Design, synthesis and characterization of bisthiazole based donor - acceptor molecular systems

Abstract

The electron withdrawing thiazole rings of bisthiazole (BTz) are constructing unit of various -conjugated donor–acceptor (D–A) molecular systems. The electron receiving nature of the thiazole ring is recognized due to the presence of electron-withdrawing nitrogen of the imine (–C=N). BTz is a good acceptor and its donor–acceptor (D–A) derivatives exhibit high fluorescence quantum yield, high photochemical and thermal stability. The photophysical and electrochemical properties of BTz based Donor–Acceptor system is a function of their HOMO–LUMO gap. The BTz based donor-acceptor systems are used in Organic photovoltaics (OPVs), Aggregation induced emission (AIE), Organic field effect transistors (OFETs), Mechanochromism, and Nonlinear optics (NLO). The HOMO–LUMO gap of D–A, D––A, D--A--D and D-A1-A-A1-D systems can be tuned by varying spacer length of -bridge. The BTz based D–A systems with strong intramolecular charge-transfer and low HOMO–LUMO gap is favorable for dye sensitized solar cells (DSSCs). A variety of donors (triphenylamine, ferrocene, etc.) and acceptors (TCNE, cynoacetic acid, malononitrile, etc.) have been explored in the design and synthesis of donor–acceptor systems. Figure 1. Bisthiazole (BTz). The substitution of the donors and acceptors at 3 and 3'-positions of the BTz perturbs the photonic properties of bisthiazole derivatives significantly. In order to tune the HOMO–LUMO gap of bisthiazole derivatives, the BTz unit was functionalized with various donors, spacers and acceptors in symmetrical manner. The influence of substitution of various D/A systems on the photophysical and electrochemical properties was studied. The main objectives of the present study are:  To synthesize donor‐substituted symmetrical BTz of the type D–A, D––A, D-A1-A-A1-D and D––A––D and to study the effect of extension of -conjugation length on the photophysical and electrochemical properties.  To design and synthesize ferrocenyl-substituted symmetrical BTz by addition of the various -spacers, linkers and their effect on HOMO–LUMO gap.  To study the influence of various D/A units on photophysical, thermal and electrochemical properties. Chapter 1: Introduction This chapter describes the synthesis and functionalization approaches of BTz derivatives, and their applications in different fields. Chapter 2: Materials and Experimental Techniques Chapter 2 summarizes the general experimental methods, characterization techniques and details of instruments used for characterization. Chapter 3: C2-Symmetric Ferrocenyl Bisthiazole: Synthesis, Photophysical, Elecrtrochemical and DFT Studies Chapter 3 describes a series of donor–acceptor ferrocenyl substituted bisthiazole, synthesized by the Pd-catalyzed Suzuki, Heck, and Sonogashira cross-coupling reactions. Their photophysical, electrochemical and computational studies reveal strong donor–acceptor interaction.Chapter 4: CS-Symmetric Triarylborane Substituted Bisthiazole for Selective Detection of F− and CN− ions Chapter 4 summarizes the synthesis of triarylborane substituted bisthiazole by using Sonogashira cross–coupling reaction for selective detection of F− and CN− ions in the presence of Cl−, Br−, I− and NO2− anions. The detection limits for F− and CN− was found to be 5.7 ×10−6 M and 2.1×10−6 M respectively. The time dependent density functional theory (TD-DFT) calculation at 6-31g (d, p)/B3LYP level on the bisthiazole triarylborane was studied for F− and CN− ions.Chapter 5: (D--A)2--D-A Type Ferrocenyl Bisthiazole Linked Triphenylamine Based Molecular Systems for DSSC: Synthesis, Experimental and Theoretical Performance Studies Chapter 5 describes the design and synthesis of ferrocenyl (donor) bisthiazole linked triphenylamine (donor) based donor--acceptor--donor-acceptor [(D--A)2--D-A] type dyes by using Pd-catalyzed Sonogashira cross-coupling and Knoevenagel condensation reactions. Their photophysical, electrochemical and computational studies reveal strong donor–acceptor interaction. The Dye sensitized solar cells (DSSCs) based on D1 and D2 exhibit the power conversion efficiency (PCE) of 6.33% and 5.03% respectively.Chapter 6: Cs-Symmetric Donor-Acceptor Systems of Bisthiazole: Synthesis, Photophysical, Electrochemical Properties and Computational Studies Chapter 6 reports the design and synthesis of Cs-Symmetric D--A--D and D-A1-A-A1-D bearing a variety of electron donating and electron withdrawing groups by using Pd-catalyzed Sonogashira cross-coupling and [2+2] cycloaddition-retroelectrocyclization reactions. Their photophysical and electrochemical properties were explored which show strong donor-acceptor interaction. The [2+2] cycloaddition-retroelectrocyclization reaction pathway was studied by theoretical calculations, which reveals that when terminal positions of bisthiazole are substituted by donor groups are favorable for the [2+2] cycloaddition reaction whereas when bisthiazole substituted by electron withdrawing groups are unfavorable for [2+2] cycloaddition-retroelectrocyclization reaction.Chapter 6: Donor-Acceptor Triphenylvinyl and Tetraphenyl Conjugates: Synthesis, Aggregation-induced emission, Aggregation-caused quenching and TD-DFT Studies. Triphenylvinyl (TPV) substituted bisthiazole 4 and tetraphenylethylene (TPE) substituted triphenylamine oxazole 5 were designed and synthesized by the Pdcatalyzed Suzuki cross-coupling reaction. The TPV 4 shows Aggregation-induced emission (AIE) behavior and TPE 5 shows aggregation-caused quenching (ACQ) effect in THF/water mixture due to the → stacking. The thermal stability of the TPV 4 and TPE 5 are reported, in which TPE 5 show better thermal stability as compared to TPV 4.Chapter 8: Conclusions and Future Scope. Chapter 8 summarizes the salient features of the work and its future prospects to develop the new materials for optoelectronic applications.