Glossary

9.3 Fluorescence and phosphorescence phenomena

7 min read•july 30, 2024

and are key phenomena in electronic spectroscopy. They involve light emission after molecules absorb energy, but differ in their mechanisms and timescales. Fluorescence happens quickly, while phosphorescence can last much longer.

These processes are crucial for understanding molecular energy levels and transitions. The Franck-Condon principle explains how molecular vibrations affect these transitions, connecting fluorescence and phosphorescence to broader concepts in electronic spectroscopy.

Fluorescence and Phosphorescence

Mechanisms and Underlying Processes

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  • Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation, occurring almost immediately after excitation
    • In fluorescence, an electron in a molecule is excited to a higher energy level by absorbing a photon, then relaxes back to its by emitting a photon of lower energy
    • The emitted photon typically has a longer wavelength (lower energy) than the absorbed photon due to the
  • Phosphorescence is a type of photoluminescence related to fluorescence, but with a slower decay rate after the excitation source is removed
    • In phosphorescence, an electron is excited to a higher energy level, then undergoes intersystem crossing to a lower energy before relaxing back to the ground state
    • This process results in a longer emission lifetime compared to fluorescence
  • Both fluorescence and phosphorescence involve electronic transitions between different energy levels within a molecule, but the underlying mechanisms and time scales differ
    • Fluorescence occurs from the singlet , while phosphorescence involves a transition from the triplet excited state to the singlet ground state
    • The is a graphical representation of the electronic states and transitions involved in fluorescence and phosphorescence, helping to visualize these processes

Quantum Yield and Efficiency

  • The of fluorescence and phosphorescence depends on the ratio of the rate to the total decay rate, which includes both radiative and non-radiative processes
    • A higher quantum yield indicates a more efficient emission process
    • Non-radiative processes, such as internal conversion, intersystem crossing, and vibrational relaxation, compete with radiative emission and can reduce the quantum yield
  • Structural factors, such as molecular rigidity and the presence of heavy atoms, can influence the rate of intersystem crossing and the efficiency of phosphorescence
    • Rigid molecular structures minimize non-radiative relaxation pathways, leading to higher quantum yields
    • Heavy atoms (bromine, iodine) facilitate intersystem crossing, increasing the probability of phosphorescence
  • The Franck-Condon principle describes the influence of vibrational overlap between the ground and excited states on the efficiency of radiative transitions
    • A larger overlap between the vibrational wavefunctions of the ground and excited states results in more efficient absorption and emission processes

Fluorescence vs Phosphorescence

Emission Lifetime and Wavelength

  • Fluorescence typically has a short lifetime, with emission occurring within nanoseconds after excitation, while phosphorescence has a longer lifetime, ranging from milliseconds to hours
    • The longer lifetime of phosphorescence is due to the forbidden nature of the triplet-to-singlet transition, which results in slower emission rates
  • Fluorescence emission generally occurs at a longer wavelength (lower energy) than the excitation wavelength due to the Stokes shift
    • The Stokes shift is caused by the rapid relaxation of the excited state to the lowest vibrational level before emission occurs
  • Phosphorescence emission is usually at an even longer wavelength than fluorescence
    • This is because the triplet excited state is lower in energy than the singlet excited state, resulting in a larger energy difference between the excited and ground states

Environmental Sensitivity

  • Fluorescence is sensitive to environmental factors such as pH, polarity, and the presence of quenchers, while phosphorescence is less affected by these factors due to the longer lifetime of the triplet state
    • Changes in pH can affect the protonation state of fluorescent molecules, altering their absorption and emission properties (fluorescein)
    • Polarity of the solvent can influence the energy levels of the excited states, leading to shifts in the emission wavelength (solvatochromism)
    • Quenchers, such as oxygen or heavy metal ions, can deactivate the excited state through collisional or static quenching mechanisms, reducing the fluorescence intensity
  • The longer lifetime of the triplet state in phosphorescence makes it less susceptible to environmental factors, as the excited molecule has more time to interact with its surroundings before emitting a photon

