Determination of the Photoisomerization Quantum Yield of a Hydrazone Photoswitch

Here we explain a set of protocols for precise measurement of the photoisomerization quantum yield of a photochromic hydrazone as a model photoswitching molecule. The methods introduced here can be applied to other families of bi-stable photoswitches. Protocols for photoswitches with different for photo-physical properties and their selection guidelines are provided in the supplementary information.

To begin, place the NMR sample at the distance of one centimeter in front of a xenon arc lamp equipped with a 436-nanometer band pass filter and start irradiation. Record a proton NMR spectrum every day till there is no exchange in the spectra as switch one reaches PSS. For another NMR sample, irradiate the solution using a 340-nanometer band pass filter and record the NMR spectrum as described previously.

Open FID files of the NMR spectra at the PSSs with NMR processing software. Integrate a distinctive set of peaks of the distinct isomers and calculate the isomeric ratios. Place the prepared sample at the distance of one centimeter in front of a xenon arc lamp equipped with a 436-nanometer band pass filter and start irradiation.

Measure the UV-visible absorption spectrum every two hours until there is no change in the spectra as switch one reaches PSS. For another sample, irradiate the solution using a 340-nanometer band pass filter and measure the UV visible spectrum at PSS in the same way. Deduce the absorbance spectra of the pure 1-Z and 1-E isomers and calculate their molar attenuation coefficients at all wavelengths as described in the text.

Heat the silicon oil filled in a heating bath circulator to 131 degrees Celsius and check whether the temperature of the bath is stabilized. Submerge two NMR sample tubes in the heating bath. After one hour of heating, transfer the NMR tubes quickly to a dry ice bath to pause the thermal relaxation caused by latent heat.

Thaw the NMR samples at room temperature and ensure dimethyl sulfoxide is defrosted. Then, record the proton NMR spectra of the samples. Again perform the heating and thawing process and record the proton NMR spectra of the samples until there is no change in the proton NMR spectra as switch one reaches thermodynamic equilibrium.

Open FID files of the NMR spectra obtained in the course of heating and calculate the concentration of 1-E based on the total sample concentration and the isomeric ratio. Then, plot the averaged concentration of 1-E as a function of the heating time. Perform an exponential fit to the data to obtain the rate constant of thermal relaxation, K, using the equation as described in the text.

Plot natural log of K versus reciprocal of T.Perform a linear fit according to the Arrhenius equation as described in the text to extrapolate the rate constant at room temperature and calculate the thermal half-life of 1-E at room temperature using the equation as described in the text. In a 20-milliliter glass vial containing 29.48 milligrams of potassium ferrioxalate trihydrate, add eight milliliters of deionized water. Add one milliliter of 0.5 molar aqueous sulfuric acid to the ferrioxalate solution and dilute to 10 milliliters with deionized water to prepare a 0.006 molar ferrioxalate in 0.05 molar aqueous sulfuric acid solution.

In another 20-milliliter glass vial containing 10 milligrams of 1, 10-phenanthroline and 1.356 grams of anhydrous sodium acetate, add 10 milliliters of 0.1 molar aqueous sulfuric acid to make a buffered 0.1%phenanthroline solution. Measure the UV-visible absorption spectrum of the ferrioxalate solution. Determine the fraction of absorbed light at 340 and 436 nanometers using the absorbances of the ferrioxalate solution as described in the text.

Place the quartz cuvette containing the ferrioxalate solution one centimeter in front of the xenon arc lamp equipped with a 436-nanometer band pass filter. Start irradiation to the sample for 90 seconds. After irradiation, add 0.35 milliliters of the phenanthroline solution and a magnetic bar into the cuvette, followed by stirring for one hour in the dark to form a ferroin complex.

Prepare a quartz cuvette containing two milliliters of non-irradiated ferrioxalate solution and 0.35 milliliters of the phenanthroline solution as a non-irradiated sample. Measure the UV-visible absorption difference between the non-irradiated and irradiated samples. Repeat the procedure for sample preparation and measurement of the UV-visible absorption spectrum described previously with a 340-nanometer band pass filter.

Calculate the molar photon flux arriving at the cuvette using this equation. Place the prepared sample one centimeter in front of the xenon arc lamp equipped with a 436-nanometer band pass filter and start irradiation. Measure the UV-visible absorption spectrum with different intervals until there is no change in the spectra as switch one reaches PSS.

Once reaching PSS, recover the cuvette from the UV-Vis spectrophotometer and irradiate the solution using a 340-nanometer band pass filter. Measure UV-visible absorption spectrum as described previously. From the obtained UV-Vis absorption spectra, calculate the values of photokinetic factor Ft using the observed absorbances at the irradiation wavelengths.

Calculate the unidirectional quantum yields for Z to E and E to Z photoisomerization processes. Upon irradiation at 436 nanometers, the proportion of 1-E increases due to the dominant Z to E isomerization of the hydrazone CN double bond. The isomeric ratio was obtained from the relative signal intensities of distinct isomers in the 1H NMR spectrum.

At 436 nanometers, the sample shows 92%of 1-E, while at 340 nanometers, 82%of 1-Z was found. The ismomeric ratios and UV-Vis absorption spectra at PSS are used to deduce the UV-Vis spectra of the pure 1-Z and 1-E isomers. These spectra of the pure isomers suggest that the incomplete photoisomerization is attributed to the reverse photochemical process.

For determining the photoisomerization quantum yield, measurement of the E to Z thermal relaxation rate and the effective molar photon flux is required. The rate constant of thermal relaxation extrapolated from the Arrhenius plot was very small at room temperature, and thus, the effect of thermal relaxation in the photoisomerization process could be ignored. The effective molar photon flux arriving at the sample was obtained from ferrioxalate actinometry and the pseudo-quantum yield of photoisomerization at the irradiation wavelength can be calculated.

Finally, the unidirectional quantum yields for Z to E and E to Z photoisomerization processes can be calculated from the pseudo-quantum yields. For determination of the photoisomerization quantum yield, it is essential to obtain precise values of the thermal relaxation rate at room temperature and the effective molar photon flux. For those who deal with bi-stable photoswitches other than hydrazones, it is important to use the proper integration method for the photokinetic factor, which is explained in the supplementary information.