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The development of new classes of molecular switches with enhanced performance and brand-new functionalities enables practitioners to push the frontiers of adaptive materials. In the realm of photoswitches, the bistability of hydrazones (i.e., molecules that incorporate the C=N-NH functional group) allows for the kinetic trapping of polymer and supramolecular assemblies resulting in multistate actuation and emergent phenomena that are not tractable with other photoswitches. This property has transformed hydrazone photoswitches into powerful tools that can be used in understanding fundamental molecular interactions and how to apply them in modulating the physicochemical properties of materials.
Progressing from disordered to organized environments, we first show how a hydrazone photoswitch template can be used in the enzymatic production of cyclodextrins (CDs) to specifically favor the formation of γ-CD in the aqueous environment.
Moving from the solution state, we studied two types of hydrazone-based solid-state materials, one in the form of polymers and the other as small molecules. In the context of polymeric materials, we carried out structure-property studies of hydrazone-containing polymers and their glass transition temperatures. We discovered a new photohardening phenomenon in the polymeric network that was corroborated using nanoindentation measurements.
For the small molecule-based materials, we reported on a series of cyclic and acyclic hydrazones, which possess varying degrees of ring strain and, hence, stability of E isomers. The photoinduced isomerization and concurrent phase transition of the cyclic hydrazones from a crystalline to a liquid phase resulted in the storage of a large quantity of energy, comparable to that of azobenzene derivatives.
Finally, we studied the switching of a photochromic hydrazone on metal surfaces. Tip-enhanced Raman spectroscopy (TERS) was used to study the photoisomerization of the self-assembled monolayers (SAMs) on Au, Ag, and Cu surfaces. It was found that the hydrazones photoisomerize on Au and Cu surfaces when irradiated with 415 nm; however, they do not undergo photoisomerization on Ag surfaces, unless higher energy light is used. The conversion efficiency (photostationary state (PSS) after irradiation at 415 or 340 nm) of the hydrazone monolayers on Au was found to rise with increased alkyl chain length, with an optimum result for n-octyl (C8) thiolate or longer linkers.
S. Yang, D. Larsen, M. Pellegrini, S. Meier, D. F. Mierke,* S. R. Beeren,* I. Aprahamian,* Dynamic enzymatic synthesis of γ-cyclodextrin using a photoremovable hydrazone template, Chem 2021, 7, 2190-2200.
S. Yang, J. D. Harris, A. Lambai, L. L. Jeliazkov, G. Mohanty, H. Zeng, A. Priimagi, I. Aprahamian,* Multistage Reversible Tg Photomodulation and Hardening of Hydrazone-Containing Polymers, J. Am. Chem. Soc. 2021, 143, 16348-16353.
Q. Qiu, S. Yang, M. A. Gerkman, H. Fu, I. Aprahamian,* G. G. D. Han,* Photon Energy Storage in Strained Cyclic Hydrazones: Emerging Molecular Solar Thermal Energy Storage Compounds, J. Am. Chem. Soc. 2022, 144, 12627–12631.
L.-Q. Zheng, S. Yang, J. Lan, L. Gyr, G. Goubert, H. Qian, I. Aprahamian,* R. Zenobi,* Solution phase and surface photoisomerization of a hydrazone switch with long thermal half-life, J. Am. Chem. Soc. 2019, 141, 17637–17645.
L.-Q. Zheng, S. Yang, S.Krähenbühl, V.V.Rybkin, J. Lan, I. Aprahamian,* R. Zenobi,* Effect of the alkyl linker length on the photoisomerization of hydrazone switches on metal surfaces, Mater. Today Chem. 2022, 24, 100797.
Yang, Sirun, "Converting Hydrazone-Based Photoswitches into Functional Materials" (2022). Dartmouth College Ph.D Dissertations. 122.