Flor modifiye tiyenotiyofen ve ditiyenotiyofen moleküllerinin elektropolimerizasyonu, elektronik ve optoelektronik özelliklerinin incelenmesi
Electropolymerization, investigation of electronic and optoelectronic properties of fluorine modified thienothiophene and dithienothiophene molecules
Yazar: SEMA TOPAL
Danışman: PROF. DR. ESMA SEZER
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Ana Bilim Dalı / Kimya Bilim Dalı
Derece: Yüksek Lisans, 2020
Konu: Kimya = Chemistry

Özet:
Keşfi ile literatürde çığır açan iletken polimerler, pek çok araştırmacının odak noktası haline gelmiş, bu konudaki çalışmalar ivmelenmiştir. Kimyasal ve elektrokimyasal yöntem ile elde edilebilmeleri, farklı fonksiyonel gruplarla özelliklerinin iyileştirilebilmesi iletken polimerlerin farklı alanlarda kullanılabilir olmasını sağlamıştır. Işık saçan diyotlar (LED, OLED), fotovoltaik hücreler, süperkapasitörler, biyosensörler, antistatik kaplama ajanları, yapay kas çalışmaları, korozyon inhibitörleri gibi farklı alanlarda kullanımı bulunan iletken polimerler, fonksiyonel gruplarına göre farklı emisyon ve absorpsiyon bantları göstererek, değişen kapasitif davranışlara sahip olabilmektedir. Son zamanlarda, özellikle enerji ve optik alanında yapılan çalışmalar farklı iletken polimerlerin kullanımının önünü açmaktadır. Artan enerji ihtiyacını karşılamak üzere son yıllarda yüksek güç ve enerji yoğunluğuna sahip malzemelere olan ihtiyaç artmıştır. Yüksek konjugasyona ve düşük bant aralığına sahip organik iletken polimerler, enerji alanındaki bu ihtiyaca cevap verebildikleri için sıklıkla tercih edilmektedir.
Tiyenotiyofen ve ditiyenotiyofen moleküllerinin pek çoğu iletken polimer forma sokulabildiği için geniş uygulama alanlarına sahip olmaktadır. Bu tez çalışması kapsamında özellikle enerji ve optik çalışmalarda oldukça önemli bir yere sahip olan ikili ve üçlü kaynaşık tiyofen halkalarından oluşan tiyenotiyofen ve ditiyenotiyfen moleküllerinin fenilin para pozisyonunda bulunan florla fonksiyonlandırılmış iki molekülü paralel karakterizasyon çalışmalarıyla incelenmiştir. Enerji ve optik alanında kullanımları ile ilgili detaylı bilgiler elde edilerek raporlanmıştır. Tiyenotiyofen (TT) ve ditiyenotiyofen (DTT) molekülleri yüksek π konjugasyonuna ve kükürt üzerinde yüksek elektron yoğunluğuna sahip olmaları ile donör karakter sergilemeleri, kolay fonksiyonlandırılabilmeleri gibi avantajlarıyla enerji ve optik alanında kullanılabilecek önemli alternatif malzemeler arasındadır.
Bu tez çalışmasında fenil TT ve fenil DTT monomerleri flor ile modifiye edilerek sırasıyla FTT ve FDTT molekülleri elde edilerek elektrokimyasal polimerizasyon yöntemi ile polimerleştirilmiştir. Elde edilen P[FTT] ve P[FDTT] polimerleri voltametri,  elektrokimyasal empedans spektroskopisi, spektroelektrokimyasal ölçümler, galvanostatik şarj-deşarj ve elektrokimyasal kuvars kristal mikroterazi (EQCM) yöntemleri kullanılarak karakterize edilmiştir.
Döngülü voltametri yöntemiyle redoks davranışları incelenmiş ve P[FTT]'nin ince film davranışına, P[FDTT]'nin ise difüzyon kontrollü polimerleşme mekanizması sahip olduğu gözlenmiştir. Her iki monomerin ITO yüzeyinde elde edilen polimer filmlerinin spektroelektrokimyasal ölçümlerinde turuncu-mavi arası renk değişimi gözlenmiş, P[FTT]'nin P[FDTT]'den daha düşük bant aralığına sahip olduğu belirlenmiştir.
P[FTT] ve P[FDTT] psedokapasitif özelliklerinin belirlenmesi amacıyla döngülü voltamogramları ve EIS ölçümleri ile karşılaştırmalı olarak incelenmiş ve her iki yöntemle elde edilen sonuçların uyumlu olduğu gözlenmiştir. P[FTT] için 1,0 V ve P[FDTT] için 1,2 V potansiyellerinde en iyi kapasitans değerleri elde edilmiştir. Ayrıca galvanostatik şarj-deşarj ölçümleri ile de malzemelerin pseudokapasitif davranışlarına ait bilgiler desteklenmiştir. Şarj-deşarj eğrileri kullanılarak polimerlerin enerji ve güç yoğunluğu değerleri hesaplanarak karşılaştırılmış, enerji ve güç depolama cihazları için oldukça umut vaad eden sonuçlar elde edilmiştir.
