Fluorescence and Phosphorescence

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What are Fluorescence and Phosphorescence?

Fluorescence and phosphorescence both are types of molecular luminescence. A photon is absorbed by a sample solution atom, which excites a species. The emission spectrum can be used to do both qualitative and quantitative research. Since both fluorescence phosphorescence are excited by the emission of a photon, they are often referred to as photoluminescence. 

Fluorescence varies from phosphorescence in that the electron spin does not change during the electronic energy transfer that causes fluorescence, resulting in short-lived electrons 10⁻⁵ s in the excited state.


Physics of Fluorescence and Phosphorescence

One of the foundations of quantum mechanics is the Pauli Exclusion Principle.

No two fermions in an atom or molecule may have the same set of quantum numbers, according to this theory. Fermions are particles with a half-integer intrinsic angular momentum or "spin" of H = \[\frac{h}{2 \pi}\], where h is Planck's constant. Electrons are “spin 1/2” fermions since they have an angular momentum of H/2. The Pauli Exclusion Principle states that two electrons in the same atomic or molecular orbital must have opposite spins, which are referred to as "up" and "down" for our purposes.

A molecule of singlet state is one in which all of the electrons are matched in up and down pairs.

A triplet state is one in which two electrons in different orbitals have the same up-down or down-down direction. 

Understanding the fluorescent and phosphorescent difference necessitates an understanding of electron spin and the singlet and triplet conditions. According to the Pauli Exclusion Theorem, no two electrons in an atom should have the same four quantum numbers (n, l, ml, ms), and only two electrons can share each orbital, with opposite spin states. Spin pairing refers to the opposing spin states. Because of this spin coupling, some molecules are diamagnetic and lack a magnetic field. The static electric field does not trap or repel electrons in diamagnetic molecules. Since unpaired electrons have magnetic moments that are drawn to the magnetic field, free radicals are paramagnetic.

When a molecule is exposed to a magnetic field, all of the electron spins in the molecular electric state are paired, and the electronic energy levels do not split.

When an unpaired electron is exposed to a magnetic field, it will take two different orientations, each of which imparts different energy to the device. When one electron is excited to a higher energy level, it may form a singlet or a triplet. The electron is promoted in the same spin direction as it was in the ground state in an excited singlet state. The promoted electron in a triplet excited state has the same spin direction as the other unpaired electron. The difference of spins of the excited singlet, ground singlet, and excited triplet is shown in the given figure,

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Jablonski Diagram Fluorescence and Phosphorescence

A molecule's energy states are shown in a Jablonski diagram.

(The man's name is pronounced Jaboski correctly.) The accented N in the Russian word“HeT,” or Nyet in English, is similar to the English “Y,” the L with a line through it is similar to the “W” in water, and the L with a line through it is similar to the “W” in water. For English spelling, Jaboski is spelt YabWoNYski, with the emphasis on the second syllable. He is one of those poor ones who, like Khrushchev and Gengis Kahn, will have their names mispronounced by English speakers for the rest of their lives.) 

The energy levels are grouped vertically, and the spin states are grouped horizontally in those diagrams. The thick lines labelled S0 and S1 are electrical levels, and the thinner lines are vibrational levels; the black lines reflect energy levels. Electrons in up or down angular momentum states are represented by the gold dots with arrows.

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Difference Between Fluorescence and Phosphorescence

  1. The main difference between fluorescent and phosphorescent is, fluorescence is the absorption of energy by atoms or molecules followed by immediate emission of light or electromagnetic radiation. On the other hand, phosphorescence is the absorption of energy by atoms or molecules followed by delayed emission of electromagnetic radiation.

  2. Fluorescent emission of radiation or light suddenly stops on the removal of the soucre of excitation. On the other hand, phosphorescence emission of radiation remains for some time even after the removal of the source of excitation.

  3. If we distinguish between fluorescence and phosphorescence, the time period or interval between the absorption and emission is the primary factor. It is very short for fluorescence and comparatively long for phosphorescence.

  4. Another differentiate between fluorescence and phosphorescence is the emitted photon (light) has lower energy than the absorbed photon and emission occurs at a longer wavelength than the incident light in fluorescence and the longer wavelength than fluorescence in phosphorescence.

  5. In fluorescent materials, gives an “an immediate flash or afterglow” on excitation. And the phosphorescent materials appear to “glow in the dark” because of the slow emission of light over time.

  6. Fluorescence and phosphorescence difference is the excited atom has a comparatively short lifetime before its transition to a low energy state in fluorescence, and for phosphorescence, the excited atom has a comparatively long time before its transition to a low energy state.


Factors Affecting Fluorescence and Phosphorescence

The common factors affecting fluorescence and phosphorescence

1. Nature of Molecule

Having conjugated double bonds.


2. Temperature/Viscosity

  • The viscosity of the medium varies as the temperature changes, affecting the number of collisions between fluorophore molecules and solvent molecules.

  • The viscosity of the medium varies as the temperature changes, affecting the number of collisions between fluorophore molecules and solvent molecules.

  • A drop in fluorescence is almost always followed by an increase in temperature.

  • Fluorescence intensity rises as viscosity rises.

  • The probability of deactivation by internal transfer and vibrational relaxation increases as the number of collisions between molecules increases.

  • The reaction must be held at a constant temperature of +/- 0.1°C.

Applications of Fluorescence and Phosphorescence Spectroscopy

Phosphorescent

Shine-in-the-dark dolls, stickers, paint, wristwatch and clock dials are all examples of phosphorescent items that glow after being filled with bright light, such as a regular reading or space light. The glow of clock dial or toys or in bulb after switching off the light in the room. Hence the glow remains for some minutes or even hours in a dark room. Phosphorescent materials in sigh board illuminate during the night. 


Fluorescence

Gemstones fluoresce, including gypsum, talc, jellyfish, chlorophyll extract, vitamins etc are the common examples of fluorescent. Mineralogy, gemology, medicine, chemical sensors (fluorescence spectroscopy), fluorescent labelling, dyes, biological detectors, cosmic-ray tracking, vacuum fluorescent screens, and cathode-ray tubes are only a few of the uses for fluorescence. 

Fluorescent coatings are used in energy-saving fluorescent lamps and LED lamps to transform short-wavelength UV or blue light into longer-wavelength yellow light, simulating the warm light of inefficient incandescent lamps.

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FAQs (Frequently Asked Questions)

Q1. Define Fluorescence and Phosphorescence.

Answer:

Fluorescence: High-energy radiation (such as ultraviolet or X-ray) is captured by electrons circling an atom and re-emitted as light energy, resulting in fluorescence.


Phosphorescence: Phosphorescence is light energy emitted by a certain form of chemical reaction in which the reactants' excess chemical energy is released as light energy. 

Q2. Is Phosphorescence Radiative or Non-Radiative?

Answer: Phosphorescence usually occurs only with heavier molecules since the spin has to be reversed with the help of spin-orbit-coupling. However electromagnetic radiation emitted at all, and with which wavelength, depends on how much energy can be released beforehand by non-radiative decay.

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