Modulation of Properties in [1]Benzothieno[3,2-b][1]benzothiophene Derivatives through Sulfur Oxidation

This study explores the impact of sulfur oxidation on the structural, optical, and electronic properties of [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives, specifically focusing on 2,7-dibromo BTBT (2,7-diBr-BTBT) and its oxidized forms, 5,5-dioxide (2,7-diBr-BTBTDO) and 5,5,10,10–tetraoxide (2,7-diBr-BTBTTO). The bromination of BTBT followed by sequential oxidation with m-chloroperoxybenzoic acid yielded the target compounds in good yields. They were characterized using a wide array of analytical techniques including different spectroscopic methods, X-ray analysis, thermal analysis, and quantum chemical calculations. The results revealed that sulfur oxidation significantly alters the crystal packing, thermal stability, and optoelectronic properties of BTBT derivatives. Notably, the oxidized forms exhibited increased thermal stability and enhanced emission properties, with quantum yields exceeding 99%. These findings provide valuable insights for designing advanced organic semiconductors and fluorescent materials with tunable properties, based on the BTBT core.


Introduction
Organic semiconductors (OSCs) have been continuously attracting attention due to their distinctive properties, such as mechanical flexibility and chemical versatility [1].Their π-conjugated systems can be readily adjusted to optimize molecular arrangement with specific optical and electronic properties [2].Among these semiconductors, [1]benzothieno [3,2-b][1]benzothiophene (BTBT) derivatives have been extensively studied and applied in organic electronics due to their favorable characteristics, including high charge carrier mobility, tunable electronic properties, thermal stability, and solutionprocessability [3][4][5].Due to these features, BTBT derivatives are highly promising materials for different applications, including organic light-emitting diodes (OLEDs) [6][7][8][9][10], organic field-effect transistors (OFET) [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27], and organic sensors [28][29][30][31][32].The structural modifications of the BTBT core have a profound influence on its electronic properties, intermolecular interactions, and crystal packing, which play a crucial role in the performance of electronic devices [33].Considering that the presence of thiophene rings in the BTBT structure allows for the modulation of its optoelectronic properties through the oxidation of sulfur atoms, this opens up opportunities to develop new materials for organic electronics.Previous studies have demonstrated that the chemical oxidation of the thienyl sulfur atom in thiophene-based poly-and oligomers is a useful strategy to modulate the electronic structure and has an influence on the fluorescence and redox properties [34][35][36].It was found that oligothiophene S,S-dioxides are recognized as outstanding candidates for use in light-emitting diodes [37][38][39][40].Moreover, the oxidation of the sulfur atom in thiophene transforms its electron-donating character into the electron-accepting sulfonyl group.This transformation indicates that oligothiophene sulfones can be interesting from the point of view of their application as bipolar or n-type semiconductors [41,42].Compared to BTBT derivatives, research concerning their oxidized counterparts is very limited, as evidenced by the negligible number of publications in this field.There have been only three publications dedicated to this class of compounds.Two of these focused on the synthesis of BTBT 5,5,10,10-tetraoxide (BTBTTO) and its reaction with amines [43,44], while one study highlighted the use of diphenylamino derivatives of BTBT dioxide and tetraoxide for the fluorescent imaging of lipid droplets [45].
In this paper, we present comparative studies of unoxidized and oxidized BTBT derivatives, focusing on the influence of sulfur oxidation on structural, physical, optical, and electronic properties.The investigations were conducted on three compounds: 2,7-dibromo [1]benzothieno [3,2b][1]benzothiophene (2,7-diBr-BTBT), its 5,5-dioxide (2,7-diBr-BTBTDO), and its 5,5,10,10tetraoxide (2,7-diBr-BTBTTO).The presence of bromine atoms in 2,7-diBr-BTBT S-oxides provides an opportunity for easy further tuning of their desired properties through straightforward functionalization via Heck or Suzuki cross-coupling reactions, paving the way to new compounds with potential optoelectronic applications.To characterize the synthesized compounds, various techniques and methods were employed.Structural verification and analysis of oxidation's impact on crystal packing were conducted using single-crystal X-ray analysis.Optical properties were investigated through emission and absorption spectroscopy.Thermal stability was assessed using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).Additionally, electronic properties, such as frontier orbital energies, ionization potential, electron affinity, and internal reorganization energy, were predicted using DFT and TDDFT methods.