Tez çalışması kapsamında ayrıca elektrokimyasal polimerizasyon mekanizması ve elde edilen polimer filmleri redoks davranışları EQCM yöntemi ile de incelenmiştir. FDTT düzenli bir şekilde polimerleşirken FTT'nin polimerizasyonu sırasında frekans düşüşlerinin düzensiz olduğu ve dolayısıyla çözeltide oluşan oligomer ve yan reaksiyonların polimerizasyona engel olduğu düşünülmektedir.
Elde edilen bütün sonuçlara göre P[FTT] ve P[FDTT]'nin enerji ve optik uygulama alanlarında kulllanılmaya uygun olduğu bulunmuştur. Çalışmanın devamında, P[FTT] ve P[FDTT]'in elektrokromik ve kapasitör cihaz denemelerinin yapılması ve sensör uygulamalarının test edilmesi planlanmaktadır.

Abstract:
Until recently, polymers were generally known as insulating materials in terms of electrical conductivity based on plastics. Different additives can be added to the polymers in order to add electrical conductivity, but the most effective method for making them conductive is created by providing a conjugated structure in the polymer chain. Conducting polymers that have become groundbreaking in the literature with their discovery has become the focus of many researchers, and studies on this subject have accelerated. The fact that conductive polymers can be obtained by chemical and electrochemical ways, and the properties of them can be easily improved by different functional groups, allows the integration of conductive polymers in different areas.   Commonly used organic conductive polymers are polyacetylene (PA), polythiophene (PTh), polyaniline (PANI), polypyrol (PPy), polyfuran (PFu), poly (para-phenylene) (PPP), polyphenylene (PPh), poly (vinyl chloride) (PVC), polyvinyl (PIn) and polyindole (PInd) are examples. It is possible to obtain an organic conductive polymer with the desired feature that these substances can be easily functionalized. Conductive polymers that are used in different fields such as luminous diodes (LED, OLED), photovoltaic cells, supercapacitors, biosensors, antistatic coating agents, artificial muscle studies, corrosion inhibitors can have varying capacitive behaviors by showing different emission absorption bands according to their functional groups. Recent studies, especially in the field of energy and optics, have paved the way for the use of different conductive polymers. Organic conductive polymers with high conjugation and bandgap are frequently preferred in the field of energy to meet the high power and energy density materials expected in the field of energy in order to meet the increasing energy demand.
Many of the thienothiophene and dithienothiophene molecules have wide application areas since they can be formed into conductive polymer form. Within the scope of this thesis, two fluorinated molecules of thienothiophene and dithiothiophene molecules consisting of binary and triple fused thiophene rings, which have an important place especially in energy and optical studies, were investigated by parallel characterization studies. Detailed information about their use in the field of energy and optics has been obtained and reported. The thienothiophene and dithiothiophene molecules are becoming more and more preferred in the field of energy and optics due to their advantages such as high π conjugation and high electron density on sulfur, and their donor character and easy functioning.
Characterization studies of the two molecules identified and the electrochemical polymers obtained from these molecules have been carried out using many methods such as voltammetric methods, electrochemical impedance spectroscopy, spectroelectrochemical measurements, galvanostatic charge-discharge, and electrochemical quartz crystal microbalance.
The electrochemical polymerization step is the most important step for reasons such as providing electronic conductivity on the chain in conductive polymers, continuing the conductivity without interruption, and performing the correct characterization processes. Electrochemical polymerization is carried out in the specific range determined for each molecule, potentially, and often in a support electrolyte solution. Electrochemical polymerization is influenced by many parameters such as the mobility of the electrolyte, the hydrophilic/hydrophobic character of the solvent, which provides charge transfer between and above the polymer chains. In this thesis study, the electrochemical polymers of both monomers were successfully obtained, and the redox behaviors were examined with the cyclic voltammetry technique. The peak currents increase steadily with the scan rate and the peak potentials remain largely constant, indicating that the P[FTT] film is electroactive. Since P[FTT] does not change the peak potential even at high scan rates, measurements up to 2000 mV/s have been taken. P[FDTT] is electroactive, but peak potentials remain constant at low scan rates only. The P[FDTT] film was found to have a low tendency to respond to high scan rates. It is thought that FTT has thin-film behavior and FDTT includes diffusion-controlled polymerization mechanism.
