Theoretical Studies on the Design of Benzo[1,2-c:4,5-c’]bis[1,2,5]selenadiazole Acceptor-Based Organic Dyes

Abstract

π-conjugated organic material, specifically, the modification of donor acceptor-based dyes, has gained significant attention over the years for its outstanding performance in efficient dye-sensitized solar cells (DSSCs) as well as contributing to overall economically sustainable solar cell technology. In the present study, density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches were executed to investigate the impact of benzobisselenadiazole (BBSD) based organic dyes with interchangeable donor and acceptor units on their optical properties. Two spacers were selected from 27 π-linker models due to their better performance on the absorption spectra. A total of 200 BSSD dyes were designed based upon the above two selected spacers linking with various donors and acceptors. The corresponding structures and properties were theoretically analyzed by DFT/TDDFT methods. D10A8-16 and D10A6-17 show the best performance and may become the best candidates to be developed as efficient dye sensitizers for DSSCs. Our results showed that structural changes can effectively improve the optoelectronic properties of the designed BSSD dyes. This provides us with an economic and fast screening way for artificial design of new dyes.

1. Introduction

Increasing energy demand poses one of the most prominent challenges for growing economical societies around the world. The sun supplies the Earth with energy at a rate nearly ten times more than the energy produced through the combustion of fossil fuels, and this makes solar energy one of the most sustainable sources of power [1,2,3]. This immense potential highlights the feasibility of solar energy as a primary solution to global energy challenges. Environmentally, this implies that covering just 0.1% to 0.16% of the Earth’s surface with solar systems, each achieving at least 10% power conversion efficiency, could meet current global energy demands [1,2,3]. In order to satisfy the rapid depletion of fossil fuels, solar cell power is hence a viable solution to effectively harvest energy from the sun for sustainable photogeneration practices. Among the various generations of solar cells, the third generation includes organic dyes and conducting polymers. It shows significant potential to become alternative photoconductive materials, with a notable trend of reducing manufacturing costs while enhancing competence. This makes it a particular promising solution for solar cells [1] through its cost efficiency and easy fabrication.

The dye-sensitized solar cell (DSSC), which was a groundbreaking invention by O’Regan and Grätzel in 1991, offers new dyes with enhanced performance and cost advantages over conventional silicon solar cells. Significant progress has been made in the search for efficient photovoltaic technologies [4,5]. A DSSC is a system that resembles the natural process of photosynthesis [6], where the dye is used as a photosensitizer to transport electrical charge from an external light source and convert it to usable solar energy. The dye sensitizer is a critical component of the DSSC. It plays a pivotal role in light absorption and the photon injection to maximize light capture in the visible to near-infrared regions (NIRs) of the electromagnetic spectrum. It also affects electron transfer through the dyes in the DSSC assembly [4,5,6,7]. An efficient dye sensitizer can ideally exhibit both the effective electron injection and dye regeneration processes to enhance light absorption.

With better performances, the efficient organic dyes may compete with traditional electrical photo generation processes based on inorganic framework. Although ruthenium-based metal dye photosensitizers have achieved a power conversion efficiency of up to 13%, detrimental environmental factors lead to significant research focused on pushing the performance of metal-free dyes beyond this threshold due to their higher molar extinction coefficients, greater flexibility, and more feasible fabrication [4,8,9,10]. The toxicity of silicon and metal-based organic dyes makes it worthwhile to computationally investigate pure metal-free donor–acceptor dyes, as their potential to overcome these disadvantages could lead to more efficient photovoltaics at a better price–performance ratio. The combination of quantum chemical property analysis with conceptual insights is a promising research approach for discovering novel organic dye-sensitized compounds [11].

Of successful DSSC applications, modifications to electron-rich and electron-deficient groups featured in the core of the structure have gained considerable attention. The most popular alternatives for organic–metal-free dye sensitizers are featured as a push-pull assembly and π-conjugated linkers [2,4,10,12]. The rational design of organic dyes for the optimum performance of advanced solar cell technology involves the proper selection of donor and acceptor moieties for photovoltaic materials to absorb visible to near-infrared regions on the electromagnetic spectrum. Several developments have emerged with selenium containing compounds, specifically fused selenium-based heterocycles. Benzoselenadiazole π-conjugated systems, due to the enhanced robustness and environmental sustainability of selenium-containing heterocycles, exhibit higher power conversion efficiencies than their sulfur analogs, making them promising for photovoltaic, biomedical, and optoelectronic applications [13,14,15]. In addition to the applications in the fields of biological sciences and organic chemistry, comparing with sulfur, selenium has a notable better performance in the synthesis of compounds computationally [16].

It was reported that 1,2,5-thia and 1,2,5-selenadiazoles are key chalcogen–nitrogen heterocycles, with 1,2,5-selenadiazoles fused with electron-withdrawing rings. They can form stable conformations with favorable electronic properties. It makes benzofused 1,2,5-selenadiazole particularly interesting in modern studies for low band gap materials in solar cells [14,17]. The stability of Benzo[c][1,2,5]selenadiazole [18] (BSD) is verified through experimental research as well as performance of derivatives with different functional groups like the ones presented in this study. Benzodiazole derivatives with sulfur or selenium can enhance optoelectronic properties and power efficiency. The widely used Benzothiadiazole (BTD) derivatives are notable for their photostability, thermostability, and tunable structures [19]. BTD is a popular electron-withdrawing compound for photovoltaic devices, while its modified derivative, BSD, containing the larger and more electron-rich selenium atom, can further enhance solar cell performance. The design of π-conjugated materials using the Benzo[c][1,2,5]selenadiazole derivative is plausible due to its tunable wavelength absorption by adjusting the polarizability of the dye backbone, and it is noteworthy for the further study in DSSCs due to the limited reports on its use as an auxiliary acceptor [18,20]. BSD has become very popular due to favorable optoelectronic properties such as electron transport, as well as their applications to photovoltaic solar cell devices [13,20].

Since the first synthesis of Benzo [1,2-c:3,4-c′]bis [1,2,5]selenadiazole (BBSD) [21], studies indicate that substituting thiophene-based derivatives with BBSD chromophores may lead to a more pronounced redshift in the NIR region of absorption than their competitive sulfur-based analogs [12,19,22,23]. Although BBSD is gaining popularity as an alternative sulfur-based system for longer wavelength emission and narrower band gaps, its synthesis is relatively complex and costly compared to simpler organic compounds [12,19,21]. The multiple steps and specific reaction conditions involved in BBSD synthesis limit experimental studies, making optimized computational methods a more feasible approach to reducing costs in BBSD-containing systems. While it is possible to synthesize BBSD using the commercially available precursor 4-nitrobenzo-2,1,3-selenadiazole, limited research has focused on improving the synthesis process [21]. This supports the theoretical study of BBSD-based dyes, where computational models can offer valuable insights without the limitations of experimental synthesis.

Given the cost and complexity of BBSD synthesis, the experimental production of BBSD-based dyes remains feasible and has been successfully reported [12,19,21]. To overcome these challenges, optimizing catalytic reactions and nucleophilic substitutions for selenium incorporation can streamline the synthesis dyes that include diselenopyrrole and biselenophene analogs [24]. Co-sensitizing BBSD with simpler, more readily synthesized dyes, such as diselenopyrrole derivatives, offers a practical approach to achieving broad-band absorption while reducing synthetic complexity. Although the development of selenium-based materials is still insufficient for the demand, these derivatives exhibit intriguing electrochemical properties that will interest further investigation. Therefore, designing the dye structure by optimizing electron-donating and electron-withdrawing groups attached to the selenium-based core will improve light absorption, the electron injection of the dyes, and overall dye performance [24]. By addressing these synthetic and structural factors, BBSD-based systems can be synthesized to be more efficient and viable for practical implementation in DSSCs.

