Negative Photoconductivity in Copper–Pyrazine Coordination Polymers

Linear-chain copper–pyrazine coordination polymers are well-known platforms for revealing the fundamental aspects of quantum magnetism, and although they have attracted attention in the high magnetic field community, less is known about their behavior under other external stimuli including light illumination. In order to explore their photophysical behavior and potential for enhanced responsivity under light on/light off conditions, we measured the photoconductivity and optical properties of copper–pyrazine dinitrate [Cu(pyz)(NO 3 ) 2 ] in both hydrogenated and deuterated forms as well as [CuL 2 (H 2 O) 2 (pyz)](ClO 4 ) 2 (L = n -methyl-2-pyridone) with different ligand positions and substitutions on the ring. Strikingly, these materials display “negative photoconductivity”, which means that conductivity decreases under light illumination─different from the traditional expectation that light creates mobile carriers. In addition to testing for and confirming a metastable trapped-state mechanism in the form of a low-lying quinoidal state of the pyrazine ligand, we use our findings to untangle structure–property relations in this family of materials. This work opens the door to controlling the properties of copper–pyrazine linear-chain compounds with light.