Controlling glass forming kinetics in 2D perovskites using organic cation isomers

The recent discovery of glass-forming metal halide perovskites (MHPs) provides opportunities to broaden the application domain beyond traditionally celebrated optoelectronic research fueled by associated crystalline counterparts. In this regard, it is crucial to diversify the compositional space of glass-forming MHPs and introduce varied crystallization kinetics via synthetic structural engineering. Here, we compare two MHPs with slightly varying structural attributes, utilizing isomer organic cations with the same elemental composition, and demonstrate how this change in functional group position impacts the kinetics of glass formation and subsequent crystallization by multiple orders of magnitude. (S)-(-)-1-(1-Naphthyl)ethylammonium lead bromide (S(1-1)NPB) exhibits a lower melting point (Tm) of 175 degrees C and the melt readily vitrifies under a critical cooling rate (CCR) of 0.3 degrees C s-1. In contrast, (S)-(-)-1-(2-naphthyl)ethylammonium lead bromide (S(1-2)NPB) displays a Tm similar to 193 degrees C and requires a CCR of 2500 degrees C s-1, necessitating the use of ultrafast calorimetry for glass formation and study of the underlying kinetics. The distinct Tm and glass-formation kinetics of the isomer MHPs are further understood through a combination of calorimetric and single-crystal X-ray diffraction studies on their crystalline counterparts, highlighting the influence of altered organic-inorganic hydrogen bonding interactions and entropic changes around melting, providing insights into the factors driving their divergent behaviors. The melting properties and kinetics of glass formation in 2D perovskites can be finely tuned using isomeric organic cations bearing distinct substitutional functional group positions, resulting in enhancement of glass-crystalline switching speed.