Electron spin polarization in an excited triplet-radical pair system: Generation and decay of the state

Abstract

A computational model to simulate electron spin polarization in the three-spin-1/2 system composed of the molecular excited triplet state of (tetraphenylporphinato)zinc(II) (ZnTPP) and the doublet ground state of the 3-(N-nitronyl-notroxide) pyridine (3-NOPy) stable radical is proposed. The model is based on numerical solutions of the stochastic Liouville equation for the diffusively rotating system where the magnetic dipolar, isotropie Heisenberg exchange, and anisotropic Zeeman electron spin interactions are taken into account in a full measure, whereas the intersystem crossing processes between the singlet and triplet states of ZnTPP are considered in terms of kinetic equations for the relevant spin density matrices. Additional longitudinal and transversal paramagnetic relaxation caused by relative rotation motions of the ZnTPP and 3-NOPy moieties is taken into consideration in the form of the generalized relaxation operator.

W-band transient EPR and photoinduced absorption on spin-labeled fullerene derivatives

“…Note that the exchange coupling is big enough for strong coupling, as shown by the g-values, but it is still small compared to other similar systems like phthalocyaninatosilicon covalently linked to a TEMPO radical (J 0 4 100 mT) 27 or the coordination of a (nitronyl-nitroxide) pyridine to Zn-tetraphenyl porphyrin (J 0 4 1000 mT). 58,71 This means that in our case the doublet and the quartet levels are nearly degenerate, very different from the other systems where the trip-quartet and the trip-doublet levels are well separated from each other. A subtle interplay between the energy differences and the amount of mixing will determine the spin polarization.…”

“…A rigorous treatment of the spin dynamics in terms of the stochastic Liouville equation for similar systems can be found in ref. 71. Here, we have analyzed the time dependence of the signals in a phenomenological way by exponential fitting of the time dependent data in order to extract the characteristic rates of the underlying processes.…”

Exploring Photogenerated Molecular Quartet States as Spin Qubits and Qudits

“…In this study, we investigate a photogenerated molecular quartet system by pulse electron paramagnetic resonance (EPR) techniques and show that it has the right properties to form a candidate qubit system. The quartet state is formed by the interaction of a stable radical and a photogenerated chromophore triplet state according to the scheme shown in Figure . ,− In this article, the use of the term “quartet state” will refer exclusively to the photogenerated electronic spin state. Further splitting of each of the sublevels by nuclear spins will be described explicitly as needed.…”

Photogenerated Quartet State Formation in a Compact Ring-Fused Perylene-Nitroxide

“…In general, stable free radicals are well-known to quench singlet excited states in a wide variety of noncovalent and flexibly linked covalent systems largely through spin exchange driven enhanced intersystem crossing (EISC); − however, there have been far fewer studies of the structural and electronic basis of such quenching in rigid systems, and these studies have often been limited to examinations of fluorescence quantum yields and lifetimes. ,, In contrast, there have been several reports of triplet excited state molecules bound to stable radicals by either covalent bonds or metal–ligand coordination, ,,,,,− ,− two of which focused on a perylene-3,4:9,10-bis­(dicarboximide) (PDI) chromophore covalently attached to a nitroxide radical. , An important aspect of these PDI-radical systems relates to PDI itselfit is a robust chromophore that is easily tailorable, particularly at the imide positions. − In previous work on these rigid, covalent PDI-radical systems, it was shown that the lowest excited singlet state ( 1* PDI) is rapidly quenched by EISC in both the weak and strong coupling regimes. − …”