Factors Affecting Fluorescence and Phosphorescence

Temperature and Viscosity Effects

  • Higher temperatures generally lead to increased non-radiative decay rates and reduced fluorescence and phosphorescence efficiency
    • Elevated temperatures promote vibrational relaxation and collisional quenching, which compete with radiative emission processes
    • The Arrhenius equation describes the temperature dependence of the non-radiative decay rate: knr=Aexp⁡(−Ea/RT)k_{nr} = A \exp(-E_a/RT), where AA is the pre-exponential factor, EaE_a is the activation energy, RR is the gas constant, and TT is the temperature
  • Higher viscosity can restrict molecular motion and reduce non-radiative decay rates, leading to increased fluorescence and phosphorescence efficiency
    • In viscous media, the rates of collisional quenching and conformational changes are reduced, favoring radiative emission
    • The Förster-Hoffmann equation relates the fluorescence quantum yield to the solvent viscosity: Φf=Cηx\Phi_f = C \eta^x, where Φf\Phi_f is the fluorescence quantum yield, CC and xx are constants, and η\eta is the solvent viscosity

Quenching and Energy Transfer

  • The presence of quenchers can significantly reduce the fluorescence and phosphorescence intensity through various mechanisms
    • Collisional quenching occurs when the excited molecule transfers its energy to the quencher upon collision, returning to the ground state without emitting a photon (oxygen quenching)
    • Static quenching involves the formation of a non-fluorescent complex between the fluorophore and the quencher, reducing the population of molecules available for emission
  • Energy transfer processes, such as Förster resonance energy transfer (FRET) and Dexter energy transfer, can also influence the fluorescence and phosphorescence properties of molecules
    • FRET is a non-radiative energy transfer mechanism that occurs through dipole-dipole interactions between a donor and an acceptor molecule, resulting in a decrease in the donor's fluorescence intensity and an increase in the acceptor's emission (fluorescent protein pairs)
    • Dexter energy transfer involves the exchange of electrons between the donor and acceptor molecules, requiring orbital overlap and typically occurring over shorter distances than FRET (triplet-triplet annihilation in OLEDs)

Applications of Fluorescence and Phosphorescence

Spectroscopy and Imaging Techniques

  • Fluorescence spectroscopy is widely used to study the structure, dynamics, and interactions of molecules, particularly in biological systems such as proteins and nucleic acids
    • Fluorescence resonance energy transfer (FRET) is a technique that measures the distance between two , providing information about molecular conformations and interactions (protein-protein interactions)
    • Time-resolved fluorescence spectroscopy can reveal the dynamics of molecular processes on picosecond to nanosecond time scales (protein folding kinetics)
  • Phosphorescence spectroscopy is used to study the triplet states of molecules and the mechanisms of intersystem crossing and energy transfer
    • Time-resolved phosphorescence spectroscopy can probe the lifetimes and energies of triplet states (photosensitizers in photodynamic therapy)
  • Fluorescence and phosphorescence are employed in various imaging techniques to visualize biological structures and processes with high spatial and temporal resolution
    • Fluorescence microscopy uses fluorescent probes to label specific cellular components or biomolecules (GFP-tagged proteins)
    • Confocal microscopy achieves high-resolution images by using a pinhole to eliminate out-of-focus light (single-molecule tracking)
    • Two-photon microscopy utilizes the simultaneous absorption of two lower-energy photons to excite fluorophores, allowing deeper tissue penetration and reduced photobleaching (neuron imaging in brain slices)

Sensors and Probes

  • Fluorescent probes and sensors are designed to detect specific analytes, such as ions, small molecules, or biomolecules, based on changes in their fluorescence properties upon binding or interaction
    • Calcium indicators, such as Fura-2 and Fluo-4, exhibit changes in their fluorescence intensity or wavelength in response to calcium ion concentration (monitoring neuronal activity)
    • pH-sensitive fluorescent dyes, like BCECF and SNARF, allow for the measurement of intracellular or extracellular pH (studying cellular metabolism and signaling)
  • Phosphorescent probes, particularly those based on transition metal complexes, offer advantages such as longer emission lifetimes and sensitivity to oxygen concentration
    • Ruthenium(II) and iridium(III) complexes are used as oxygen sensors, exploiting the quenching of their phosphorescence by molecular oxygen (measuring oxygen levels in biological samples)
    • Lanthanide complexes, such as europium(III) and terbium(III) chelates, exhibit long-lived phosphorescence and are used in time-resolved fluorescence immunoassays (detecting biomarkers and pathogens)

