Interaction between Polarized Triplets and Stable Radicals in Liquid Solutions

Blank, Aharon; Levanon, Haim

doi:10.1021/jp010033g

Cited by 39 publications

(58 citation statements)

References 22 publications

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“…The results presented above are in qualitative agreement with previous theoretical results [9,19,20,[24][25][26]29]. For the quartet-bom RTPs, the radical spin polarization was found in refs.…”

Section: Perturbation Theory Calculationssupporting

confidence: 92%

“…Here ]S) describes the singlet excited state of the molecule. Note that the problem was considered in the framework of this extended basis by several authors (see, e.g., [19,21]). …”

Section: Radicai-triplet Pair Mechanism Of Cidepmentioning

confidence: 99%

“…Due to the hfi a coherence terrn (see Eqs. (16), (17), (19)) can contribute to the spin polarization.…”

Section: M(t) = F Mr(t')exp(-(t --T')/r)dt'mentioning

confidence: 99%

“…11. Systems containing stable free radicals and triplets produced by light were also studied experimentally and theoretically in recent years (see, e.g., [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]). It was demonstrated that in these systems additional mechanisms contribute to the radical spin polarization.…”

Section: Introductionmentioning

confidence: 99%

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Time-resolved EPR study of electron spin polarization and spin exchange in mixed solutions of porphyrin stable free radicals

et al. 2003

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The time-resolved electron paramagnetic resonance (EPR) spectra are studied in the temperature range of 110-300 K for two mixed solutions of porphyrins, ZnTPP and H2TPP, in toluene and the stable free radical 2,2,6,6-tetramethyl-l-piperidinyloxyl (TEMPO). The EPR spectra and their kinetic behavior were studied for concentrations of TEMPO vafied in the interval from 0.51 to 7.68 mM, while the porphyrin concentration was fixed as I mM. The EPR spectra of triplet-state porphyrins and free radicals manifest the chemically induced spin polarization. For the relatively short-lived radical-triplet pairs, with the perturbation theory up to the fourth order, the theoretical expressions ate obtained for the triplet and radical spin polarization induced by the enhanced intersystem crossing (ISC) due to the interaction of excited singlet-state porphyrins with free radicals and by the triplet quenching by free radicals. The time-dependent EPR spectra of the t¡ are simulated taking into account the spin-lattice relaxation. Ir is shown that the variation of the triplet EPR spectra shape, when the time of observation increases, arises from the spin-lattiee relaxation kineties. The kinetic behavior of the TEMPO EPR spectrum was simulated on the basis of the kinetie scheme suggested earlier in the literature. The triplet spin-lattice relaxation time, the tate of the ISC and the lifetime of the excited singlet state were estimated by fitting the kinetic curves for the triplet EPR spectra intensity. For the mixed porphyrin-TEMPO solutions, a possible set of the rate constants of important bimolecular proeesses were determined. For this set of parameters, it turns out that the spin polarization transfer has a smaller rate constant than the rate constant of the diffusion collisions of the triplet and radical. It appears that the rate constant of the ISC catalyzed by radieals is relatively high in the solutions close to the melting point of the solvent and in the soft-glassy state. In the triplet porphyrins the initial spin pola¡ induced by the spin-selective ISC was found to exceed the equilibrium spin polarization by up to two orders of magnitude.

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Section: Perturbation Theory Calculationssupporting

confidence: 92%

“…Here ]S) describes the singlet excited state of the molecule. Note that the problem was considered in the framework of this extended basis by several authors (see, e.g., [19,21]). …”

Section: Radicai-triplet Pair Mechanism Of Cidepmentioning

confidence: 99%

“…Due to the hfi a coherence terrn (see Eqs. (16), (17), (19)) can contribute to the spin polarization.…”

Section: M(t) = F Mr(t')exp(-(t --T')/r)dt'mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Time-resolved EPR study of electron spin polarization and spin exchange in mixed solutions of porphyrin stable free radicals

et al. 2003

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“…In the last decade the spin polarization of triplets was studied intensively [2,4,[11][12][13][14][15][16][17][18][19][20]. The experimental data are interpreted taking into account the polarization due to the spin-selective intramolecular intersystem crossing, the spinselective triplet quenching by free radicals and the spin-selective intramolecular intersystem crossing enhanced by free radicals.…”

Section: Discussion and Summarymentioning

confidence: 99%

Mechanism of the electron spin polarization of excited triplet states caused by the mutual annihilation of triplet states

Salikhov

2004

Appl. Magn. Reson.

