Positive Solvatochromism

Demonstration Experiment on Video

Positive Solvatochromism

Objectives: Bathochromic Shift, Solvent Polarity

Peter Keusch

German version

Chemicals:
4,4'-bis(dimethylamino)fuchsone
toluene
acetone
methanol
piperidine

Glass wares:

3 conical measures, graduated, 500 mL
3 glass stirring rods
volumetric pipette, 3 mL
pipette bulb
3 snap-cap vials, 10 mL
watch glass
spatula

Hazards and safety precautions:

Methanol may be a reproductive hazard. Ingestion may be fatal. Risk of very serious, irreversible damage if swallowed. Exposure may cause eye, kidney, heart and liver damage. Chronic or substantial acute exposure may cause serious eye damage, including blindness.
Toluene is toxic by inhalation, ingestion or by absorption through skin. Serious irritant. Experimental teratogen.

Piperidine is a poison. May be fatal if inhaled or swallowed. Severe irritant. Skin contact may cause severe irritation or burns. Contact with the eyes may lead to permanent damage.

Acetone is highly flammable. Irritating to eyes.

Safety glasses and protective gloves must be worn. The experiment should be performed under a portable fume cupboard giving all-round visibility!

Experimental procedure:

Three conical measures are filled with 400 mL of toluol, acetone and methanol, respectively. The solvents are mixed with 3 mL of piperidine (removal of trace amounts of acid). A "spatula-tip" full of 4,4'-bis(dimethylamino)fuchsone is dissolved in each of the three different solvents while stirring.

Results:

	Solvent	Color of solution
conical measure 1	toluol	orange
conical measure 2	acetone	red
conical measure 3	methanol	purple

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 used dye exhibits positive solvatochromism. With increasing polarity of the solvent, the absorption maximum is shifted to longer wavelengths.

Resonance structures of 4,4'-bis(dimethylamino)fuchsone

· 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 excited state is more polar than the ground state (1), it will be better stabilized by polar solvation and its energy lowered (2) so that the transition will occur at longer wavelength i.e. there will be a bathochromic shift ("red" shift) with increasing solvent polarity.

References:
Computer-Interfaced Experiments Dyes - Positive Solvatochromism
Computer-Interfaced Experiments Dyes - Negative Solvatochromism
Demonstration Experiment on Video Negative Solvatochromism

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