Projection Experiment: Positive Solvatochromism

Demonstration of Microscale Projection Experiments - Chemistry en miniature

Positive Solvatochromism

Objectives: Bathochromic Shift, Solvent Polarity

Peter Keusch

German version

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

Hazards and safety precautions:

Benzene is known to be a human carcinogen on sufficient evidence of carcinogenicity in humans. Breathing benzene can cause drowsiness, dizziness, and unconsciousness; long-term benzene exposure causes effects on the bone marrow and can cause anemia and leukemia. Severe eye irritant. Skin and respiratory irritant.
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.
Dioxane is probable carcinogen. Toxic. Harmful by inhalation, ingestion and through skin contact. Irritant.

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

Experimental procedure:

1. Some crystals of the fuchson dye are dissolved in 1 mL of benzene (test tube 1), acetone (test tube 2) and methanol (test tube 3), respectively. The solvents are mixed with a drop piperidine (removal of trace amounts of acid).
A Pasteur pipette containing some crystals of 4,4'-bis (dimethylamino) fuchsone is placed in each of the three test tubes. The dye is introduced into the solvent by gentle squeezing of the pipette bulb.

2. The dye is dissolved in dioxane. The solution is mixed dropwise with water.

Results:

Experiment 1:

Solvent Color of solution
Test tube 1 benzene orange
Test tube 2 acetone red
Test tube 3 methanol purple

Experiment 2:

The procedure leads to solution colors ranging from yellow to red-violet.

Discussion:

Solvatochromism is the ability of a chemical substance to change color due to a change in solvent polarity. The dye used 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.

· Non-polar solvents, however, destabilize the excited state. The energy difference between ground state and excited state becomes larger, shifting the absorption maximum to the blue.

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

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