Design and synthesis of push-pull functionalized benzothiadiazole chromophores

Abstract

2,1,3-Benzothiadiazole (BTD) is a strong electron withdrawing unit, widely employed in organic electronics due to its strong electron affinity (EA) and high reduction potential. BTD is a fused bicyclic-heteroarene in which benzene ring is connected with diazole based five-membered ring, where one of the C atom in the diazole ring is replaced by an S atom (Figure. 1). The 2,1,3-benzothiadiazole unit acts as a powerful electron deficient unit due to the presence of imine functional unit in the diazole ring, and contains four reactive sites (4-, 5-, 6- and 7-positions) (Figure 1), which can be easily functionalized by the incorporation of donor or acceptor units. The substitution of donor or acceptor units at the 4,7- positions of BTD unit results in a linear shape, whereas substitution of donor or acceptor units at the 5,6-positions exhibits a butterfly shape. The functionalization at 4,7-positions of BTD unit results in molecular systems with excellent photophysical properties including broad absorption spectra, higher molar absorptivities, better quantum yield and solvatofluorochromic effects, whereas functionalization at the 5,6- positions of the BTD core results in inefficient -conjugation due to high torsional angle. The introduction of donor-acceptor units at 4,7-positions and 5,6-positions of BTD unit offers push-pull systems with excellent solid state photoluminescence quantum yield, which are suitable candidates for organic light emitting diodes (OLEDs). The BTD chromophores exhibit outstanding performance in photovoltaic devices like dye sensitized solar cells (DSSCs), bulk heterojunction organic solar cells (BHJOSCs), nonlinear optics (NLOs), organic field-effect transistors (OFETs), hole transporting materials (HTMs) owing to its high reduction potential and strong electron-withdrawing capacity. The BTD chromophores exhibit strong fluorescence quantum yield and good photostability due to which the chromophores have been used for the detection of metal ions (Cu2+, Hg2+ , Fe3+) and acetate anions. In the recent years, BTD based fluorescent chromophores have been utilized as a new class of bio-probes due to its favorable photophysical properties including large stokes shifts and better thermal stability.