The open molecular framework of paramagnetic lithium octabutoxy-naphthalocyanine: implications for the detection of oxygen and nitric oxide using EPR spectroscopy

Abstract

The oxygen-induced broadening of the electron paramagnetic resonance (EPR) spectrum of lithium octa-n-butoxy-naphthalocyanine (LiNc-BuO) is interpreted in terms of its open molecular framework crystal structure. LiNc-BuO was prepared as a microcrystalline powder and its structure analyzed using X-ray powder diffraction techniques. The structure contains strongly coupled dimers of LiNc-BuO molecules, which favors a high degree of spin exchange, and results in a single sharp EPR line. The molecular packing leads to a structure with open channels large enough (10 × 6 Ų) for the penetration of small diatomic paramagnetic molecules such as oxygen (O₂) and nitric oxide (NO), as well as the larger triatomic species, nitrogen dioxide (NO₂). The EPR linewidth of LiNc-BuO is extremely sensitive to the concentration of paramagnetic gases in the pressure range of 0–760 mmHg. The effect of oxygen on LiNc-BuO is reversible without any signs of permanent adsorption or chemical oxidation. The time response of the effect of oxygen is extremely rapid (0.24 s). The paramagnetic gas-sensing properties of LiNc-BuO are attributed to the open molecular framework of the crystal structure. The oxygen-sensing property, combined with the previously established biostability and biocompatibility of this material, should enable precise and accurate measurements of oxygen concentration in biological systems using EPR spectroscopy.