A Multi-Responsive Bismuth-Based Hybrid Exhibiting Dual Thermochromism, SHG Activity, and Dielectric Switching Induced by Sequential Phase Transitions.

Hybrid organic-inorganic halides combining heavy p-block metals and flexible organic cations emerging as versatile frameworks for multifunctional stimuli-responsive materials. Here, we report a simple bismuth-based iodide hybrid, (But)3[BiI6], that exhibits coupled thermochromic, dielectric, and nonlinear optical responses governed by sequential temperature-induced phase transitions. Single-crystal X-ray diffraction reveals three orthorhombic polymorphs (Pnnm, Pccn, and P21212), stabilized through two reversible phase transitions at 260/274 K and 130/150 K upon cooling/heating. The structural evolution is driven by the interplay between discrete BiI6 octahedra and highly flexible butylammonium cations interconnected via N-H···I hydrogen bonding and I···I interactions. Temperature-dependent optical measurements reveal pronounced and reversible thermochromism (orange → green → orange), which is well reproduced by density of states calculations, confirming the electronic origin of the color modulation. Broadband dielectric spectroscopy uncovers strong dipolar dynamics in the high-temperature phase and a sharp dielectric switching behavior associated with the first phase transition, while low-temperature ordering leads to a nearly frequency-independent dielectric response. Second-harmonic generation measurements further demonstrate the emergence of a noncentrosymmetric phase below 130 K, enabling nonlinear optical activity. The coexistence of thermochromism, dielectric switching, and switchable symmetry breaking establishes (But)3[BiI6] as a lead-free multifunctional hybrid, highlighting a viable strategy for designing environmentally benign materials with coupled optical, electrical, and nonlinear optical functionalities for future optoelectronic and photonic applications.