The terms fluorescence and phosphorescence are frequently used to describe the emission of light by substances. Fluorescence refers to the rapid emission of light by a substance shortly after it has been excited by light. Whereas, phosphorescence refers to the slow emission of light that persists even after the light excitation has ceased.

The fluorescence phenomenon happens due to the excitation of electrons in a substance to a higher energy level, followed by their relaxation quickly back to the ground state. However, phosphorescence is a phenomenon that takes place when a substance absorbs energy and emits it slowly over a more extended period, ranging from seconds to hours.

Fluorescence has various applications in science, including medical imaging, forensics, and environmental monitoring, due to its sensitivity, specificity, and non-invasive nature. Phosphorescence has various applications in imaging, displays, and sensors.

Singlet and Triplet States

Singlet state: All electron spins are paired. It has only one possible orientation in space, therefore, referred to as a singlet state.

Triplet state: One of the electron spins is unpaired. It has three possible orientations in space, therefore, known as the triplet state.

 Singlet or triplet excited states can be formed when an electron is excited to a higher energy level. In a singlet excited state, the spin of the electron does not change. Whereas, in a triplet excited state, the spin of the electron is flipped. Therefore, according to Hund’s rule of maximum multiplicity, the triplet state is energetically more favorable as compared to the singlet.

Singlet state and triplet state of electrons

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Allowed and Forbidden Transitions

Allowed transitions: The transitions that have a high probability of occurring are called the allowed transitions. Singlet-to-singlet transitions are allowed. In allowed transitions, the spin of the electron does not change. Therefore, the lifetime of singlet to singlet transition is shorter than triplet to singlet transition.

Forbidden Transitions: The transitions with a low probability are called forbidden transitions. Singlet-to-triplet and triplet-to-singlet transitions are forbidden. This transition involves a change in spin multiplicity. Any change in the spin multiplicity during a transition has a low probability of happening. Therefore, the lifetime of triplet to singlet transition is longer than singlet to singlet.

Reason Behind Fluorescence

Fluorescence occurs when an electron transitions from the singlet ground state (S0) to the singlet excited state (S1) through spin-allowed transitions. The absorption corresponds to the S0 → S1 transition. The electronic spins in S1 and S0 are anti-parallel i.e. against Hund’s rule. Therefore, the singlet excited state is not energetically favorable and an immediate transition from S1 → S0 takes place. This process results in the emission of a photon, known as fluorescence, which has a brief lifetime of around a nanosecond.

Jablonski diagram of fluorescence
Jablonski Diagram for Fluorescence

Reason Behind phosphorescence

In phosphorescence electron(s) then move from S1 to the excited triplet state T1 through a process called intersystem crossing (ISC). The electron spins in Sand T1 are parallel i.e. according to Hund’s rule. Therefore, the Tstate is energetically favorable and the transition from Tto Sis delayed.

Jablonski Diagram for Phosphorescence


  • S=  singlet ground state
  • S= 1st excited singlet state
  • S= 2nd excited singlet state
  • ISC = intersystem crossing
  • T= excited triplet state

Fluorescence vs. Phosphorescence

Fluorescence  Phosphorescence
The process in which atoms or molecules absorb energy and then promptly emit electromagnetic radiation The process in which atoms or molecules absorb energy,  followed by the delayed emission of electromagnetic radiation
The time interval between absorption and emission is very short around nanoseconds The time interval between absorption and emission is relatively longer around seconds to thousands of seconds
The excited atom has a comparatively shorter lifetime The excited atom has a comparatively longer lifetime
Electron spin does not flip in excited state Electron spin is flipped in excited state
It involves singlet-singlet transitions It involves singlet-triplet transitions
Its excited singlet state is against Hund's rule of maximum multiplicity Its excited triplet state is according to Hund's rule of maximum multiplicity
The wavelength of emitted radiations is longer than the absorbed radiations The wavelength of emitted radiations is longer than the fluorescence and the absorbed radiations
Energy change is smaller as compared to incident radiations Energy change is smaller as compared to fluorescence
Fluorescent lamps, fluorescent dyes, fluorescent minerals, fluorescent proteins, fluorescent inks, etc are some examples Glow-in-the-dark toys, phosphorescent paint, watch dials and instrument panels, jellyfish, luminescent exit signs, etc

Concepts Berg

What is the difference between fluorescence, phosphorescence, photoluminescence, and luminescence?

Luminescence is a broader term encompassing fluorescence phosphorescence and photoluminescence. This phenomenon doesn’t require high temperatures. Therefore, often referred to as “cold light.”
Fluorescence occurs when a substance absorbs photons and promptly emits light.
Phosphorescence involves the emission of light after the excitation source is removed, resulting in a delayed glow.
Photoluminescence involves the excitation of electrons by the absorption of photons in the visible region. The final step involves the emission of light during the relaxation of electrons.

Is glow-in-the-dark fluorescence or phosphorescence? 

Glow-in-the-dark materials typ exhibit phosphorescence. Fluorescent materials don’t glow in the dark.

Is black light a fluorescence? 

Black lights are typically fluorescent lamps that have been modified to emit primarily UVA radiation along with a limited amount of visible light.

What is the principle of fluorescence and phosphorescence?

Fluorescence takes place when a substance absorbs photons and rapidly emits lower-energy photons, usually in the visible range.
Phosphorescence involves a delayed emission of photons after the excitation source is removed, due to energetically favorable excited states.

What wavelength is phosphorescence? 

Phosphorescence typically takes place at longer wavelengths as compared to absorbed radiations.

Are fluorescence and phosphorescence radiative or non-radiative? 

Fluorescence and phosphorescence are radiative processes where energy is released as light.

Which is better: luciferase or fluorescence proteins? 

Fluorescent proteins are brighter than luciferase. However, autofluorescence and light scattering can make them problematic in tissues or live animals. The choice depends on the application: luciferase for sensitive detection, and fluorescence proteins for direct visualization of targeted proteins.

What are the units for fluorescence intensity? 

MESF (Molecules of Equivalent Soluble Fluorophore) is the stoichiometric unit for fluorescence intensity.

What is the temperature effect on the phosphorescence lifetime? 

The phosphorescence lifetime of a material is generally inversely proportional to temperature. As temperature increases, the phosphorescence lifetime tends to decrease due to increased thermal energy and faster relaxation processes.

Why does ZnS show a fluorescence effect? 

ZnS exhibits a fluorescence effect due to the presence of impurities, such as transition metal ions. These impurities have different energy levels as compared to pure Zn. These impurities can absorb and re-emit photons, leading to the fluorescence phenomena in ZnS crystals. Pure ZnS is non-fluorescent.

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