Synthesis and characterization of quinoidal diketopyrrolopyrrole derivatives with exceptionally high electron affinities

Abstract

Open-shell singlet biradicaloids are short-lived intermediates, but they exhibit fascinating properties for spin-based devices. Therefore, understanding the nature of their electronic structure and stability is critical for harnessing them in optoelectronic or spintronic devices. Toward this goal, we have synthesized a series of diketopyrrolopyrrole-based quinoidal molecules to investigate the contribution and relative importance of the biradical form on the ground-state electronic structure and distribution of spin density. Possibility of crossover from a closed-shell to an open-shell structure with increase in the C=C/C-C conjugation length was investigated. The ground-state properties were systematically investigated by nuclear magnetic resonance (NMR) spectroscopy, single-crystal X-ray diffraction, and electrochemical studies. Furthermore, n-doping has been carried out in solution at ambient conditions to understand the nature of doped species and demonstrate air stability. Doped species were probed by UV-visible and electron spin resonance (ESR) spectroscopy to unambiguously establish the generation of anionic species in solution. Experimental results are complemented by theoretical calculations to provide insight into the trend toward biradicaloid spin states with increasing conjugation length.