In general, spectroelectrochemical methods include techniques in which the two methods are used together in cases where spectroscopic or electrochemical methods are insufficient. Spectroelectrochemical measurements are carried out in a quartz cuvette using a transparent working electrode. Electrochemical changes on the transparent electrode surface are the evaluation of the refraction and reflection of the light sent from the light source on the electrode with the detector. UV-visible absortion measurements of the films have been completed by applying constant potential at each value in 0.1 V intervals. Based on the spectroelectrochemical results, a peak was seen at 450 nm for P[FTT] and 455 nm for P[FDTT] due to π-π * transitions in the neutral form (0.0 V) of the polymers. With its potential increase (with the transition to oxidized form), the absorption of the peak around 450 nm was observed to decrease. Isobestic points were detected around 590 nm for P[FTT] and around 550 nm for P[FDTT]. New peaks are formed that support the polaron-bipolaron structure at ∼700 nm and ∼1000 nm after the isobestic point. Orange-blue color change was observed in the spectroelectrochemical measurements of polymer films obtained on the ITO surface of both monomers, and it was concluded that FTT has a lower bandgap than FDTT from the calculated bandgap values.
Cyclic voltammograms have been examined to determine the capacitive properties of molecules, and this thought has been consistent with the capacitance values obtained from EIS measurements considering that it exhibits pseudocapacitive behavior. Impedance is defined as the measure of resistance against alternating current (AC). From EIS analysis, which has two different methods as potentiostatic and galvanostatic, galvanostatic method measures the AC potential as a result of the AC current applied to the system and gives information. In the potentiostatic method, AC current response given by the system is measured by applying AC potential. The impedance value at each frequency can be calculated using the current and potential values by repeating the measurement in the specified frequency ranges. The impedance is given by the Nyquist graph showing the imaginary resistance on the y-axis and the real resistance on the x-axis. This graph gives information about the real resistance of the electrochemical system in the high-frequency range, mass transfer, and ion/electron diffusion in the low-frequency range. EIS measurements were studied in a wide frequency range, low (10 mHz - 1 Hz), medium (1 Hz - 100 Hz), and high (100 Hz - 10 MHz). EIS is also a very useful method for verifying the capacitive behavior of materials. Capacitance values in the low frequency range can be calculated to determine the charge storage capacities of polymers. Looking at the Nyquist graph, a decrease in resistance is observed with the potential increase for both polymers. Phase angle graph also confirm the Nquist graph, as the potential increases in the low frequency region, the phase angle approaches 90º. The best capacitance values were obtained at potentials of 1.0 V for P[FTT] and 1.2 V for P[FDTT].
The method by which the potential change of the working electrode is measured overtime during the application of constant current between the opposite electrode and the working electrodes is called chronopotentiometry, and is often used to determine the capacitive properties of the electrodes. According to this method, the electrode is charged by applying constant current, and then the potential-time graph is obtained with its discharge. In GCD measurements, the potential response of the electrode over time at constant current (0.5-1-2-3-4-5 A / g) values determined using the chronoamperometric technique was recorded. Pseudocapacitive behaviors obtained from CVs of P[FTT] and P[FDTT] are supported by galvanostatic charge-discharge measurements. In addition, the galvanostatic charge-discharge technique is used to determine whether the material shows stable behavior and the number of cycles (cycle life of the electrode) with stable performance. In addition, energy density and power density can be calculated by using galvanostatic charge-discharge measurement in other characteristics that are very important in determining the capacitive characteristics and performance of the materials. Energy and power densities of both polymers were calculated at a current density of 0.5 A / g, energy densities were 0.884 Wh / kg for P[FTT], 1.568 Wh / kg for P[FDTT], and power densities were 233.5 W / kg for P[FTT], 225.8 W / kg for P[FDTT] was found. Energy and power density values of polymers were calculated by using charge-discharge curves and compared, and very promising results were obtained for energy and power storage devices.
Within the scope of the thesis, electrochemical polymerization and the redox behaviors of the polymer films obtained were also investigated by the electrochemical quartz crystal microbalance method. Electrochemical quartz crystal microbalance, which can perform mass measurements with high sensitivity, is called a microsensor type. It is a method used to investigate electrolyte-electrode interface changes. In the EQCM technique, the quartz crystal is placed between two metal electrodes that create an alternating electric field between them. Resonance frequency formed between metals causes vibration movement in crystal. Since the resulting frequency is sensitive to mass change, the mass change in the crystal is measurable. With this technique based on the measurement of the change in the vibration frequency of the crystal, using the Sauerbrey equation makes the mass change on the crystal surface computable. Molecular verification can be performed by calculating the molecular weight or degree of doping from the equation obtained by combining Sauerbrey with Faraday equations. A regular decrease in frequency is observed due to the regular accumulation of P[FDTT] on the surface of the quartz electrode. When the EQCM measurement performed simultaneously with the electrochemical polymerization of P[FTT] is observed, the frequency shows a decreasing increase. An irregular increase in frequency suggests that different factors such as side reactions and oligomer formation affect the polymerization of FTT.
As a result of the study, the characterization of the two monomers studied was made using different methods, and their effectiveness in the field of energy and optics was examined and reported. The study will be continued by conducting electrochromic device trials and capacitor device trials and sensor applications of both molecules.