Reversible Excited-State Proton Geminate Recombination: Revisited

Area of Science:

• Photochemistry
• Physical Chemistry
• Chemical Kinetics

Background:

• Pines and Huppert discovered excited-state proton transfer and reversible geminate recombination in photoacids.
• The Debye-Smoluchowski equation modeled this reaction, fitting time-resolved fluorescence data for 8-hydroxy-1,3,6-pyrene trisulfonate (HPTS).
• A t^-3/2 power law decay in fluorescence was predicted for the long-time tail due to reversible proton recombination.

Purpose of the Study:

• To reaffirm the power-law behavior of HPTS fluorescence decay.
• To investigate the influence of molecular-level complexities on proton recombination kinetics.
• To theoretically analyze rebinding kinetics and their approach to asymptotic behavior.

Main Methods:

• Analysis of time-resolved fluorescence data for HPTS.
• Theoretical modeling using the Debye-Smoluchowski equation with appropriate boundary conditions.
• Investigation of molecular factors including symmetry, rotational dynamics, and proton binding.
• Analysis of rebinding kinetics via the number of extrema in the logarithmic derivative.

Main Results:

• The study reaffirms the power-law decay of HPTS fluorescence.
• Molecular complexities were shown to affect kinetics only at intermediate times, not asymptotically.
• The asymptotic behavior of the fluorescence tail decay was confirmed.
• Subtle effects on the direction of approach to the asymptotic line were observed and experimentally corroborated.

Conclusions:

• The long-time fluorescence tail decay of HPTS is robust and not significantly altered by molecular-level complexities.
• Theoretical analysis provides insights into the subtle deviations from idealized diffusion models.
• The findings support the established understanding of reversible geminate recombination in photoacid systems.