At room temperature, majority of the molecules in a solution are in this state. The upper lines represent the energy state of the three excited electronic states: S 1 and S 2 represent the electronic singlet state left and T 1 represents the first electronic triplet state right. The upper darkest line represents the ground vibrational state of the three excited electronic state. The energy of the triplet state is lower than the energy of the corresponding singlet state.
There are numerous vibrational levels that can be associated with each electronic state as denoted by the thinner lines. This transition leads to a change in multiplicity and thus has a low probability of occurring which is a forbidden transition.
The knowledge of forbidden transition is used to explain and compare the peaks of absorption and emission. The table below compares the absorption and emission rates of fluorescence and phosphorescence. The rate of photon absorption is very rapid. Fluorescence emission occurs at a slower rate.
Since the triplet to singlet or reverse is a forbidden transition, meaning it is less likely to occur than the singlet-to-singlet transition, the rate of triplet to singlet is typically slower. Therefore, phosphorescence emission requires more time than fluorescence. The favored deactivation process is the route that is most rapid and spends less time in the excited state.
If the rate constant for fluorescence is more favorable in the radiationless path, the fluorescence will be less intense or absent. After discussing all the possible deactivation processes, variable that affect the emissions to occur. Molecular structure and its chemical environment influence whether a substance will fluoresce and the intensities of these emissions. The quantum yield or quantum efficiency is used to measure the probability that a molecule will fluoresce or phosphoresce.
For fluorescence and phosphorescence is the ratio of the number of molecules that luminescent to the total number of excited molecules. For highly fluoresce molecules, the quantum efficiency approaches to one. Molecules that do not fluoresce have quantum efficiencies that approach to zero. They are related by the quantum yield equation given below:. The magnitudes of kf , kd, and kpd depend on the chemical structure, while the rest of the constants ki, kec, and kic are strongly influenced by the environment.
Fluorescence rarely results from absorption of ultraviolet radiation of wavelength shorter than nm because radiation at this wavelength has sufficient energy to deactivate the electron in the excited state by predissociation or dissociation. Molecules that are excited electronically will return to the lowest excited state by rapid vibrational relaxation and internal conversion, which produces no radiation emission.
Fluorescence arises from a transition from the lowest vibrational level of the first excited electronic state to one of the vibrational levels in the electronic ground state. A few aliphatic, alicyclic carbonyl, and highly conjugated double-bond structures also exhibit fluorescence as well. Most unsubstituted aromatic hydrocarbons fluoresce in solution too.
The quantum efficiency increases as the number of rings and the degree of condensation increases. Simple heterocycles such as the structures listed below do not exhibit fluorescence. Although simple heterocyclics do not fluoresce, fused-ring structures do. For instance, a fusion of a benzene ring to a hetercyclic structure results in an increase in molar absorptivity of the absorption band.
The lifetime of the excited state in fused structure and fluorescence is observed. Examples of fluorescent compounds is shown below. Benzene ring substitution causes a shift in the absorption maxima of the wavelength and changes in fluorescence emission.
The table below is used to demonstrate and visually show that as benzene is substituted with increasing methyl addition, the relative intensity of fluorescence increases.
The relative intensity of fluorescence increases as oxygenated species increases in substitution. The values for such increase is demonstrated in the table below. Influence of a halogen substitution decreases fluorescence as the molar mass of the halogen increases. As demonstrated in the table below, as the molar mass of the substituted compound increases, the relative intensity of the fluorescence decreases. In heavy atom substitution such as nitro derivatives or heavy halogen substitution such as iodobenzene, the compounds are subject to predissociation.
These compounds have bonds that easily rupture that can then absorb excitation energy and go through internal conversion. Therefore, the relative intensity of fluorescence and fluorescent wavelength is not observed and this is demonstrated in the table below. Fluorescence is particularly favored in molecules with rigid structures.
The table below compares the quantum efficiencies of fluorine and biphenyl which are both similar in structure that there is a bond between the two benzene group. The difference is that fluorene is more rigid from the addition methylene bridging group. By looking at the table below, rigid fluorene has a higher quantum efficiency than unrigid biphenyl which indicates that fluorescence is favored in rigid molecules.
This concept of rigidity was used to explain the increase in fluorescence of organic chelating agent when the compound is complexed with a metal ion. The fluorescence intensity of 8-hydroxyquinoline is much less than its zinc complex. The explanation for lower quantum efficiency or lack of rigidity in caused by the enhanced internal conversion rate k ic which increases the probability that there will be radiationless deactivation.
Nonrigid molecules can also undergo low-frequency vibration which accounts for small energy loss. Quantum efficiency of Fluorescence decreases with increasing temperature. As the temperature increases, the frequency of the collision increases which increases the probability of deactivation by external conversion. 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:. Unlike phosphorescent products, fluorescent pigments stop glowing once the light source is removed.
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength and therefore lower energy, than the absorbed radiation. In fluorescence, an electron is raised from a certain baseline energy known as ground level to an excited level by a light photon or other radiation. Transition of the electron back to the ground level can occur spontaneously with radiation of the same energy as that which was absorbed.
The most common example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the spectrum and thus invisible to the human eye, while the emitted light is in the visible region, which gives the fluorescent a distinct color that can be seen only when exposed to UV light.
Fluorescent materials cease to glow nearly immediately when the radiation source stops, unlike phosphorescent materials, which continue to emit light for some time after.
Phosphorescence is the absorption of energy by atoms or molecules followed by delayed emission of electromagnetic radiation. In other words, it is emission of light from a substance exposed to radiation and persisting as an afterglow after the exciting radiation has been removed.
In phosphorescence, light is absorbed by a material, bumping up the energy levels of electrons into an excited state. Fluorescent and phosphorescence are only two ways light may be emitted from a material.
Other mechanisms of luminescence include triboluminescence , bioluminescence , and chemiluminescence. Actively scan device characteristics for identification. Use precise geolocation data.
Select personalised content. Create a personalised content profile. Measure ad performance. Select basic ads. Create a personalised ads profile. Select personalised ads. Apply market research to generate audience insights. Measure content performance. Develop and improve products. List of Partners vendors. Share Flipboard Email. Anne Marie Helmenstine, Ph. Chemistry Expert. Helmenstine holds a Ph. She has taught science courses at the high school, college, and graduate levels.
Facebook Facebook Twitter Twitter. Key Takeaways: Fluorescence Versus Phosphorescence Both fluorescence and phosphorescence are forms of photoluminescence. In a sense, both phenomena cause things to glow in the dark. In both cases, electrons absorb energy and release light when they return to a more stable state. Fluorescence occurs much more quickly than phosphorescence. When the source of excitation is removed, the glow almost immediately ceases fraction of a second. The direction of the electron spin does not change.
Phosphorescence lasts much longer than fluorescence minutes to several hours.
0コメント