Surface-relief diffraction gratings of various geometries were fabricated from the molecular spin crossover complex [FeII(hptrz)3](OTs)2 using the soft lithographic method described in Part 1 of this paper [Quintero et al., J. Mater. Chem., DOI: 10.1039/c2jm15662h]. The grating geometry and optical properties were obtained from atomic force microscopy and spectroscopic ellipsometry measurements. The grating diffraction efficiency (η) was determined as a function of the temperature at various wavelengths (400–700 nm) using a conoscopic microscopy setup. A significant effect of the molecular spin state change on η is demonstrated (ca. 3% modulation). Using scalar transmission theory the changes in the diffraction pattern were quantitatively traced back to the decrease of the real part of the refractive index (ΔnHL = –0.01) accompanying the 1A → 5T spin transition. These stimuli-responsive photonic devices exhibit useful properties for various applications. Here a proof of concept gas sensor application is demonstrated for the detection of alcohol vapor.