Synthesis of 2,7-diBr-BTBT S-oxides
The synthesis of 2,7-diBr-BTBT S-oxides, as depicted in Scheme 1, commenced with BTBT as the starting material, which was obtained using a one-pot procedure from commercially available o-chlorobenzaldehyde [46].Next, BTBT was subjected to a bromination reaction to produce 2,7-diBr-BTBT [47].The resulting dibromo BTBT derivative was oxidized with m-chloroperoxybenzoic acid (m-CPBA) at room temperature, affording the dioxide 2,7-diBr-BTBTDO in 76% yield.This compound was subsequently further oxidized to the corresponding tetraoxide 2,7-diBr-BTBTTO.To avoid issues with separating the product from the unreacted substrate (both compounds have low solubility and similar retardation factors), the reaction was driven to 100% conversion.To ensure this, the reaction was conducted at an elevated temperature with a large excess of m-CPBA.The reaction proceeded cleanly, yielding 2,7-diBr-BTBTTO as the sole product in 83% yield.

Structural Analysis
X-ray diffraction analysis was performed to confirm the structures of the synthesized 2,7-diBr-BTBT S-oxides.Single crystals of 2,7-diBr-BTBTDO and 2,7-diBr-BTBTTO for SC-XRD analysis (CCDC: 2368892 and 2368896) were obtained by heating the dissolved compounds in the corresponding solvents followed by slow cooling of the samples.All crystallographic data are listed in Tables S1-S3.5 in the Supplementary Materials.The analyzed molecules crystallized in different space groups, indicating that the degree of sulfur oxidation has a significant effect on their crystal packing (Figure 1).Furthermore, the crystal packing of the 2,7-diBr-BTBT S-oxides differed from the well-known herringbone arrangement observed for the unoxidized parent compound [47].2,7-diBr-BTBT forms a crystal structure where molecules are π-stacked, maintaining an interplanar spacing typical for flat halogenated OSCs of approximately ~3.46 Å [48].2,7-diBr-BTBTDO and 2,7-diBr-BTBTTO also crystallize in an π-stacked molecular arrangement, but the oxidation of one or two sulfur atoms has a significant effect on the arrangement of the crystals obtained.2,7-diBr-BTBTDO forms π-stacked molecular columns with an alternating arrangement of the oxidized sulfur atoms and an interplanar distance of ~3.55 Å between adjacent molecules.In contrast, 2,7-diBr-BTBTTO forms oppositely oriented planes with a distance of ~3.46 Å, and each molecule is π-stacked with four neighboring molecules.

Physical and Optical Properties
The obtained compounds were white (2,7-diBr-BTBT), yellowish (2,7-diBr-BTBTDO), and yellow (2,7-diBr-BTBTTO) solids.To evaluate the thermal stability of 2,7-diBr-BTBT and its oxidized derivatives, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out.The results show that the thermal stability increases with the number of oxygen atoms in the molecule (Table 1 and Figure S1 in the Supplementary Materials).The fully oxidized 2,7-diBr-BTBTTO undergoes decomposition approximately 18 • C and 38 • C above the decomposition temperatures of its partially oxidized counterpart 2,7-diBr-BTBTDO and its unoxidized parent compound 2,7-diBr-BTBT, respectively.In addition, the compounds do not exhibit liquid crystal properties.DSC measurements showed that they do not undergo any phase transitions up to their melting point or decomposition temperature (Figure S2 in the Supplementary Materials).We performed UV-Vis and photoluminescence (PL) measurements on the synthesized compounds in both solution (dichloromethane (DCM)) and the solid state.The findings, detailed in Table 1 and Figure 2, demonstrate that the oxidation of sulfur atoms within the thiophene rings has a substantial impact on their optical properties.The increase in the number of oxygen atoms in the molecule results in noticeable shifts in both the absorption (λ abs ) and emission maxima (λ em ) to the longer wavelengths.Furthermore, the Stokes shift for oxidized derivatives in solution is significantly higher, ranging from 79 to 90 nm, whereas 2,7-diBr-BTBT exhibits a much smaller shift of about 25 nm.The quantum yield (ϕ) of 2,7-diBr-BTBT is less than 1%, which is lower than the 3.4% quantum yield observed for BTBT [49].This result is in line with the expected heavy atom effect [50,51].In contrast, for 2,7-diBr-BTBTDO and 2,7-diBr-BTBTTO, the quantum yields are significantly higher, surpassing 99% for both compounds.The emission maxima of the investigated compounds in the solid state are in the range of 399-495 nm and exhibit a smaller red shift of 9 to 37 nm in comparison to their emission maxima in solution.

