Molecular packing and magnetic properties of lithium naphthalocyanine crystals: hollow channels enabling permeability and paramagnetic sensitivity to molecular oxygen

Abstract

The synthesis, structural framework, magnetic and oxygen-sensing properties of a lithium naphthalocyanine (LiNc) radical probe are presented. LiNc was synthesized in the form of a microcrystalline powder using a chemical method and characterized by electron paramagnetic resonance (EPR) spectroscopy, magnetic susceptibility, powder X-ray diffraction analysis, and mass spectrometry. X-Ray powder diffraction studies revealed a structural framework that possesses long, hollow channels running parallel to the packing direction. The channels measured approximately 5.0 × 5.4 Å(2) in the two-dimensional plane perpendicular to the length of the channel, enabling diffusion of oxygen molecules (2.9 × 3.9 Å(2)) through the channel. The powdered LiNc exhibited a single, sharp EPR line under anoxic conditions, with a peak-to-peak linewidth of 630 mG at room temperature. The linewidth was sensitive to surrounding molecular oxygen, showing a linear increase in pO(2) with an oxygen sensitivity of 31.2 mG per mmHg. The LiNc microcrystals can be further prepared as nano-sized crystals without the loss of its high oxygen-sensing properties. The thermal variation of the magnetic properties of LiNc, such as the EPR linewidth, EPR intensity and magnetic susceptibility revealed the existence of two different temperature regimes of magnetic coupling and hence differing columnar packing, both being one-dimensional antiferromagnetic chains but with differing magnitudes of exchange coupling constants. At a temperature of ∼50 K, LiNc crystals undergo a reversible phase transition. The high degree of oxygen-sensitivity of micro- and nano-sized crystals of LiNc, combined with excellent stability, should enable precise and accurate measurements of oxygen concentration in biological systems using EPR spectroscopy.

Figures

Structure 1

Scheme 1

Fig. 1

X-Ray powder diffraction pattern of LiNc. The points represent the experimental data points, while the calculated pattern from the simulated annealing is shown by the solid line. The tick marks indicate the allowed peak positions for this specific structure, while the difference curve is shown underneath the patterns.

Fig. 2

Slip-stacking of LiNc along the a-axis. d1 represents the distance between the LiNc planes and d2 represents the slip-stacking distance along the a-axis. The white spheres represent lithium, the green spheres carbon, the blue spheres nitrogen and the grey spheres hydrogen.

Fig. 3

Three-dimensional representation of the packing of LiNc molecules showing the large channels running parallel to the a-axis. The color scheme is the same as in Fig. 2.

Fig. 4

Effect of molecular oxygen on the EPR linewidth of LiNc crystals. (A) Variation of EPR linewidth of microcrystals with pO2. A linear variation of linewidth with the pO2 is observed. The probe sensitivity (as measured by the slope) is 31.3 mG per mmHg. Inset shows the EPR spectra of LiNc microcrystals measured under vacuum, in the presence of 20% O2–80% N2, or 70% O2–30% N2 using an X-band EPR spectrometer at room temperature. The instrumental settings were: microwave power, 1 mW; modulation amplitude, 1 G (250 mG for 0% oxygen); modulation frequency, 100 kHz; receiver time constant, 41 ms; receiver gain, variable; acquisition time, 10 s (single scan); A single sharp peak with a peak-to-peak width (ΔBpp) of 0.63, 6.3, and 17.5 G in 100% N2, 20% O2 and 70% O2, respectively, is observed. (B) Effect of molecular oxygen on the EPR linewidth of LiNc nanocrystals. A linear variation of linewidth with the pO2 is observed. The probe sensitivity (as measured by the slope) is 27.1 mG per mmHg. The inset shows particle-size distribution in nano-particulate suspension of LiNc.

Fig. 5

Effect of incident microwave power on the signal intensity of X-band EPR spectrum of LiNc microcrystals under anoxic conditions. The inset shows an expanded view of the variation of signal intensity in the low microwave power region

Fig. 6

Temperature dependence of EPR spectra of LiNc crystals. (A) Typical EPR spectra of LiNc crystals obtained under vacuum at 20, 40, 60, 80 and 270 K are shown. Spectrum at room temperature is given in Fig. 5. The EPR instrumental settings were: microwave frequency, 9.7 GHz; microwave power, 0.5 mW; modulation amplitude, 0. 25 G; modulation frequency, 100 kHz; receiver time constant, 41 ms; acquisition time, 15 s (single scan). (B) Temperature dependence of linewidth of LiNc crystals under vacuum. The peak-to-peak linewidth of LiNc crystals decreases with increasing temperatures. The inset shows the same variation on heating the sample.

Fig. 7

Simulation of the EPR spectra and temperature dependence of EPR intensity. The simulations of spectra measured at (A) 40 K and (B) 20 K show their decomposition into two isotropic lines. Though the intensities of both lines increase on decrease of temperature, the relative intensity of the low-field high-g line is higher on lowering the temperature. (C) and (D) show temperature dependence of integrated EPR intensity of LiNc crystals under vacuum on cooling and heating.

Fig. 8

Magnetic susceptibility (high-temperature regime) and inset (low-temperature regime) of LiNc crystals as a function of temperature for an applied magnetic field of 1000 G. The experimental (·) and theoretical values (—) using a Bonner–Fischer model for LiNc crystals are given for comparison. The experimental susceptibility values have been corrected for diamagnetism.

Fig. 9

Effective magnetic moment (high-temperature regime) and inset (low-temperature regime) of LiNc crystals as a function of temperature for an applied magnetic field of 1000 G. The experimental (·) and theoretical values (—) using a Bonner—Fischer model for LiNc crystals are shown for comparison.