Modulating alkyl groups in hybrid quaternary ammonium salts to design polar glass-ceramic with strong nonlinear optical response

Organic-inorganic hybrid glass-ceramics have received ever-growing attention as nascent research hotspots due to their extraordinary optical properties, e.g., nonlinear optical response, whereas the documented hybrid glass-ceramics are associated with underlying steric effects of quaternary phosphonium cations with a large triphenylphosphanyl group (–PPh₃). In contrast, numerous hybrid salts based on more easily accessible quaternary ammonium cations have been reported, but they have rarely been fabricated as glass-ceramics or even glasses. Herein, we present four hybrid quaternary ammonium salts, (R₃NBn)[FeCl₄] (Bn = benzyl; R = Me for 1, Et for 2, Pr for 3, and Bu for 4, respectively), which belong to space groups P-1, Pbca, Pna2₁, and P3₁, respectively, and undergo intriguing solid-liquid phase transitions at melting temperatures of 355, 356, 368, and 330 K, respectively. Unlike 1–3 revealing fast recrystallization during the subsequent cooling processes, 4 with the longer and more flexible butyl groups shows a much slower recrystallization rate due to the reduced attractive interactions between organic cations and inorganic anions and the increased repulsion from the adjacent cations. As a result, 4 exhibits thermally-induced reversible crystal-liquid-glass-crystal transformations with vitrified temperature of 248 K and recrystallized on-set temperature of 286 K. Strikingly, a glass-ceramic thin film of 4 with inner polar micro-crystals embedded in glass was manufactured under ambient conditions. This thin film unfolds great enhancement of second-harmonic generation response, being approximately 12 times larger than its poly-crystalline form, without any poling treatments. The investigation provides the first access to polar glass-ceramic in hybrid quaternary ammonium salts and offers promising insights for designing glass-ceramics in diverse hybrid salts.