BENZOTHIADIAZOLE-BASED ORGANIC MATERIALS FOR ELECTRONIC APPLICATIONS

Abstract

Among the molecular building blocks used in the organic electronic materials, benzothiadiazole is found to be efficient moiety. Benzothiadiazole showed strong electron withdrawing capability due to existence of two imine groups. This property improves the distribution of donor electrons and broadens the absorption spectra into the NIR region. It also acts as low band chromophore and narrows the band gap of the molecule. This ensures the efficient light-harvesting ability and facile charge transport in device. In this thesis, we extensively investigated the design, synthesis and optoelectronic applications of benzothiadiazole-based functional materials. The thesis contains seven chapters. The first chapter presents the aim and scope for the work on benzothiadiazole derivatives and its scope for utility in optoelectronic applications such as organic light emitting diodes (OLEDs) and dye sensitized solar cell (DSSCs). The second chapter describes the literature background related to synthesis, physicochemical, thermal, electrochemical characterization and electronic applications of benzothiadiazole-based compounds used as sensitizer in DSSCs and as luminescent materials in OLEDs. In chapter 3, a series of 5-methylbenzo[d][1,2,3]thiadiazole (MBTD)-based derivatives have been synthesized using palladium catalyzed Suzuki-Miyaura coupling reaction in moderate to good yields. The selective Suzuki coupling reactions resulted in eight regioisomers. The effect of methyl substitution on electronic absorption, emission, thermal, electrochemical and electroluminescent characteristics was studied in detail and compared with non-methylated benzothiadiazole derivatives. The dyes (5 and 9) containing methyl unit near tert-butylpheny vi showed red-shifted absorption when compared to its analogous regioisomer(6 and 8) due to interrupted π-conjugation between donor and benzothiadiazole in later. The geometrical changes in the ground (non-planar) and excited (more planar) states manifested in the absorption and emission properties. All the dyes displayed positive solvatochromism in the emission spectra, whereas their absorption profiles are not affected with increase of polarity which indicates that dyes are non-polar in ground state. The dyes restrained the formation of molecular aggregates in the solid state. All of the compounds exhibited exceptional thermal stability attributable to the rigid (MBTD) building block. The compound containing disubstituted triphenylamine exhibited high lying HOMO in the series while the LUMO is stabilized in tert-butylpheny substituted compound. The electronic structure correlations are supported by density functional calculations. Solution processed multi-layered OLED device were fabricated employing these compounds either as host emitters or dopant emitters in suitable host matrix and exhibited green/yellowish green electroluminescence In chapter 4, we demonstrate the tuning of optical, electrochemical and electroluminescent properties in the benzimidazole-benzothiadiazole conjugates. Effect for functionalization if the nuclear propertion of benzothiadiazole was studied by synthesized 4,7- disubstituted and 5,6 substituted derivatives. The former has linear conjugation while the later offers “V” shaped vii molecular structures. Additionally the effect of donor in the conjugation was also investigated. The introduction of N-phenylbenzimidazole unit is believed to impart rigidity to molecule and generate anti-aggregation property. Also it may have additional effects on the electron affinity and transporting properties. The effect of different substitution pattern on electronic absorption, emission, thermal, electrochemical and electroluminescent characteristics was studied in detail and compared with benzothiadiazole derivatives which lacks lateral substitution. Interestingly, the linear benzimidazole derivatives substituted at C4 and C7 of benzothiadiazole showed bathochromic shift when compared to its lateral substituted regioisomer with substitution at C5 and C6 position. Moreover the tri- benzimidazole substituted chromophore showed red shift when compared to substituted at C4 and C7. This suggests the lateral substituents may be participating in the conjugation. The dye containing lateral benzimidazole exhibits distinct HLCT nature with moderate quantum yield of 25-75%. The device with 5% dopant concentration exhibited satisfactory performance with violet, green, yellow and orange-red emission. The thiophene based compound (22) as orange-red emitter exhibited excellent performance in the series which is attributed to the utilization of both energies of LE and CT excitons (i.e., hybridized local and charge transfer (HLCT)) In chapter 5, we have designed and synthesized a series of compounds based on 5-methyl benzothiadiazole to demonstrate the variation of properties by changing the position of methyl group with respect to spacer for dye sensitized solar cell applications. The substitution of methyl group at C5 or C6 position leads to synthesis of two types of isomeric dyes. The compound containing methyl unit substituted near donor exhibited high molar extinction coefficient, strong light harvesting ability, and better photovoltaic performances compared to their congeners where methyl unit is positioned near acceptor. The compounds tethered with benzene or thiophene as spacer modulates the electronic communication, influencing light harvesting and in turn efficiency. Consequently, investigation of DSSCs performances under viii standard global AM 1.5 solar conditions, the dyes containing methyl unit substituted near donor with thiophene as spacer exhibited two fold increment in power conversion efficiency (PCE) value upto 4.33% when compared to its congener (2.71%). In chapter 6, a series of lateral 5, 6-substituted benzothiadiazole hybrids are presented. The effect of substitution at C5 and C6 position of benzothiadiazole nucleus have been analysed by photophysical, electrochemical, thermal and electroluminescence properties. The compound containing triphenylamine substitution (33) exhibited bathochromic shift in absorption spectra when compared to other derivatives. Among the carbazole containing dyes, simple modification of attachment with phenyl spacer (para and meta) para linked N-phenyl carbazole compound exhibited red shift in absorption and emission spectra due to extended π- conjugation. This work provides the photophysical, electrochemical insight into lateral functionalization with classical donor-acceptor energy pair in comparison with linear isomers. In chapter 7, a summary of the work is presented critical analyses and established structure property correlation for new materials. Moreover, by considering recent developments, the scope for development of benzothiadiazole-based covalent organic framework (COF) materials is presented for optoelectronic Applications.