Study of Spin-Correlated Radical Ion Pairs in Irradiated Solutions by Optically Detected EPR and Related Techniques

Probing an Isolated Conjugated Polymer Molecule with Radiation-Generated Spin-Correlated Polaron Pairs

“…There is the electron spin correlation between the PP’s partners which originates from the fact that the spins of electrons composing the closed electronic shell of the ionized matrix molecules are strongly entangled due to Pauli’s principle. The initial spin entanglement is kept in the created PPs and the spin state of the latter varies with time between the singlet and triplet states. − Such singlet–triplet transitions occur mainly due to Zeeman and hyperfine spin interactions which are different for the positive and negative polarons located at different fragments of the polymer chain. The recombination of PPs, therefore, results in either triplet or singlet electronically excited states of the conjugated polymer molecule that depends in part on the external magnetic field.…”

“…Thus, the fluorescence intensity decay reflects the recombination rate of polarons composing PP in conjugated polymers that may allow, in particular, to estimate the rate of polaron migration along the polymer chain (see Supporting Information, section S1). Furthermore, it is possible to gain information on spin interactions in the recombining spin-correlated polarons by observing the spin evolution of the PP with temporal resolution. , However, such evolution, in the conditions of a radiation experiment, changes the recombination fluorescence intensity within a factor of about 1.5 or less , while, as illustrated in Scheme , the intensity is reduced by the orders of magnitude for several tens of nanoseconds. That is why the spin correlation effects can be much easier visualized using a ratio of fluorescence decays recorded in different magnetic fields.…”

“…In this study, the focus was made on the time-resolved magnetic field effect (TR MFE) in the radiation-induced fluorescence. , This is determined as the I B ( t )/ I 0 ( t ) ratio, where I B ( t ) and I 0 ( t ) denote the delayed fluorescence intensity decay recorded at a comparatively strong and nearly zero external magnetic fields, respectively. Some theoretical backgrounds of the method are considered in Supporting Information.…”

“…The samples under study were placed between the poles of electromagnet. Fluorescence of the samples was excited by X-ray pulses with a photon energy of approximately 20 keV and detected by time-correlated single photon counting using setup described elsewhere. ,, The shape of the kinetic curves of the fluorescence from PVC doped with p -TP and other fluorescent compounds is briefly discussed in the Supporting Information (Section S1).…”

“…The calculations were performed assuming that the ratio obeys where ρ SS ( t ) is the singlet spin state population of the PP ensemble and the indices B and 0 denote strong and zero magnetic fields, respectively. The parameter θ characterizes the effective fraction , of singlet-born PPs among the pairs that recombine at a given moment. This approximation is valid if the rate constants for the recombination of singlet and triplet PP are the same.…”