Compared with the experimental bandgap of BTD (5.52 eV), the BBSD analog exhibits a lower bandgap of 5.43 eV [19], and thus appears to be a more suitable candidate for construction of near-infrared absorbing materials. Theoretical and experimental investigations suggest that incorporating the BBSD chromophore with D-π-A-π-D and D-A-π-A morphology shows a near-infrared light harvesting structure–property relationship beneficial for the functionality of photovoltaic cells and optoelectronic devices [12,20,22,25], The BBSD-based π-conjugated backbone used in DSSCs demonstrates a favorable broad absorption spectrum, which influences the interpretation of spectroscopic properties for the design of advanced dyes in efficient organic photovoltaics. Ab initio approaches are utilized to perform an in silico analysis of intriguing optoelectronic properties of DSSCs. density functional theory (DFT) and time-dependent density functional theory (TD-DFT) are traditional first-principles techniques with the computational ability to analyze the ground state and the excited states of dye sensitizers. These reveal that structural and electronic properties may contribute to improving the photoconductivity of the novel environmentally sustainable compounds [9,26,27]. DFT and TDDFT are suitable computational methodologies that offer a robust and cost-effective theoretical framework for providing crucial insights into the organic dye-sensitized interface and revealing important properties related to photovoltaic cell performance such as charge transport and electron injection [11].

The experimental study on the acceptor groups BBTD, BSBT, and BBSD by Qian et al. reveals that a combination of TPA, thiophene, and BBSD in a donor–acceptor (D-A) arrangement leads to a significant reduction in the band gap for the near-infrared-absorbing compounds [22]. Although there is still a lack of research, studies on this topic are steadily increasing as researchers recognize the impact of BBSD as an auxiliary acceptor within the chromophore structure, which influences the optical properties [12,13,20] and the structural design of the photovoltaic materials studied herein. As a derivative of benzoselenadiazole, the BBSD asymmetric chromophore offers significant advantages for achieving stable and high-performance organic solar cells [28]. BBSD’s intrinsic molecular architecture contributes to its increased chemical stability. Moreover, BBSD exhibits a smaller bandgap than the sulfur analog. The substitution at its 4- and 8-sites can improve BBSD dyes through the precise tuning of photophysical properties, optimizing electron density and charge transfer [19]. BBSD also shows a smaller bandgap than the other selenium-containing chromophores. It can promote the broader solar light absorption to enhance photoconversion efficiency, including the near-infrared region. Additionally, the strong electron-accepting properties of BBSD make it superior for creating efficient D-A motifs that surpass other selenium-based systems in stability, light absorption, and tunability. Extending the quantum parameters of studied dyes is achievable through strategic design and enhancement of the fundamental electron-hole flow process in DSSCs. Electron donor and acceptor moieties attached to the dye’s spacer unit can significantly influence the power conversion efficiency of DSSC devices. Lower optical band gap and stronger intermolecular charge transfer are essential for the high performance of the D-π-A and/or D-A-π-A frameworks [6,10,29].

The selection of the π-conjugated spacer plays a critical role in D-A systems, acting as a bridge that facilitates efficient intramolecular charge transfer, which is essential for optimal solar cell performance. By fine-tuning the spacer, photophysical and electrochemical properties can be optimized to achieve enhanced conjugation and planarity. This tuning adjusts the HOMO and LUMO energy levels to promote efficient charge delocalization and improve light absorption in the visible to NIR. BBSD was selected as a unit within the spacer for its intrinsic aromaticity, due to its planar structure and the delocalized π-conjugated electron system contributed by nitrogen and selenium atoms, which stabilize the molecule and enhance its electronic properties. This further illustrates that BBSD is an effective building block for designing high-performance D-A dyes, establishing its relevance in this study’s design strategy.

In this paper, we adopted the (D-(A-π)₂-A) design for the models. Out of 27 designed organic dye spacers, 2 were selected as the π-linkers to pair with 10 different donor and 10 acceptor groups, respectively. A total of 200 distinct complexes were designed and studied. DFT/TDDFT approaches were applied for the theoretical analysis of all the designed dyes. Preliminary theoretical calculations lay the groundwork for developing advanced BBSD-based NIR-absorbing dyes with exceptional optoelectronic performance. By elucidating the structure-property relationships among selected donor groups, acceptor groups and BBSD-based conjugated spacers, this study sheds light on their potential application in organic DSSCs and other high-performance optoelectronic devices. Optical and electrochemical parameters were predicted to analyze how the conjugation arrangement of the dyes affects the overall efficiency of the solar cell. This work tests the known limits of selenium chemistry and further provides effective design strategies for DSSCs with selenium-containing heterocycles. Our results reveal that the proposed theoretical design for organic dyes is reasonable for advanced dye modifications and screening with BBSD-acceptor.

2. Materials and Methods

In the present study, three density functional theory (DFT) functionals were employed, including the hybrid B3LYP [12], its long-range corrected derivative CAM-B3LYP [30], and the meta-hybrid M06-2X functional with Hartree–Fock exchange correlations of 20%, 65%, and 54%, respectively [31,32]. A standard triple zeta basis set augmented by polarization functions was employed, i.e., 6-311G(d,p) [7,33]. Time-dependent density functional theory (TDDFT) [9,33] was developed to theoretically study excitation energies, absorption wavelengths, and oscillator strengths. TDDFT has proved to be a useful tool in our previous work to explore optical properties for large and medium size molecules. The combination of DFT and TDDFT can provide efficient and reasonably accurate predictions of excited state properties of the BBSD-acceptor based dyes. All computational calculations associated with BBSD-acceptor based dyes were performed by employing DFT/TD-DFT approaches in the Gaussian 16 programs [29,34]. The ground state geometries of all the designed dyes were fully optimized using the above-mentioned theoretical level. All considered structures were proved to be minimum energy geometries by harmonic vibrational frequency analysis. The absorption spectra of all the models were investigated based upon the optimized structures of the TDDFT method at the same theoretical level.

A study on BBSD chromophores demonstrated that the hybrid meta-GGA functional M06-2X, with an exchange–correlation coefficient (R²) of 0.992 [12], offers highly accurate estimations of charge transfer and optical absorption in large organic compounds in comparison with experimental values, thus validating its performance in identifying the best-performing dyes based in this study on its superior exchange–correlation results. Furthermore, comprehensive research based on the Minnesota density functional, M06-2X hold that the method results give good estimation accounting for dispersion and is one of the best to study non covalent interactions of D-A based dyes, while the split-valence set, 6-311G(d,p), is utilized with polarizable and diffused functions added for increased efficacy [12,29,30]. Hence, all the discussions below will be focused on the results obtained by M06-2X functionals. The results obtained by B3LYP and CAM-B3LYP are also shown as comparisons in the SI as references. (See Supplemental Information).

3. Results and Discussion

3.1. Spacer Selection

In order to accurately identify the electronic and optical properties related to the studied dyes, preliminary calculations were performed with 27 π-spacer units to compare and establish an optimal functional for use in this theoretical investigation. The molecular geometries of π linkers 1–27 are shown in Figure 1. The geometrical structures of linkers are based on the existing literature. Synthesized molecules from recently reported DSSC studies [4,7,10,12,19,22,35] with sulfur- and selenium-containing compounds were theoretically combined in different frameworks to analyze optical behavior. Selenium is recognized for its beneficial biological properties in medicinal chemistry, including antibacterial and antioxidant effects, as well as its effectiveness in treating various disorders, while in material science, it is used to develop organic conductors and photovoltaic materials through modifications of selenium heterocycles and selenophene analogs to optimize their properties [13,15]. As seen in Figure 1, sulfur atoms were substituted with selenium for the sulfur-containing linkers in efforts to increase the absorption and strength of the first electronic excitation transition. Moreover, the fusion of electron-accepting constituents with selenadiazole was proven to yield favorable results in experimental synthesis [17]. All linkers therefore have selenium heterocyclic units. Spacers 5, 7–15, 18–20, and 22–23 all contain one BBSD-acceptor unit, while spacers 16, 17, 21, and 24–27 include two BBSD units. In order to achieve better performance of the π-conjugated spacers, the related dyes were designed by linking the designed π-conjugated spacers with varying electron-accepting and -donating groups, increasing the corresponding conjugation length.

