Ditiyeno[3,2-b;3',2'-d]tiyofen (dtt) – Bor İçeren Donör Akseptör Sistemlerin Sentezleri Ve Özellikleri

Abstract

Günümüzde π- konjuge sistemler materyal kimyasında önemli bir yer almaktadırlar. Yarı-iletkenliklerinden dolayı elektronik ve optoelektronik uygulamalar için uygun özelliklere sahiptirler. Bunun sebebi organik materyallerin yapıları kolayca değiştirilebilir ve tasarlanabilir. Yapısı değiştirilen organik π-konjuge sistemlerin özellikleri değişir ve uygulama alanlarıda çeşitlilik gösterebilir. Organik ışık saçan diyotlar (OLED), transistörler, güneş hücreleri, organik lazerler gibi uygulamalarda π- konjuge sistemler kullanılır. 30 yılı aşkın süredir OLED'ler için organik materyaller tasarlanıyor. Bunların temel özellikleri elektroda yük taşıyabilmeleri, elektrolüminesans ve floresans yapabilmeleridir. Bu özelliklere sahip birçok materyal tasarlanmış ve yüksek verimlilikte, iyi renk stabilizasyona sahip kırmızı, mavi, yeşil renklerde OLED'ler yapılmıştır. Fakat beyaz organik ışık veren diyotlar (WOLED) hala ilgi çekici durumdadırlar. Çünkü cihaz verimlilikleri ve cihazın yaşam ömrü OLED'lerde elde edilen seviyelere ulaşamamıştır. Cihazın verimini arttırmak için birçok yöntem geliştirilmiştir. Donör-akseptör (D-A) sistemlerin kullanılması bu yöntemlere önemli bir örnek niteliğindedir. Bu sistemlerde en yüksek enerjili dolu orbital (HOMO) ve en düşük enerjili boş orbital (LUMO) arasındaki enerji farkı (band-gap) azalır, böylece iletkenlik yani cihaz verimliliği arttırılmış olur. Akseptörün ve donörün kullanılışı yöntem bakımından çeşitlilik gösterebilir. Örneğin cihazda ayrı ayrı katmanlar olarak bulanabilirler, D-A veya A-D-A gibi tasarlanmış bir π-konjuge molekül ile tek bir katman olarak kullanılabilirler ya da D-A sistemleri içeren oligomerler, polimerler kullanılabilir. Donör olarak seçilecek molekülün π-elektron yoğunluğunun fazla olması gerekir. Ditiyeno[3,2-b;3',2'-d]tiyofen (DTT), 3 kükürt atomu içerdiğinden dolayı elektronca zengindir ve aromatik yapıdadır. DTT'ler S,S-dioksitlerine dönüştürüldüklerinde aromatiklikleri bozunmasına rağmen floresanslarında büyük bir artış meydana gelmektedir. DTT ve DTT-S,S-dioksitlerinin literatürde birçok elektronik ve optoelektronik uygulamaları mevcuttur. Akseptör olarak seçilecek materyalin ise π-elektron yoğunluğu düşük olmalıdır. 3-koordineli-bor, pz orbitalinin boş olmasından dolayı iyi akseptör özellik gösterir. Fakat 3-koordineli-borun boş orbitali su ve havadan çok çabuk etkilenir ve 4-koordineli-bora dönüşür. Bundan dolayı borlu materyallerin gelişimi yavaş olmuştur. Büyük geniş sübstitüentler kullanarak boş pz orbitalinin ataklara karşı sterik olarak korunması sağlanmıştır. Grubumuz tarafından 2004 yılında DTT sentezinde yeni bir metot geliştirmiştir. Bu metot kullanılarak donör karakterdeki DTT ve DTT-S,S-dioksit türevleri sentezlenmiştir. Bunun yanı sıra bor içeren 2,4,6-timetilfenildimetoksiboran, literartürdeki sentez yöntemi modifiye edilerek sentezlenmiş ve akseptör olarak kullanılmıştır. D-A polimer sistemi, bromlanmış 4 farklı donörün n-BuLi ile reaksiyona girerek karbanyon oluşturmasının ardından tek basamakta akseptör olan 2,4,6-timetilfenildimetoksiboran ortama ilave edilmesi ile sentezlenmiştir. Oluşan D-A polimerlerinin ilk olarak elektronik ve optik özellikleri incelenmiştir. Döngülü Voltametri (Cyclic Voltammetry) ile oksidasyon ve indirgeme potansiyelleri ölçülmüş, elektronik band-gapleri hesaplanmıştır. UV-VIS ve FL spektrofotometre cihazları ile polimer filmi kaplanmış İTO'ların ve tetrahidrofuran (THF) içindeki polimerlerin absorpsiyonları ve floresansları ölçülmüştür, ele geçen verilerden kuantum verimleri ve optik band-gapleri hesaplanmıştır. Ayrıca OLED uygulamaları katı halde yapıldıklarında çökmeye bağlı emisyon artışı (aggregation induced emission) olup olmadığını görmek için THF-SU karışımlarında UV-VIS ve FL spektrofotometri ölçümleri alınmıştır. Bu analizlerden farklı olarak 11Bor-Nükleer Manyetik Rezonans (NMR), termogravimetrik analiz (TGA), Light Scattering ölçümleri alınarak yapısal incelemeleri tamamlanmıştır. D-A polimerlerin incelenmesi sonucunda dört polimerinde band-gap aralıkları OLED uygulamaları için uygun bulunmuştur. Hepsinin TGA sonuçlarında termal kararlılıkları iyi çıkmıştır. Fakat çökmeye bağlı sönümlenme (aggregation caused quenching) olduğu gözlenmiştir. Buna bağlı olarak dört D-A polimerin de kuantum verimleri %10'nun altında elde edilmiştir. Cihaz çalışmaları kuantum verimlerinin düşük olmasından dolayı yapılmamıştır. Sönümlenmenin engellenmesi için yapısal değişikliklerin yapılması gerekmektedir.

