Structural properties and lattice phonons evolution in phenothiazine/iminostilbene solid solutions

Together with co-crystals, solid solutions of molecular systems are vital in the design of multicomponent solids that exhibit improved physical and chemical properties compared to those of pure substances. In this work, both the bulk and thin film phases of the molecular solid solutions of the active pharmaceutical ingredients (APIs) phenothiazine (PTZ) and iminostilbene (ISB) are characterized structurally, while low frequency Raman spectroscopy coupled with DFT simulations is employed to understand the impact of the loss of perfect periodicity of the mixed system on its lattice dynamics. X-ray diffraction methods show the statistical distribution of the two molecules in the structure, and the steady variation of the structural parameters with solution composition, confirming that we are dealing with monophasic mixtures. The spectroscopic properties are demonstrated to be different depending on the nature of the vibrational mode. While the vibrational spectra of molecules can always be decomposed into a superposition of the spectra of the two pure compounds, the lattice phonons exhibit a continuous evolution throughout the solution series.