The electronic absorption data for dye π-spacers 1–27 with M062X/6-311G(d,p) are shown in Table 1 to reveal the reference set for study. The investigation of spacers with results obtained using the B3LYP and CAM-B3LYP exchange–correlation functionals with the same basis set is provided in Table S1. (See Supplemental Information). Previous D-A reports, specifically with BBSD-acceptor-based moieties, utilize this range of functionals to properly account for charge transfer and excitations [12]. To examine the performance of the 27 π-spacers, dyes were modeled by linking the spacers with anchoring substituents of carbazole linked to benzene (donor 1 (D1)) and 2-cyanoacrylic acid (acceptor 1 (A1)), respectively. This will be served as reference molecules. The predicted absorption spectra suggest that dyes with spacers 16 and 17 show the longest first maximum absorption wavelength.

The internal conjugated units of spacers 16 and 17 are derived from a reported DFT/TDDFT study of a D-π-A dye framework, with triphenylamine as the donor and 2-cyanoacrylic acid as the acceptor. In this study, various exchange–correlation functionals, including B3LYP, PBE0, BHandHLYP, and CAM-B3LYP, were applied to explore the dyes and their binding interactions with (TiO₂)₉ clusters [36]. Ground-state optimizations using the B3LYP functional with the 6-31G basis set, along with TDDFT calculations at the TD-BHandHLYP/6-31G* level, showed good agreement with experimental data [36]. To further enhance the absorption properties of the dyes, we propose incorporating a BBSD unit into the conjugated system.

Testing a range of functionals established a reliable computational framework with qualitative results in agreement with experimental studies. Quantum chemical calculations of π-spacers demonstrate that the DFT/TD-DFT framework provides an optimal exchange–correlation method, yielding reliable hyperpolarizability data [32]. Performed at the M06-2X/6-311G(d,p) level of theory, this approach successfully establishes a reference library for this study.

Based on the performance of preliminary calculations and experimental comparison of BBSD-based chromophores, internal units of 16 and 17 that are sandwiched between two BBSD-acceptor groups illustrated the best absorption spectra among all the 27 π-linkers. Therefore, these two spacers (16 and 17) were chosen to be the base to build up the studied library of BBSD dyes. They are paired with different terminal donor and acceptor groups in order to search for better candidates of the high performance BBSD-acceptor dyes. BBSD NIR-absorbing compounds can lead to a bathochromic shift with the increase in quinoidal character with selenium substitution, along with the contribution of the BBSD unit to the extension of chain length [37]. Modifications have been made to BBSD-based π-linkers by alternating the terminal donor and acceptor groups. A total of 200 dyes based upon spacers 16 and 17 were thus designed.

3.2. Geometrical Structures of D-[A- π-A-π]-A Type Dyes

All the studied D-[A-π-A-π]-A type dyes included four main parts: (1) terminal donor group, (2) π-conjugated spacer sandwiched by two internal acceptor moieties, (3) benzene, and (4) terminal acceptor group. The geometrical structures of spacers 16 and 17 with D1 and A1 are shown in Figure 2. The π-linkers within dyes 16 and 17 separating the two BBSD units are methyl-substituted cyclopentadiselenophene and 3-methylselenophene, respectively.

The dye sensitizers were built through exchanging the anchoring donor and acceptor groups while keeping the internal (A-π)² spacer substituents unchanged. Figure 3 depicts the set of anchoring electron-withdrawing and electron-donating fragments of the studied dyes. Both the donors (D1–D10) and the acceptors (A1–A10) include some recognized functional groups and some newly synthesized fragments of selenium that have conjugated structures with a low-bandgap [4,7,12,19,36]. These compounds were anchored on the terminal ends of the dyes to evaluate the effects of substituent modifications on the corresponding photovoltaic activity. D1 and A1 are the anchoring substituents as the corresponding reference dyes [12] for the remaining designed dyes.

Among the donors, electron-rich groups such as carbazole and fluorene are designated as D1 and D7, respectively. Dimethoxy- and dicarbonyl-substituted diphenyl amines are represented as D2 and D4. N-phenyl amine analogs correspond to D5, D8, and D9. Phenoxazine is classified as D6. Selenium-containing donors include D3, which features a methoxy-substituted phenyl selenophene, and D10, identified as diselenopyrrole. The sp³-hybridized nitrogen-bridged diselenopyrrole donor (D10) exhibits a coplanar structure that enhances conjugation, resulting in red-shifted absorption spectra and a high molar extinction coefficient due to the quinoidal character of its selenophene units [24]. The nitrogen atom further increases its strong electron-donating ability, enabling efficient charge transfer and superior dye performance. Similarly, selenium-containing donors like D3 demonstrate longer absorption wavelengths, highlighting the role of selenium in extending light-harvesting capabilities. Donor D6 (Phenoxazine), with its stable heterocyclic core and strong electron-donating properties, outperforms analogs such as Phenothiazine and TPA, leading to higher power conversion efficiencies [38]. These findings emphasize the critical role of specific electronic features in donors like D10 and their synergistic interaction with acceptors in superior optoelectronic performance in BBSD-based dyes.

Acceptors A1–A5 include common electron-releasing functional groups: A1 as 2-cyanoacrylic acid, A2 as nitro, A3 as cyano, A4 as carboxylic acid, and A5 as nitropyridine. A6 is selected from the terminal group of a newly synthesized NIR-absorbing dye (BTPSeV-4F) by substituting the terminal thiophene with selenophene [39]. Acceptors A7 and A8, derived from a recent study on highly efficient photosensitizers [40], demonstrate enhanced visible light absorption (300–750 nm) and improved photovoltaic efficiency (5.53–7.56%) due to sensitization by SFA-5–8, while the selection of 2-cyano-acrylamide and phenyl acetonitrile units is based on their strong electron-accepting capabilities, which facilitate effective interaction with TiO₂. Acceptors A9 and A10 are chosen from a group of low-band gap materials based on thiadiazolo[3,4-g]quinoxaline, which was reported to improve the power conversion efficiencies of up to 11% [41]. The structural effect of the alternating substituents is analyzed through characteristics of bond lengths and dihedral angles between the donor–acceptor and the π-linker-acceptor, as depicted in Figure 4. The geometric parameters from the optimized geometry of the studied dyes are listed in Table 2 and Table 3, respectively.

The bonding interactions between the donor units and π-system, as well as between the π-system and acceptor units, were investigated for conjugated spacers 16 and 17, in relation to donor 10 and acceptors 1–10. The ’D-A’ bonding interactions in Table 2 correspond to the bond length nearest to the R₁ substitution, as indicated by the red line in Figure 4, while the ‘π-A’ bond corresponds to the red line closest to the R₃ substitution in the molecular framework. Shorter bond lengths are indicative of enhanced bond strength and overall molecular stability [42]. The bond lengths between the donor and π-groups for spacers 16 and 17 range from 1.433 Å to 1.436 Å. A slight decrease in bond length was observed for spacer 17 compared to the reference structure D1A1, as well as D10A1-D10A10 for spacer 16, aligning with the predicted stability of the configuration. Additionally, the bond lengths between the π-system and the terminal acceptor for spacer 16 range from 1.450 Å to 1.485 Å, while those for spacer 17 range from 1.450 Å to 1.484 Å. While these values are slightly greater than those of the reference model, a subtle decrease in bond length is detected in spacer 17 compared to spacer 16 for the π–acceptor interactions. The bond length from donor to acceptor is favorable for effective charge transfer across the dyes.

