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 (T_m) of 175 °C and the melt readily vitrifies under a critical cooling rate (CCR) of 0.3 °C s^-1. In contrast, (S)-(-)-1-(2-naphthyl)ethylammonium lead bromide (S(1-2)NPB) displays a T_m ∼193 °C and requires a CCR of 2500 °C s^-1, necessitating the use of ultrafast calorimetry for glass formation and study of the underlying kinetics. The distinct T_m 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.