Photoinduced Trans‐to‐cis Phase Transition of Polycrystalline Azobenzene at Low Irradiance Occurs in the Solid State

Abstract

The ability to produce large-scale, reversible structural changes in a variety of materials by photoexcitation of a wide variety of azobenzene derivatives has been recognized for almost two decades. Because photoexcitation of trans-azobenzene produces the cis-isomer in solution, it has generally been inferred that the macroscopic structural changes occurring in materials are also initiated by a similar large-amplitude trans-to-cis isomerization. This work provides the first demonstration that a trans-to-cis photoisomerization occurs in polycrystalline azobenzene, and is consistent with the previously hypothesized nature of the trigger in the photoactuated mechanisms of the materials in question. It is also demonstrated that under low irradiance, trans-to-cis isomerization occurs in the solid (not via a pre-melted phase); and the presence of the cis-isomer thus lowers the melting point of the sample, providing a liquid phase. A variety of experimental techniques were employed, including X-ray diffraction measurements of polycrystalline azobenzene during exposure to laser irradiation and fluorescence measurements of the solid sample. A practical consequence of this work is that it establishes trans-azobenzene as an easily obtainable and well-defined control for monitoring photoinduced structural changes in X-ray diffraction experiments, using easily accessible laser wavelengths.

Nanosecond, Time-Resolved Shift of the Photoluminescence Spectra of Organic, Lead-Halide Perovskites Reveals Structural Features Resulting from Excess Organic Ammonium Halide

“…Additionally, we measured the crystal structure with in situ laser illumination (Figure 7). 55 We did not observe any other XRD peaks in the presence of laser illumination. Thus, since the nanosecond shift in PL maximum was observed only in the scheme-A perovskites and not in the .…”

Synthetic Control of the Photoluminescence Stability of Organolead Halide Perovskites

“…A quartz sample holder was used as a substrate for drop-casted toluene solvated samples. Details of the in situ measurements were as previously reported [51]. Transmission Electron Microscopy (TEM) images were collected using a FEI Tecnai G2 F20 field-emission TEM capable of 200 kV with a point-to-point resolution of less than 0.25 nm having a 0.10-nm line-to-line resolution.…”

“…In order to rule out phase changes as the cause of the photoinduced spectral changes, we conducted in situ XRD experiments to search for possible structural changes in the nanocrystalline perovskites during illumination with a pulsed laser. An open window was used to irradiate the samples while they were mounted in the XRD instrument as we have previously reported for other in situ XRD measurements [51]. The nanocrystalline material was in the solid state so no solvent effects were present.…”

Using Fluorescence Spectroscopy To Identify Milk from Grass-Fed Dairy Cows and To Monitor Its Photodegradation

“…To quantify accurately the amount of chlorophyll metabolites in the milk spectrum, it is necessary to obtain the metabolite spectrum after subtracting the riboflavin spectrum in milk. The resulting difference spectra were described by a linear combination of three spectra, which is a well-established method , for determining an individual spectrum from a mixture of fluorophores, where the fluorophores do not perturb each other. , The difference fluorescence spectra can be written as where I p , I c2 , and I c3 are the spectra of pheophorbide a and two other components in milk, respectively (see eqs 1–3 of the Supporting Information) and the A values are the weighted amplitudes of these spectra. I p is well-described by a sum of two log-normal line shapes, , as apparent from the pheophorbide a fluorescence spectrum in methanol.…”

“…The resulting difference spectra were described by a linear combination of three spectra, which is a well-established method 13,14 for determining an individual spectrum from a mixture of fluorophores, where the fluorophores do not perturb each other. 15,16 The difference fluorescence spectra can be written as…”