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some_origins_of_multiexponetial_decays_for_single_dyes [2019/03/06 12:43] – adminsome_origins_of_multiexponetial_decays_for_single_dyes [2019/03/06 12:44] – admin
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 //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 before the excited-state reaction could take place. The decay measured would be 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 would decay to the ground-sate before the excited-state reaction could take place. The decay measured would be single exponential and coincident with $\tau_A$.
  
-//Case B) The forward reaction constant $k_{AB}$ is fast, but the back-reaction constant $k_{BA}$ 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, with decay time $\tau_1$. However $\tau_1$ would be shorter than $\tau_A$, and it would be dependent on the concentration of X $(\tau_1 = 1/ (kr_A+knr_A + k_{AB}[x]), where [x] denotes the concentration of X and $kr$ and $knr$ the intrinsic radiative and non-radiative rate constants of A, respectively) . The lifetime measured in the spectral region of B would be biexponential with times $\tau_1$ and $\tau_2$. $\tau_1$ would have a negative pre-exponential factor (rising component) and it would be coincident with the decay time measured in the spectral region of A. The decaying component $\tau_2$ would be coincident with the original lifetime of compound B, $\tau_B$.+//Case B) The forward reaction constant $k_{AB}$ is fast, but the back-reaction constant $k_{BA}$ 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, with decay time $\tau_1$. However $\tau_1$ would be shorter than $\tau_A$, and it would be dependent on the concentration of X $(\tau_1 = 1/ (kr_A+knr_A + k_{AB}[x])$, where [x] denotes the concentration of X and $kr$ and $knr$ the intrinsic radiative and non-radiative rate constants of A, respectively) . The lifetime measured in the spectral region of B would be biexponential with times $\tau_1$ and $\tau_2$. $\tau_1$ would have a negative pre-exponential factor (rising component) and it would be coincident with the decay time measured in the spectral region of A. The decaying component $\tau_2$ would be 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.
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 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 would be established. This means that the concentrations of A and B with respect to each other would always be constant prior to their decay, and hence the whole system could be treated as a single dye. The decay would be single exponential, being an average of $\tau_A$ and $\tau_B$ weighted by their fraction in the equilibrium.+//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 would be established. This means that the concentrations of A and B with respect to each other would always be constant prior to their decay, and hence the whole system could be treated as a single dye. The decay would be single exponential, being an average of $\tau_A$ and $\tau_B$ weighted by their fraction in the equilibrium.
  
 This situation may happen if compounds A and X were directly in contact prior to excitation, for example throgh ground-state interactions. This situation may happen if compounds A and X were directly in contact prior to excitation, for example throgh ground-state interactions.
some_origins_of_multiexponetial_decays_for_single_dyes.txt · Last modified: 2019/03/19 12:31 by oschulz