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Preparation of some Phosphorescent Compounds

the Experiment

It is quite simple to prepare compounds that present the phosphorescence phenomenon. Starting from fluorescent organic molecules and inserting them into a glass matrix, the efficiency of non-radiative relaxation processes is greatly reduced. In practice, the molecule is trapped in a glassy or crystalline matrix and its mobility is greatly reduced. In these conditions (conditions similar to those obtained by reducing the temperature) the phosphorescence of the molecule is “turned on”.

In our experiment we used two widely used fluorescent dyes : fluorescein and coumarin. The glassy matrix was obtained by melting boric acid and incorporating a small quantity of the fluorophore into the molten mass. Subsequently the obtained mixture is allowed to cool and is reduced to powder. If all went well, what you get is a phosphorescent material, green in the case of fluorescein, light blue for coumarin.

The obtained compounds were examined with our “phosphorimeter”, as seen in the image below :

Fluorescein

Fluorescein is a synthetic organic compound available as a dark orange/red powder slightly soluble in water and alcohol. It is widely used as a fluorescent tracer for many applications.
Fluorescein is a fluorophore commonly used in microscopy, in a type of dye laser as the gain medium, in forensics and serology to detect latent blood stains, and in dye tracing. Fluorescein has an absorption maximum at 494 nm and emission maximum of 521 nm (in water).
The major derivatives are fluorescein isothiocyanate (FITC). The disodium salt form of fluorescein is known as uranine or D&C Yellow no. 8. The fluorescence of this molecule is very intense; peak excitation occurs at 494 nm and peak emission at 521 nm.

In the image on the side you can see the cuvette with the compound obtained with fluorescein and boric acid. In the image below you can see the fluorescence spectrum excited by UV source.

The graphs presented below show the trend of the emission of phosphorescence for the fluorescein fused in boric acid. Measurements were made with the sample at two different temperatures: one at room temperature (20 ° C) and the other at low temperature (<0 ° C).
The graphs show clearly that at low temperatures the phosphorescence lasts longer, the maximum intensity remains the same but the decay is slower. This is in accord with the phosphorescence physical model that predicts that the non-radiative decay pathways – in competition with the radiative mode – strongly depend on the temperature: at lower temperatures the thermal decay is less efficient and therefore the phosphorescence lasts longer .

Turning to the logarithmic scale, the exponential trend of the decay is evident: in the two graphs below, the exponential law that best approximates the experimental data is also shown.

Coumarin

Coumarin is an aromatic compound. At room temperature is in the form of colorless crystals, with characteristic odor.
Isolated for the first time from Dipteryx odorata, whose name was indeed coumarin, coumarin is present in more than 27 families of plants, and is responsible for sweet smell of freshly cut grass.
It is the first of a class of compounds – called coumarins – that share the coumarin chemical structure.
Coumarin is also used as a gain medium in some dye laser and as a sensitizer in photovoltaic technologies.
Coumarin absorbs wavelengths less than 400 nm and gives strong fluorescence at about 460 nm.

In the image on the side you can see the cuvette with the compound obtained with coumarin and boric acid. In the image below you can see the fluorescence spectrum excited by UV source.

The graph below shows the trend of phosphorescence decay for boric acid + coumarin. Also in this case, as for fluorescein there is a good agreement with an exponential trend.

Phosphorescence kinetics

In both cases the decay of the phosphorescence emission follows an exponential law : thus we are in the case of the first-order system.

In first-order kinetics the light emission is proportional to the concentration of excited electrons. Exponential phosphorescence decay is expected when the concentration of valence band holes is much larger than the concentration of excited electrons in the conduction band. First-order decay is described by the following differential equation :

Emission = −dn/dt = kn

Where n is the concentration of carriers excited to a higher energy level. Integrating with respect to time, the usual exponential decay law is obtained :

n(t) = n(0)e-t/τ 

Such law applies to radioactive phenomena, fluorescent decay, and in some cases to phosphorescent phenomena. This model works as long as the decay rate is proportional to the population.

Conclusions

The preparation of these phosphorescent materials has proved to be very simple and has allowed us to obtain an example of phosphorescent compounds with exponential decay (first order systems) also allowed us to experiment, in a qualitative way, the strong temperature dependence of the persistence of the emission.

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