UV‐vis‐Induced Vitrification of a Molecular Crystal

A charge-transfer complex of 2,5-dimethyl-N,N'-dicyanoquinonediimine (DM) with silver (crystalline Ag(DM) 2 , defined as a) is irreversibly transformed by UV-vis illumination. Depending on the illumination conditions, three new types of solids (defined as γ, δ, and e) with different structural and physical properties are obtained and examined by a variety of analytical techniques, including solid-state, high-resolution, cross-polarization magic angle spinning (CP-MAS) 13 CNMR, elemental analysis (EA), mass spectrometry (MS), X-ray absorption fine structure (XAFS), and powder X-ray diffraction (XRD). The CP-MAS, EA, MS, and XAFS results indicate that compound y is a glass state of Ag(DM) 2 . The transformation from crystalline (a) to amorphous (y) solid Ag(DM) 2 is an irreversible exothermic glass transition (glass-transition temperature 155.2 °C; ΔH=-126.8 kJ mol -1 ), which implies that the glass form is thermodynamically more stable than the crystalline form. Compound δ (Ag(DM) 1.5 ) consists of silver nanoparticles (diameter (7±2) nm) dispersed in a glassy matrix of neutral DM molecules. The =N-CN-Ag coordination bonds of the a form are not maintained in the δ form. Decomposition of a by intense illumination results in a white solid (e), identified as being composed of silver nanoparticles (diameter (60±10) nm). Physical and spectroscopic (XAFS) measurements, together with XRD analysis, indicate that the silver nanoparticles in both δ and e are crystalline with lattice parameters similar to bulk silver; however, the magnetic susceptibilities differ from bulk silver.