How can I compare the UV/Vis absorption difference after metal ion chelating of crown ether-containing polymer?

Recently I have synthesized the polymer that contains 1-aza-18-crown-6 in the repeating unit. I want to know if there is any UV/Vis absorption difference after the chelation of metal ion. Considered metal ions are Cs, Co, Sr. In the other papers that compare the UV/Vis absorption difference, they use some metal salts that are soluble in some organic solvents like $\ce{CH\_3CN}$, EtOH, or MeOH. And in this case the crown ether-containing compounds are also dissolved in $\ce{CH\_3CN}$, EtOH, or MeOH. But the problem is, in my case, the synthesized polymer does not dissolve in $\ce{CH\_3CN}$, EtOH, or MeOH. It dissolves in $\ce{CH3Cl}$, $\ce{CH2Cl2}$, toluene, chlorobenzene, THF, and dioxane, but does not dissolve in $\ce{CH3CN}$, EtOH, MeOH, acetone, hexane, ether, and EtOAc. I tried to find some Cs, Co, or Sr salts that are soluble in THF, but I couldn't find it. Some said that $\ce{Cs2CO3}$ and CsF dissolves in THF, but it failed as I tried. In this situation How can I compare UV/Vis absorption difference after metal ion chelating based on solution measurement?


* I want to measure the difference for UV/Vis absorption, not other spectroscopy!




About the salt: Instead of $\ce{CsCO3}$ I would try $\ce{CsCl}$ for which [sciencemadness](https://www.sciencemadness.org/smwiki/index.php/Caesium_chloride) reports a solubility in ethanol of $\pu{0.76 g/100 mL}$ (at $\pu{25 ^\circ{}C}$). Perhaps this little already is enough. If your institution is subscriber, Pabsch *et al.* report comparison of experimentally determined solubilities of inorganic salts (including $\ce{CsCl}$, $\ce{CsHCO3}$, and $\ce{Cs2CO3}$) in organic solvents methanol, ethanol, and *N*-methyl-2-pyrrolidone (NMP). According to the freely available SI (Table S4), both $\ce{CsCl}$ and $\ce{CsHCO3}$ worked well enough for MeOH, EthOH, NMP. If your institution is subscriber to the Reaxys database, solubility of salts (expressed in Hill formulae) is an indexed property, too.


To the spectroscopic experiment:


* Does / do your polymer(s) without the metal cation loading dissolve well enough in these three solvents (perhaps especially NMP)? If so, record their spectra once "empty", and once loaded.
* Often (early level of teaching lab report), the UV spectra's ordinate is absorption; this helps to note if the data are recorded within the linear range of the detectors. For the subsequent comparison, it can become more meaningful if substituted by $\log \varepsilon$ as function either over $\lambda$ [nm], or $E$ [eV] instead. With this in mind, you don't need to record the UV-Vis spectrum in one sweep either; if suitable/necessary prepare e.g., one solution suitable for the interval of $200 \dots \pu{300 nm}$, and a second one for the range of $300 \dots \pu{700 nm}$ to both stay well within the dynamic range of the detectors. But equally to take full advantage of the linear range of the detectors, too.
* Third, with the data in hand, compare both the position (energy scale) and $\varepsilon{}$ of the absorption peaks for the unloaded vs the loaded ligand. There will be some variation of the spectra's bands because of the solvent ([solvatochromism](https://en.wikipedia.org/wiki/Solvatochromism), red / batho-, or blue / hypsochromic shift). Hence compare the spectra pairwise (both in MeOH, or both in NMP) about the "empty" and "loaded" ligand. You can facilitate their determination with spectra recorded with good resolution / small increments while sweeping the wavelength, and processing the collected data by derivative spectroscopy. Agilent's application note ([link to a pdf](http://www.youngin.com/application/AN-0608-0115EN.pdf)) can serve as a primer to this useful technique. Depending on the spectrometer software, this functionality is not promptly seen (invest some time to read the documentation). Alternatively, Tom O'Haver's [pragmatic introduction to signal processing](https://terpconnect.umd.edu/%7Etoh/spectrum/TOC.html) dedicates a [special section](https://terpconnect.umd.edu/%7Etoh/spectrum/Differentiation.html#Spectroscopy) to this topic (with templates for spreadsheet Excel .xls / Open Office Calc .ods, and MATLAB/Octave scripts ready to use).


Pabsch, D.; Figiel, P.; Sadowski, G.; Held, C. Solubility of Electrolytes in Organic Solvents: Solvent-Specific Effects and Ion-Specific Effects. *J. Chem. Eng. Data* **2022**, *67*, 2706-2718. <https://doi.org/10.1021/acs.jced.2c00203>.


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