The dihedral angles indicate the coplanarity of the models, which in turn affects both the wavelength of absorptions in the electromagnetic spectrum and the intramolecular charge transfer properties [43]. Increased dihedral angles can lead to instability in organic dye molecules due to heightened steric hindrance. Conversely, lower dihedral angle counters reduced dye aggregation through distortion, facilitating coplanarity and enhancing charge transfer between donor and acceptor units through the π-conjugated system [42]. This relationship underscores the importance of dihedral angles in the design of efficient organic dyes.

The dihedral angle 1 of BBSD-based dyes is defined as the angle measured from donor D10 to the first BBSD unit within π-spacer, while the dihedral angle 2 is measured from the benzene unit in the π-spacer to the terminal acceptor. The angles studied for torsion are described as θ₁ and θ₂ in Figure 4. For spacers 16 and 17 in conjunction with D10, the dihedral angles range from 142.35° to 179.96° and 142.23° to 179.99°, respectively. It is noted that the dihedral angle for the structure D10A8-17 is affected by steric hindrance between the benzene and the cyano group of the terminal acceptor, leading to deviations in the estimated angles. The conjugation effect significantly influences the coplanar structure of the dyes and induces charge transfer [42,43]. The studied structures exhibit favorable dihedral angles for potential integration into DSSC technologies with reduced dihedral angles related to reference molecules.

3.3. Electronic Properties of D-[A-π-A-π]-A Type Dyes

The electronic properties of the dyes are critical for intramolecular charge transfer. Structural modifications to BBSD-based dyes using specific D-A arrangements enable a detailed examination of the electrochemical properties related to charge generation. The energy difference between the HOMO and LUMO defines the optical energy band gap in the ground state and is analyzed alongside the contributions to the frontier molecular orbitals. The HOMO–LUMO gaps for D10 arrangements with A1–A10 linking with the studied spacers are depicted in Figure 5. The corresponding data for other studied structures can be found in Tables S2–S5. (See Supplemental Information). The band gap is related to the dye’s photo absorbing capacity. Smaller gaps will enhance charge transfer to obtain longer absorption wavelength. The frontier molecular orbitals before excitation (S⁰) and after excitation (S*) for the D10A1–A10 arrangements with spacers 16 and 17 are illustrated in Figure 6 and Figure 7, respectively. The molecular orbitals for D1–D9 with A1–A10 for spacers 16 and 17 are depicted in Figures S1–S18. (See Supplemental Information).

The energy level of HOMO is from −5.6 eV to −5.77 eV and from −5.37 eV to −5.48 eV for spacers 16 and 17, respectively. While the energy level of LUMO is evaluated from −3.55 eV to −3.99 eV and from −3.62 to −3.77 eV for spacers 16 and 17, respectively. All the LUMO values of the studied dyes lie above the energy of the TiO2 conduction band, while all the HOMO values lie under the redox potential of the iodine electrolyte in a DSSC mechanism. The orbital energy levels indicate that the studied dye arrangements have efficient thermodynamic potential for electron injection and can be regenerated by the electrolyte, with the HOMO positioned away from the TiO₂ semiconductor to prevent back reactions in the dye-sensitized solar cell mechanism and the LUMO located closer to the TiO₂ [4,10,44]. The goal of molecular engineering of dyes is to mitigate charge transfer from the semiconductor back to the dye material after initial photoexcitation and charge separation for effective injection and dye regeneration. Compared to reference models, the substitution of the terminal donor and acceptor raises the HOMO energy level and lowers the LUMO energy level of the investigated dyes.

The orbital band gaps of the benchmark dyes D1A1–16 and D1A1–17 are 2.37 eV and 1.97 eV, respectively. D10 arrangements with A1–A10 exhibit the most significant decrease in band gap energy compared to the other D-A motifs. The average band gaps are 2.01 eV and 1.72 eV for spacers 16 and 17, respectively. With the exceptions of D5 and D7, all other dyes show a slight decrease in band gap compared to the reference dye. The reductions in band gaps for D3 and D6 are close to that of D10, with ranges for spacer 16 of 2.09 eV to 2.14 eV for D3A1–A10 and 2.07 eV to 2.14 eV for D6. For spacer 17, D3A1–A10 motifs have a range of 1.76 eV to 1.81 eV, while D6 varies from 1.75 eV to 1.81 eV. These results indicate that BBSD-based dyes have lower energy gaps than the reference model (D1A1) for both spacers. Furthermore, the overall band gap energy values for spacer 17 are significantly lower than those for spacer 16. Compared with methyl-substituted cyclopentadiselenophene for spacer 16, a larger π-conjugated system in the core of spacer 17 (3-methylselenophene) increases the coplanarity of the designed molecules with the π-linker and facilitates better electron transfer at lower energy levels. A smaller band gap energy along with the substitution of donor and acceptor suggests that designed dyes are good candidates for DSSCs compared to the reference molecule.

In the FMO images from DFT calculations, the purple cloud represents regions of high electron density, while the blue cloud represents regions of low or negative electron density. As shown in Figure 6, for spacer 16 with D10, before the excitation, the electrons are located on the diselenopyrrole donor and extend across the BBSD-containing π-linker. After excitation, the electrons are redistributed mainly on the BBSD-acceptor unit within the π-conjugated spacer. For spacer 17, electrons are also heavily populated across D10 donor and π-conjugated linker before excitation. For D10A4-17 and D10A5–17, electrons are distributed only on units within π-spacer. After excitation, similar to that of spacer 16 models, electrons are mostly delocalized on the BBSD-acceptor molecules for spacer 17 models. Upon excitation, the electrons are redistributed on the BBSD unit near the donor or acceptor, or on both.

3.4. Absorption Spectra

Properties of light absorption can be assessed through the UV–Vis absorption spectra. The theoretical results of the absorption spectra of the designed models are shown in Figure 8. The left panel of the figure shows the calculated absorption spectra for the reference molecules D1A1-16 and D1A1-17, while the right panel displays the simulated absorption spectra for all D10 arrangements combined with acceptors A1–A10, using spacers 16 and 17, respectively.

Table 4 lists the results of the excitation energy, oscillator strengths, wavelength of absorption, dipole moment for all D10A1–D10A10 structures and the reference models (D1A1-16 and D1A1-17). All the results for other designed dyes can be found in Tables S2–S5. (See Supplemental Information).

The 10 lowest singlet excited states through TDDFT quantum chemical calculations were explored. The first vertical excitation observed for the studied dyes, excluding reference models, reveals a $S_0\to S_1$ excitation transition. Reference models for spacers 16 and 17 with D1A1 had a maximum absorption of 1053.7 nm and 1234.5 nm, respectively. The results suggest that the absorption wavelength exhibits greater variation with the donor substitutions than the acceptor substitutions. As donor units are substituted from D2 to D10, the change in the maximum absorption wavelength is obvious, from 1294.2 nm to 1313.7 nm for spacer 16, and from 1503.4 nm to 1536.5 nm for spacer 17. On the other hand, for acceptor substitutions A1–A10, the maximum absorption wavelengths have subtle changes with a fixed donor. The incorporation of the BBSD-acceptor unit within the π-conjugated system of spacers 16 and 17 leads to a significant red-shifted wavelength, attributed to the alternating donor–acceptor configurations (D-A groups 1–10).

