UV-vis-Induced Vitrification of a Molecular Crystal

• Published in: Advanced functional materials 2007-07, Vol.17 (10), p.1663-1670
• Main Authors: Naito, T., Sugawara, H., Inabe, T., Kitajima, Y., Miyamoto, T., Niimi, H., Asakura, K.
• Format: Article
• Language: English
• Subjects: Charge transfer; Glasses; inorganic; Metal-organic materials; Nanoparticles; Nanoparticles, inorganic; Photochemistry; Silver
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.200600583

A charge‐transfer complex of 2,5‐dimethyl‐N,N′‐dicyanoquinonediimine (DM) with silver (crystalline Ag(DM)2, defined as α) is irreversibly transformed by UV‐vis illumination. Depending on the illumination conditions, three new types of solids (defined as γ, δ, and ϵ) 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) 13C NMR, 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 γ is a glass state of Ag(DM)2. The transformation from crystalline (α) to amorphous (γ) 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 α form are not maintained in the δ form. Decomposition of α by intense illumination results in a white solid (ϵ), 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 ϵ are crystalline with lattice parameters similar to bulk silver; however, the magnetic susceptibilities differ from bulk silver. The application of UV‐vis illumination or heat to a highly conductive opaque molecular crystal irreversibly alters its appearance and physical properties (see figure). The vitrification is related to the unique one‐dimensional structural and electronic characteristics of this material, such as metal instability and strong electron–phonon interactions. Several related solids are also characterized in detail.