Fluorescence vs. Phosphorescence: Understanding Light Emission in Molecules

When a molecule absorbs light, it may emit light through fluorescence or phosphorescence, two types of luminescence with distinct mechanisms and timescales.

Fluorescence occurs when a molecule absorbs a photon and is promoted to an excited singlet state (S₁). After rapid nonradiative relaxation to the lowest vibrational level of S₁, it emits a photon as it returns to the ground singlet state (S₀). This process is fast—typically between 10⁻⁸ and 10⁻⁴ seconds​.

In contrast, phosphorescence involves a forbidden transition. After intersystem crossing from S₁ to the triplet state T₁, the molecule eventually emits a photon as it transitions from T₁ to S₀. Since this involves a change in electron spin, it is slower—ranging from 10⁻⁴ to 10² seconds, often requiring cooling to be observable​​.

The emitted light in phosphorescence is of lower energy (i.e., longer wavelength) than in fluorescence, due to greater energy loss during internal conversion and intersystem crossing​. Both processes are commonly used in analytical chemistry, fluorescent labeling, and even laundry detergents that make clothes appear whiter under UV light​

Quiz

·  Fluorescence is a transition from the _______ excited state to the ground state.

·  Phosphorescence involves a transition from the _______ state to the ground singlet state.

·  The timescale of phosphorescence is _______ than that of fluorescence.

·  A nonradiative transition between states of the same spin is called _______.

·  Emission of light during phosphorescence occurs on the order of _______ seconds.

Answer

signlet , triplet , longer , internal conversion , 10⁻⁴ to 10²