Syntheses of Thiophene and Thiazole-Based Building Blocks and Their Utilization in the Syntheses of A-D-A Type Organic Semiconducting Materials with Dithienosilolo Central Unit

Dithienosilole moiety is an electron donating unit, and it has been applied, for example, as a part of small molecular and polymeric electron donors in high performance organic photovoltaic cells. Herein, we report efficient synthetic routes to two symmetrical, dithienosilolo-central-unit-based A-D-A type organic semiconducting materials DTS(Th2FBTTh)2 and DTS(ThFBTTh)2. Fine-tuned conditions in Suzuki–Miyaura couplings were tested and utilized. The effect of inserting additional hexylthiophene structures symmetrically into the material backbone was investigated, and it was noted that contrary to commonly accepted fact, the distance between electron donor and acceptor seems to play a bigger role in lowering the Egap value of the molecule than just extending the length of the conjugated backbone. We searched for precedent cases from the literature, and these are compared to our findings. The optical properties of the materials were characterized with UV–vis spectroscopy. Majority of the intermediate compounds along the way to final products were produced with excellent yields. Our results offer highly efficient routes to many heterocyclic structures but also give new insights into the design of organic semiconducting materials.


■ INTRODUCTION
In the syntheses of conjugated organic semiconductor materials, the utilization of different types of coupling reactions is of paramount importance, as constructing the chain-like conjugated structures using alternative synthetic strategies would turn out to be an extremely laborious task. It can be safely stated that coupling reactions like Suzuki−Miyaura have laid the foundation for these types of organic molecules to the extent that can be seen today. In addition, Suzuki−Miyaura cross-coupling is an established and widely used tool of organic synthetic chemistry, especially in organic materials chemistry and medicinal chemistry. 1 Akira Suzuki, the original developer of the Suzuki coupling, was one of the three scientists awarded with Nobel prize in chemistry in 2010 "for palladium-catalyzed cross couplings in organic synthesis". 2 Organic semiconductors have been successfully applied in organic light emitting diode (OLED) technology, 3 organic biosensors, 4 and organic field-effect transistors (OFET), 5 as well as in organic photovoltaics (OPV), a topic that has drawn considerable research interest in recent times. This could be attributed to the many attractive properties of OPVs, e.g., ease of processing (inkjet printing, roll-to-roll processing) and the possibility to produce lightweight and flexible devices. Most of the aforementioned properties are common also with other organic semiconductor applications.
Ability to fine-tune the electronic and optical properties of materials is one of the greatest strengths of organic semiconductors. Alternating the donor−acceptor sequence in organic semiconductors has had a beneficial impact on HOMO−LUMO levels of the material and promoting, e.g., charge carrier properties. 6 For example, in small molecular OPV active layers, the most successful composition has been acceptor−donor−acceptor, where the central part of the molecule was an electron donating moiety and end groups work as electron acceptors (also known as push−pull structure). 7 Benzothiadiazole (BT) is an abundantly present building block among organic semiconductors. In addition, several structural modifications of benzothiadiazole in organic semiconductor applications can be found from the literature. For example, fluorination of benzothiadiazole (fluorobenzothiadiazole, FBT) fragment has been shown to be an effective way to lower HOMO and LUMO energy levels of the molecule. 8 Another highly important moiety is thiophene which has extensively been used as a building block in organic semiconductors. In fact, thiophene can be found, as a separate fragment or part in a fused ring system, from most of the recently reported active layers of the high-performance OPVs. 9 The first thiophene-containing semiconducting polymer to gain widespread popularity was P3HT (poly(3-hexylthiophene). On the other hand, the other pentacyclic heteroaromatic compound, thiazole (Tz), is far less utilized in organic semiconducting materials, even though it is present in many natural products, and it has found various uses in the fields of material and medicinal chemistry. Our previous studies showed that the thiazole unit had a major role in the regioselectivity of bromination reactions which could be affected by pH control. 10 The utilization of the alkylated dithienosilole (DTS) moiety in OPV application dates to 2006 to the work of Usta et al. 11 Even though the number of publications concerning dithienosilole as a central unit has been recently decreasing, dithienosilole moiety is still a valid building block when developing high-performance electron donors in OPV applications. 12 In this paper, we report a synthetic route for two novel organic A-D-A type semiconductor materials, DTS-(Th 2 FBTTh) 2 and DTS(ThFBTTh) 2 . These molecules differ in structure so that DTS(Th 2 FBTTh) 2 contains two additional, symmetrically placed n-hexylthiophene moieties. UV−vis spectroscopy was used to characterize the optical properties of the compounds. Differential scanning calorimetry was utilized to determine melting points and the thermal stability of the materials. Most of the intermediates along the synthetic routes could be produced with good to excellent yields. The developed procedures can be applied in fine-tuning the properties of small molecular semiconductors, as well as with their polymeric counterparts from the viewpoint of monomer synthesis strategies.

