Heavier Group 15 Elements: A New Frontier in Molecular Switch Development

Molecular switches—compounds capable of reversibly interconverting between distinct states in response to external stimuli—are foundational to the design of dynamic functional materials. Classical switches based on carbon and lighter pnictogen frameworks, such as stilbenes, azobenzenes, and imines, have long dominated the field owing to their well-defined photophysical properties, synthetic accessibility, and reversible E/Z-isomerization or related transformations. In recent years, significant efforts have been devoted to designing molecular switches incorporating main-group elements—not only to harness the unique attributes of these elements in expanding the frontier of stimuli beyond light and heat, but also to unlock novel mechanistic pathways. In this context, heavier group 15 elements—particularly phosphorus—have emerged as promising platforms for designing responsive molecular frameworks. Advances over the past decade in the synthesis and stabilization of unsaturated phosphorus species—including phosphaalkenes (P=C), diphosphenes (P=P), their heavier analogues (E=E, P=E; E = Sb, Bi), a variety of hypervalent phosphorus compounds, and phosphorus-based (di)radicals—have opened new opportunities in this field. These systems are not limited to classical photo- or thermally induced E/Z isomerization, but also respond to alternative triggers such as metal coordination, redox inputs, and chemical stimuli. Moreover, reactivity modes such as tautomerism, ligand rearrangement, and conformational dynamics provide further avenues for structural interconversion, enriching the scope of pnictogen-based molecular motion. Building upon the well-established paradigms of C=C, N=N, and C=N-based switching systems, this Perspective highlights the evolution and future potential of heavier pnictogen-based molecular switches, with a particular focus on phosphorus-containing frameworks. We examine how E/Z-isomerization, tautomerism, and coordination-driven transformations can be strategically harnessed to develop multifunctional, stimuli-responsive materials. Furthermore, we compare these systems with their lighter main-group analogues and showcase recent advances in their integration into molecular motors, photoresponsive ligands, and other related applications. In doing so, we outline a forward-looking roadmap for the rational design of main-group-based molecular switches and underscore the promise of heavier pnictogens in expanding the molecular design toolkit. We also highlight key challenges that must be addressed to enhance the efficiency of these systems and position them as viable alternatives to classical molecular organic switches.