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Over the last two decades, organic and polymer light-emitting diodes (OLEDs and PLEDs) have attracted remarkable interest because of their potential applications in full- colour flat-panel displays and portable electronic devices [126-130]. It is very crucial to properly design molecule for improved OLEDs performance as well as to develop highly efficient materials with desirable properties [131-134]. Due to the strong electron withdrawing capabilities, relatively better chemical, thermal and photo-chemical stabilities and excellent fluorescence properties [135-139], heterocyclic molecules have received tremendous attention in recent years. Benzothiazole is a heteroaromatic compound with electron rich sulphur and nitrogen atoms in its molecular constituent and exhibits strong fluorescence in solutions as well as in solid state [140-142]. As a result, benzothiazole incorporated compounds have attracted overwhelming research interest in OLEDs application owing to their unique electro-optical properties [35-38]. Recently, a lot of attention has been paid on benzothiazole incorporated molecules to study their LC as well as photophysical properties together

Dutta et al. [56] synthesized donor-acceptor type two LC compounds (a and b) having benzothiazole moiety (Figure 2.26) and studied their optical, electrochemical and field- effect transistor properties. Both of the molecules exhibited SmA LC phase in the temperature range from 160oC to 240oC and the fabricated device showed p-channel behaviour with hole mobilities of 10-2 cm2/V s. Iwan et al. [44] prepared two unsymmetrical azomethines, BTA1 and BTA2 (Figure 2.27) having benzothiazole core and investigated their optical, electrochemical and photovoltaic performances. Both compounds exhibited blue photoluminescence with electrochemical band gap values as estimated by cyclic voltammetry were in the range of 2.31-2.81 eV and that band gap values were influenced by the introduction of fluorine atoms.

Figure 2.27: Azomethines BTA1 and BTA2 with benzothiazole core

BTA2 also showed very low photovoltaic performance while BTA1 exhibited

photovoltaic response and could have potential as an acceptor in bulk heterojunction (BHJ) devices. Wang et al. [42] synthesized benzothiazole-based derivatives with various π-electron donors as novel bipolar fluorescent compounds. These compounds exhibited high fluorescence quantum yield, desirable HOMO levels and high thermal stability. Optical study revealed that increasing conjugation length reduced the optical band gap up to 3.05 eV. The electrochemical and UV-vis spectroscopic results were used to estimate the actual HOMO and LUMO levels, which showed good match with the data obtained from density functional theory (DFT) calculations. It was concluded

that the newly synthesized compounds would be promising candidate in the application of OLEDs as a multifunctional material. Fu et al. [41, 43, 143] synthesized some benzothiazole derivatives (BBPA, BBNA, BPPA, BPPA-2CN, BPNA and BPNA-2CN) and investigated electroluminescent property (Figure 2.28). Compounds BBPA and BBNA exhibited excellent blue electroluminescent property and highly efficient blue- emitting electroluminescent devices were fabricated with BBPA or BBNA as the blue emitter. Considering the broad emission characteristics and electroluminescent performance, compounds BBPA and BBNA would be excellent candidates for blue light-emitting diodes in future OLED application compared to the other reported blue emitters [144, 145].

Figure 2.28: Molecular structure of BBPA, BBNA, and BPNA

On the other hand, compound BPNA showed bright yellowish-white emission and may be expected to serve as a new promising emission material for white organic electroluminescent devices. Raposo et al. [146] designed and synthesized two series of

novel thienyl pyrrole azo dyes bearing benzothiazole acceptor groups. The electrochemical as well as the linear and non-linear optical properties of well-defined asymmetric push-pull -conjugated systems can be readily tuned by varying the linkage position of benzothiazole heterocycle (2 or 6) to the azo bridge. New dendrimers with benzothiazole as surface group and triazole as branching unit have also been synthesized and investigated for their photophysical, electrochemical and dye-sensitized solar cell (DSCC) performance [147]. It was shown that the presence of more number of benzothiazole and triazole units increased the molar absorption coefficient and altered the fluorescence as well as electrochemical behaviours in the dendrimers. Dye- sensitized solar cell (DSSC) studies revealed that dendrimers with more number of benzothiazole and triazole groups exhibited better current generating capacity than the dendrimers with lesser number of benzothiazole and triazole groups.

In recent years, benzothiazole derivatives have gained great importance in bio- molecules detection due to their favourable spectral properties, particularly long wavelength absorption and fluorescence maxima positions, high values of molar extinction coefficient and fluorescence quantum yields [148]. Zheng et al. [49] synthesized three fluorine contained benzothiazole derivatives (F-N-Me, F-N,N-Me and O-FEt-PIB) (Figure 2.29) and studied their activity as potential tracers for -amyloid plaques in Alzheimer‘s disease (AD).

The binding affinity of each compound was higher than that of PIB and fluorescent staining of senile plaques in human AD brain sections also exhibited the specific binding of these compounds to -amyloid plaques. Binding assay also showed that compound F-N-Me was capable of either bind to -amyloid plaques or tangle while F-N,N-Me and O-FEt-PIB did not exhibit this ability. Ono et al. [50] designed and synthesized two benzothiazole derived push-pull dyes (PP-BT-1 and PP-BT-2) (Figure 2.30) and evaluated their biological potential as probes for detecting β-amyloid (Aβ) plaques in the brain. In vitro binding experiments, these benzothiazole compounds showed high affinity for Aβ (1-42) aggregates. Compound PP-BTA-1 and PP-BTA-2 clearly stained Aβ plaques in both mouse and human brains, reflecting their affinity for Aβ aggregates in vitro. They concluded that the prepared compounds could be potential as fluorescent tracers for detecting βA in Alzheimer‘s disease.

Figure 2.30: Structure of benzothiazole derivatives PP-BT-1 and PP-BT-2 for optical imaging of βA in AD

A series of amidine, thiourea, and guanidine derivatives of 2-amino-6- (trifluoromethoxy)benzothiazole have been synthesized and evaluated for their effectiveness as neuroprotective agents in brain diseases [149]. The biological activity of these compounds tested by means of an in vitro protocol of ischemia/reperfusion injury demonstrated that some compounds significantly attenuated neuronal injury. Some benzothiazole derivatives based on bithiophene structure have also been synthesized as potential radiotracers for -amyloid plaques in Alzheimer‘s disease [51]. Recently, benzothiazole incorporated compounds and their metal complexes have been

considered to be potential as antimicrobial, anticancer and photosensitizing agents [150- 153].