Modular access to π-conjugated planar chiral diradicaloids using redox-active pillar[5]arenes
摘要
Open-shell chiral diradicals that feature both the unique chiroptical properties and tunable spin states have kindled numerous interests in organic synthesis and material science. We present a new class of such model systems based on the π-extended planar chiral pillar[5]arene derivatives P5Flu and P5Cz, where the pillar[5]arene-based chiral scaffolds are coupled with sterically encumbered fluorenyl and carbazolyl moieties as highly luminescent redox-active emitters, respectively. Both molecules in neutral states can readily undergo chemical oxidations leading to π-conjugated chiral diradicals of the dehydrogenated P5Flu2• and a positively charged dication P5Cz2+2•. Characterization of the resulting Müller’s open-shell species by variable-temperature electron spin resonance (VT-ESR), UV-vis-NIR absorption, VT-NMR and by computational studies revealed a singlet (ΔES-T = −1.63 kcal mol−1) and a nearly degenerate singlet-triplet ground state (ΔES-T = 0.05 kcal mol−1, with a diradical character index y0 = 0.89) for P5Cz2+2• and P5Flu2•, respectively. We particularly show in this work a new journey for pillar[5]arenes from the conventionally available closed-shell structures to different open-shell spin states with potential applications in chirospintronics and NIR-active chiroptoelectronic materials.