to both calculated and experimental ionization energies ( Table 1). The efficient small molecule donors should have the HOMO energy levels around −5.2 eV to ensure acceptable device open circuit voltages [32]. All compounds, except 2c, fulfill this condition. The lowest HOMO orbital has 2b (−5.24 eV), then 2d (−5.12 eV), 2a (−5.12 eV), and 2c (−4.78 eV). Moreover, the value of the ionization potential suggests the donor properties of the compounds (ionization potential (IP) < 5.7 eV) ( Table 1) [33]. Compound 2c has the lowest IP ad cal (5.7 eV) compared to compounds 2a (6.07 eV), 2d (6.08 eV), and 2b (6.25 eV), and therefore has the strongest donor properties. These results differ by approximately 0.5 eV (2d by 0.2 eV) from the experimental results, respectively: 5.24 eV (2c), 5.60 eV (2a), 5.86 eV (2d), and 5.76 eV (2b). Furthermore, the structure of compounds 2a-d is planar, which ensures close intermolecular contacts and high charge carrier mobilities [32]. Figure 2 shows the experimental and calculated UV-vis absorption and photoluminescence spectra of 2a-d. The corresponding data are summarized in Table 2. It is observed that the λ max cal of all the structures corresponds to HOMO → LUMO transitions (range in yellow and red light) and to HOMO→LUMO + 1 transitions (in the violet range). The calculated λ max value of molecule 2a shows the broadest wavelength with maximum absorption at 413 and 623 nm, which is red-shifted by 58 and 140 nm when compared with experimental results. The theoretical UV-vis spectra of 2b, c, and d are also red-shifted (110-170 nm) in comparison with experimental results. Moreover, 2c and 2d show similar absorption behavior, but 2c shows broader and red-shifted spectra with the maximum of absorbance at 418 and 652 nm. The compounds 2a-d have high oscillator strength (f), which corresponds to the appearance of a high experimental absorption coefficient ( Table 2). device open circuit voltages [32]. All compounds, except 2c, fulfill this condition. The lowest HOMO orbital has 2b (−5.24 eV), then 2d (−5.12 eV), 2a (−5.12 eV), and 2c (−4.78 eV). Moreover, the value of the ionization potential suggests the donor properties of the compounds (ionization potential (IP) < 5.7 eV) ( Table 1) [33]. Compound 2c has the lowest IPad cal (5.7 eV) compared to compounds 2a (6.07 eV), 2d (6.08 eV), and 2b (6.25 eV), and therefore has the strongest donor properties. These results differ by approximately 0.5 eV (2d by 0.2 eV) from the experimental results, respectively: 5.24 eV (2c), 5.60 eV (2a), 5.86 eV (2d), and 5.76 eV (2b). Furthermore, the structure of compounds 2a-d is planar, which ensures close intermolecular contacts and high charge carrier mobilities [32]. Figure 2 shows the experimental and calculated UV-vis absorption and photoluminescence spectra of 2a-d. The corresponding data are summarized in Table 2. It is observed that the λmax cal of all the structures corresponds to HOMO → LUMO transitions (range in yellow and red light) and to HOMO → LUMO + 1 transitions (in the violet range). The calculated λmax value of molecule 2a shows the broadest wavelength with maximum absorption at 413 and 623 nm, which is red-shifted by 58 and 140 nm when compared with experimental results. The theoretical UV-vis spectra of 2b, c, and d are also red-shifted (110-170 nm) in comparison with experimental results. Moreover, 2c and 2d show similar absorption behavior, but 2c shows broader and red-shifted spectra with the maximum of absorbance at 418 and 652 nm. The compounds 2a-d have high oscillator strength (f), which corresponds to the appearance of a high experimental absorption coefficient ( Table 2).

Photophysical Studies
The optical properties of synthesized compounds 2a-d were studied b fluorescence spectroscopy. The UV-vis absorption and emission spectra o 2a-d in dichloromethane (DCM) are shown in Figure 2, and the correspon summarized in Table 2. Compounds 2a-d show two absorption peaks at 311 to the overlap of the n-π* and π-π* transition, whereas absorption maxima a are due to a push-pull system, which allows for intramolecular electron de from donor unit to acceptor. Compounds 2a, b, and d show luminescence in light from green to red light, with the maximum of emission at 609, 592, respectively, while compound 2c is batochromically shifted to near IR, with t of emission at 667 nm. This red shift in the emission spectrum of 2c, as comp and d, can be explained in terms of the stronger donor character of ethyle phene.

