Initial-state preparation effects in time-resolved electron paramagnetic resonance experiments.

Area of Science:

• Chemical Physics
• Spectroscopy
• Organic Chemistry

Background:

• Time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy is a powerful tool for studying radical pair mechanisms in organic molecules.
• Optical excitation is commonly used to generate radical pairs, but the relationship between excitation wavelength and TR-EPR signal intensity is not always straightforward.

Purpose of the Study:

• To explain the experimental observation of similar TR-EPR spectra intensities for optical excitation in both highly absorbing and nonabsorbing regions of an organic molecule.
• To elucidate the underlying photophysical mechanisms responsible for this phenomenon.

Main Methods:

• Theoretical analysis of photoexcitation processes and intersystem crossing (ISC) rates.
• Modeling of initial triplet state populations following optical excitation.
• Comparison of simulated TR-EPR spectra with experimental data.

Main Results:

• The study demonstrates that photoexcitation in different spectral regions can lead to similar initial populations of triplet states.
• Low intersystem crossing rates in organic molecules prevent significant perturbation of these initial triplet populations on experimental timescales.
• The relative intensities of TR-EPR spectra are primarily determined by these initial triplet state populations.

Conclusions:

• The observed phenomenon is attributed to an initial-state preparation effect, where the excitation wavelength influences the initial distribution of radical pair spin states.
• This effect is significant in systems with long intersystem crossing times compared to experimental timescales.
• The findings highlight the importance of considering initial-state preparation in the interpretation of TR-EPR experiments, especially for organic molecules with weak spin-orbit coupling.