Time-Resolved Fluorescence Wiki

User Tools

Site Tools

Trace:
some_origins_of_multiexponetial_decays_for_single_dyes

Differences

This shows you the differences between two versions of the page.

Link to this comparison view

Both sides previous revisionPrevious revision
Next revisionBoth sides next revision
some_origins_of_multiexponetial_decays_for_single_dyes [2014/05/14 16:09] – veigasome_origins_of_multiexponetial_decays_for_single_dyes [2014/05/19 16:00] – veiga
Line 15: Line 15:
 The fluorophore (A) may be reacting with the medium or with diffusing molecules dissolved in it (X) to form a product (B). Typical cases are dissociation reactions. These reactions can be followed by Absorption spectroscopy, where the different bands can be identified and assigned. Excitation in regions where the two species (A and B) absorb may lead to biexponential decays if the fluorescence of both compounds is observed at the same wavelength.  The fluorophore (A) may be reacting with the medium or with diffusing molecules dissolved in it (X) to form a product (B). Typical cases are dissociation reactions. These reactions can be followed by Absorption spectroscopy, where the different bands can be identified and assigned. Excitation in regions where the two species (A and B) absorb may lead to biexponential decays if the fluorescence of both compounds is observed at the same wavelength. 
  
-{{:scheme1.jpg?400|}}+{{:scheme1.png?400|}}
  
 Scheme 1 Scheme 1
Line 25: Line 25:
  
  
-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 intrinsic fluorescent lifetime, the observed decay time measured with a TCSPC spectrometer may be single- or multiexponential. +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 fluorescent lifetime, the observed decay time measured with a TCSPC spectrometer may be single- or multiexponential. 
  
 Let us consider the reaction in Scheme 2 in different situations: Let us consider the reaction in Scheme 2 in different situations:
  
  
-{{:scheme2.jpg?400|}}+{{:scheme2.png?400|}}
  
 Scheme 2 Scheme 2
Line 38: Line 38:
 Case A) The constant kAB is too slow with respect to $\tau_A$ and $\tau_B$. In this case the compound A decays before the excited-state reaction can take place. The decay measured is single exponential and coincident with $\tau_A$. Case A) The constant kAB is too slow with respect to $\tau_A$ and $\tau_B$. In this case the compound A decays before the excited-state reaction can take place. The decay measured is single exponential and coincident with $\tau_A$.
  
-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, with decay time $\tau_1$. However $\tau_1$ is shorter than $\tau_A$, and it depends on the concentration of X ($\tau_1$ = 1/ (krA+knrA + kAB[x]), where [x] denotes the concentration of X and kr and knr the intrinsic radiative and non-radiative rate constants) . The lifetime measured in the spectral region of B is biexponential with times $\tau_1$ and $\tau_2$. $\tau_1$ has a negative pre-exponential factor (rising component) and it is coincident with the decay time measured in the spectral region of A. The decaying component $\tau_2$ is coincident with the intrinsic lifetime of compound B, $\tau_B$.+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, with decay time $\tau_1$. However $\tau_1$ is shorter than $\tau_A$, and it depends on the concentration of X ($\tau_1$ = 1/ (krA+knrA + kAB[x]), where [x] denotes the concentration of X and kr and knr the intrinsic radiative and non-radiative rate constants) . The lifetime measured in the spectral region of B is biexponential with times $\tau_1$ and $\tau_2$. $\tau_1$ has a negative pre-exponential factor (rising component) and it is coincident with the decay time measured in the spectral region of A. The decaying component $\tau_2$ is coincident with the original lifetime of compound B, $\tau_B$.
  
 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.
some_origins_of_multiexponetial_decays_for_single_dyes.txt · Last modified: 2019/03/19 12:31 by oschulz