Supramolecular π···π Stacking Interactions: Overlooked Forces in Thermo/Photochromic Materials

Displaying the contribution of π···π stacking interactions in a supramolecular assembly is of importance for understanding the intrinsic nature of the molecular assembly and for tailoring strategies to fabricate novel functional materials. Since π···π stacking interactions are facile, nondestructive, and reversible, these interactions are particularly promising in materials engineering and stimuli-responsive materials. Stimuli-responsive behavior is an attractive feature of smart materials, which endows them with programmability by showing prospective potentials in diverse fields, including but not limited to sensors, actuators, optoelectronic devices, medical, and so forth. Thermochromic and photochromic materials are among stimuli-responsive materials that can show reversible or irreversible visible change in response to external stimuli, including heat and light. While thermochromism is a phenomenon in which a color change occurs in response to the change in temperature, photochromism is an incident where a color change happens in response to ultraviolet (UV) or visible photoirradiation. This review underscores the role of π···π stacking interactions in thermochromic and photochromic materials such as small organic compounds, crystals, polymorphs, and polymers. We explore thermo- and/or photochromic compounds in which π···π stacking interactions promote or in some cases impede thermo/photochromism, along with assessing role of substituents in π···π stacking interactions, and its impact on these phenomena. We then provide our own perspective with regard to the contribution of these interactions in the design of future thermochromic and photochromic materials.