Effect of nitrogen dioxide on the EPR property of lithium octa-n-butoxy 2,3-naphthalocyanine (LiNc-BuO) microcrystals

Lithium octa-n-butoxy-naphthalocyanine (LiNc-BuO) is a stable free radical that can be detected by electron paramagnetic resonance (EPR) spectroscopy. Previously we have reported that microcrystals of LiNc-BuO exhibit a single sharp EPR peak, whose width varies linearly with the partial pressure of paramagnetic molecules such as oxygen and nitric oxide. In this report, we present the effect of nitrogen dioxide (NO₂), which is also a paramagnetic molecule, on the EPR properties of LiNc-BuO. The gas-sensing property of LiNc-BuO is attributed to the open molecular framework of the crystal structure which is arranged with wide channels capable of accommodating large molecules such as NO₂. The EPR linewidth of LiNc-BuO was highly sensitive to the partial pressure of NO₂ in the gas mixture. The line-broadening was quick and reversible in the short-term for low concentration of NO₂. However, the EPR signal intensity decreased with time of exposure, apparently due to a reaction of NO₂ with LiNc-BuO crystals to give diamagnetic products. The results suggested that LiNc-BuO may be a useful probe for the determination of trace amounts of NO₂ using EPR spectroscopy.