The trend in maximum absorption wavelength of the donor substitution for spacer 16 is in the following order D10 > D6/D3 > D8/D9 > D2 > D4 > D5 > D7 > D1. Donor D10 shows the highest absorption values for spacer 16 (depicted in Figure 8), ranging between 1294.16 nm and 1313.70 nm. Donors D6A1-D6A3 and D6A8 demonstrate higher absorption values compared to those associated with D3. Conversely, D3A4–D3A7 and D3A9–D3A10 show superior absorption compared to related configurations with D6. Similar trends are observed with donors D8 and D9; specifically, only D8A1–D8A4 and D8A8–D8A10 exceed the absorption values of their D9 counterparts, while the majority of other D8-acceptor combinations yield lower absorption than those involving D9 for spacer 16. The relationship between D2 and D4 remains consistent, although D4A6–16 displays higher absorption than D2A6–16. Additionally, donor D7 does not significantly enhance absorption relative to the benchmark. D7A1–16 shows an absorption of 1039.59 nm, which is lower than that of the reference molecule. Overall, the designed D-A models with spacer 16 exhibit a red-shift in absorption compared to the reference molecule, indicating a promising potential for application in DSSCs.

The trend observed in molecules with spacer 17 is similar to that of spacer 16, in the following order: D10 > D6 > D3 > D8 > D9 > D2 > D4 > D5 > D7 > D1. However, the absorption values associated with the selected donor–acceptor structural motifs exhibit clearer correlations. D10 yields the highest absorption range for spacer 17, from 1503.39 nm to a peak of 1536.54 nm (D10A6-17). Donors D6 and D3 provide the second and third highest spectral values, respectively, with absorption ranges of 1442.90 nm to 1506.96 nm and 1442.11 nm to 1501.14 nm for the corresponding A1–A10 configurations, respectively. On the other hand, dye structures incorporating donors D5 and D7 show delicate increases in absorption wavelengths compared to the reference (around 1200 nm). The absorption values for the studied dyes are comparable to those of BBSD-based chromophores analyzed using the same theoretical methodology. Brymora reported a maximum absorption wavelength of 1393 nm for their studied BBSD-based chromophore at M06-2X/6-311G(d) computational level [12]. Spacer 17 demonstrates significantly higher absorption wavelengths than the reference molecule and several dye structures with spacer 16, implying its potential for enhanced performance in DSSC applications.

Stronger oscillator strengths are associated with better charge transition character indicated by longer wavelength photo absorption [7]. Oscillator strengths for spacer 17 (1.03–1.20) are greater than those for spacer 16 (0.72–0.87). This indicates that the designed dyes with spacer 17 are better candidates for enhanced photovoltaic performance. Dyes with higher oscillator strengths also exhibit larger transition dipole moments, indicating improved efficiency for better optical performance, as shown in Tables S3 and S4. (See Supplemental Information).

3.5. Dye Efficiency

Ligand harvesting efficiency (LHE) is an important photophysical parameter that is effective in efficient dye sensitizer design [7]. The relation of LHE is calculated as follows:

$\mathrm{LHE}=1-{10}^{-\mathrm{f}}$

where f is the oscillator strength of the molecule. The electronic phase transition from $S_0\to S_1$ gives rise to the maximum wavelength of absorption, where the oscillator strength is obtained from the excited molecular orbital contribution. LHE is solely dependent on oscillator strength related to the maximum absorption. The value of LHE should be relatively high to maximize photovoltaic response in a DSSC [7]. Modifications are made to the reference structures to increase optical absorption and efficiency. LHE for D10A1–D10A10 for spacer 16 and spacer 17 are estimated to be 0.809 to 0.866 and 0.906–0.937, respectively (Table 4). Given the narrow range of LHE values, it can be inferred that the studied dyes will exhibit similar absorption behavior to the corresponding spacers, as supported by the reported optical properties [7]. The LHE results suggest that D10A8 with spacer 16 and D10A6 with spacer 17 are the most efficient dye structures. Favorable LHE values of designed sensitizers are capable of promoting electron injection. Therefore, it is reasonable to assume that the chosen donor and acceptor fragments are responsible for enhancing photophysical properties for photovoltaic use.

4. Conclusions

In the present study, we comprehensively explored in silico 200 BBSD-based D-[A-π-A-π]-A type dyes by employing DFT/TDDFT approaches. The photophysical parameters and structure-property relationships were evaluated at the DFT//TDDFT//M06-2X/6-311G(d,p) theoretical level. The highly parameterized M06-2X functional effectively captures medium-range electron correlation in noncovalent interactions but may underestimate interaction energies in donor–acceptor-based dyes, where long-range electron correlation is critical [45]. This limitation could affect the accuracy of energy predictions and photophysical properties in the studied BBSD systems. To improve precision, alternative methodologies, such as long-range corrected functionals, should be considered for analyzing the best-performing dye systems.

The selection of the electron donors plays a significant role in the absorption wavelengths for the studied dyes. All the dyes with spacer 17 enhance the absorption wavelengths to the near-infrared region (above 1500 nm). Notably, the dye featuring the diselenopyrrole donor (D10) paired with spacer 17 induces a bathochromic shift of approximately 302 nm compared to the D1A1 structure, exhibiting favorable parameters for integration into DSSCs. The D10A6-17 with spacer 17 model appears to be the highest near-infrared-absorbing dye (maximum absorption wavelength of 1536.5 nm with oscillator strength of 1.20). Our results suggest that out of the 200 structures, D10A8-16 and D10A6-17 have the best performance and are the best candidates for development as efficient dye sensitizers for DSSCs. The present study provides a plausible way to efficiently screen and design potential candidates for DSSCs dyes and represent further evidence of the advantageous properties of D-(A-π)₂-A type dyes.

Author Contributions

Conceptualization, T.D. and J.W.; methodology, T.D.; software, J.L.; calculations and analysis, T.D.; writing—original draft preparation, T.D.; writing—review and editing, T.D., J.W. and J.L.; visualization, T.D.; supervision, J.L.; funding acquisition, J.L. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article or Supplementary Materials.

Acknowledgments

We would like to acknowledge the Mississippi Center for Supercomputing Research (MCSR) providing computing support.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Chemical structures of 27 BBSD-based π-conjugated spacer.

Figure 2. Geometrical structures of D-[A-π-A-π]-A type dyes: spacer 16/17 with D1/A1. (red represents donor moiety, green represents the π-spacer, and blue represents the acceptor).

Figure 3. Electron-donor (D1 to D10, in red) and electron-acceptor (A1 to A10, in blue) units used to design D-π-A chromophores.

Figure 4. Substitution pattern of BBSD-based dyes with spacers 16 and 17 for all dye arrangements. Red lines depict chosen bond lengths in Table 2 and purple arrows depict chosen dihedral angles in Table 3.

Figure 5. Energy diagram of HOMO (green) and LUMO (purple) energies and energy gaps (red) in eV for donor D10 and acceptors A1−A10 for spacer 16 (a) and spacer 17 (b). Energy for TiO2 and iodine electrolyte are also shown for reference.

Figure 6. Frontier molecular orbitals before excitation (S0) and after the excitation (S*) of donor 10-designed dyes with spacer 16.

Figure 7. Frontier molecular orbitals before excitation (S0) and after the excitation (S*) of donor 10-designed dyes with spacer 17.

Figure 8. UV–Vis absorption spectra of reference model dyes D1A1 in panel (a,b) in relation to D10A1-A10 for spacers 16 and 17 in relation to the reference model of D1A1.