Computational Analysis
To provide insight into the electronic changes accompanying the oxidation of sulfur atoms in 2,7-diBr-BTBT, TDDFT calculations were performed using the PBE0/6-311+G(2d,p)//D3-M06-2X/def2-TZVP method.The results of these calculations are summarized in Table 2 and graphically presented in Figure 3.To account for solvent-solute interactions in the calculation of the electronic properties, the IEFPCM model [52-54] for DCM was applied.The calculated absorption maxima perfectly match the experimental data, with deviations ranging from 0.06 to 0.14 eV.The largest deviation was observed for 2,7-diBr-BTBTTO, where the absorption band was red-shifted by approximately 18 nm compared to the experimental value.In all the compounds examined, the lowest energy bands were predominantly associated with the HOMO⟶LUMO transitions (>97%).During the oxidation process, the HOMO energy gradually decreased from -6.230 eV for 2,7-diBr-BTBT to -7.048 eV for 2,7-diBr-BTBTTO.The same trend, but to a greater extent, was observed for the LUMO energy.In this case, the energy difference between the LUMO of 2,7-diBr-BTBT and 2,7-diBr-BTBTTO reached 1.48 eV.As the energy of the frontier orbitals decreased, the energy band gap also reduced, ultimately reaching around 3.83 eV for 2,7-diBr-BTBTTO.These results are in strong agreement with the findings reported for thiophene dioxides [36,55] and support our observation that the emission maxima of the investigated compounds exhibit an increasing red shift with a greater number of oxygen atoms.Hole-electron analysis [56] for individual atoms in 2,7-diBr-BTBT revealed that during excitation to the first excited state, two sulfur atoms and two carbon atoms connecting them provide a considerable input to the hole.The sum of their contribution is almost 50%.During electron excitation, these atoms lose 0.25

