Dyes: Positive Solvatochromism

Computer-Interfaced Experiments - Absorption Measurement

Dyes
Positive Solvatochromism

Objectives: Shift of the Spectrum, 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'-bis (dimethylamino)fuchsone (MG: 344.46)

acetone

methanol

toluene

Dye solutions: 5 · 10 ^-5 molar solution of 4,4'-bis(dimethylamino)fuchsone in methanol and 10 ^-4 molar solutions in acetone and toluol are available.

Apparatus and glass wares:
photometer fitted with a recorder output: Spectronic 20 Bausch & Lomb
test tube cuvettes (Spectronic)

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.

Acetone is highly flammable. Irritating to eyes.

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:

Spectronic 20 (Download) features an analog output on the bottom of the instrument. The analog output of the photometer is connected to the input B of the INTERFACE.

The photometer has been designed so that when it displays 100 % transmittance, the analog signal at its output connector is 1 VDC; when the instrument displays 0 % transmittance, the output voltage is 0 VDC.

Matching of the program 'Multimeter':
- In the program 'Measuring and Evaluating' the subprogram 'Multimeter' is activated and via the menu item <F3> 'Select measur. quantities' ®'Reselect channel B' the quantity 'Voltage DC'is selected.
- After the program has been switched to <F4>'Automatic/Param./Select formula'® Enter parameter wavelength is entered. Also l and nm are entered. l stands for 'Physical symbol' and nm for 'Physical unit'.
- Under the menu item 'Enter formula' A is entered. A stands for 'Physical symbol' . The 'No. decimal places ' is set to 4 .
According to A = 2 - logU · 100 the beginning of the formula A (n, l, U) = is completed by entering - lnU / ln10 (Fig. 1)

Fig. 1: Menu screen - Matching of the program

Calibration of the photometer: The absorbance is measured in the inervall from 360 nm to 640 nm. Using the wavelength control knob the wavelength is set to 360 nm. After the photometer has been zeroed, a cuvette filled about 3/4 full with water is placed into the sample compartment. Using the light control knob the needle is set to "0" on the absorbance scale (100% T).

Measurements: The cuvette (filled with water) is replaced by a cuvette, those to 3/4 with the solution of the dye solution filled is. After pressing the function key <F1> the appropriate wavelength is entered. The measured value is confirmed with ¿ . Afterwards the wavelength is changed by increasing the wavelength by 5 nm. The cuvette filled with distilled water again is placed into the sample holder. The needle of the measuring device is regulated to the 100 % T. A cuvette filled with the dye solution is inserted into the sample compartment and the absorption at the appropriate wavelength is stored.

Before the data are stored by means of <F7> 'Select representation'® 'Display' l is selected for the x-axis and A for the y1-axis. Under 'Select graph options' is being ensured that the data points are displayed as crosses.

Graphical analysis:

A direct comparison of the measurements is possible in an overlay mode, which can be activated by switching to <F8> 'Disc operations'® 'Multigraph on' . The desired series of measurements are selected individually, in order to represent them together in the main menu under <F6>'Evaluate in graph'. By pressing the function key <F3> and by the input of the number of the appropriate graph a fitted smooth curve is drawn through the data points (Fig. 2).

absorption spectra

Fig. 2: Absorption spectra of 4,4’-bis(dimethylamino)fuchsone
in methanol (1) acetone (2) toluene (3)

Solvent	Color of the solution	Absorption maxima
methanol	purple	555 nm / 365 nm
acetone	red	500 nm / 470 nm
toluene	orange	485 nm / 445 nm

Result:

The dye exhibits negative solvatochromism. With increasing polarity of the solvent, the absorption maximum is shifted to longer wavelengths (bathochromic or "red" shift).

Discussion:

resonance

Fig. 3: Resonance structures of 4.4’-bis-dimethylaminofuchson

· The Förster's model allows an interpretation. The model involves the following, highly simplifying assumptions: Molecules in the ground and excitation state have different polarities (1). Dyes with non-polarized ground state (Fig. 3) are more strongly polarized in polar solvents, because the high-energy, polar structure of the excitation state is stabilized. The excited state is lowered. The ground state is hardly affected. The energy difference between ground state and excited state becomes smaller with increasing solvent polarity. The decrease of the transition energy results in a bathochromic shift (Fig. 4).

Fig. 4: Depiction of the solvatochromic shift

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