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A new mechanism of the triplet spin polarization is considered qualitatively and quantitatively. The spin polarization arises due to the spin-selective triplet-triplet annihilation and the subsequent spin dynamics in the spin-correlated pairs of t¡ With the time-dependent permrbation theory the simple rules are found which specify the polarization pattern of triplets. lntroductionNowadays the nonequilibrium electron spin pola¡ is intensively studied. These investigations are motivated by several reasons. Indeed, the spin polarization increases the sensitivity of the electron paramagnetic resonance (EPR) detection of paramagnetic particles. Of particular interest is that the nonequilibrium spin polarization induced in the course of photochemical and photophysical processes allows one to investigate intermediate short-lived paramagnetic species [1,2]. The electron spin polarization pattern reflects details of the photochemical and photophysical processes responsible for the polarization. Therefore, spectroscopic manifestations of the spin polarization provide powerful methods to study elementary photoinduced processes. The polarized electron spins are of p¡ interest due to their applicati0ns in spintronics, etc.Various mechanisms of electron spin polarization have been studied. One of the most interesting among them is the so-called chemically induced dynamic electron polarization (CIDEP) effect [1,2]. The CIDEP formation proceeds in two stages. The first stage is a spin-selective molecular process. This might be, e.g., the spin-selective recombination of free radicals (the radical pair mechanism [RPM] of the CIDEP formation [3]) or the spin-selective quenching of triplet states by free radicals (the radical-triplet pair mechanism [RTPM] of the CIDEP formation [4]). In this stage the spin-correlated pairs of two paramagnetic spe-

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Radical‐Enhanced Intersystem Crossing in a Bay‐Substituted Perylene Bisimide−TEMPO Dyad and the Electron Spin Polarization Dynamics upon Photoexcitation**

et al. 2020

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A 4‐amino‐2,2,6,6‐tetramethyl‐1‐piperidinyloxyl (TEMPO) radical was attached to the bay position of perylene‐3,4 : 9,10‐bis(dicarboximide) (perylenebisimide, PBI) to study the radical‐enhanced intersystem crossing (REISC) and electron spin dynamics of the photo‐induced high‐spin states. The dyads give strong visible light absorption (ϵ=27000 M−1 cm−1at 607 nm). Attaching a TEMPO radical to the PBI unit transforms the otherwise non‐radiative decay of S1 state (fluorescence quantum yield: ΦF=2.9 %) of PBI unit to ISC (singlet oxygen quantum yield: ΦΔ=31.8 %, ΦF=1.6 %). Moreover, the REISC is more efficient as compared to the heavy atom effect‐induced ISC (ΦΔ=17.8 % for 1,8‐dibromoPBI). For the dyad, ISC takes 245 ps and triplet state lifetime is 1.5 μs, much shorter than the native PBI (τT=126.6 μs). X‐ and Q‐band time‐resolved electron paramagnetic resonance spectroscopy shows that the exchange interaction in the photoexcited radical‐chromophore dyad is larger than the triplet zero‐field splitting (ZFS) and the difference of Zeeman energies of the radical and chromophore. The inversion of electron spin polarization from emissive to absorptive was observed and attributed to the initial completion of the quartet state population and the subsequent depopulation processes induced by the zero‐field splitting.

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Interaction between Polarized Triplets and Stable Radicals in Liquid Solutions

Cited by 39 publications

References 22 publications

Time-resolved EPR study of electron spin polarization and spin exchange in mixed solutions of porphyrin stable free radicals

Time-resolved EPR study of electron spin polarization and spin exchange in mixed solutions of porphyrin stable free radicals

Mechanism of the electron spin polarization of excited triplet states caused by the mutual annihilation of triplet states

Radical‐Enhanced Intersystem Crossing in a Bay‐Substituted Perylene Bisimide−TEMPO Dyad and the Electron Spin Polarization Dynamics upon Photoexcitation**

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