# Reversal of Photoinduced Bending of Crystals Due to Internal Refraction of Light
## 2 Materials and Methods  
To study the photoinduced bending of the crystal, a thermostated chamber with optical windows was used, mounted on Neophot 21 (Carl Zeiss) inverted optical microscope. The crystal, cantilevered to a thin glass holder, was placed in the chamber in a flow of dry nitrogen at a temperature of 0 \({}^{\circ}\)C maintained with an accuracy of \(\pm\)1 \({}^{\circ}\)C (detailed scheme is described in [39]). To carry out photoisomerization, the crystal was preheated at 80 \({}^{\circ}\)C and cooled down to 0 \({}^{\circ}\)C, and then irradiated with LED sources with \(\lambda\) = 523 nm, 465 nm, 403 nm and a high-pressure xenon arc lamp with a 350 nm bandpass filter.  
During the transformation, time lapse images of the crystal were recorded with a digital camera, using additional 625 nm LED source for crystal illumination, which does not cause photochemical transformations. To determine the time dependence of the crystal curvature the time lapse series were analyzed using specially developed software, \"Bending crystal track\" ([[http://imagej.net/PhotoBend](http://imagej.net/PhotoBend)]([http://imagej.net/PhotoBend](http://imagej.net/PhotoBend)) (accessed on 21 November 2022)), a plugin for ImageJ, image analysis program [40]. The method is based on the recognition of crystal image elements and determination of the coordinates of three specific points on its axis: two located at the ends of the crystal and one in the middle, from which the current length and curvature of the crystal are calculated [24].  
In order to obtain a thin crystal suitable for studying in the setup described above, the method of dissolving a thicker crystal attached to a holder was used. The initial crystal of the complex, having an approximately square cross section \(\sim\)10 \(\upmu\)m thick, was glued to a glass capillary with a thin end (\(\sim\)40 \(\upmu\)m in diameter). A cuvette with HCONH\({}_{2}\) (formamide) used to thin the crystal was placed under optical microscope. Since [Co(NO\({}_{3}\))\({}_{5}\)NO\({}_{2}\)Cl(NO\({}_{3}\)) dissolves much more slowly in formamide than in water, formamide can work as a polishing etchant for these crystals. Another cuvette with dried dimethyl sulfoxide (DMSO) was placed nearby, which was used to quickly interrupt dissolution and wash the crystal from formamide. The crystal on the holder was carefully immersed in formamide, while rotational, vertical and horizontal movements were performed with constant monitoring of the dissolution process under the microscope. Dissolution occurs most rapidly from the edges of the crystal, and it acquires a rounded shape in the cross section. Periodically, the dissolution process was stopped by immersion in DMSO to inspect and control the result. If necessary, the procedure is repeated until the desired thickness and geometry quality is achieved. Using this technique, it was possible to obtain a crystal sample with a thickness of \(\sim\)2 \(\upmu\)m, shown in Figure 1. The natural loss of the faceting of crystals during dissolution results in rounded shape of the crystal's cross-section, which became a critical feature of this method for the experimental result obtained in the study. An example of a cross section image of a crystal that acquires a rounded shape after the etching is shown in Figure S1 of the Supplementary Materials.  
Figure 1: Optical microscopic image of the 2 \(\upmu\)m thick [Co(NH\({}_{3}\))\({}_{5}\)NO\({}_{2}\)]Cl(NO\({}_{3}\)) crystal obtained by etching of a thicker crystal cantilevered to a holder (the edge of the glass capillary can be seen on the left side of the image).