Photophysical Studies
The optical properties of synthesized compounds 2a-d were studied by UV-vis and fluorescence spectroscopy. The UV-vis absorption and emission spectra of compounds 2a-d in dichloromethane (DCM) are shown in Figure 2, and the corresponding data are summarized in Table 2. Compounds 2a-d show two absorption peaks at 311-370 nm due to the overlap of the n-π* and π-π* transition, whereas absorption maxima at 452-526 nm are due to a push-pull system, which allows for intramolecular electron density transfer from donor unit to acceptor. Compounds 2a, b, and d show luminescence in the range of light from green to red light, with the maximum of emission at 609, 592, and 563 nm, respectively, while compound 2c is batochromically shifted to near IR, with the maximum of emission at 667 nm. This red shift in the emission spectrum of 2c, as compared to 2a, b, and d, can be explained in terms of the stronger donor character of ethylenedioxythiophene.

Electrochemical Properties
During the electrochemical oxidation, an irreversible process of polymerization occurs for all monomers. First, Cyclic Voltammetry (CV) scans in the anodic range are presented in Figure 3b. The oxidation potential of 2c was much lower than that of compounds 2a, 2b, and 2d, which subsequently oxidized within a narrow range. This trend for changing substituents is consistent with the literature [34,35] and with calculated values of HOMO.
During reverse polarization, reduction of oligomers occurs. For 2d, the first oxidation scan's sharp reduction peak suggests dimer formation.
During the electrochemical oxidation, an irreversible process of polymerization occurs for all monomers. First, Cyclic Voltammetry (CV) scans in the anodic range are presented in Figure 3b. The oxidation potential of 2c was much lower than that of compounds 2a, 2b, and 2d, which subsequently oxidized within a narrow range. This trend for changing substituents is consistent with the literature [34,35] and with calculated values of HOMO. During reverse polarization, reduction of oligomers occurs. For 2d, the first oxidation scan's sharp reduction peak suggests dimer formation. CV of three reduction scans is presented in Figure 3a. The similar reduction potential of all compounds is caused by the connection of the changing donor unit by the thiophene β position and its long distance from the benzothiadiazole unit, where the LUMO is localized. Differently calculated values of LUMO vary between compounds. That indicates that the communication between the D-A parts is slightly overestimated by the density functional theory (DFT), probably by the ideal planarity of the optimized structure. Additionally, the absolute values of LUMO are significantly lower than Electron Affinities, which is usual for the LUMO calculated by the DFT method [36]. Reduction potentials of first quasi-reversible peaks are slightly less electronegative than pure dithienylbenzothiadiazole (−1.74 V at onset) [34]. Monomer electrochemical results are summarized in Table 1 Electropolymerization of investigated compounds (Figure 4) forms insoluble films on platinum wire. Polymer CV was conducted under the same conditions as monomers. Three scans of separately registered oxidation and reduction are presented in Figure 5. Polymers p2b, p2c, and p2d undergo rapid degradation while reducing 2a also degrades, but slower. During oxidation, p2b and p2c remain stable, and p2a is stable after the first scan. Only p2d clearly degraded under oxidation. A high drop in p2c oxidation onset in comparison with 2c (0.83 V) is characteristic for 3,4-ethylenedioxythiophene (EDOT)ended monomers and results in a very low bandgap (0.72 V) [37]. The oxidation potential of polymers p2d and p2b is similar, but the potential of the latter is lower, like for the corresponding monomers. However, p2a's oxidation potential decreased after  Figure 3a. The similar reduction potential of all compounds is caused by the connection of the changing donor unit by the thiophene β position and its long distance from the benzothiadiazole unit, where the LUMO is localized. Differently calculated values of LUMO vary between compounds. That indicates that the communication between the D-A parts is slightly overestimated by the density functional theory (DFT), probably by the ideal planarity of the optimized structure. Additionally, the absolute values of LUMO are significantly lower than Electron Affinities, which is usual for the LUMO calculated by the DFT method [36]. Reduction potentials of first quasi-reversible peaks are slightly less electronegative than pure dithienylbenzothiadiazole (−1.74 V at onset) [34]. Monomer electrochemical results are summarized in Table 1. All compounds show a narrow band gap (1.75-2.38 eV). The narrowest band gap (∆E g el ) has 2c (−1.75 eV), then 2a (2.01 eV), 2b (2.28 eV), and 2d (2.38 eV). These results correlate well with the theoretical values (E g cal ). The largest difference between the experimental and theoretical results was observed for compound 2c (0.44 eV), and the smallest for compound 2d (0.09 eV).

