Formation and decay of pyrene radical cation and pyrene dimer radical cation in the absence and presence of cyclodextrins during resonant two-photon ionization of pyrene and sodium 1-pyrene sulfonate

Abstract

Formation and decay of pyrene radical cation and pyrene dimer radical cation during resonant two-photon ionizations (TPI) of pyrene (Py) and sodium 1-pyrene sulfonate (NaPySA) in the absence and presence of cyclodextrins were studied with visible and near IR transient absorption measurement using 355-nm laser flash photolysis technique. Ionization of Py and 1-pyrene sulfonate anion (PySA⁻) occurred within the laser flash (5 ns) to give Py radical cation (Py˙⁺) and PySA⁻ radical cation (Py˙⁺SA⁻) having Py˙⁺ chromophore. Dimerizations of Py˙⁺ and Py and of Py˙⁺SA⁻ and PySA⁻ were observed to give the dimer radical cations (Py₂˙⁺ and Py₂˙⁺(SA⁻)₂) based on time-resolved measurements of the charge resonance (CR) band. The TPI of PySA⁻ was also examined in the presence of β- and γ-cyclodextrins (βCD and γCD, respectively). Py₂˙⁺(SA⁻)₂ in the cavity of two γCDs having fully overlapped structure showed the CR band around 1700 nm, while Py₂˙⁺(SA⁻)₂ in the cavity of two βCD having partially and fully overlapped structures indicated the CR bands around 1500 or 1700 nm, respectively. On the basis of formation and decay rates of Py₂˙⁺(SA⁻)₂, bimolecular formation of Py₂˙⁺(SA⁻)₂ and the neutralization are inhibited in the cavities of CDs. Selective formation of the fully overlapped structure of Py₂˙⁺(SA⁻)₂ in the cavity of two γCDs within a laser flash is explained by direct TPI of two PySA⁻s with the parallel structure of two Py chromophores in two PySA⁻s in the cavity of two γCDs.