Analysis of time-resolved EPR spectra observed in the photolysis of disulphonated anthraquinones included in cyclodextrins – evidence of the S-T_ mixing of radical pair mechanism

ABSTRACT

Cyclodextrins (CDs) are well known hosts of inclusion compounds, and they can be used for molecular identification and separation. Several studies have already been reported from the point of view of photchemistry and spin-chemistry. These reports showed that the inside environment of CDs such as their bore-size works for their specific properties. In this report, the time-resolved EPR spectra observed in the photolysis of aqueous solution of disulphonated anthraquinones included in β-CD and γ-CD are carefully analysed. The spectral simulation employing Radical Pair Mechanism (RPM) of Chemically Induced Dynamic Electron Polarisation is carried out. Finally, it is concluded that the peculiar hyperfine-dependence observed in these particular systems is exclusively caused by the $\text{S - }{\text{T}}_{-}$ mixing RPM, despite the radical pairs (RPs) having small hyperfine coupling (HFC) constants (less than 3.4 mT). Another finding is the quenching of the spin polarisation of the RP conveyed from the excited triplet state of anthraquinones before $\text{S - }{\text{T}}_{-}$ mixing. This is explained as arising from slow relative diffusional motion of the RP members when included in the interior of the CDs. General expression of the $\text{S - }{\text{T}}_{-}$ and S-T₊ mixing of multiple nuclei (I = 1/2) with different conditions such as the spin multiplicity of the precursor molecule and the sign of the HFC constants, is also suggested.

GRAPHICAL ABSTRACT

KEYWORDS:

• Cyclodextrins
• time-resolved EPR
• radical pair
• $\text{S - }{\text{T}}_{-}$ mixing
• sulphonated anthraquinone

Acknowledgements

We thank Ms. Moe Sasaki, Ms. Rina Sato, and Mr. Akira Naruoka, former students of our group in Shizuoka University, for their enthusiastic help.

Disclosure statement

No potential conflict of interest was reported by the authors.