Nuclear Spin Singlet States in Photoactive Molecules: From Fluorescence/NMR Bimodality to a Bimolecular Spin Singlet State Switch

Nuclear spin singlet states are silent states in nuclear magnetic resonance (NMR). However, they can be probed indirectly and offer great potential for the development of contrast agents for magnetic resonance imaging (MRI). Here, we are introducing two novel concepts: Firstly, we investigate the bimodal NMR/fluorescence properties of 13C2‐tetraphenylethylene. It possesses a long‐lived singlet state in organic solvents, which shortens upon the addition of water. This simultaneously increases the aggregation‐induced emission (AIE) of the molecule, resulting in a substantial enhancement of fluorescence. Secondly, we introduce a bimolecular singlet state switch based on 3‐2H‐coumarin containing an isolated proton. Upon UV‐light exposure, a dimer forms leading to a coupling between two previously isolated protons. A nuclear spin singlet state can now be populated. Excitation with a wavelength of 254 nm results in partly cleavage of the molecule back into its monomer.

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