Probing local structure of glass with orientation-dependent luminescence

The local ordering of particles is considered an important process in glass transition. Ordering is usually observed in simulation and micrometer-sized colloid. However, clear information on local ordering at the molecular level is difficult to obtain experimentally. In this study, we prepared an easily glass-forming fluorophore with a color change owing to the intermolecular arrangement in the liquid, glass, and crystal states. The bathochromic shifts of the photoluminescence spectra indicated a change in the intermolecular orientation upon immediate cooling of the melt. The recovery of the spectra by successive heating indicated that rotation contributed to the change in the intermolecular orientation. The orientation in the glass was distinct from that during crystal growth, which was observed as a slow bathochromic shift by maintaining the temperature between the melting points of the blue- and green-luminescent crystals obtained from dichloromethane/ethanol and dichloromethane/hexane, respectively. Our results demonstrate that the anisotropic interaction between glass-forming luminophores is useful for uncovering molecular-level events in the glassy state. A rigid glass-former revealed local structures of glass and super-cooled liquid by luminescence related to globally and locally stabilized packing.

Automated summary

The local ordering of particles in glass transition is considered important but difficult to observe experimentally at the molecular level. In this study, a glass-forming fluorophore was used to reveal changes in intermolecular arrangement and orientation through color changes in photoluminescence spectra. The results showed that the orientation in glass differs from that during crystal growth, suggesting that anisotropic interactions are valuable for uncovering molecular-level events in the glassy state.