Organic electronics have been the focus of growing number of the researchers particularly in the fields of chemistry for more than 50 years. The main attraction of this field comes from the ability to modify the chemical structures of the organic compounds in a way that the properties of the materials could directly be affected. Until the mid-1980s, their stability and performance fell short of those devices based on materials such as silicon or gallium arsenide. This situation was changed since then with the demonstration of a low voltage and efficient thin film light emitting diode, which opened the door of the possibility of using organic thin films for a new generation of optoelectronics devices. It has now been proven that organic thin films are useful in a number of applications. Among them, organic light emitting device (OLED) is the most successful one, which is used now in color displays. Organic thin film transistors and low cost and efficient organic solar cells are not far behind OLEDs. Moreover, devices such as organic lasers and memories might be seen eventually. In general, two groups of organic materials, namely small molecules and polymers, are used in electronic and optoelectronic devices. Understanding of their electronic structure is the key to the design of high performance optical and electronic organic devices, and some important tunings in structure or composition of an organic material can markedly alter its bulk properties. Currently, modification of the molecular structures of the conjugated materials to tune their optoelectronic properties is a challenging topic. Thiophene-based organic materials are among the most promising compounds with tuneable functional properties by proper molecular engineering. For example, tiophenes; their oligomers and polymers are not proper materials for applications in light emitting devices as they have low electron affinities and low solid-state photoluminescence efficiencies. On the other hand, converting oligothophenes into the corresponding oligothiophene-S,S-dioxides has been shown to be useful for increasing both thin film photoluminescence efficiencies and molecular energy levels. The use of boron to alter the properties of organic electronics and optoelectronics materials has started recently and given interesting results. The reason for that is the presence of empty pz orbital of boron which behaves as a strong electron withdrawing atom when it makes three bonds. It delocalizes electrons strongly when integrated to “π” systems. In organic material chemistry, conjugated organoborane polymers are now considered as a new class of organic materials with their widespread applications in electronics, optoelectronics and sensors. The method developed by our group for the synthesis of ditihenothiophene (DTT) having aromatic groups is among the best methods available in the literature. DTTs are the heterocyclic rings, formed by fusing three thiophene rings. In this work, polymers of DTTs and DTT-S,S-dioxides, prepared by this methods, consisting of boron atoms will be prepared. Cyclic voltammetry (CV), ultra-violet (UV) and fluorescent (FL) properties of the materials will be studied to understand their sensor, electronics and optoelectronics properties. The aim of this study is to enhance the use of boron in new developing organoborane material chemistry and to develop new organic electronics, optoelectronics and sensor materials as our country is rich in boron element. In this work, four DTT and DTT-S,S-dioxide derivatives were synthesized as a donor. • Synthesis of 2,6-Dibromo-3,5-diphenyldithieno[3,2-b;2',3'-d]thiophene (Donor 1): To a solution of 3,5-diphenyldithieno[3,2-b;2',3'-d]thiophene (500 mg, 1.43 mmol) in DMF (60 mL) was added NBS (560 mg, 3.20 mmol) protected from light portionwise at 0 °C. After 4 h, the reaction mixture was poured into cold water to precipitate the crude product, which was purified by column chromatography over silica gel using a mixture of n-hexane/DCM (3:1) as eluent to give the Donor 1 (581 mg, 1.14 mmol) in 80% yield as a white crystal. • Synthesis of 2,6-Dibromo-3,5-diphenyldithieno[3,2-b;2',3'-d]thiophene-S,S-dioxide (Donor 2): To a solution of 2,6-dibromo-3,5-diphenyldithieno[3,2-b;2',3'-d]thiophene (400 mg, 790 µmol), dissolved in DCM (60 mL), was added m-CPBA (680 mg, 2.76 mmol, 70%) at ambient temperature. The reaction was then left stirring overnight at room temperature for 2 days. The solution was extracted with 10% KOH, 10% NaHCO3 and brine. Organic layer was dried over Na2SO4, filtered and the solvent was evaporated under atmospheric pressure. The crude product was purified by column chromatography over silica gel using a mixture of hexane/DCM (4:1) as eluent to give the Donor 2 (332 mg, 0.62 µmol) in 78% yield as a yellow powder. • Synthesis of 2,6-Bis(5-bromothiophen-2-yl)-3,5-diphenyldithieno[3,2-b;2',3'-d]thiophene (Donor 3): In a reactor, a mixture of 2,6-dibromo-3,5-diphenyldithieno[3,2-b;2',3'-d]thiophene (500 mg, 987 µmol), 2-pinacolborylthiophene (518 mg, 987 µmol), Pd0(PPh3)4 (56.9 mg, 49 µmol), and K2CO3 (1 mL, 2 M) in THF (50 mL) was degassed with N2 for 20 minutes. Then, the mixture was sealed and stirred at 66 °C for 2 days. It was filtered through Celite and the solution was extracted with water. Organic layer was dried over Na2SO4, filtered and the solvent was evaporated under reduced pressure. The crude product was purified by column chromatography over silica gel, eluting with n-hexane. The product was subsequently dissolved in DMF and reacted with NBS. After 4 h, the reaction mixture was poured into cold water to precipitate the crude product, which was purified by column chromatography over silica gel, eluting with n-hexane to give Donor 3 (423 mg, 0.63 µmol) in 64% yield as a yellow powder. • Synthesis of 2,6-Bis(5-bromothiophen-2-yl)-3,5-diphenyldithieno[3,2-b;2',3'-d]thiophene-S,S-dioxide (Donor 4): In a reactor, a mixture of 2,6-dibromo-3,5-diphenyldithieno[3,2-b;2',3'-d]thiophene-S,S-dioxide (500 mg, 929 µmol), 2-pinacolborylthiophene (488 mg, 2.32 mmol), Pd0(PPh3)4 (53.6 mg, 46.0 µmol), and K2CO3 (1 mL, 2 M) in THF (50 mL) was degassed with N2 for 20 minutes. Then, the mixture was sealed and stirred at 66 °C for 2 days. It was filtered through Celite and the solvent was evaporated under reduced pressure. The crude product was dissolved in DCM and the solution was extracted with water. Organic layer was dried over Na2SO4, filtered and the solvent was evaporated under atmospheric pressure. The crude product was purified by column chromatography over silica gel, eluting with a mixture of n-hexane/DCM (3:1). The product was subsequently dissolved in DMF and reacted with NBS. After 4 h, the reaction mixture was poured into cold water to precipitate the crude product, which was purified by column chromatography over silica gel, eluting with a mixture of hexane/DCM (3:1) to give Donor 4 (329 mg, 0.47 µmol) in 50% yield as a orange powder. In addition, a boron derivatives was synthesized as an acceptor in this work. • Synthesis of 2,4,6-trimethylphenyldimethoxyborane: Trimethoxyborane was distilled over CaH2 to avoid water unwatering. Later, 2-mesitylmagnesium bromide (15 mL, 1M) was added dropwise to an diethyl ether solution (80 mL) containing trimethoxyborane (6.44 mL, 57.7 mmol) at -78°C under N2 atmosphere. After the reaction was stirred at -78° C for 3 hours, it was allowed to warmup to room temperature and left to stir overnight. The solution was then filtered under N2 and the precipitated salts were washed off with dry n-pentane (20 mL). The filtrates were combined, concentrated and distilled under vacuum (1 mmHg, 72 oC) to give dimethoxymesitylborane (11.1 g, 57.79 mmol) in 57% yield as colorless viscous liquid. Furthermore, four donor-acceptor (D-A) polymers were synthesized using one pot synthesis for WOLED application. • n-BuLi (2.05 eq.) was added dropwise to a THF solution (40 mL) of donors (1 eq), which were all brominated, at -78°C under N2. The reaction was stirred at -78° C for an hour. Then 2,4,6-trimethylphenyldimethoxyborane in THF (20 mL) was added dropwise to the reaction medium. The reaction was warmed up to room temperature, and left to stir overnight. The solvent was evaporated under reduced pressure. The crude product was adsorbed on silica gel and then purified by removing donors and oligomers by column chromatography over silica gel, eluting with a mixture of n-hexane/DCM (2:1). D-A polymers were extracted from silica gel using THF. The THF solutions were concentrated under reduced condition. The D-A polymers were precipitated into cold n-hexane to give the long chain polymer. The all D-A were analyzed with Cyclic voltammetry (CV), ultra-violet (UV) and fluorescent (FL) to understand their sensor, electronics and optoelectronics properties. Apart from these measurements, their TGA, LS, 11Boron NMR analyses were carried out. Based on TGA analysis, they were thermally stable up to app. 350oC. Their molecular weight were approximate 30000. Their calculated optical and electronical band gap energies were suitable for WOLED applications, but aggregation-caused quenching was observed for all D-A polymers. Therefore, they had small quantum efficiencies of < 0.10. Due to low quantum yields, none of them were designes to be used for diod applications. In the future work, our efforts will be devoted to getting rid of aggregation-caused quenching (AQC) and improving the quantum yields through aggregation induced emission (AIE).