Yifan Su,† Xiang Li,† Dexin Zheng,† Joakim Andreásson, Hong Wang, Le Yu,* Jian Chen,* Jiani Ma,* and Yu Fang. Chem. Sci., 2025, DOI: 10.1039/d5sc06627a

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 light, 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.

Figure 1. The photochemical pathways of ad-MC.

Steady-state absorption spectroscopy and ¹H-NMR analysis revealed that ad-MC undergoes trans→cis isomerization upon 300 nm irradiation. More significantly, under 254 nm light irradiation, ad-MC efficiently releases the stable fluorescent MC form (2MC), achieving a photolysis yield of 0.19—markedly higher than the 0.12 yield observed for in-MC.

Figure 2. (a) fs-TA spectra of ad-MC in EtOH (l_ex = 266 nm). (b) The PES of the photochemical reaction of ad-MC (PBE0/def2-TZVP (EtOH)). (c) Photorelease reaction mechanism of ad-MC.

Femtosecond transient absorption spectroscopy combined with theoretical calculations revealed the photochemical reaction mechanism of ad-MC. The study demonstrates that the photorelease reactions of both ad-MC and in-MC follow a similar multi-step mechanism, involving sequential hydrogen transfer, proton transfer, cyclization, and C–O bond cleavage processes.

First Authors: Su Yifan, doctoral candidate, Shaanxi Normal University; Li Xiang, master’s student, Hunan University of Science and Technology; Zheng Dexin, Li Xiang, master’s student, Shaanxi Normal University

Correspondence Authors: Prof. Ma Jiani, Shaanxi Normal University; Prof. Chen Jian, Hunan University of Science and Technology; Assoc. Prof. Yu Le, Northwest University

Full Text Link: https://pubs.rsc.org/en/content/articlelanding/2025/sc/d5sc06627a