Synthesis of 5-[3-Hexyl-5-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)thiophen-2-yl]-2-(3-hexylthiophen-2-yl)-1,3-thiazole (12).
Magnesium chips (1.3 equiv, 35.3 mg, 1.45 mmol) and an iodine crystal were added in a reaction tube with a magnetic stirring bar. The sealed reaction system was heated until iodine sublimed. The reaction system was allowed to cool to 25°C which after the system was purged with argon for 5 min. Compound 11a (491.9 mg, 0.99 mmol) was dissolved in dry THF (2.8 mL) and added through the septum in the reaction system. Under constant stirring, pinacolborane (1.1 equiv, 0.16 mL, 1.10 mmol) was added dropwise through the septum. The reaction mixture was stirred at 25°C overnight. The reaction mixture was cooled with ice bath, 2 M aqueous HCl (3 mL) was added dropwise, and the mixture was stirred for 10 min. During that period, the released H 2 gas escaped from the reaction system through the open needle. The reaction mixture was extracted with toluene (3 × 10 mL). The combined organic layers were dried with Na 2 SO 4 , filtered, and evaporated to dryness. The product was isolated by using flash chromatography (SiO 2 ). The column was eluated with toluene until the impurities run out. In the second stage, the column was eluated with acetone to isolate the desired product. Finally, evaporation gave compound 12 as a viscous oil (405.3 mg) in 75% yield. 1
Compound DTS(ThTzThFBTTh) 2 (Scheme 2) was selected for the next target after facing problems with the synthesis of compound DTS(ThTzFBTTh) 2 (Scheme 1). To achieve the target, needed building blocks had to be designed and synthesized. Suzuki−Miyaura cross-coupling between 9 and 2-(3-hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane gave 10 in high yield (96%). The bromination of 10 with NBS in an ultrasonic bath gave selectively monobrominated product 11a in high yield (92%). The chemical structure of 11a was confirmed by 1 H, 13 C, and 2D NMR measurements (see Supporting Information) to verify the site of bromination. A small amount of dibrominated byproduct 11b along with the unreacted starting material 10 could be separated from the product mixture. The borylation method applied for the synthesis of 7 was also efficient for 11a affording 12 in good yield (75%). Unfortunately, several attempted Suzuki−Miyaura cross-couplings between 12 and 2 were unsuccessful, and the desired product 15 could not be separated. Based on TLC and NMR analyses, the major problem seems to be unwanted deborylation of 12 in the presence of Pd 2 (dba) 3 and (t-Bu) 3 P·HBF 4 or Xantphos. XPhosPdG2, which has previously been shown to act as an efficient catalyst for labile pinacol esters, showed only low conversion of 2 into product 15 at 30°C. Increasing temperature up to 60°C was shown to facilitate deborylation of 12.
