| Demonstration Experiment on Video Negative Solvatochromism Objectives: Hypsochromic Shift, Solvent Polarity Peter Keusch |
Hazards and safety precautions:
Safety glasses and protective gloves must be worn. The experiment should be performed under a portable fume cupboard giving all-round visibility! Experimental procedure: Experiment 1: Three conical measure are filled with 400 mL of aceton, isopropanol and ethanol, respectively. The solvents are each mixed with 3 mL of piperidine (removal of trace amounts of acid). A small "spatula-tip" full of pyridinium betaine (dye I) is dissolved in the different solvents while stirring. Experiment 2: Three conical measures are filled with 400 mL of n-butanol, methanol or dist. water, respectively. The solvents are each mixed with 3 mL of piperidine (removal of trace amounts of acid). A "spatula-tip" full of pyridiniumiodide (dye II) is dissolved in each of the three solvents while stirring.
|  Experiment 3: 200 mL of dist. water are mixed with 3 mL of piperidine and one "spatula-tip" full of quinolinium betaine (dye III) in a beaker. The dye solution is added to a graduated cylinder filled with 200 mL of chloroform. The stoppered cylinder is shaken. Results: Experiment 1
N-(4'-Hydroxy-3',5'-diphenyl)-2,4,6-triphenyl-pyridinium-betaine - Reichardt' s Dye (I) Experiment 2:
4-(4'-hydroxystyryl)-N-methyl-pyridinium-iodide (II) Experiment 3: The betaine is blood-red in water and ink-blue in chloroform. 
2-(4'-hydroxystyryl)-N-methyl-quinolinium-betaine (III)  Video clip (Download RealPlayer .rm file)Discussion: Solvatochromism is the ability of a chemical substance to change color due to a change in solvent polarity. The dyes used exhibit negative solvatochromism. As the solvents become more polar, the light absorbed by the dye shifts from the low energy, long wavelength to the high energy, short wavelength end of the spectrum.  · The phenomenon of solvatochromism arises from a change in the electronic structure and distribution of charge of the excited state as compared with the ground state. If the ground state is more polar than the excited state (1), it will be better stabilized by polar solvation and its energy lowered (2) so that the transition will occur at a shorter wavelength i.e. there will be a hypsochromic shift ("blue" shift) with increasing solvent polarity..  Summary: The sign of the solvatochromism depends on the polarity of the dye molecules in the ground state. Two fields of application result from the phenomenon of the solvatochromism: · Determination of the polarity of organic dyes in the ground state. · Determination of solvent polarities. References: Computer-Interfaced Experiments Negative Solvatochromism Computer-Interfaced Experiments Positive Solvatochromism Demonstration Experiment on Video
Positive Solvatochromism
Index of Lecture Experiments 
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