Table 1

Vertical transition maximum absorption wavelengths and oscillator strengths of spacer 1–27 calculated at the DFT/TDDFT//M06-2X/6-311G(d,p) level.

Spacer	λabs (nm)	OS (f)
1	533.4	1.81
2	545.2	2.03
3	549.6	2.22
4	470.3	0.76
5	741.8	0.47
6	476.9	1.68
7	733.3	0.48
8	963.8	0.74
9	961.8	0.77
10	814.2	0.52
11	840.0	0.63
12	921.6	0.59
13	921.0	0.75
14	863.6	0.35
15	910.5	0.65
16	1053.7	1.03
17	1234.5	1.49
18	749.3	0.46
19	759.7	0.49
20	1058.3	0.15
21	956.9	1.13
22	773.2	0.51
23	924.6	0.56
24	794.1	1.10
25	924.1	0.79
26	961.8	1.04
27	879.3	0.82

Table 2

Selected bond length of donor D10 with acceptors 1–10 and spacers 16 and 17, compared to reference molecule D1A1 (in Angstrom (Å) at the M062X/6-311G(d,p) level).

π–Spacer	Spacer 16		Spacer 17
Compound	D–A	π–A	D–A	π–A
D1A1	1.469	1.394	1.468	1.387
D10A1	1.434	1.454	1.433	1.452
D10A2	1.434	1.459	1.433	1.458
D10A3	1.436	1.462	1.434	1.461
D10A4	1.436	1.485	1.435	1.484
D10A5	1.436	1.480	1.436	1.480
D10A6	1.436	1.458	1.435	1.457
D10A7	1.434	1.452	1.433	1.450
D10A8	1.433	1.450	1.433	1.460
D10A9	1.435	1.451	1.435	1.450
D10A10	1.435	1.466	1.435	1.466

Table 3

Selected dihedral angles of dye D10 with acceptors 1–10 and spacers 16(a) and 17(b), compared to reference molecule D1A1 (in degrees (°) at the M062X/6-311G(d,p) level).

(a)
Compound	Φ1	Φ2
D1A1-16	138.7	−179.3
D10A1-16	–179.3	−179.6
D10A2-16	−179.8	−179.5
D10A3-16	−179.7	176.8
D10A4-16	−179.7	180.0
D10A5-16	179.2	142.4
D10A6-16	−179.8	148.4
D10A7-16	179.9	−179.7
D10A8-16	179.9	−179.5
D10A9-16	−179.8	180.0
D10A10-16	−179.4	147.9
(b)
Compound	Φ1	Φ2
D1A1-17	138.7	−102.2
D10A1-17	−179.9	180.0
D10A2-17	179.5	−178.3
D10A3-17	−179.4	177.6
D10A4-17	−179.5	180.0
D10A5-17	179.5	142.2
D10A6-17	180.0	−148.7
D10A7-17	−179.9	179.9
D10A8-17	179.7	−19.4
D10A9-17	−179.9	179.4
D10A10-17	179.7	148.5

Table 4

Calculated maximum absorption wavelengths (in nm and eV), oscillator strengths, light-harvesting efficiencies, and excitation energies for D10 dyes compared to the reference molecule D1A1. All results were obtained at the DFT//TDDFT//M062X/6-311G(d,p) level.

Compound	Excitation	λ (nm)	λmax (eV)	f	LHE
D1A1-16	S2	1053.7	1.18	1.03	0.907
D10A1-16	S1	1309.4	0.95	0.82	0.848
D10A2-16	S1	1309.2	0.95	0.80	0.840
D10A3-16	S1	1300.0	0.95	0.76	0.826
D10A4-16	S1	1294.2	0.96	0.72	0.809
D10A5-16	S1	1300.7	0.95	0.75	0.820
D10A6-16	S1	1309.9	0.95	0.85	0.857
D10A7-16	S1	1310.1	0.95	0.85	0.860
D10A8-16	S1	1313.7	0.94	0.87	0.866
D10A9-16	S1	1311.1	0.95	0.83	0.853
D10A10-16	S1	1306.9	0.95	0.80	0.842
D1A1-17	S2	1234.5	1.00	1.49	0.968
D10A1-17	S1	1523.8	0.81	1.14	0.928
D10A2-17	S1	1519.6	0.82	1.11	0.922
D10A3-17	S1	1520.6	0.82	1.10	0.921
D10A4-17	S1	1503.4	0.82	1.03	0.906
D10A5-17	S1	1515.5	0.82	1.06	0.913
D10A6-17	S1	1536.5	0.81	1.20	0.937
D10A7-17	S1	1528.9	0.81	1.19	0.936
D10A8-17	S1	1521.7	0.81	1.15	0.930
D10A9-17	S1	1530.5	0.81	1.17	0.933
D10A10-17	S1	1523.4	0.81	1.12	0.924

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst15010030/s1. Figures S1–S18: Electron density distribution pattern for dyes D1A1-D9A10 with spacers 16 and 17; Table S1: Vertical transition wavelengths of spacers with B3LYP and CAM-B3LYP; Tables S2–S5: Electronic Transition Data for Dyes D1A1-D10A10 with spacers 16 and 17.

References

1. Saranya, K.; Rameez, M.; Subramania, A. Developments in Conducting Polymer Based Counter Electrodes for Dye-Sensitized Solar Cells—An Overview. Eur. Polym. J.; 2015; 66, pp. 207-227. [DOI: https://dx.doi.org/10.1016/j.eurpolymj.2015.01.049]

2. Lee, C.-P.; Lin, R.Y.-Y.; Lin, L.-Y.; Li, C.-T.; Chu, T.-C.; Sun, S.-S.; Lin, J.T.; Ho, K.-C. Recent Progress in Organic Sensitizers for Dye-Sensitized Solar Cells. RSC Adv.; 2015; 5, pp. 23810-23825. [DOI: https://dx.doi.org/10.1039/C4RA16493H]

3. Grätzel, M. Solar Energy Conversion by Dye-Sensitized Photovoltaic Cells. Inorg. Chem.; 2005; 44, pp. 6841-6851. [DOI: https://dx.doi.org/10.1021/ic0508371] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/16180840]

4. Roohi, H.; Mohtamadifar, N. The Role of the Donor Group and Electron-Accepting Substitutions Inserted in π-Linkers in Tuning the Optoelectronic Properties of D–π–A Dye-Sensitized Solar Cells: A DFT/TDDFT Study. RSC Adv.; 2022; 12, pp. 11557-11573. [DOI: https://dx.doi.org/10.1039/D2RA00906D] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/35425060]

5. Ikpesu, J.E.; Iyuke, S.E.; Daramola, M.; Okewale, A.O. Synthesis of Improved Dye-Sensitized Solar Cell for Renewable Energy Power Generation. Sol. Energy; 2020; 206, pp. 918-934. [DOI: https://dx.doi.org/10.1016/j.solener.2020.05.002]

6. Krishna, J.G.; Ojha, P.K.; Kar, S.; Roy, K.; Leszczynski, J. Chemometric Modeling of Power Conversion Efficiency of Organic Dyes in Dye Sensitized Solar Cells for the Future Renewable Energy. Nano. Energy.; 2020; 70, 104537. [DOI: https://dx.doi.org/10.1016/j.nanoen.2020.104537]

7. Sathyanarayana Moorthi, V.; Joe, J.; Kumar, S. Density Functional Theory Study of 2,1,3-Benzoxadiazole-5-Carboxylic Acid as Photosensitizers for Dye-Sensitized Solar Cells. Orient. J. Chem.; 2015; 31, pp. 149-157. [DOI: https://dx.doi.org/10.13005/ojc/310116]