Computational Analysis
To provide insight into the electronic changes accompanying the oxidation of sulfur atoms in 2,7-diBr-BTBT, TDDFT calculations were performed using the PBE0/6-311+G(2d,p)//D3-M06-2X/def2-TZVP method.The results of these calculations are summarized in Table 2 and graphically presented in Figure 3.To account for solvent-solute interactions in the calculation of the electronic properties, the IEFPCM model [52][53][54] for DCM was applied.The calculated absorption maxima perfectly match the experimental data, with deviations ranging from 0.06 to 0.14 eV.The largest deviation was observed for 2,7-diBr-BTBTTO, where the absorption band was red-shifted by approximately 18 nm compared to the experimental value.In all the compounds examined, the lowest energy bands were predominantly associated with the HOMO−→LUMO transitions (>97%).During the oxidation process, the HOMO energy gradually decreased from -6.230 eV for 2,7-diBr-BTBT to -7.048 eV for 2,7-diBr-BTBTTO.The same trend, but to a greater extent, was observed for the LUMO energy.In this case, the energy difference between the LUMO of 2,7-diBr-BTBT and 2,7-diBr-BTBTTO reached 1.48 eV.As the energy of the frontier orbitals decreased, the energy band gap also reduced, ultimately reaching around 3.83 eV for 2,7-diBr-BTBTTO.These results are in strong agreement with the findings reported for thiophene dioxides [36,55] and support our observation that the emission maxima of the investigated compounds exhibit an increasing red shift with a greater number of oxygen atoms.Hole-electron analysis [56] for individual atoms in 2,7-diBr-BTBT revealed that during excitation to the first excited state, two sulfur atoms and two carbon atoms connecting them provide a considerable input to the hole.The sum of their contribution is almost 50%.During electron excitation, these atoms lose 0.25 electrons, which are transferred to the remaining carbon atoms.The oxidation of thiophene sulfur alters the electron-donating thienyl sulfur atom into a strong electron-accepting group, which is reflected in the electron distribution during excitation.Consequently, in 2,7-diBr-BTBTTO, the two sulfonyl groups and their connecting carbon atoms gain 0.31 electrons, primarily from the bromine atoms.In the case of asymmetrical 2,7-diBr-BTBTDO, electron excitation increases electron density at the sulfonyl group and the phenyl ring fused with the thiophene oxidized moiety.To estimate the ambient stability of investigated compounds and their charge transport properties, the adiabatic ionization potential (IP), electron affinity (EA), and internal reorganization energy (λ) were calculated (Table 3).IP and EA are crucial for understanding the ease with which molecules can gain or lose electrons, directly influencing their electronic properties and reactivity.Furthermore, these parameters also allow us to estimate the energy barriers involved in the injection of holes and electrons into molecules, which is particularly valuable in the design and development of materials for electronic and optoelectronic applications.On the other hand, the hole (λh) and electron (λe) reorganization energies, which are associated with the process of transferring an electron or hole between two adjacent molecules, are the key parameters in the study of charge transport in organic semiconductors.Calculations at the D3-M06-2X/def2-TZVPD level revealed that the IP of the investigated compounds increased with oxidation, from 7.69 eV for unoxidized 2,7-diBr-BTBT to 8.73 eV for 2,7-diBr-BTBTTO.These higher IPs for the oxidized 2,7-diBr-BTBT counterparts indicate greater oxidation stability, making To estimate the ambient stability of investigated compounds and their charge transport properties, the adiabatic ionization potential (IP), electron affinity (EA), and internal reorganization energy (λ) were calculated (Table 3).IP and EA are crucial for understanding the ease with which molecules can gain or lose electrons, directly influencing their electronic properties and reactivity.Furthermore, these parameters also allow us to estimate the energy barriers involved in the injection of holes and electrons into molecules, which is particularly valuable in the design and development of materials for electronic and optoelectronic applications.On the other hand, the hole (λ h ) and electron (λ e ) reorganization energies, which are associated with the process of transferring an electron or hole between two adjacent molecules, are the key parameters in the study of charge transport in organic semiconductors.Calculations at the D3-M06-2X/def2-TZVPD level revealed that the IP of the investigated compounds increased with oxidation, from 7.69 eV for unoxidized 2,7-diBr-BTBT to 8.73 eV for 2,7-diBr-BTBTTO.These higher IPs for the oxidized 2,7-diBr-BTBT counterparts indicate greater oxidation stability, making these compounds less susceptible to oxidation-related degradation.The same trend was observed for the electron affinity energy, which gradually increased from 0.72 eV for 2,7-diBr-BTBT to 2.42 eV for 2,7-diBr-BTBTTO.Thiophene sulfur oxidation also alters the internal reorganization energy (the sum of λ h and λ e ), which increases in the order: 2,7-diBr-BTBT < 2,7-diBr-BTBTDO < 2,7-diBr-BTBTTO.While the values of λ h and λ e are similar for 2,7-diBr-BTBT, they differ significantly in the oxidized derivatives.For 2,7-diBr-BTBTDO, the difference between λ h and λ e is about 0.07 eV, suggesting a greater propensity to conduct electrons rather than holes.The opposite situation is observed for 2,7-diBr-BTBTTO, for which this difference is approximately -0.26 eV, indicating a preference for hole transport.

