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Computer-aided Experiments - Measurement: Absorbance

Absorption Maxima of Triphenylmethane Dyes

Determination of the absorption maxima: fuchsin, crystal violet and malachite green

Peter Keusch



Datalogging using a digital diode array photometer
LEYBOLD DIDACTIC





German version



Chemicals:
5 · 10-5 M aqueous fuchsin solution (17 mg fuchsin dissolved in 1000 mL H2O)
5 · 10-5 M aqueous crystal violet solution (21 mg crystal violet dissolved in 1000 mL H2O)
5 · 10-5 M aqueous malachite green solution ( 18 mg malachite green dissolved in 1000 mL H2O)


Apparatus:
digital diode array photometer
computer-connector cable
adapter cable
date output module
cuvettes


Hazards and safety precautions:

Crystal violet may cause cancer. Severe eye irritant. Harmful by inhalation, ingestion and through skin contact.
Malachite green is harmful if swallowed. Contact with skin or eyes may cause irritation.

Safety goggles and gloves must be worn. Good ventilation!


Experimental procedure:

The Digital Diode Array Photometer provided by Leybold Didactic is connected to the computer via a data output module. The apparatus and the software supplied with the device under Windows permit fast and precise recordings of spectra. The individual spectra of the triphenylmethane dyes are overlayed and the respective absorption maxima are determined (Fig. 1).


Graphical analysis:







Fig. 1: Evaluation screen - absorbance maxima
  fuchsin (1)   crystal violet (2)    malachite green (3)


Discussion:

The extent of the bathochromic shift increases in the order fuchsin (1) < crystal violet (2) < malachite green (3). The position of the absorption maxima of the mentioned triphenylmethane dyes depends on the geometry of the chromophore and on the electronic nature of the para-substituents on the phenyl rings. The presence of a single band in the absorption spectrum of both fuchsin (lmax = 547 nm) and crystal violet (lmax = 590 nm) is usually a good indication of the involvement of all three terminal amino groups in the resonance system. Malachite green, which has only two out of three phenyl rings substituted with dimethylamino groups shows two absorption bands in the visible region. The intense absorption band at longest wavelength (x band) is shifted to the red (lmax = 619 nm). The second band (y band) is in the shorter-wavelength part of the spectrum (lmax = 425 nm)and is significantly weaker (Fig. 1).


· From a strictly electronic point of view, the lmax values of structurally symmetric triphenylmethane dyes should increase with increasing electron-donating ability of the terminal amino groups. The electron donating resonance effect of the dimethylamino group is stronger than that of the amino group. Therefore, the absorption maximum of crystal violet is higher than that of fuchsin (Fig. 1).

· X-ray studies indicate that the geometrical structure of crystal violet (like fuchsin) resembles a three-bladed propeller. The planes of the phenyl rings are twisted out of the plane defined by the central carbon atom and its three bonds. The dihedral angle between the phenyl rings and the central coordination plane is 27.7°. The twisting can be understood as a compromise between the effect of p-electron conjugation and the ortho-ortho steric repulsion involving aromatic hydrogens on adjacent rings (Fig. 2). The p-electron conjugation (maximum resonance stabilization) favours a planar structure. The steric interaction between the ortho-hydrogen atoms favours a non-planar conformation. Due to the non-planar structure, the p-electron conjugation in crystal violet is not so extensive as in a p-conjugated planar sytems.



Fig. 2: Ring twisting
steric repulsion of ortho hydrogen atoms (blue)

            

            Oppositely twisted enantiomeric conformations of crystal violet ion

· Unlike crystal violet, malachite green has only two out of three phenyl rings substituted with dimethylamino groups. One ring bears an auxochrome (dimethylamino group) and the other has an anti-auxochrome (protonated dimethylamino group). The two substituted rings are arranged in one plane (2). The degree of p-electron conjugation between the two coplanar rings is at a maximum. Hence, the absorption maximum (x band = main absorption band) of malachite green is shifted to the longer wavelength region of the visible spectrum. The third aromatic is considerably turned out of the plane of the other two rings. A comparison between the absorption spectra of Michler's hydrol blue and malachite green shows, that the unsubstituted ring in malachite green has only a small influence on the absorption maximum.



References:
Video clip - Crystal Violet - a pH Indicator
Light Absorption of Triphenylmethylium Salts
Light Absorbance of Triphenylmethane Dyes
Kinetics: Fading of Triphenylmethane Dyes - Pseudo First Order Reaction






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