Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a specific type of luminescence that occurs when a molecule or atom relaxes from an excited electronic state to a lower energy state, releasing a photon in the process.
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Fluorescence is a type of photoluminescence, where the absorbed energy is in the form of light, typically in the ultraviolet or visible range.
The wavelength of the emitted light is typically longer (and therefore, lower in energy) than the wavelength of the absorbed light, a phenomenon known as the Stokes shift.
Fluorescent molecules or atoms are often used as markers or probes in various scientific and medical applications, such as fluorescence microscopy and flow cytometry.
The efficiency of fluorescence is measured by the quantum yield, which is the ratio of the number of photons emitted to the number of photons absorbed.
Conjugated pi systems, as described in the topic 30.1 Molecular Orbitals of Conjugated Pi Systems, can exhibit strong fluorescence due to the delocalization of electrons within the pi system.
Review Questions
Explain how the process of fluorescence is related to the concept of molecular orbitals in conjugated pi systems.
In conjugated pi systems, the delocalization of electrons within the pi orbitals allows for the absorption of light and the subsequent emission of fluorescent light. When a molecule in a conjugated pi system absorbs a photon, the energy promotes an electron from the ground state to an excited state. As the electron relaxes back to the ground state, it releases the absorbed energy in the form of a photon, resulting in the observed fluorescence. The specific wavelength of the emitted light is determined by the energy gap between the excited and ground states, which is influenced by the extent of conjugation and the molecular orbitals within the pi system.
Describe how the quantum yield of a fluorescent molecule is related to the efficiency of the fluorescence process.
The quantum yield of a fluorescent molecule is the ratio of the number of photons emitted through fluorescence to the number of photons absorbed by the molecule. This metric directly reflects the efficiency of the fluorescence process. A high quantum yield indicates that a large fraction of the absorbed energy is converted into emitted photons, making the molecule a more effective fluorescent probe or marker. Factors that can affect the quantum yield include the structure of the molecule, the solvent environment, and the presence of quenching agents that can compete with the fluorescence relaxation pathway. Understanding the quantum yield is crucial when selecting or designing fluorescent probes for applications in areas like fluorescence microscopy, where the brightness and sensitivity of the fluorescent signal are key performance criteria.
Analyze how the Stokes shift, the difference between the wavelengths of absorbed and emitted light, is important in the context of fluorescence and conjugated pi systems.
The Stokes shift, the difference between the wavelength of the absorbed light and the wavelength of the emitted fluorescent light, is a critical characteristic of fluorescent molecules, including those with conjugated pi systems. The Stokes shift arises due to the energy loss that occurs during the relaxation process from the excited state to the ground state. In conjugated pi systems, the delocalization of electrons allows for efficient light absorption, but the subsequent emission of light occurs at a lower energy, resulting in a longer wavelength. This Stokes shift is important because it allows for the separation of the excitation and emission signals, enabling the use of fluorescent probes and markers in various analytical techniques, such as fluorescence microscopy and flow cytometry, where the ability to distinguish the excitation and emission wavelengths is crucial for effective signal detection and analysis.