Determination of the Photoisomerization Quantum Yield of a Hydrazone Photoswitch

Summary

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.

Abstract

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.

Introduction

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....

Protocol

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 day until there is no change in the spectra as switch 1 reaches PSS. After reaching PSS, keep the NMR tube in the dark at room temperature ....

Results

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.......

Discussion

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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
1,10-phenanthroline	Sigma-Aldrich	131377-2.5G
340 nm bandpass filter, 25 mm diameter, 10 nm FWHM	Edmund Optics	#65-129
436 nm bandpass filter, 25 mm diameter, 10 nm FWHM	Edmund Optics	#65-138
Anhydrous sodium acetate	Alfa aesar	A13184.30
Dimethyl sulfoxide	Samchun	D1138	HPLC grade
Dimethyl sulfoxide-d6	Sigma-Aldrich	151874-25g
Gemini 2000; 300 MHz NMR spectrometer	Varian
H2SO4	Duksan	235
Heating bath	JeioTech	CW-05G
MestReNova 14.1.1	Mestrelab Research S.L., https://mestrelab.com/
Natural quartz NMR tube	Norell	S-5-200-QTZ-7
Potassium ferrioxalate trihydrate	Alfa aesar	31124.06
Quartz absorption cell	Hellma	HE.110.QS10
UV-VIS spectrophotometer	Scinco	S-3100
Xenon arc lamp	Thorlabs	SLS205	Fiber adapter was removed