General Remarks
Commercial-grade reagents and solvents were used as received without further purification.The 1 H, 13 C, and 2D NMR spectra were recorded using Bruker AV Neo 400 or Bruker Avance III 500 spectrometers (Bruker, Billerica, MA, USA).Chemical shifts for 1 H and 13 C NMR spectra are reported in parts per million (ppm) relative to the residual proton signal of the deuterated solvent.High-resolution mass spectrometry (HRMS) analyses were conducted on a Waters Synapt HDMS mass spectrometer (Waters Corporation, Milford, MA, USA).The synthesis of BTBT [46] and 2,7-diBr-BTBT [47] was carried out according to established literature procedures.The NMR spectra of all new compounds are included in the Supplementary Materials.

X-ray Analysis
Suitable crystals of 2,7-diBr-BTBTDO, and 2,7-diBr-BTBTTO were selected, transferred to mineral oil, and mounted on cryo loops.The crystals were then flash-cooled directly in a stream of N 2 .Diffraction intensities were recorded using a Rigaku XtaLAB Synergy-S diffractometer (Rigaku Europe SE, Neu-Isenburg, Germany) equipped with a Cu Kα radiation source (λ = 1.5418Å) and a HyPix-6000HE hybrid photon counting detector (Rigaku Europe SE, Neu-Isenburg, Germany).The total number of runs and images was based on the strategy calculation of the CrysAlisPro program (Rigaku Oxford Diffraction, v 1.171.43.111a, 2024).The molecular models of the structures were created with the structure solution program SHELXT 2018/2 [57] using intrinsic phasing with Olex2 v. 1.5 [58] as the graphical interface and refined by least squares with the 2018/3 version of SHELXL [59].All non-hydrogen atoms were refined anisotropically.The positions of the hydrogen atoms were calculated geometrically and refined using the riding model.The structures were validated with CheckCif (http://checkcif.iucr.org,accessed on 9 July 2024) and deposited in the Cambridge Crystallographic Data Centre (CCDC) under the accession numbers 2368892 and 2368896 for 2,7-diBr-BTBTDO and 2,7-diBr-BTBTTO, respectively.

Optical Characterization
Electronic absorption spectra were recorded on a Shimadzu UV-VIS spectrophotometer UV 2700 (Shimadzu Scientific, Kyoto, Japan).Photoluminescence and quantum yield measurements were performed using a Horiba Scientific FluoroMax+ spectrofluorometer (Horriba Scientific, Kyoto, Japan).All measurements were conducted at room temperature in a 1 cm cuvette.Spectrophotometric grade dichloromethane (DCM) was used for measurements in solution.Photoluminescence spectra of 2,7-diBr-BTBTDO and 2,7-diBr-BTBTTO were recorded after excitation at their respective maximum absorption wavelengths, and a 300 nm excitation wavelength was used for 2,7-diBr-BTBT.Photoluminescence quantum yields (ϕ) were determined using an integrating sphere.

Thermal Properties
Thermogravimetric analysis (TGA) was conducted using a TGA 2950 analyzer (TA Instruments, Eden Prairie, MN, USA).The experiments were performed under a nitrogen atmosphere in the temperature range of 50 • C to 600 • C at a heating rate of 10 • C min −1 .The decomposition temperature was determined at 5% weight loss.The thermal properties of the synthesized compounds were studied using differential scanning calorimetry (DSC) on a DSC 2920 (TA Instruments), heating from 0 • C to 220 • C at a rate of 5 • C min −1 .