8. Kar, S.; Roy, J.K.; Leszczynski, J. In Silico Designing of Power Conversion Efficient Organic Lead Dyes for Solar Cells Using Todays Innovative Approaches to Assure Renewable Energy for Future. NPJ Comput. Mater.; 2017; 3, 22. [DOI: https://dx.doi.org/10.1038/s41524-017-0025-z]

9. Roy, J.K.; Kar, S.; Leszczynski, J. Insight into the Optoelectronic Properties of Designed Solar Cells Efficient Tetrahydroquinoline Dye-Sensitizers on TiO₂(101) Surface: First Principles Approach. Sci. Rep.; 2018; 8, 10997. [DOI: https://dx.doi.org/10.1038/s41598-018-29368-9]

10. Semire, B.; Oyebamiji, A.K.; Odunola, O.A. Electronic Properties’ Modulation of D–A–A via Fluorination of 2-Cyano-2-Pyran-4-Ylidene-Acetic Acid Acceptor Unit for Efficient DSSCs: DFT-TDDFT Approach. Sci. Afr.; 2020; 7, e00287. [DOI: https://dx.doi.org/10.1016/j.sciaf.2020.e00287]

11. Azaid, A.; Raftani, M.; Alaqarbeh, M.; Kacimi, R.; Abram, T.; Khaddam, Y.; Nebbach, D.; Sbai, A.; Lakhlifi, T.; Bouachrine, M. New Organic Dye-Sensitized Solar Cells Based on the D–A–π–A Structure for Efficient DSSCs: DFT/TD-DFT Investigations. RSC Adv.; 2022; 12, pp. 30626-30638. [DOI: https://dx.doi.org/10.1039/D2RA05297K]

12. Brymora, K.; Ducasse, L.; Delaure, A.; Hirsch, L.; Jarrosson, T.; Niebel, C.; Serein-Spirau, F.; Peresutti, R.; Dautel, O.; Castet, F. Computational Design of Quadrupolar Donor-Acceptor-Donor Molecules with near-Infrared Light-Harvesting Capabilities. Dyes Pigment.; 2018; 149, pp. 882-892. [DOI: https://dx.doi.org/10.1016/j.dyepig.2017.12.003]

13. Hellwig, P.S.; Peglow, T.J.; Penteado, F.; Bagnoli, L.; Perin, G.; Lenardão, E.J. Recent Advances in the Synthesis of Selenophenes and Their Derivatives. Molecules; 2020; 25, 5907. [DOI: https://dx.doi.org/10.3390/molecules25245907] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33322179]

14. Konstantinova, L.S.; Knyazeva, E.A.; Rakitin, O.A. Recent Developments in the Synthesis and Applications of 1,2,5-Thia- and Selenadiazoles. A Review. Org. Prep. Proced. Int.; 2014; 46, pp. 475-544. [DOI: https://dx.doi.org/10.1080/00304948.2014.963454]

15. Elsherbini, M.; Hamama, W.S.; Zoorob, H.H. Recent Advances in the Chemistry of Selenium-Containing Heterocycles: Five-Membered Ring Systems. Coord. Chem. Rev.; 2016; 312, pp. 149-177. [DOI: https://dx.doi.org/10.1016/j.ccr.2016.01.003]

16. Balaguez, R.A.; Krüger, R.; Iepsen, B.; Schumacher, R.F.; Oliboni, R.S.; Barcellos, T.; Junqueira, H.C.; Baptista, M.S.; Iglesias, B.A.; Alves, D. Bisarylselanylbenzo-2,1,3-selenadiazoles: Synthesis, Photophysical, Electrochemical and Singlet-Oxygen-Generation Properties. Eur. J. Org. Chem.; 2018; 2018, pp. 6507-6514. [DOI: https://dx.doi.org/10.1002/ejoc.201801186]

17. Rakitin, O.A. Fused 1,2,5-Thia- and 1,2,5-Selenadiazoles: Synthesis and Application in Materials Chemistry. Tetrahedron Lett.; 2020; 61, 152230. [DOI: https://dx.doi.org/10.1016/j.tetlet.2020.152230]

18. Li, H.; Guo, Y.; Lei, Y.; Gao, W.; Liu, M.; Chen, J.; Hu, Y.; Huang, X.; Wu, H. D-π-A Benzo[c][1,2,5]Selenadiazole-Based Derivatives via an Ethynyl Bridge: Photophysical Properties, Solvatochromism and Applications as Fluorescent Sensors. Dyes Pigments; 2015; 112, pp. 105-115. [DOI: https://dx.doi.org/10.1016/j.dyepig.2014.06.035]

19. Ye, F.; Chen, W.; Pan, Y.; Liu, S.H.; Yin, J. Benzobisthiadiazoles: From Structure to Function. Dyes Pigments; 2019; 171, 107746. [DOI: https://dx.doi.org/10.1016/j.dyepig.2019.107746]

20. Lu, F.; Qi, S.; Zhang, J.; Yang, G.; Zhang, B.; Feng, Y. New Benzoselenadiazole-Based D–A–π–A Type Triarylamine Sensitizers for Highly Efficient Dye-Sensitized Solar Cells. Dyes Pigments; 2017; 141, pp. 161-168. [DOI: https://dx.doi.org/10.1016/j.dyepig.2017.02.013]

21. Cillo, C.M.; Lash, T.D. Benzo[1,2-c:3,4-c.’]Bis[1,2,5]Selenadiazole, [1,2,5]Selenadiazolo-[3,4 e.]-2,1,3-benzothiadiazole, Furazanobenzo-2,1,3-thiadiazole, Furazanobenzo-2,1,3-selenadiazole and Related Heterocyclic Systems. J. Heterocycl. Chem.; 2004; 41, pp. 955-962. [DOI: https://dx.doi.org/10.1002/jhet.5570410616]

22. Qian, G.; Dai, B.; Luo, M.; Yu, D.; Zhan, J.; Zhang, Z.; Ma, D.; Wang, Z.Y. Band Gap Tunable, Donor−Acceptor−Donor Charge-Transfer Heteroquinoid-Based Chromophores: Near Infrared Photoluminescence and Electroluminescence. Chem. Mater.; 2008; 20, pp. 6208-6216. [DOI: https://dx.doi.org/10.1021/cm801911n]

23. Liang, X.; Chen, L.; Wang, Y.; Ding, Y.; Xu, Q.; Zhang, X.; Li, P.; Yang, G.; Yin, C.; Zhou, H. et al. Acceptor Engineering of Semiconducting Oligomers for Improved NIR-II Fluorescence Imaging and Therapy. Dyes Pigments; 2023; 220, 111756. [DOI: https://dx.doi.org/10.1016/j.dyepig.2023.111756]

24. Pao, Y.-C.; Chen, Y.-L.; Chen, Y.-T.; Cheng, S.-W.; Lai, Y.-Y.; Huang, W.-C.; Cheng, Y.-J. Synthesis and Molecular Properties of Tricyclic Biselenophene-Based Derivatives with Nitrogen, Silicon, Germanium, Vinylidene, and Ethylene Bridges. Org. Lett.; 2014; 16, pp. 5724-5727. [DOI: https://dx.doi.org/10.1021/ol502793e]

25. Knyazeva, E.A.; Wu, W.; Chmovzh, T.N.; Robertson, N.; Woollins, J.D.; Rakitin, O.A. Dye-Sensitized Solar Cells: Investigation of D-A-π-A Organic Sensitizers Based on [1,2,5]Selenadiazolo[3,4-c]Pyridine. Sol. Energy; 2017; 144, pp. 134-143. [DOI: https://dx.doi.org/10.1016/j.solener.2017.01.016]

