Excitation happens in the same way as in fluorescence, namely through electromagnetic radiation. The release of energy through vibrational relaxation and internal conversion while maintaining the same spin is the same here, as well, but only until the S 1 state is reached.
Alongside the singlet states, a triplet state exists and so-called intersystem crossing ISC can occur since the T 1 state is energetically more favorable than the S 1 state. This crossing, like internal conversion, is an electronic transition between two excited states. But contrary to internal conversion, ISC is associated with a spin reversal from singlet to triplet. This ISC process is described as "spin-forbidden". It is not completely impossible — due to a phenomenon called "spin-orbit coupling" — however, it is rather unlikely [7].
In the T 1 state, non-radiative decay is possible as well. However, a transition between the lowest energy level of the triplet state and the S 0 state is not readily possible, because that transition is spin-forbidden, too.
Still, it can happen anyway with a small possibility. It causes a rather weak emission of photons because the electron spin has to be reversed again. The energy is trapped in this state for a while and can only be released slowly [6]. After all energy has been released, the electrons are back in the ground state [6,7,10].
The spin-allowed and -forbidden processes serve as explanations for an immediately ceasing glow of fluorescence and for the afterglow of phosphorescence. Phosphorescence usually occurs only with "heavier" molecules since the spin has to be reversed with the help of spin-orbit-coupling.
Whether electromagnetic radiation is emitted at all, and with which wavelength, depends on how much energy can be released beforehand by non-radiative decay [6,7]. It also depends on the properties of so-called quenchers that are surrounding molecules and are able to take up larger amounts of energy. All processes that can lead to an inhibition of radiative decays can cause fluorescence quenching. Examples are non-radiative decay processes, but also the destruction of the fluorescent molecule [10].
The quantum efficiency describes the efficiency of the process and is defined as the ratio of absorbed and emitted photons [13]. This property is different for each substance. Even though this text focuses on photoluminescence, the photo-physical processes are the same for all types of luminescence [4]. In addition to products like glow sticks, fluorescence and phosphorescence are used in many other ways. Further examples are guideposts leading to an emergency exit that need no electric supply but glow at night due to phosphorescence.
Even plants can be made fluorescent: Spinach can be modified with the help of nanotechnology so that it can detect traces of explosive substances in the groundwater. The leaves contain carbon nanotubes to which nitroaromatics can bond. If they do, a fluorescent signal is released by the plant and can be detected with infrared cameras [14].
The video demonstrates different types of luminescence. Solvents with lower viscosity have higher possibility of deactivation by external conversion.
Fluorescence of a molecule decreases when its solvent contains heavy atoms such as carbon tetrabromide and ethyl iodide, or when heavy atoms are substituted into the fluorescing compound.
Orbital spin interaction result from an increase in the rate of triplet formation, which decreases the possibility of fluorescence. Heavy atoms are usually incorporated into solvent to enhance phosphorescence. The fluorescence of aromatic compound with basic or acid substituent rings are usually pH dependent. The wavelength and emission intensity is different for protonated and unprotonated forms of the compound as illustrated in the table below:.
The emission changes of this compound arises from different number of resonance structures associated with the acidic and basic forms of the molecule.
The additional resonance forms provides a more stable first excited state, thus leading to fluorescence in the ultraviolet region. The resonance structures of basic aniline and acidic anilinium ion is shown below:. An example of this type of fluorescence seen in compound as a function of pH is the phenolic form of 1-naphtholsulfonic acid.
This compound is not detectable with the eye because it occurs in the ultraviolet region, but with an addition of a base, it becomes converted to a phenolate ion, the emission band shifts to the visible wavelength where it can be visually seen. Acid dissociation constant for excited molecules differs for the same species in the ground state. These changes in acid or base dissociation constant differ in four or five orders of magnitude.
Dissolved oxygen reduces the intensity of fluorescence in solution, which results from a photochemically induced oxidation of fluorescing species. Quenching takes place from the paramagnetic properties of molecular oxygen that promotes intersystem crossing and conversion of excited molecules to triplet state. Paramagnetic properties tend to quench fluorescence.
The equation below best describes this relationship. The table below defines the variables in this equation. Rewriting Equation 2 gives:. If the equation below were to be plotted with F versus c, a linear relation would be observed.
F then lies below the extrapolation of the straight-line plot. This excessive absorption is the primary absorption. Another cause of this negative downfall of linearity is the secondary absorption when the wavelength of emission overlaps the absorption band. This occurs when the emission transverse the solution and gets reabsorbed by other molecules by analyte or other species in the solution, which leads to a decrease in fluorescence.
Dynamic Quenching is a nonradiative energy transfer between the excited and the quenching agent species Q. The requirements for a successful dynamic quenching are that the two collision species the concentration must be high so that there is a higher possibility of collision between the two species.
Temperature and quenching agent viscosity play a role on the rate of dynamic quenching. Dynamic quenching reduces fluorescence quantum yield and the fluorescence lifetime. Dissolved oxygen in a solution increases the intensity of the fluorescence by photochemically inducing oxidation of the fluorescing species. Quenching results from the paramagnetic properties of molecular oxygen that promotes intersystem crossing and converts the excited molecules to triplet state. Paramagnetic species and dissolved oxygen tend to quench fluorescence and quench the triplet state.
Static quenching occurs when the quencher and ground state fluorophore forms a dark complex. Fluorescence is usually observed from unbound fluorophore. Static quenching can be differentiated from dynamic quenching in that the lifetime is not affected in static quenching.
An example of the three types of photoluminescence absorption, fluorescence and phosphorescence is shown for phenanthrene in the spectrum below. In the spectrum, the luminescent intensity is measure in a wavelength is fixed while the excitation wavelength is varied.
The spectrum in red represents the excitation spectrum, which is identical to the absorption spectrum because in order for fluorescence emission to occur, radiation needs to be absorbed to create an excited state.
The spectrum in blue represent fluorescence and green spectrum represents the phosphorescence. Fluorescence and Phosphorescence occur at wavelengths that are longer than their absorption wavelengths. Phosphorescence bands are found at a longer wavelength than fluorescence band because the excited triplet state is lower in energy than the singlet state. The difference in wavelength could also be used to measure the energy difference between the singlet and triplet state of the molecule.
Introduction Fluorescence can occur in gaseous, liquid, and solid chemical systems. Singlet and Triplet Excited State Understanding the difference between fluorescence and phosphorescence requires the knowledge of electron spin and the differences between singlet and triplet states. Subscribe with us Enter your email address:.
Recent Posts Pharm, M. Sc, Pharm. Opportunity for M. Sc in the research project at Jamia Hamdard. Malaria : New knowledge about naturally acquired immunity may improve vaccines. Jobs by Qualification D. Tags Articles. Pharma News. Both fluorescence and phosphorescence are spontaneous emissions of electromagnetic radiation. The difference is that the glow of fluorescence stops right after the source of excitatory radiation is switched off whereas for phosphorescence, the glow does not stop after the source of excitatory radiation is switched off.
The glow continues for durations of fractions of a second up to hours. Viva Differences. What Is Fluorescence? It is the absorption of energy by atoms or molecules followed by immediate emission of light or electromagnetic radiation.
It is the absorption of energy by atoms or molecules followed by delayed emission of electromagnetic radiation. The emission of radiation remains for some time even after the removal of source of excitation.
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