Determination of the Photoisomerization Quantum Yield of a Hydrazone Photoswitch

Photoisomerization quantum yield is a fundamental photophysical property that should be accurately determined in the investigation of newly developed photoswitches. Here, we describe a set of procedures to measure the photoisomerization quantum yield of a photochromic hydrazone as a model bistable photoswitch.

Photoswitching organic molecules that undergo light-driven structural transformations are key components to construct adaptive molecular systems, and they are utilized in a wide variety of applications. In most studies employing photoswitches, several important photophysical properties such as maximum wavelengths of absorption and emission, molar attenuation coefficient, fluorescence lifetime, and photoisomerization quantum yield are carefully determined to investigate their electronic states and transition processes. However, measurement of the photoisomerization quantum yield, the efficiency of photoisomerization with respect to the absorbed photons, in a typical laboratory setting is often complicated and prone to error because it requires the implementation of rigorous spectroscopic measurements and calculations based on an appropriate integration method. This article introduces a set of procedures to measure the photoisomerization quantum yield of a bistable photoswitch using a photochromic hydrazone. We anticipate that this article will be a useful guide for the investigation of bistable photoswitches that are being increasingly developed.

Photochromic organic molecules have attracted considerable attention in a wide range of scientific disciplines as light is a unique stimulus that can drive a system away from its thermodynamic equilibrium non-invasively¹. Irradiation of light with appropriate energies allows structural modulation of photoswitches with high spatiotemporal precision^2,3,4. Thanks to these advantages, various types of photoswitches based on configurational isomerization of the double bonds (e.g., stilbenes, azobenzenes, imines, fumaramides, thioindigos) and ring opening/cl....

1. ¹H NMR spectrum acquisition at photostationary state (PSS)

1. In a natural quartz NMR tube containing 4.2 mg (0.01 mmol) of hydrazone switch 1, add 1.0 mL of deuterated dimethyl sulfoxide (DMSO-d₆). Transfer half of the solution to another NMR tube.
2. Place one of the NMR tubes 1 cm in front of a Xenon arc lamp equipped with a 436 nm bandpass filter. Start irradiation to the NMR sample and record a ¹H NMR spectrum every d.......

Upon irradiation of 1 in an NMR tube with 436 nm light (Z:E = 54:46 in the initial state), the proportion of 1-E increases due to the dominant Z-to-E isomerization of the hydrazone C=N bond (Figure 1). The isomeric ratio can be readily obtained from the relative signal intensities of distinct isomers in the ¹H NMR spectrum (Figure 2). After 5 days of irradiation at 436 nm, the sampl.......

Various strategies to tune the spectral and switching properties of photoswitches have been developed, and the register of photoswitches is rapidly expanding²⁸. It is thus crucial to correctly determine their photophysical properties, and we anticipate the methods summarized in this article will be a helpful guide to experimenters. Provided that the thermal relaxation rate is very slow at room temperature, measurement of PSS compositions at different irradiation wavelengths, molar attenuation coef.......

This work was supported by the Chung-Ang University Research Grants in 2019 and the National Research Foundation of Korea (NRF-2020R1C1C1011134).