Exchange Interaction in Radical−Triplet Pairs:  Evidences for CIDEP Generation by Level Crossings in Triplet−Doublet Interactions

Abstract

Chemically induced dynamic electron polarization (CIDEP) generated through interaction of the excited triplet state of 1-chloronaphthalene, benzophenone, benzil, and Buckminsterfullerene (C60) with 2,2,6,6,-tetramethyl-1-piperidinyloxyl (TEMPO) radical was investigated by using time-resolved ESR spectroscopy. We carefully examined what factors affect the CIDEP intensities. By comparing CIDEP intensities of TEMPO in the 1-chloronaphthalene, benzophenone, and benzil systems with that obtained in the C60−TEMPO system, the absolute magnitude of net emissive polarization was determined to be −2.2, −6.9, and −8.0, respectively, in the units of Boltzmann polarization. In the 1-chloronaphthalene−TEMPO system, the viscosity effect on the magnitude of net polarization was studied by changing the temperature (226−275 K) in 2-propanol. The emissive polarization was concluded to result from the state mixing between quartet and doublet manifolds in a radical−triplet pair induced by the zero-field splitting interaction of the counter triplet molecule. The magnitude of net polarization is much larger than the polarization calculated with the reported theory that the CIDEP is predominantly generated in the region where the exchange interaction is smaller than the Zeeman energy. Our experimental results are quantitatively explained by the theory that the CIDEP is generated predominantly in the regions where the quartet and doublet levels cross. We propose a theoretical treatment to calculate the magnitude of net polarization generated by the level crossings in the radical−triplet pair mechanism under highly viscous conditions and perform a numerical analysis of the net RTPM polarization with the stochastic-Liouville equation. The viscosity dependence of the net polarization indicates that the back transition from the doublet to quartet states sufficiently occurs in the level-crossing region under highly viscous conditions. The estimated large exchange interaction suggests that the quenching of the excited triplet molecules by TEMPO proceeds via the electron exchange interaction.

Excited-state Dynamics and Spin-exchange Coupling of Anthracene–Verdazyl Radical in Frozen Glass Matrix Investigated by Transient Absorption Spectroscopy

“…Actually, the magnitude of the exchange coupling between the photoexcited triplet state and radical spin in the weak exchangecoupled spin systems were determined using time-resolved ESR (TRESR) spectroscopy. 6 When the target system is emissive, the magnitude of the stronger spin-exchange coupling in the photoexcited-tripletdoublet coupled spin systems can be also determined by the temperature dependence of the emission intensity as demonstrated for a fluorescent metal-complex radical hybrid; 7 the lifetime of the spin polarization in this hybrid was also temperature-dependent, as obtained for the emission lifetime. In our previous papers, 8 we reported the first observation of a quartet (S = 3/2) photoexcited state and a quintet (S = 2) photoexcited state in organic π-radicals that were generated by a robust spin alignment through π-conjugation between stable iminonitroxide radicals (S = 1/2) and the triplet (S = 1) excited state of a phenyl-or diphenylanthracene derivative, which are ideal model systems in order to study the relationship between the π-topology and photoinduced spin alignment.…”

Time Resolved CW-EPR Spectroscopy of Powdered Samples: Electron Spin Polarization of a Nitroxyl Radical Adsorbed on NaY Zeolite, Generated by the Quenching of Excited Triplet Ketones

“…The spin polarization lifetimes of both nitroxides 2-14 N and 3 are the same within experimental error (3.9 and 3.8 µs, respectively), and are in agreement with reported values of TEMPO spin polarization lifetimes in solution. 1,29 To study the benzophenone triplet quenching kinetics by nitroxides inside the zeolites in more detail, diffuse reflectance laser flash photolysis was performed. Laser flash photolysis (308 nm, 10 ns) of 1 loaded on NaY affords a readily detectable transient absorption at 525 nm corresponding to the triplet state of 1 (Figure 5a).…”

Triplet porphyrins as donors in intramolecular electron transfer and their intermolecular interaction with free radicals

“…Following the widespread use of lasers in EPR experiments, there have been some new observations of CIDEP of stable radicals in solution in the presence of photoexcited triplet chromophores. This polarization could not be explained by the above mechanisms and was treated theoretically [29][30][31][32][33][34] by the RTPM and ESPT, which are the main themes of this part of the paper.…”

“…Current theories provide analytical [29,[35][36][37] and numerical [34] results for the dependence of the spin polarization upon the zero-field splitting (ZFS) parameter (D), solvent viscosity (Z) and radical-triplet electron spin exchange (J). According to RTPM theory, the observed net polarization, which is equal for all hyperfine lines, depends on the precursor state (photoexcited singlet or triplet) and sign of J [38][39][40][41].…”