Materials and Devices

  • , such as organometallic complexes and persistent , find applications in organic light-emitting diodes (OLEDs), bioimaging, and sensing
    • Iridium(III) and platinum(II) complexes are used as phosphorescent dopants in OLEDs, enabling efficient electroluminescence and a wide color gamut (displays and lighting)
    • Persistent phosphors, which exhibit long-lasting afterglow, are employed in bioimaging and theranostics (in vivo cell tracking and drug delivery monitoring)
  • Fluorescent materials are widely used in solar cells, security inks, and optical sensors
    • Fluorescent organic dyes, such as perylene diimides and BODIPY derivatives, are incorporated into dye-sensitized solar cells to enhance light harvesting and energy conversion efficiency
    • Quantum dots, which are semiconductor nanocrystals with size-dependent fluorescence properties, are used in display technologies and as fluorescent tags in biological imaging (multiplexed cell labeling)

Key Terms to Review (18)

Excited State: The excited state of an atom or molecule refers to a condition where one or more electrons have absorbed energy and moved to a higher energy level, away from their ground state configuration. This state is critical in understanding various phenomena such as absorption spectra, emission of light, and electron transitions, reflecting how energy interactions can influence molecular behavior and stability.
Fluorescence: Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. This phenomenon occurs when a molecule absorbs a photon, gets excited to a higher energy state, and then quickly returns to its ground state, releasing energy in the form of visible light. Fluorescence is essential in understanding absorption, emission, and scattering processes, as it illustrates how materials interact with light and can serve various applications in fields like biology and materials science.
Fluorescent Microscopy: Fluorescent microscopy is a powerful imaging technique that uses fluorescence to visualize and study biological specimens at a cellular and molecular level. This method involves exciting fluorescent dyes or proteins with specific wavelengths of light, causing them to emit light at longer wavelengths, which can then be captured to create detailed images of the sample. It plays a critical role in biological and medical research, allowing for the observation of specific cellular components and processes in real-time.
Fluorophores: Fluorophores are molecules that can absorb light at a specific wavelength and then re-emit it at a longer wavelength, exhibiting fluorescence. This property makes them incredibly useful in various applications, especially in biological imaging and assays, as they allow researchers to visualize specific structures or processes within cells and tissues.
Ground State: The ground state refers to the lowest energy level of a quantum mechanical system, where the system is in its most stable configuration. It serves as a reference point for understanding various phenomena, including electronic transitions, vibrational states, and the behavior of multi-electron systems. The ground state is crucial for explaining how systems interact with external energy sources and how they can be excited to higher energy levels.
Hugh Stoddart's Work: Hugh Stoddart's work primarily focuses on the research and development of fluorescence and phosphorescence phenomena, particularly in the realm of molecular physics. His contributions have greatly advanced the understanding of how these luminescent processes operate at the molecular level, influencing fields such as biochemistry, materials science, and nanotechnology. By exploring the mechanisms behind these phenomena, Stoddart has helped uncover applications in imaging, sensing, and information storage.
Jablonski Diagram: A Jablonski diagram is a graphical representation that illustrates the electronic states of a molecule and the transitions between these states, particularly in the context of fluorescence and phosphorescence. It depicts energy levels, such as ground and excited states, along with the different types of transitions that can occur, including absorption, fluorescence emission, and phosphorescence emission. Understanding this diagram is crucial for analyzing how molecules absorb and emit light, which ties directly into the phenomena of fluorescence and phosphorescence.
Lifetime measurement: Lifetime measurement refers to the determination of the duration of an excited state of a molecule or atom before it returns to its ground state, typically through processes like fluorescence or phosphorescence. This concept is essential for understanding how long a molecule can emit light after being excited, which helps in characterizing its behavior and properties in various applications such as spectroscopy and imaging.
