some_origins_of_multiexponetial_decays_for_single_dyes
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some_origins_of_multiexponetial_decays_for_single_dyes [2014/05/19 16:00] – veiga | some_origins_of_multiexponetial_decays_for_single_dyes [2015/11/03 14:08] – admin | ||
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- | The fluorescence lifetime of a fluorophore | + | The fluorescence lifetime of a dye measured with a TCSPC spectrometer can be multiexponential due to many reasons. The most obvious cases are due to scattering or presence of impurities. Although less obvious, it is also widely known that in an inhomogeneous media a pure dye will also exhibit a multiexponential decay. Advanced users know that if the decay is not measured with polarizers at magic angle, the rotation correlation time shows up as a second exponential in the decay (the second exponential is in fact the product of the rotation correlation time by the fluorescence lifetime, divided by their sum). And they also know that measuring without polarizers is not equivalent to measure at magic angle... |
But it is worth noting that even at magic angle in a perfectly aligned spectrometer pure dyes in homogeneous media may exhibit a multiexponential decay. The origin may be physical, like solvent relaxation, or chemical, when the fluorescent molecule undergoes a ground or excited state reaction. In this brief article | But it is worth noting that even at magic angle in a perfectly aligned spectrometer pure dyes in homogeneous media may exhibit a multiexponential decay. The origin may be physical, like solvent relaxation, or chemical, when the fluorescent molecule undergoes a ground or excited state reaction. In this brief article | ||
- | ===== 1) Ground state reaction with the medium | + | ===== 1) Ground-state reactions |
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- | Note that this situation corresponds to the typical case of Static-Quenching in which the intensity of A decreases with the concentration of X, but where the lifetime | + | Note that this situation corresponds to the typical case of Static-Quenching in which the intensity of A decreases with the concentration of X, but where the lifetime |
===== 2) Excited-state reactions ===== | ===== 2) Excited-state reactions ===== | ||
- | The electronic redistribution of electrons due to optical excitation leads in many cases to a different reactivity in the ground and excited states. In other words, a fluorescent molecule which is boring under the dark may become reactive upon excitation. Common excited state reactions are redox (electron transfer) and acid-base (proton transfer) reactions. Depending on the rate of the excited-state rection relative the the original | + | The electronic redistribution of electrons due to optical excitation leads in many cases to a different reactivity in the ground and excited states. In other words, a fluorescent molecule which is boring under the dark may become reactive upon excitation. Common excited state reactions are redox (electron transfer) and acid-base (proton transfer) reactions. Depending on the rate of the excited-state rection relative the the original |
Let us consider the reaction in Scheme 2 in different situations: | Let us consider the reaction in Scheme 2 in different situations: | ||
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Scheme 2 | Scheme 2 | ||
- | The molecule A is prompted to te excited-state where it can react with a molecule X to form the compound B, through a rate constant kAB. Once the compound B is formed the back-reaction can occur, with a rate constant kBA. Compounds A and B are fluorescent with intrinsic | + | //Starting point: |
- | Case A) The constant kAB is too slow with respect to $\tau_A$ and $\tau_B$. In this case the compound A decays | + | //Case A) The constant kAB is too slow with respect to $\tau_A$ and $\tau_B$.// In this case the compound A would decay to the ground-sate |
- | Case B) The forward reaction constant kAB is fast, but the back-reaction constant kBA is too slow in comparison to $\tau_A$ and $\tau_B$. In this case the decay time measured in the spectral region of A will be single exponential, | + | //Case B) The forward reaction constant kAB is fast, but the back-reaction constant kBA is too slow in comparison to $\tau_A$ and $\tau_B$.// In this case the decay time measured in the spectral region of A would be single exponential, |
Note that this situation is the typical case of dynamic quenching with the particular case that the product being formed is fluorescent. | Note that this situation is the typical case of dynamic quenching with the particular case that the product being formed is fluorescent. | ||
