Glossary

5.3 Excited state lifetime and quantum yield

2 min read•july 24, 2024

Excited state dynamics are crucial in photochemistry, determining how long molecules stay excited and how efficiently they use absorbed light. These factors influence everything from energy transfer to reaction rates.

Understanding excited state lifetimes and quantum yields helps predict and control photochemical processes. By measuring these properties, we can optimize reactions, design better sensors, and improve photocatalysts for various applications.

Excited State Dynamics

Excited state lifetime significance

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  • Excited state lifetime measures time molecule spends in excited state before returning to ground state typically denoted as τ (tau) measured in seconds ( or )
  • Determines time available for photochemical reactions influences energy transfer probability to other molecules affects competition between radiative and processes
  • Molecular structure solvent environment temperature and presence of quenchers (oxygen) impact excited state lifetime

Calculation of excited state lifetime

  • Excited state lifetime formula τ=1/(kr+knr)τ = 1 / (k_r + k_{nr}) where krk_r is rate constant and knrk_{nr} is non-radiative decay rate constant
  • Relaxation pathways include radiative decay (fluorescence phosphorescence) and non-radiative decay (internal conversion intersystem crossing)
  • Longer lifetime corresponds to smaller decay rate constants shorter lifetime indicates faster decay processes
  • Time-resolved spectroscopy techniques and measurements experimentally determine excited state lifetime

Quantum yield concept

  • measures efficiency of photophysical or photochemical process ratio of photons emitted to photons absorbed
  • Formula Φ=NumberofeventsNumberofphotonsabsorbedΦ = \frac{Number of events}{Number of photons absorbed} quantifies process efficiency
  • Competition between radiative and non-radiative processes influences quantum yield higher values indicate more efficient radiative decay
  • Molecular structure temperature and presence of quenchers (triplet oxygen) or energy transfer acceptors (FRET pairs) affect quantum yield

Determination of quantum yield

  • Chemical actinometry and comparative methods using standard fluorophores (rhodamine 6G) experimentally determine quantum yield
  • Absorption spectra emission spectra and concentration of reactants and products required for calculation
  • Quantum yield calculation steps:
  1. Measure number of photons absorbed
  2. Determine number of product molecules formed or reactant molecules consumed
  3. Apply formula Φ=molesofproductformedmolesofphotonsabsorbedΦ = \frac{moles of product formed}{moles of photons absorbed}
  • Ensure complete light absorption account for inner filter effects use appropriate excitation wavelength range for accurate measurements
  • Quantum yield measurements evaluate photocatalyst efficiency (TiO2) optimize photochemical reactions (photopolymerization) design fluorescent probes and sensors (BODIPY dyes)

Key Terms to Review (18)

