Design, synthesis, and properties of benzobisthiadiazole-based donor–π–acceptor–π–donor type of low-band-gap chromophores and polymers

A novel low-band-gap chromophore (5, 0.86 eV) having fluorene as a donor, benzobisthiadiazole (BBTD) as an acceptor, and pyrrole as a π-spacer was successfully designed and synthesized, to probe the effect of π-spacer on the band-gap level of the donor–π–acceptor–π–donor type of chromophores. Compared with the thiophene spacer analogue (in compound 3), the intramolecular hydrogen bonding between the pyrrole and the neighboring BBTD unit pushes the absorption maximum and fluorescence emission of chromophore 5 into the near-infrared spectral region with a red shift of 172 and 158 nm, respectively. The same red-shift phenomenon can also be realized by addition of Lewis acid (e.g., BF3) to the BBTD-containing chromophores with other spacers. Attempt of using low-band-gap chromophore 5 in bulk heterojunction (BHJ) solar cells was made, showing a non-optimized photovoltaic device with the power conversion efficiency of 0.01%. A precursor approach to introduction of the alkaline-labile BBTD acceptor into the polymer backbone has been demonstrated by successful synthesis of low-band-gap polymer P2. The same strategy can be in principle applied to the synthesis of a series of low-band-gap chromophores or polymers with strong acceptors.