Tuning the photocaged spiropyran photoswitch with a sterically hindered adamantane group: releasing the stable merocyanine

Journal article, 2025

Molecular photoswitches isomerize between two or more forms by exposure to light or by thermal processes, accompanied by color changes. Spiropyran (SP) derivatives are widely studied. The stable colorless SP form photoisomerizes to the colored merocyanine (MC) form by UV exposure, whereas the reverse reaction is triggered by visible light or in a thermal reaction. Based on our previously reported (J. Am. Chem. Soc., 2018, 140, 14069) photolabile o-nitrobenzyl caged merocyanine switch (denoted as in-MC) and a negative photochromic spiro[azahomoadamantane-pyran] (Anal. Bioanal. Chem., 2023, 415, 715), we have designed a novel multiphotochromic molecule, ad-MC, by combining an adamantane-containing negative photochromic unit (2MC) and o-nitrobenzyl. A wavelength dependent photorelease of the stable 2MC form is demonstrated for the first time. The detailed photochemical reaction mechanisms of ad-MC and in-MC were investigated using femtosecond transient absorption spectroscopy and DFT/TD-DFT calculations. The results show that the photochemical mechanisms are the same for both molecules, but the variation in excited state hydrogen transfer barriers' heights leads to the observed differences in photolysis yields and rates of decaging. The structure-reactivity relationships revealed for ad-MC and in-MC expand the diversity and functionality of SP-based photoswitches for promising utilization in chemical biology applications.