- | Case C) The interconversion rate constants kAB and kBA are in the same order of $\tau_A$ and $\tau_B$. In this case, the decay curves measured for species A and B is biexponential for both, with common lifetimes $\tau_1$ and $\tau_2$. However, $\tau_1$ and $\tau_2$ | + | //Case C) The interconversion rate constants kAB and kBA are in the same order of $\tau_A$ and $\tau_B$.// In this case, the decay curves measured for species A and B would be biexponential for both, with common lifetimes $\tau_1$ and $\tau_2$. However, $\tau_1$ and $\tau_2$ |
A(t) = A1 e^{-t/ | A(t) = A1 e^{-t/ | ||
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being A0 the concentration of A at t=0. | being A0 the concentration of A at t=0. | ||
- | **Note from the equations | + | **Note from A(t) that, even if B would not be fluorescent, |
A situation like this may occur with molecules dissolved in an aprotic but hygroscopic media, like acetonirile. Water molecules may diffuse (diffusion happens in the nanosecond time scale) and react with fluorophores bearing proton-transfer groups like -OH or -NH2. | A situation like this may occur with molecules dissolved in an aprotic but hygroscopic media, like acetonirile. Water molecules may diffuse (diffusion happens in the nanosecond time scale) and react with fluorophores bearing proton-transfer groups like -OH or -NH2. | ||
- | Case D) Interconversion rate constants kAB and kBA are very quick compared to the intrinsic lifetimes $\tau_A and $\tau_B. In this case the equations of case C would still apply. But in practice a quick equilibrium between reactants and product | + | //Case D) Interconversion rate constants kAB and kBA are very quick compared to the intrinsic lifetimes $\tau_A and $\tau_B.// In this case the equations of case C would still apply. But in practice, a quick equilibrium between reactants and product |
- | This situation may happen if compounds A nd B are directly in contact prior to excitation | + | This situation may happen if compounds A and X were directly in contact prior to excitation, for example throgh ground-state interactions. |
===== 3) Solvation dynamics ===== | ===== 3) Solvation dynamics ===== | ||
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Scheme 3. Left: energetic representation of solvation dynamics. The fluorophore is represented as a sphere with a pointing dipole. Solvent molecules are represented in gray around the fluorophore. Right up : Spectral consequence of solvation dynamics. The fluorescence spectrum shifts in time to lower energies. Right bottom: Decay traces measured in different spectral regions. Blue flanks are multiexponential with positive pre-exponentila factors, red flanks are multiexponential with rising components. | Scheme 3. Left: energetic representation of solvation dynamics. The fluorophore is represented as a sphere with a pointing dipole. Solvent molecules are represented in gray around the fluorophore. Right up : Spectral consequence of solvation dynamics. The fluorescence spectrum shifts in time to lower energies. Right bottom: Decay traces measured in different spectral regions. Blue flanks are multiexponential with positive pre-exponentila factors, red flanks are multiexponential with rising components. | ||
- | Before the excitation, the fluorophore is in the ground state S0 , which has a characteristic dipole moment. Solvent molecules, which also have their characteristic dipole moment are oriented in such a way that the interactions dipole-dipole with the fluorophore are as favorable possible. When the fluorophore is prompted to the excited state, its electronic distribution switches almost instantly. At time zero after excitation, the solvent molecules remain in their " | + | Before the excitation, the fluorophore is in the ground state S0 , which has a characteristic dipole moment. Solvent molecules, which also have their characteristic dipole moment are oriented in such a way that the interactions dipole-dipole with the fluorophore are as favorable possible. When the fluorophore is prompted to the excited state, its electronic distribution switches almost instantly. At time zero after excitation the solvent molecules remain in their " |
- | + | ||
- | In fluid media, solvation dynamics can be described with a multiexponential function spanning from the fs time scale to tens of picoseconds. Hence, the tail of this process can be monitored with a TCSPC spectrometer equipped with fast detectors such as a MCP or a Hybrid-PMT. In viscous media or at low temperatures, | + | |
+ | In fluid media, solvation dynamics can be described with a multiexponential function spanning from the femtosecond time-scale to tens of picoseconds. Hence, the tail of this process can be monitored with a TCSPC spectrometer equipped with fast detectors such as a MCP or a Hybrid-PMT. In viscous media or at low temperatures, |
some_origins_of_multiexponetial_decays_for_single_dyes.txt · Last modified: 2019/03/19 12:31 by oschulz