CO 2 -Sensitive Porous Magnet: Antiferromagnet Creation from a Paramagnetic Charge-Transfer Layered Metal-Organic Framework.

Porous magnets that undergo a magnetic phase transition in response to gaseous adsorbates are desirable for the development of sustainable sensing and memory devices. Familiar gases such as O ₂ and CO ₂ are one class of target adsorbates because of their close association with life sciences and environmental issues; however, it is not easy to develop magnetic devices that respond to these ubiquitous gases. To date, only three examples of gas-responsive magnetic phase transitions have been demonstrated: (i) from a ferrimagnet to an antiferromagnet, (ii) its vice versa (i.e., change of magnetic phase), and (iii) from a ferrimagnet to a paramagnet (i.e., erasure of the magnetic phase). However, the creation of a magnet, meaning the change from a nonmagnet to a magnet by O ₂ or CO ₂ gas adsorption and magnetic switching by this phenomenon have not yet been explored. Herein, we report a CO ₂ -induced antiferromagnet modified from a paramagnetic charge-flexible layered compound, [{Ru ₂ (2,4-F ₂ PhCO ₂ ) ₄ } ₂ TCNQ(OEt) ₂ ] ( 1 ; 2,4-F ₂ PhCO ₂ ^- = 2,4-difluorobenzoate; TCNQ(OEt) ₂ = 2,5-diethoxy-7,7,8,8-tetracyanoquinodimethane), where three molar equivalents of CO ₂ was accommodated at a CO ₂ pressure of 100 kPa. The magnetic change originates from charge fluctuation due to the transfer of electrons moving from the electron-donor to the electron-acceptor unit or vice versa, resulting in a change in the electron distribution induced by CO ₂ adsorption/desorption in the donor-acceptor-type charge transfer framework. Owing to the reversible electronic state change upon CO ₂ adsorption/desorption, these magnetic phases are switched, accompanied by modification of the electrical conductivity, which is boosted by the CO ₂ accommodation. This is the first example of the creation of a CO ₂ -responsive magnet, which is promising for novel molecular multifunctional devices.