Arrhenius Equation: The Arrhenius equation is a mathematical formula that describes how the rate of a chemical reaction depends on temperature and activation energy. It highlights the relationship between temperature and reaction rate, showing that as temperature increases, the rate of reaction typically increases due to more molecules having enough energy to overcome the activation barrier. This concept connects to various aspects like excited state lifetime, quantum yield, and kinetics of photochemical reactions.
Eyring Equation: The Eyring Equation describes the relationship between the rate constant of a chemical reaction and the activation energy, linking the rate of reaction to the temperature and entropy of the transition state. This equation is crucial for understanding how factors like temperature and molecular interactions affect reaction rates, which connects to concepts like excited state lifetime and quantum yield, as well as steady-state and time-resolved kinetics.
Fluorescence lifetime: Fluorescence lifetime is the average time a molecule stays in its excited state before emitting a photon and returning to the ground state. This concept is crucial for understanding various processes, as it is directly related to the efficiency of fluorescence, which can be influenced by factors such as radiative decay processes, collisional quenching, and energy transfer mechanisms.
Frank-Condon Principle: The Frank-Condon Principle states that electronic transitions between different vibrational states of a molecule occur with a preference for those vibrational levels where the nuclear positions of the initial and final states are similar. This principle explains why certain transitions are more likely than others and directly impacts the excited state lifetime and quantum yield, as it influences how efficiently a molecule can absorb and emit light.
Jablonski diagram: A Jablonski diagram is a graphical representation that illustrates the electronic states of a molecule and the transitions between these states, particularly during the absorption and emission of light. It is crucial for understanding processes like fluorescence and phosphorescence, as it highlights the different energy levels and the various pathways for relaxation from excited states to ground states.
Nanoseconds: A nanosecond is one billionth of a second, or 10^-9 seconds. This time scale is crucial for understanding the dynamics of excited states in molecules and the processes that occur after a molecule absorbs light. In photochemistry, the excited state lifetime often falls within the range of nanoseconds, which influences the quantum yield and the efficiency of photochemical reactions.
Non-radiative decay: Non-radiative decay refers to the process by which an excited state returns to a lower energy state without the emission of photons. This phenomenon plays a crucial role in light-matter interactions and is important for understanding how energy is transferred and dissipated in various systems, affecting absorption and emission characteristics. By studying non-radiative decay, one can gain insights into excited state dynamics and their implications for excited state lifetimes and quantum yields.
Phosphorescence delay time: Phosphorescence delay time refers to the duration between the absorption of light by a substance and the subsequent emission of light after the initial excitation. This time period is significant because it encompasses the transition from an excited singlet state to a triplet state, leading to a slower release of energy in the form of light, which is what characterizes phosphorescence. The length of this delay can provide insight into the excited state lifetime and quantum yield of the phosphorescent material.
Photochemical efficiency: Photochemical efficiency refers to the effectiveness with which absorbed light energy is converted into chemical energy during photochemical reactions. This term is closely linked to concepts like excited state lifetime and quantum yield, which help to evaluate how well a system utilizes light for chemical transformations. The higher the photochemical efficiency, the more productive the system is in driving reactions using light energy.
Photodissociation: Photodissociation is the process by which a chemical bond is broken due to the absorption of light, leading to the formation of two or more products from a single reactant. This phenomenon is crucial in understanding excited state lifetimes, as the time a molecule remains in an excited state influences the likelihood of bond cleavage. Additionally, photodissociation plays a vital role in various chemical reactions and atmospheric chemistry.
Photoisomerization: Photoisomerization is the process by which a molecule undergoes a structural change when exposed to light, resulting in different isomers. This transformation is significant as it can affect the physical and chemical properties of the substance, leading to various applications in fields like materials science, photopharmacology, and biochemistry.
Photosynthesis efficiency: Photosynthesis efficiency refers to the effectiveness with which plants convert light energy into chemical energy during the process of photosynthesis. This efficiency is influenced by several factors, including the excited state lifetime of chlorophyll molecules and the quantum yield of photosynthetic reactions, both of which determine how well absorbed light can be utilized for converting carbon dioxide and water into glucose and oxygen.
Photovoltaic Cells: Photovoltaic cells are devices that convert light energy directly into electrical energy through the photovoltaic effect. This process is crucial for harnessing solar energy, a renewable resource, and plays a significant role in addressing energy needs sustainably while also reflecting the fundamental principles of photochemistry.
Picoseconds: A picosecond is a unit of time equal to one trillionth of a second, or 10^-12 seconds. This incredibly short timescale is crucial in photochemistry, as it often relates to the lifetimes of excited states and the dynamics of molecular processes that occur after the absorption of light.
Quantum Yield: Quantum yield is a measure of the efficiency of a photochemical process, defined as the ratio of the number of events (like the formation of a product) to the number of photons absorbed. This concept is crucial in understanding how light interacts with matter, as it helps quantify how effectively light energy is converted into chemical energy or emitted as light, linking absorption and emission phenomena.
Radiative Decay: Radiative decay is the process by which an excited atom or molecule returns to its ground state by emitting a photon, resulting in the release of energy. This phenomenon is essential for understanding how light interacts with matter, specifically in terms of emission and absorption. The mechanism plays a crucial role in various relaxation pathways, influencing excited state lifetimes and quantum yields, and underpins the principles of fluorescence and phosphorescence.
Solvent Effects: Solvent effects refer to the influence that the solvent has on the properties and behavior of solute molecules, especially during electronic transitions. The interactions between the solvent and solute can alter absorption and emission spectra, affect transition probabilities, and modify excited state lifetimes and decay pathways. Understanding solvent effects is crucial for predicting how molecules behave in different environments, impacting excited states and non-radiative processes.
Temperature dependence: Temperature dependence refers to how various physical and chemical processes, such as reaction rates, excited state lifetimes, and quantum yields, change with temperature. This concept is crucial in understanding how temperature variations can affect the behavior of molecules and their interactions, influencing phenomena like non-radiative decay, collisional quenching, and electron transfer mechanisms.
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