26. Adamo, C.; Jacquemin, D. The Calculations of Excited-State Properties with Time-Dependent Density Functional Theory. Chem. Soc. Rev.; 2013; 42, pp. 845-856. [DOI: https://dx.doi.org/10.1039/C2CS35394F] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/23117144]

27. Arı, H.; Büyükmumcu, Z. Comparison of DFT Functionals for Prediction of Band Gap of Conjugated Polymers and Effect of HF Exchange Term Percentage and Basis Set on the Performance. Comput. Mater. Sci.; 2017; 138, pp. 70-76. [DOI: https://dx.doi.org/10.1016/j.commatsci.2017.06.012]

28. Abdullah,; Kim, E.; Akhtar, M.S.; Ameen, S. Highly Stable Bulk Heterojunction Organic Solar Cells Based on Asymmetric Benzoselenadiazole-oriented Organic Chromophores. Int. J. Energy Res.; 2022; 46, pp. 7825-7839. [DOI: https://dx.doi.org/10.1002/er.7683]

29. Tripathi, A.; Kumar, V.; Chetti, P. Impact of Internal (Donor/Acceptor) Moieties and π-Spacer in Triphenylamine-Based Dyes for DSSCs. J. Photochem. Photobiol. A Chem.; 2022; 426, 113738. [DOI: https://dx.doi.org/10.1016/j.jphotochem.2021.113738]

30. Srivastava, R.; Al-Omary, F.A.M.; El-Emam, A.A.; Pathak, S.K.; Karabacak, M.; Narayan, V.; Chand, S.; Prasad, O.; Sinha, L. A Combined Experimental and Theoretical DFT (B3LYP, CAM-B3LYP and M06-2X) Study on Electronic Structure, Hydrogen Bonding, Solvent Effects and Spectral Features of Methyl 1H-Indol-5-Carboxylate. J. Mol. Struct.; 2017; 1137, pp. 725-741. [DOI: https://dx.doi.org/10.1016/j.molstruc.2017.02.084]

31. Waheed, A.; Muhammad, S.; Gilani, M.A.; Adnan, M.; Aloui, Z. A Systematic and Comparative Analysis of Four Major Classes of DFT Functionals to Compute Linear and Nonlinear Optical Properties of Benchmark Molecules. J. Comput. Biophys. Chem.; 2021; 20, pp. 517-528. [DOI: https://dx.doi.org/10.1142/S2737416521500307]

32. Zouaoui-Rabah, M.; Sekkal-Rahal, M.; Djilani-Kobibi, F.; Elhorri, A.M.; Springborg, M. Performance of Hybrid DFT Compared to MP2 Methods in Calculating Nonlinear Optical Properties of Divinylpyrene Derivative Molecules. J. Phys. Chem. A; 2016; 120, pp. 8843-8852. [DOI: https://dx.doi.org/10.1021/acs.jpca.6b08040] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/27749050]

33. Ouahine, H.; Hasnaoui, A.; Hdoufane, I.; Idouhli, R.; Abouelfida, A.; Ait Ali, M.; El Firdoussi, L. Benzo[c][1,2,5]Selenadiazole Organoselenium Derivatives: Synthesis, X-Ray, DFT, Fukui Analysis and Electrochemical Behavior. J. Mol. Struct.; 2020; 1199, 126914. [DOI: https://dx.doi.org/10.1016/j.molstruc.2019.126914]

34. Available online: https://gaussian.com/ (accessed on 10 February 2023).

35. Zhang, Z.; Wang, J. Structures and Properties of Conjugated Donor–Acceptor Copolymers for Solar Cell Applications. J. Mater. Chem.; 2012; 22, 4178. [DOI: https://dx.doi.org/10.1039/c2jm14951f]

36. Zhang, J.; Kan, Y.-H.; Li, H.-B.; Geng, Y.; Wu, Y.; Su, Z.-M. How to Design Proper π-Spacer Order of the D-π-A Dyes for DSSCs? A Density Functional Response. Dyes Pigments; 2012; 95, pp. 313-321. [DOI: https://dx.doi.org/10.1016/j.dyepig.2012.05.020]

37. Li, B.; Zhao, M.; Zhang, F. Rational Design of Near-Infrared-II Organic Molecular Dyes for Bioimaging and Biosensing. ACS Mater. Lett.; 2020; 2, pp. 905-917. [DOI: https://dx.doi.org/10.1021/acsmaterialslett.0c00157]

38. Al-Ghamdi, S.N.; Al-Ghamdi, H.A.; El-Shishtawy, R.M.; Asiri, A.M. Advances in Phenothiazine and Phenoxazine-Based Electron Donors for Organic Dye-Sensitized Solar Cells. Dyes Pigments; 2021; 194, 109638. [DOI: https://dx.doi.org/10.1016/j.dyepig.2021.109638]

39. Jia, Z.; Ma, Q.; Chen, Z.; Meng, L.; Jain, N.; Angunawela, I.; Qin, S.; Kong, X.; Li, X.; Yang, Y. et al. Near-Infrared Absorbing Acceptor with Suppressed Triplet Exciton Generation Enabling High Performance Tandem Organic Solar Cells. Nat. Commun.; 2023; 14, 1236. [DOI: https://dx.doi.org/10.1038/s41467-023-36917-y] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/36871067]

40. Badawy, S.A.; Abdel-Latif, E.; Fadda, A.A.; Elmorsy, M.R. Synthesis of Innovative Triphenylamine-Functionalized Organic Photosensitizers Outperformed the Benchmark Dye N719 for High-Efficiency Dye-Sensitized Solar Cells. Sci. Rep.; 2022; 12, 12885. [DOI: https://dx.doi.org/10.1038/s41598-022-17041-1]

41. Irfan, A.; Mahmood, A. Computational Designing of Low Energy Gap Small Molecule Acceptors for Organic Solar Cells. J. Mex. Chem. Soc.; 2018; 61, pp. 309-316. [DOI: https://dx.doi.org/10.29356/jmcs.v61i4.461]

42. Vuai, S.A.H.; Khalfan, M.S.; Babu, N.S. DFT and TD-DFT Studies for Optoelectronic Properties of Coumarin Based Donor-π-Acceptor (D-π-A) Dyes: Applications in Dye-Sensitized Solar Cells (DSSCS). Heliyon; 2021; 7, e08339. [DOI: https://dx.doi.org/10.1016/j.heliyon.2021.e08339] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/34816038]

43. Roy, J.K.; Kaur, R.; Daniel, A.; Baumann, A.; Li, Q.; Delcamp, J.H.; Leszczynski, J. Photophysical Properties of Donor–Acceptor−π Bridge–Acceptor Sensitizers with a Naphthobisthiadiazole Auxiliary Acceptor: Toward Longer-Wavelength Access in Dye-Sensitized Solar Cells. J. Phys. Chem. C; 2022; 126, pp. 11875-11888. [DOI: https://dx.doi.org/10.1021/acs.jpcc.2c02117]

44. David Ndaleh, D.N.; Nugegoda, D.; Watson, J.; Cheema, H.; Delcamp, J.H. Donor Group Influence on Dye-Sensitized Solar Cell Device Performances: Balancing Dye Loading and Donor Size. Dyes Pigments; 2021; 187, 109074. [DOI: https://dx.doi.org/10.1016/j.dyepig.2020.109074]

45. Hohenstein, E.G.; Chill, S.T.; Sherrill, C.D. Assessment of the Performance of the M05−2X and M06−2X Exchange-Correlation Functionals for Noncovalent Interactions in Biomolecules. J. Chem. Theory. Comput.; 2008; 4, pp. 1996-2000. [DOI: https://dx.doi.org/10.1021/ct800308k] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/26620472]