What is Fluorescence? Explain with example.
Fluorescence is a temporary physical phenomenon where the atoms or molecules of a substance get energized and excited by the incidence of high-energy electromagnetic radiation and emit light radiation rapidly through transitions to lower energy states.
When a fluorescent substance absorbs energy due to the incidence of X-rays, light waves, or beams of electrons, it starts to emit visible light (of longer wavelength) and quickly reaches the ground state from the partially excited state. This phenomenon is called fluorescence. The substances that show such activities are called fluorescent substances. Crucially, as soon as the incident light radiation is cut off, fluorescence ceases immediately.
Chlorophyll is an excellent example of a fluorescent substance occurring naturally in green plants. In nature, this pigment is used to trap solar rays and convert them into chemical energy by initiating biochemical pathways. If you extract chlorophyll in a test tube and shine UV light on it, it will emit a deep red fluorescence. This behavior only shows up when the substance interacts with a specific range of exciting wavelengths.
Fluorescence has many practical applications, including mineralogy, gemology, medicine, chemical sensors (fluorescence spectroscopy), fluorescent labelling, dyes, biological detectors, cosmic-ray detection, vacuum fluorescent displays, and cathode-ray tubes. Its most common everyday application is in gas-discharge fluorescent lamps and LED lamps, in which internal fluorescent coatings convert UV or blue light into longer-wavelength combinations, resulting in balanced white light.
Fluorescence Quantum Yield
Fluorescence quantum yield gives the total quantitative efficiency of the fluorescence process. It is defined as the ratio of the number of photons emitted to the total number of photons absorbed by the chemical system:
The maximum possible fluorescence quantum yield is 1.0 (100%), where every single photon absorbed results in a photon emitted. Compounds with quantum yields around 0.10 are still considered quite fluorescent by laboratory standards.
Related Topics:
Fluorescence Quenching
Phosphorescence
Test Your Understanding: Fluorescence Principles
1. Which statement correctly highlights the practical behavior of a typical fluorescent material when the exciting light source is suddenly removed?
- (A) The material continues to glow brightly for hours due to energy storage.
- (B) The light emission ceases almost instantaneously (typically within 10-8 seconds).
- (C) The color of the emitted light shifts completely into the infrared spectrum.
- (D) The substance undergoes irreversible thermal decomposition.
View Answer
Explanation: Fluorescence occurs via a quantum mechanically 'allowed' singlet-to-singlet electronic transition (S1 → S0). Because it does not require a spin flip, the lifetime of the excited state is incredibly short, meaning emission stops the moment the incoming light stops.
2. Compared to the electromagnetic radiation initially absorbed by a molecule, the light emitted during fluorescence typically possesses:
- (A) A shorter wavelength and higher energy
- (B) The exact same wavelength and identical energy
- (C) A longer wavelength and lower energy
- (D) High-frequency gamma radiation characteristics
View Answer
Explanation: Due to non-radiative vibrational relaxation processes that happen after excitation (known as the Stokes Shift), some energy is always lost as heat to the environment. Consequently, the emitted photon has less energy and a longer wavelength than the absorbed photon.
3. If a fluorescent organic dye sample absorbs exactly 5,000 photons of light and subsequently emits 1,500 photons as fluorescence, what is its calculated Quantum Yield (Φ)?
- (A) 0.15
- (B) 0.30
- (C) 0.45
- (D) 1.00
View Answer
Explanation: Quantum Yield (Φ) is calculated using the formula:
Photons Emitted / Photons Absorbed. Putting the values gives 1,500 / 5,000 = 0.30.