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

Abstract

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.

Efficient Radical‐Enhanced Intersystem Crossing in an NDI‐TEMPO Dyad: Photophysics, Electron Spin Polarization, and Application in Photodynamic Therapy

“…[37][38][39][40][41][42] Concerning this aspect, stable nitroxide free radicals,f or example, 2,2,6,6-tetramethylpiperidinyloxy (TEMPO)a nd nitronyl nitroxide, have been used to connect to organic chromophores. [38,40,[43][44][45][46][47][48] In the case of strong spin-spin exchange interactions,t he eigenstates of such systemsi nclude doublet states (D 0 state, namely 2 [S 0 , R]; D 2 state, namely 2 [S 1 , R]; D 1 state, namely 2 [T 1 , R]) and quartet state (Q state, namely 4 [T 1 , R]). That is to say, the electronic state of the entire molecule could be a doublet or aq uartets tate, and the chromophorec ould be in its singlet or triplet state.…”

“…On the other hand, controlling the spin dynamics in organic molecules is pivotal for energy and charge transport . Concerning this aspect, stable nitroxide free radicals, for example, 2,2,6,6‐tetramethylpiperidinyloxy (TEMPO) and nitronyl nitroxide, have been used to connect to organic chromophores . In the case of strong spin–spin exchange interactions, the eigenstates of such systems include doublet states (D 0 state, namely 2 [S 0 , R ]; D 2 state, namely 2 [S 1 , R ]; D 1 state, namely 2 [T 1 , R ]) and quartet state (Q state, namely 4 [T 1 , R ]).…”

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

“…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.…”

“…Note that according to Eqs. (11) and (12) in the second order, the spin polarization pattern of triplets does not depend on the orientation of the external magnetic field. However, the fourth-order contribution depends on the orientation of the external magnetic field In irregular systems like glasses the principal axes of the zfs tensors of the two triplets in the pair do not coincide.…”

“…(14) and (15) with Eqs. (11) shows that the average values of Qzz(A) and Qxx(A) -Qrr(A) for triplets in glasses are about two times less than those for t¡ in single crystals.…”