CAREER: Understanding and Controlling Fatigue in Photo-Responsive MOFs: Characterizing Photochromic Transformations in Single Crystals

Non-technical Abstract: In this project funded by the Solid-State Materials Chemistry program of the Division of Materials Research, the research team is developing crystalline porous materials that are activated by UV light. The photo-active chemical groups change shape when exposed to light. These light-driven transformations cause the pores (holes) and channels of the crystalline frameworks to change shape leading to materials capable of selectively absorbing or releasing guest molecules, for example pharmaceutical drugs, but only when exposed to the proper wavelength of light. In addition to advancing our understanding of the fundamental photochemistry that occurs in these novel materials, the research establishes design principles to understand and ultimately control and prevent fatigue (failure) in a variety of photo-responsive materials. The education and outreach activities bring crystallography and X-ray science into U.S. classrooms through the continued development of an X-ray science course for advanced undergraduate and graduate students. The educational projects, including a nation-wide crystal growing competition, bring concepts of crystals and light to K-12 classrooms through the development of hands-on experiments and curricular development for teachers and students. Technical Abstract: This research focuses on identifying and understanding the root causes of fatigue in photochromic materials with the ultimate goal of developing design principles to control fatigue in any photo-responsive system. The primary objective is developing a detailed molecular level understanding of the photochemistry occurring within diarylethene-based materials, an exciting niche in the nascent field of photo-responsive metal-organic frameworks. Specific aims of the research include: (1) the synthesis and characterization of new diarylethene-based photoswitchable linkers and metal-organic frameworks prepared from these linkers, (2) quantifying the photophysics of photochromic molecules in crystalline environments using spectroscopic and diffraction methods to determine reaction energetics and limitations, (3) determining the factors and triggers that govern fatigue in these materials, (4) developing new techniques to assess the distribution of guest species within the metal-organic framework nanopores and determining their impact on the photophysical properties of the crystalline frameworks, (5) using the knowledge gained to rationally design and engineer advanced photochromic materials with enhanced properties, namely improved resistance to fatigue. The primary research tools include traditional and in situ single crystal X-ray diffraction (photo-crystallography), steady-state and time-resolved absorption spectroscopy/microscopy, nuclear magnetic resonance (NMR), mass spectrometry, thermogravimetric analysis, and computational modeling.