Differential Scanning Calorimetry. To determine the melting temperatures and heat stability of DTS(Th 2 FBTTh) 2 and DTS(ThFBTTh) 2 , DSC measurements were performed for the materials. DSC curves of the materials can be found in Supporting Information (S53 and S54). DTS(ThFBTTh) 2 showed a higher melting temperature of 164°C compared to the melting temperature of DTS(Th 2 FBTTh) 2 , which was measured to be 133°C. This result can be rationalized by the lack of two n-hexyl chains in DTS(ThFBTTh) 2 , which could be responsible for better intermolecular π−π -interactions of the material.
After the DSC measurements, 1 H NMR spectra from both material samples were recorded, and it was noted that heating under an inert atmosphere to 280°C had no effect on the integrity of the materials. Thus, the materials can withstand temporary heating to 280°C under an inert atmosphere without degradation, far exceeding the temperatures encountered in, e.g., typical OPV applications.
UV−vis Measurements. In order to study the effect of structural variation on electrical and spectral properties, UV− vis absorption spectra of DTS(Th 2 FBTTh) 2 and DTS-(ThFBTTh) 2 were measured with a spectrophotometer. The normalized absorption spectra of the compounds in CHCl 3 solution are presented in Figure 1. DTS(Th 2 FBTTh) 2 shows an absorption maximum at 510 nm, whereas the absorption maximum of DTS(ThFBTTh) 2 is red-shifted 30 nm located at 540 nm.
Optical band gaps (E gap ) were determined from the absorption edges of the lowest energy absorption band. 18 The calculated values were 2.00 and 1.93 eV for compounds DTS(Th 2 FBTTh) 2 and DTS(ThFBTTh) 2 , respectively. In Table 1, the results are compared with the previous studies found in the literature. 19−22 The observed E gap values are higher compared to the value of DTS(FBTTh 2 ) 2 (1.85 eV). By comparing the results of DTS(Th 2 FBTTh) 2 and DTS-(ThFBTTh) 2 to the results of DTS(FBTTh 2 ) 2 (entries 1− 3), it can be observed, in addition, that both absorption maxima and edges are blue-shifted with the increasing distance of DTS and FBT units which results in simultaneous increase of E gap values. On the other hand, E gap can be decreased by increasing the number of thiophene (Th) end units as demonstrated in the series of DTS(PTTh) 2 , DTS(PTTh 2 ) 2 , and DTS(PTTh 3 ) 2 (entries 4−6). The series of DTGe-(FBTTh 2 ) 2 , DTGe(FBTTh 3 ) 2 , and DTGe(ThFBTTh 2 ) 2 (entries 7−9) show also that E gap can be decreased by increasing the Th end units and decreasing the distance between central donor unit (DTGe) and electron accepting FBT units, even though the effect is much less pronounced. For benzodithiophene (BDT) central donors (entries 10−16), this effect seems to significantly increase with smaller distances between central donor and acceptors as in the case of the DTS central unit. 23,24 From the results presented here and others found from literature, it can be concluded that bringing the acceptor moiety closer to the central donor unit by shortening the intermediate π-bridge decreases the E gap -value of the compound most efficiently.

■ CONCLUSIONS
In summary, two novel A-D-A type organic semiconductors were designed and synthesized. Incorporation of additional thiophene spacers into the backbone structure led to higher E gap value, thus decreasing the absorption onset wavelength. Lowering the optical band gap is considered beneficial in most applications, and these results suggest that a careful optimization of the moiety sequence in organic semiconductors plays a potentially larger role in decreasing the optical band gap than the overall size of the conjugated structure alone.
Even though only two out of four target compounds were eventually isolated, a significant amount of information concerning optimized synthetic procedures for novel building blocks was produced along the way. Our findings offer efficient routes to access a variety of small molecular units indispensable especially in the field of organic semiconductors.
NMR spectra of synthesized compounds with assignments and DSC curves of compounds DTS-(Th 2 FBTTh) 2 and DTS(ThFBTTh) 2 (PDF) ■ AUTHOR INFORMATION