Computational Details
All quantum-mechanical calculations were performed using Gaussian 09, revision D.01 [60].For simulating optical properties, the geometries of the investigated compounds were optimized using the M06-2X [61] functional with the D3 version [62] of dispersion correction and the Karlsruhe valence triple-zeta def2-TZVP basis set [63].Calculations were conducted in a dichloromethane (DCM) dielectric medium, applying tight convergence criteria for geometry and energy.The "ultrafine" integration grid was used for all calculations.The character of the equilibrium point on the potential energy surface was identified through frequency calculations.TDDFT calculations were performed for the first six singlet excited states using the PBE0 functional [64] in conjunction with the Polple 6-311+G(2d,p) basis set [65].Calculations of the internal reorganization energy, adiabatic ionization potential (IP), and adiabatic electron affinity (EA) were conducted at the D3-M06-2X/def2-TZVPD level in the gas phase.Adiabatic IPs were calculated as the energy difference between the relaxed cationic state (E + (M + )) and the geometry-optimized neutral molecule (E 0 (M)) (IP = E + (M + ) − E 0 (M)).EAs were determined by the energy difference between the optimized neutral molecule (E 0 (M)) and the anionic state (E − (M − ))(EA = E 0 (M) − E − (M − )).The hole (λ h ) and electron (λ h ) reorganization energies were calculated using the following equations: ) where E + (M)/E − (M) is the energy of the cation/anion calculated at the optimized geometry of the neutral molecule, and E 0 (M + )/E 0 (M − ) is the energy of the neutral molecule calculated at the optimized geometry of the corresponding cation/anion.A graphical plot for the calculation of reorganization energy is presented in Figure 4. Hole-electron analysis using Hirshfeld partitions was conducted with the Multiwfn 3.8 program [66].Basis sets def2-TZVP [63] and def2-TZVPD [67] were downloaded from the Basis Set Exchange (BSE) [68].The optimized geometrical parameters of the investigated compounds are provided in the Supplementary Materials.
of the neutral molecule, and E 0 (M + )/E 0 (M − ) is the energy of the neutr lated at the optimized geometry of the corresponding cation/anion.A the calculation of reorganization energy is presented in Figure 4. Hole-electron analysis using Hirshfeld partitions was conducted 3.8 program [66].Basis sets def2-TZVP [63] and def2-TZVPD [67] were the Basis Set Exchange (BSE) [68].The optimized geometrical parame gated compounds are provided in the Supplementary Materials.

Conclusions
In summary, the sulfone derivative 2,7-diBr-BTBTDO and the d 2,7-diBr-BTBTTO have been successfully synthesized via the oxidation with m-CPBA and fully characterized using various analytical and th Comparative analysis of 2,7-diBr-BTBT and its oxidized derivatives r tial of sulfur oxidation as a strategy to tune the properties of BTBT d dized compounds are characterized by red-shifted absorption and em a Stokes shift reaching 90 nm and an impressive quantum yield above compounds also show a decrease in both HOMO and LUMO energi duced energy band gap.This increased stability is further supported thermal properties, which show an increase in decomposition tempe creasing number of oxygen atoms.Changes in the electronic propertie molecules also affect their tendency to form unique crystal patterns.A

Conclusions
In summary, the sulfone derivative 2,7-diBr-BTBTDO and the disulfone derivative 2,7-diBr-BTBTTO have been successfully synthesized via the oxidation of 2,7-diBr-BTBT with m-CPBA and fully characterized using various analytical and theoretical methods.Comparative analysis of 2,7-diBr-BTBT and its oxidized derivatives revealed the potential of sulfur oxidation as a strategy to tune the properties of BTBT derivatives.The oxidized compounds are characterized by red-shifted absorption and emission spectra with a Stokes shift reaching 90 nm and an impressive quantum yield above 99%.The oxidized compounds also show a decrease in both HOMO and LUMO energies, leading to a reduced energy band gap.This increased stability is further supported by their improved thermal properties, which show an increase in decomposition temperature with an increasing number of oxygen atoms.Changes in the electronic properties of the individual molecules also affect their tendency to form unique crystal patterns.Although all investigated compounds form π-stacked molecular arrangements, they differ significantly, as reflected by their different space group symmetry.All the above findings are instrumental in designing new materials based on the BTBT skeleton with optimized properties for organic electronic applications and fluorescence-based techniques.

Figure 4 .
Figure 4. Diagram illustrating the calculation of the hole (λ h ) and electron (λ e ) reorganization energies.

Table 1 .
Physical and optical properties of 2,7-diBr-BTBT and its S-oxides in DCM and solid state.
1Decomposition temperature determined by TGA corresponding to 5% weight loss at 10 • C min −1 under nitrogen flow.