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Computer-Interfaced Experiments - Absorption Measurement

Dyes
Negative Solvatochromism

Objectives: Hypsochromic Shift, Solvent Polarity

Peter Keusch


Datalogging and data analysis using the Program "Measuring and Evaluating" and the Analog-Digital-Converter CASSY-E - LEYBOLD DIDACTIC



German version



Chemicals:
4-(4'-hydroxystyryl)-N-methyl-pyridinium-iodide (I)  (m.w. = 197.02)
2-(4'-hydroxystyryl)-N-methyl-quinolinium-betaine (II)  (m.w. = 261.32)
n-butanol
chloroform
methanol
n-propanol
piperidine

Apparatus and glass wares:
photometer fitted with a recorder output
cuvettes


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.
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. Highly flammable. Vapours may flow along surfaces to a distant source of ignition.
Chloroform causes cancer in laboratory animals, and is IARC listed as a probable human carcinogen. Inhalation and ingestion are harmful and may be fatal. May cause reproductive damage. Irritant. Exposure to alcohol may increase toxic effects. Prolonged or repeated skin contact may cause dermatitis.
n-Propanol is very flammable.

Safety glasses and gloves must be worn. The preparation of the corresponding solutions and the experimental procedure are carried out in a fume hood!


Experimental procedure:

The following solutions are supplied.

Dye I:   5 · 10 -5 molar in water,  5 · 10 -5 in methanol,  1.25 · 10 -4 molar in n-propanol,  5 · 10 -4 molar in n-butanol.
Dye II:   5 · 10 -5 molar in water,  2.5 · 10 -5 molar in chloroform.

All the dye solutions are mixed with some drops of piperidine, in order to eliminate acid traces in the solvent.















Graphical analysis:

The experimental procedure and the data analysis are carried out as described in experiment Positive Solvatochromism.


spectra
Fig. 1: Absorption spectra of 4-(4'-hydroxystyryl)-N-methyl-pyridinium-iodide
in   n-butanol  (1)    n-propanol  (2)    methanol  (3)   water (4)
Solvent Color of the Solution Absorption maxima
n-butanol purple 535 nm / 410 nm
n-propanol red 405 / 525 nm
methanol orange 395 nm / 485 nm
water yellow 385 nm


Fig. 2: Absorption spectra of 2-(4'-hydroxystyryl)-N-methyl-quinolinium-betaine
in    chloroform  (1)    water  (2)
Solvent Color of the Solution Absorption maxima
chloroform ink-blue 585 nm / 440 nm
water blood-redly 410 nm



Result:

The dyes 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 (hypsochromic or "blue" shift).


Discussion:



Fig. 3: Resonance structures

·   Polar solvents stabilize the ground state of the dipolar (zwitterionic) dye molecules relative to the excited state  (Fig. 3).  The energy difference between ground state and excited state becomes larger with increasing solvent polarity. The increase of the transition energy results in a hypsochromic shift  (Fig. 4).




Fig. 4: Depiction of the solvatochromic shift

·   Non-polar solvents, however, destabilize however the polar ground state. The energy difference between ground state and excited state becomes smaller leading to a bathochromic shift of the absorption maximum.


Summary:

The sign of the solvatochromism depends on the polarity of the dye molecules in the ground state. Due to the phenomenon of the solvatochromism two fields of application are given:

·   Determination of the polarity of organic dyes in the ground state

·   Determination of solvent polarities.


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


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