Physical Chemistry

Large, tunable and reversible pH changes by spiropyran photoacids



Molecular photoswitches capable of generating pH changes with precision will allow pH-dependent processes to be controlled remotely and non-invasively with light. We introduce a series of new spiropyran photoswitches, delivering reversible bulk pH changes up to 3.2 pH units (pH 6.5 to pH 3.3) upon irradiation with 450 nm light, displaying tunable and predictable timescales for thermal recovery. We present models to show that the key parameters for optimizing the bulk pH changes are measurable: the solubility of the photoswitch, the acidity of the merocyanine form influenced by the thermal equilibrium position between the spiropyran and the merocyanine isomers, and the increased acidity under visible light irradiation. Using ultrafast transient absorption spectroscopy, we determine quantum yields for the ring closing reaction and observe the lifetime of the transient cis-merocyanine isomer ranging from 30 to 550 ns. Quantum yields did not appear to be a limitation of bulk pH switching. The models we present use experimentally determined parameters and are, in principle, able to predict the change in pH obtained for any related spiropyran photoacid.

Version notes

Minor corrections to the manuscript. Corrected mistakes in chemdraw figures.


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Supplementary material

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Supplementary Information
Synthetic procedures, spectroscopic data, all photoswitching and fitted data.
Thumbnail image of 1) NMR
NMR data
All NMR data assigned in MNova format.
Thumbnail image of 2) UV-vis data + fitted origin
UV-vis data
All UV-vis absorption data and data fitting.
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HR-MS data
All high-resolution mass spectrometry data
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Transient absorption data
All transient absorption data.
Thumbnail image of 5) Models - excel
Models used to calculate pH drops and timescales of thermal recovery.