N-Methylation and Fluorination of the Arylammonium Cation Enable Melting Point Suppression in Layered Hybrid Metal...

Achieving low melting points (T_m) in 2D hybrid metal halides (HMHs) is of great significance in glass formation and single-crystal film-based device fabrication for optoelectronic applications. Compared with organic cations with flexible alkyl chains or rings, the HMHs with rigid aryl rings as the organic cations are more conducive to charge transfer and glass formation. However, their T_m values are generally high and the difference between the T_m and decomposition temperature is small, which is unfavorable for melting processing. Here, an N-methylation design strategy of the phenylammonium cation is reported to lower the T_m and increase the fusible processing window to address these challenges. Three 2D HMHs (4-F/Cl/Br-NMA)₂PbI₄ (4F/4Cl/4Br; 4-F/Cl/Br-NMA = 4-fluoro/chloro/bromo-N-methylaniline cation) are synthesized with aromatic amines as the A-site cations, and their T_m values reach as low as 138 °C. Among them, 4F has a large fusible processing window of 65 °C and a good glass conversion performance with a glass transition temperature to T_m ratio of 0.69. The mechanism to achieve the lowered T_m is ascribed to the methyl substitution on the nitrogen atom weakens the residual movement of the ─NH₂─ group through the conjugated delocalization effect between the benzene ring and the amino group, and thus reduces the hydrogen bonding interactions between the cations and inorganic layers. At the same time, the substitution of fluorine atoms on the benzene ring also helps to lower the T_m.