Cocrystals of 10-Methylphenthiazine and 1,3-Dinitrobenzene: Implications for the Optical Sensing of TNT-Based Explosives

The
evaporation of an ethanol solution containing an equimolar
mixture of 10-methylphenothiazine and 1,3-dinitrobenzene gave red-purple
crystals. The diffuse reflection spectrum for the cocrystals showed
a low reflectance from the UV through the visible spectrum until the
reflectance increased at the red end of the visible spectrum. The
crystal structure showed alternating π stacking of the electron-rich
10-methylphenothiazine and the electron-poor 1,3-dinitrobenzene. There
were also hydrogen bonding interactions between the nitro groups from
1,3-dinitrobenzene and the aromatic hydrogen atoms from 10-methylphenothiazine.
The infrared spectrum showed a shift to lower wavenumbers for the
symmetric and antisymmetric stretching modes for the nitro groups.
Thin films containing 10-methylphenothiazine in polystyrene were exposed
to 1,3-dintrobenzene vapor, and spectroscopic ellipsometry showed
an average increase in the refractive index of 0.006 through the entire
range of wavelengths from 1000 to 300 nm.