Rules for Magnetic Exchange in Azulene-Bridged Biradicals: Quo Vadis?

Electronic coupling through organic bridges facilitates magnetic exchange interactions and controls electron transfer and single-molecule device electron transport. Electronic coupling through alternant pi-systems (e.g., benzene) is better understood than the corresponding coupling through nonalternant pi-systems (e.g., azulene). Herein, we examine the structure, spectroscopy, and magnetic exchange coupling in two biradicals (1,3-SQ(2)Az and 1,3-SQ: Az-NN; SQ = the zinc(II) complex of spin-1/2 semiquinone radical anion, NN = spin-1/2 nitronylnitroxide; Az = azulene) that possess nonalternant azulene pi-system bridges. The SQ radical spin density in both molecules is delocalized into the Az pi-system, while the NN spin is effectively localized onto the five-atom ONCNO pi-system of NN radical. The spin distributions and interactions are probed by EPR spectroscopy and magnetic susceptibility measurements. We find that J = +38 cm(-1) for 1,3-SQ(2)Az and J = +9 cm(-1) for 1,3-SQ: Az-NN (H = -2J (S) over cap (SQ).(S) over cap (SQorNN)). Our results highlight the differences in exchange coupling mediated by azulene compared to exchange coupling mediated by alternant pi-systems.

Automated summary

Electronic coupling through nonalternant azulene pi-system bridges influences magnetic exchange interactions, unlike the coupling through alternant pi-systems. The differences in exchange coupling mediated by azulene are highlighted compared to exchange coupling mediated by alternant pi-systems.