Photoswitchable Nitrogen Superbases: Using Light for Reversible Carbon Dioxide Capture

Abstract

Using light as an external stimulus to alter the reactivity of Lewis bases is an intriguing tool for controlling chemical reactions. Reversible photoreactions associated with pronounced reactivity changes are particularly valuable in this regard. We herein report the first photoswitchable nitrogen superbases based on guanidines equipped with a photochromic dithienylethene unit. The resulting N‐heterocyclic imines (NHIs) undergo reversible, near quantitative electrocyclic isomerization upon successive exposure to UV and visible irradiation, as demonstrated over multiple cycles. Switching between the ring‐opened and ring‐closed states is accompanied by substantial p K _a shifts of the NHIs by up to 8.7 units. Since only the ring‐closed isomers are sufficiently basic to activate CO ₂ via the formation of zwitterionic Lewis base adducts, cycling between the two isomeric states enables the light‐controlled capture and release of CO ₂.

Abstract

Light‐controlled CO ₂ activation was achieved by the development of photoswitchable nitrogen superbases equipped with a photochromic dithienylethene unit. The reversible electrocyclic isomerization of the N‐heterocyclic imine backbone is accompanied by substantial p K _a shifts of up to 8.7 units.