Shuang Xu+, Le Yu+, Yang Cheng, Yidan Li, David Lee Phillips, Tomáš Slanina,* Jiani Ma,* and Yu Fang. Chem. Sci. 2026. DOI: https://doi.org/10.1039/D6SC05127H

Molecular photoswitches are molecules that alter their structures and properties in response to light irradiation, and have found widespread applications across various chemical fields in recent years, including chemical biology, materials science, catalysis, and the study of molecular machines. Integrating multiple photoswitchable units within a single molecule and achieving precise regulation of molecular geometry through selective activation of individual photoresponsive groups represents a challenging endeavor. To realize truly orthogonal photoswitching, sufficient spectral separation between any two of the four states is required. Moreover, the lack of mechanistic understanding in this field has hindered further development and practical applications of such systems.

By covalently linking an aryl azopyrazolium (PZ) moiety with a spiropyran (SP) moiety, the authors designed and synthesized an ionic orthogonal photoswitch, PZ-SP-MeSO₄. Reversible switching among the four states of this molecule was achieved using three wavelengths of light—365 nm, 420 nm, and 450 nm. The ionic character endows the molecule with excellent water solubility up to 18.4 mM, and the cation-π interaction between the pyrazolium positive charge and the benzene ring of the spiropyran extends the thermal half-life of the Z-isomer at room temperature to 717 days. This system also exhibits pH-cooperative tunability, enabling the construction of multi-valued molecular logic gates, which offers a novel strategy for optical information storage and molecular computing.

Figure 1. (a) Photoswitch properties of typical mono-heteroaryl, bis-heteroaryl azo molecules and ionic photoswitches, (b) comparison of the reported orthogonal photoswitches, (c) ionic orthogonal photoswitches designed in this work.

By combining transient absorption spectroscopy experiments with theoretical calculations, this work elucidates the wavelength-dependent regulation mechanism of PZ-SP-MeSO₄. Upon 450 nm excitation to the S₁ state, the electronic transition is entirely localized on the PZ unit, and the N=N double-bond twisting isomerization proceeds through a barrierless process. Upon 355 nm excitation to the high-lying S₃ state, the C-O bond cleavage of the spiropyran moiety occurs in the excited state, followed by isomerization after spiropyran ring-opening in the ground state, achieving exclusively the SP→MC conversion. Upon 420 nm excitation to the S₂ state, the molecule first undergoes rapid E→Z isomerization of the PZ unit in the excited state; subsequently, the excess vibrational energy in the hot ground-state species (Z)-PZ-SP-MeSO₄ further drives C-O bond cleavage in the ground state, realizing the SP→MC conversion. Mechanistic studies reveal that the spatial localization and delocalization of excited-state electron density are key to the selective activation of the two photoresponsive units. This principle is general in nature and provides theoretical guidance for the molecular design of various orthogonal photoswitches in the future, filling the mechanistic gap in high-lying excited-state-mediated multiphotochromic regulation.

Figure 2. TAS and potential energy profiles at B3LYP-D3BJ/6-311G(d, p) SMD (H2O) level of theory of PZ-SP-MeSO4 in H2O after excitation with (a) 450 nm, (b) 355 nm and (c) 420 nm light.

First Authors: Xu Shuang, master’s student, Shaanxi Normal University; Assoc. Prof. Yu Le, Northwestern University

Correspondence Authors: Prof. Ma Jiani, Shaanxi Normal University; Prof. Tomáš Slanina, Czech Academy of Sciences

Full Text Link: https://doi.org/10.1039/D6SC05127H