CV of three reduction scans is presented in
Electropolymerization of investigated compounds ( Figure 4) forms insoluble films on platinum wire. Polymer CV was conducted under the same conditions as monomers. Three scans of separately registered oxidation and reduction are presented in Figure 5. Polymers p2b, p2c, and p2d undergo rapid degradation while reducing 2a also degrades, but slower. During oxidation, p2b and p2c remain stable, and p2a is stable after the first scan. Only p2d clearly degraded under oxidation. A high drop in p2c oxidation onset in comparison with 2c (0.83 V) is characteristic for 3,4-ethylenedioxythiophene (EDOT)-ended monomers and results in a very low bandgap (0.72 V) [37]. The oxidation potential of polymers p2d and p2b is similar, but the potential of the latter is lower, like for the corresponding monomers. However, p2a's oxidation potential decreased after polymerization by only approximately 0.33 V. All results for polymers are shown in Table 3.   Moreover, compound 2c was electropolymerized, and the obtained film, poly(4,7bis(5-(3,4-ethylenedioxythiophene)thiophen-2-yl)benzothiadiazole), served as a matrix for an enzyme, horseradish peroxidase (HRP), for 17β-estradiol detection in an electrochemical biosensor [30]. The detection limit for 17β-estradiol was set to 105 nM, the sensitivity of the proposed biosensor was found to be 1.16 × 10 −4 A•μM −1 •cm −2 , and the lifetime  Table  3.  Moreover, compound 2c was electropolymerized, and the obtained film, poly(4,7bis(5-(3,4-ethylenedioxythiophene)thiophen-2-yl)benzothiadiazole), served as a matrix  Moreover, compound 2c was electropolymerized, and the obtained film, poly(4,7bis(5-(3,4-ethylenedioxythiophene)thiophen-2-yl)benzothiadiazole), served as a matrix for an enzyme, horseradish peroxidase (HRP), for 17β-estradiol detection in an electrochemical biosensor [30]. The detection limit for 17β-estradiol was set to 105 nM, the sensitivity of the proposed biosensor was found to be 1.16 × 10 −4 A·µM −1 ·cm −2 , and the lifetime of the system can be determined for 5 weeks. Compound 2b also was used in an amperometric, tyrosinase-based biosensor for epinephrine detection [38]. The sensitivity of the proposed biosensor was found to be 3.08 × 10 −7 A·µM −1 ·cm −2 . Poly(2c) and poly(2b) serve as electron mediators to improve the flow of electrons between the enzyme's active center and the electrode surface, and they act as a transducer during the transfer of electric charge [30,38]. Both of these sensors show a good sensitivity, which confirms the semiconductor nature of the obtained compounds.

Computational Details
The theoretical studies were performed by applying the density functional theory (DFT) method [39]. The calculations were performed utilizing the B3LYP functional [40][41][42] and the standard cc-pVDZ atomic basis set [43]. The basis set was adopted based on the former experience [44][45][46]. This basis provides orbital HOMO energies reasonably reproducing directly calculated ionization energies (Table 1). Optical transition of all compounds was studied by conducting Time-dependent density-functional theory (TD-DFT) computations [47]. To simulate the UV-vis absorption, spectra and oscillator strength were estimated at their ground-state optimized geometries for a maximum of 200 excited states. All the calculations were carried out using the Gaussian16 suite of codes [48]. The computational resources were provided by the Wroclaw Centre for Networking and Supercomputing (http://wcss.pl). The molecular graphics were produced by applying the GausView program (Gaussian, Inc. Wallingford, Connecticut, USA) [49].

Optical Measurements
UV-vis spectra were recorded on the Spectroquant Pharo 300 spectrophotometer (Merck, Darmstadt, Germany). The luminescence spectra were recorded on the Hitachi F-2500 fluorescence spectrophotometer (Hitachi High Technologies America, Inc., Pleasanton, CA, USA).

Cyclic Voltammetry
For the cyclic voltammetry (CV) experiments, a three-electrode glass cell was used with a platinum wire as a working electrode, a platinum wire spiral as a counter electrode, and a silver wire reference using 0.1 M NBu 4 PF 6 (TCI Europe) electrolyte solution in dichloromethane (DCM) (Sigma-Aldrich (Poznan, Poland), Chromasolv, HPLC). The potential sweeps were controlled by a MetrohmAutolab PGSTAT 100 N potentiostat. The potential of the silver electrode was determined using a ferrocene redox couple (Fc/Fc + ) for each measurement set, under the same conditions as the measured samples. The solutions were de-aerated with argon before, and argon kept flowing into the cell, above the solution surface, during measurements. A concentration of 1 mmol/dm 3 of the monomers was used for both the measurements and polymerization.

Conclusions
In summary, we have designed and synthesized a series of benzothiadiazole derivatives as donor small molecules with good yield. In the case of the 2c derivative, we obtained up to 85% yield, which gives an excellent result in the Stille reaction. The 2a and 2c compounds absorb the light in the UV-yellow range, and the 2b and 2d compounds in the UV-green range. Moreover, these compounds emit in a very wide range (2a, b, and d from green to red and 2c from green to near IR); therefore, these compounds are very promising optoelectronics materials, e.g., in White OLED (WOLED). In addition, they show a narrow energy gap (1.75-2.38 eV), especially as polymer films (0.72-1.64 eV), and low IP values decreasing for polymers (5.24-5.86 eV for monomers and 4.41-5.27 eV for polymers), which proves their electron-donating nature and semiconductor properties. Furthermore, compound 2c and 2b served as a conductive matrix for an enzyme in biosensors.