BODIPY or 4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene is a class of organic dyes with excellent optical properties which also can be tuned by chemical modifications on the BODIPY core. In this work we have designed and synthesized four novel D-A (P1 - P4) copolymers with a thiophene fused BODIPY as the acceptor subunit and Benzodithiophene as the donor subunit. Unlike P1and P2, P3 and P4 have alkylated thiophene as spacer between the donor and acceptor moieties. Synthesis of the polymers were done using standard Stille coupling reactions and yielded high molecular weight polymers. DFT calculations performed on the polymer backbones suggest a great degree of planarity in P1 and P2 which explains the low solubility of the pair as compared to P3 and P4. The extended conjugation in BODIPY core leads to ultra-narrow band gap in the polymers as seen from UV-vis absorption spectra of the polymers. The polymers P3 and P4 show intense charge transfer bands center at ~ 950 nm with the absorption band edge extending beyond 1100 nm suggesting band gaps of ~ 1 eV. The polymers 3 and 4 show large Stokes shift of ~ 200 nm with emission maxima at ~ 1200 nm. The HOMO of the polymers were calculated to be ~ -5.5 eV from cyclic voltammetry studies and hence are stable in ambient conditions. Foster resonance energy transfer (FRET) is observed in blends of these polymers with PTB7 where a complete quenching of PTB7 fluorescence is observed and further studied using Time resolved photoluminescence spectroscopy (TRPL). This suggests potential use of these polymers as acceptor material in NIR absorbing solar cells along with PTB7 donor and in NIR photodetectors.

BODIPY based ultra-low band gap D-A polymer with NIR absorption and emission

2019 Spring National Meeting

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