Non-radiative transition: A non-radiative transition is a process in which an excited state of a molecule returns to its ground state without the emission of photons. This phenomenon plays a crucial role in understanding energy transfer processes, where absorbed energy is dissipated as thermal energy instead of light. In the context of fluorescence and phosphorescence, non-radiative transitions help explain why certain materials may not fluoresce or phosphoresce efficiently despite absorbing energy.
Phosphorescence: Phosphorescence is the process by which certain materials absorb light and then re-emit it over an extended period of time, creating a glow that can last from milliseconds to hours after the excitation source is removed. This phenomenon is closely related to fluorescence but differs primarily in the duration of light emission and the underlying mechanisms involved in energy transition.
Phosphorescent materials: Phosphorescent materials are substances that can absorb light energy and re-emit it over an extended period, even after the source of excitation is removed. This process involves the transition of electrons to a metastable state, allowing the material to glow for a while, distinguishing them from fluorescent materials, which emit light only while being excited. The phenomenon showcases how certain materials can store energy and release it gradually, highlighting important principles of energy transfer and electronic states.
Phosphors: Phosphors are materials that exhibit the phenomenon of luminescence, which means they can absorb energy and re-emit it as visible light. This property is particularly important in applications such as fluorescent lights, cathode ray tubes, and glow-in-the-dark materials. Understanding phosphors involves recognizing their ability to undergo electronic transitions, contributing to both fluorescence and phosphorescence, where the latter refers to a delayed emission of light after the excitation source is removed.
Quantum Yield: Quantum yield is a measure of the efficiency of a photophysical or photochemical process, defined as the ratio of the number of photons emitted (or reacted) to the number of photons absorbed. It is particularly significant in understanding processes like fluorescence and phosphorescence, as it indicates how effectively absorbed light is converted into emitted light or chemical change. A high quantum yield means that most absorbed photons result in the desired output, while a low quantum yield suggests that many absorbed photons do not contribute to the intended outcome.
Radiative decay: Radiative decay is the process by which an excited atomic or molecular state returns to a lower energy state by emitting a photon. This phenomenon is crucial for understanding fluorescence and phosphorescence, where the emitted light results from transitions between energy levels in atoms or molecules, leading to visible effects in various materials.
Self-Quenching: Self-quenching refers to the process in which a molecule that has absorbed energy becomes less efficient at re-emitting that energy, often due to the presence of multiple excited states or interactions among excited molecules. This phenomenon is particularly significant in fluorescence and phosphorescence, where the emitted light can be diminished by nearby molecules that can absorb or dissipate energy instead of emitting it. Understanding self-quenching helps to explain why certain fluorescent materials might exhibit reduced brightness under specific conditions.
Stokes Shift: Stokes shift refers to the phenomenon where the emission of light from a fluorescent substance occurs at a longer wavelength than the absorbed light. This shift in wavelength arises due to the energy lost during the relaxation of the excited electronic states, leading to fluorescence or phosphorescence. The Stokes shift is crucial for distinguishing between different fluorescent species and helps in applications like imaging and sensing.
The discovery of fluorescence: The discovery of fluorescence refers to the phenomenon where certain substances emit light when exposed to radiation, typically ultraviolet light. This process involves the absorption of photons, which elevates electrons to a higher energy state, followed by the release of energy in the form of visible light as the electrons return to their ground state. This remarkable property has led to significant advancements in various fields, such as biology, chemistry, and physics, influencing everything from fluorescent dyes to medical imaging techniques.
Triplet state: A triplet state is an excited state of a molecule or atom where two electrons are in parallel spin orientation, resulting in a total spin quantum number of 1. This configuration makes triplet states relatively long-lived compared to singlet states, as the forbidden nature of the spin transition means that relaxation back to the ground state occurs slowly. These properties are significant in understanding phenomena like phosphorescence, where the system can remain in the triplet state before emitting light.
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