CAREER: Advancing the Mechanistic Understanding of Naphthopyran Mechanochemistry

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With the support of the Macromolecular, Supramolecular and Nanochemistry (MSN) and Chemical Structure, Dynamics and Mechanisms-B (CSDM-B) programs in the Division of Chemistry, Maxwell J. Robb of the California Institute of Technology (Caltech) is advancing the understanding of the chemical reactivity of mechanophores embedded within long chain macromolecules or polymers. Mechanophores are organic compounds that undergo chemical transformations in response to external mechanical force. When incorporated into polymer chains, they can act as sensors and provide valuable information about the behavior of polymers under stress. This characteristic of mechanophores is important in guiding future design of stimuli-responsive polymers with applications in a variety of areas including adhesives, sensors, drug delivery, and sustainability. In this research, experimental and computational techniques will be used to systematically study the chemistry of mechanophores that change color when exposed to stress. Reactivity will be analyzed and correlated with the chemical structure of the mechanophore to understand and control response when exposed to external stress. The synthesized molecules will be incorporated into polymers to design unique responsive materials that change color under stress. Several educational initiatives will be integrated with the research to broaden participation and enable training of undergraduate and graduate students in polymer chemistry, as well as physical and synthetic organic chemistry. Summer research experiences will support the scientific development of undergraduate students from underrepresented groups majoring in chemistry. In addition, a hands-on polymer mechanochemistry module will be integrated into K-12 outreach programs in collaboration with the Center for Learning, Teaching, and Outreach at Caltech. This activity will engage numerous members of the Pasadena community each year and introduce important concepts related to polymers and materials chemistry. Lastly, a webinar series will be developed to educate students about the different career paths available in chemistry to help to build the next generation of scientists.This research will focus on the development of an improved mechanistic understanding of the mechanochemical reactivity of naphthopyran mechanophores. Comprehensive mechanistic and activity studies will be performed to determine the effect of substituents on the mechanochemical ring-opening reaction of naphthopyran by establishing linear free energy relationships. These studies will be complemented by computation to provide detailed insights into the mechanism of merocyanine formation under force. Additionally, the Robb team will work toward developing a detailed understanding of force-coupled merocyanine isomerization, which is a critical component of naphthopyran mechanochemistry. An improved fundamental understanding of structure-reactivity relationships in these systems would serve to guide the development of bis-naphthopyran mechanophores that achieve distinct ring-opening reactions under varying mechanical loads in polymers, leading to different merocyanine states with uniquely coupled visual responses. This project has the potential to advance knowledge of mechanochemical reactivity and empower innovation in the rapidly expanding field of polymer mechanochemistry and in related cross-cutting disciplines.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.