In this review, we present an overview of the application of time-resolved electron paramagnetic resonance (TREPR) to the study of excited triplet states. After a brief discussion of background and experimental methods, triplet properties clarified by TREPR are reviewed to show how TREPR provides rich information about electronic and molecular structures and dynamic properties of the lowest excited triplet states. The review includes discussion of the properties of non-phosphorescent triplet states, various interactions (configurational, vibronic and spin–orbit) and triplet properties, molecular distortions, and triplet states related to excited-state proton transfer. For each topic, typical examples are taken mainly from work carried out in the authors’ laboratories in Kyoto and Sendai over the last two decades. Finally, recent new advances are reviewed briefly by focusing attention on two topics: excited triplet states in liquid solution and multiplet excited states generated by triplet–radical interactions.

Short-lived excited triplet states studied by time-resolved EPR spectroscopy

New trends of electron paramagnetic resonance (EPR) are reviewed in studies of the electronic excited states conducted during this decade. The studies include EPR of excited triplet states in fluid...

Recent Developments in Studies of Electronic Excited States by Means of Electron Paramagnetic Resonance Spectroscopy

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-1-piperidinyloxyl (TEMPO). The EPR spectra and their kinetic behavior were studied for concentrations of TEMPO varied in the interval from 0.51 to 7.68 mM, while the porphyrin concentration was fixed as 1 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 are 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 triplets are simulated taking into account the spin-lattice relaxation. It is shown that the variation of the triplet EPR spectra shape, when the time of observation increases, arises from the spin-lattice relaxation kinetics. The kinetic behavior of the TEMPO EPR spectrum was simulated on the basis of the kinetic scheme suggested earlier in the literature. The triplet spin-lattice relaxation time, the rate 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 processes 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 radicals 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 polarization induced by the spin-selective ISC was found to exceed the equilibrium spin polarization by up to two orders of magnitude.

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

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, isotropic 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.

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

Our recent developments in highly sensitive high-frequency electron spin resonance (ESR) using a microcantilever are reviewed. ESR signals of a Co Tutton salt microcrystal (<1 μg) have been detected at low temperature at frequencies up to 315 GHz under a static magnetic field using a microcantilever and a modulation technique. The achieved sensitivity is about 109 spins/G at 4.5 K. Moreover, we have shown that similar ESR detection using a microcantilever is possible up to 130 GHz under a pulsed magnetic field without using a modulation technique. The achieved sensitivity is about 1011 spins/G at 1.7 K. These results suggest that the ESR detection using a microcantilever is promising for applications to high-resolution and high-sensitivity terahertz ESR.

Recent Advances in High-Frequency Electron Spin Resonance Detection Using a Microcantilever

The electron paramagnetic resonance (EPR) signals of photoexcited quartet (Q1) states for zinc(II) tetra-tert-butyl-phthalocyanine (ZnPc) ligated by 3- and 4-(N-nitronyl-nitroxide) pyridine radicals (3-NOPy, 4-NOPy) were observed in toluene solution at room temperature by means of X-band (9.4 GHz) time-resolved EPR (TREPR) spectroscopy. Theg values of Q1 in the ZnPc-3-NOPy and ZnPc-4-NOPy complexes were found to beg=2.0025 andg=2.0036, respectively. The obtainedg value (2.0036) for ZnPc-4-NOPy is in good agreement with the value (g=2.0037) of the Q1 state calculated under the strong-exchange limit. Theg value (2.0025) is just an average of the Q1 and D1 (g=2.0013) states for ZnPc-3-NOPy. Theg value of Q1 for zinc(II) meso-tetraphenylporphine (ZnTPP) ligated by 3-NOPy showed a slight shift (g=2.0027) at X-band and no shift (g=2.0031) at W-band from the calculatedg value (g=2.0031) (J. Fujisawa, Y. Iwasaki, Y. Ohba, S. Yamauchi, K. Koga, S. Karasawa, M. Fuhs, K. Mobius, S. Weber, Appl. Magn. Reson. 21, 483–493, 2001). These changes in theg value were found to originate from an averaging of the TREPR spectra over the Q1 and photoexcited doublet (D1) states via a fast intersystem crossing (ISC) process. The ISC rates between these two states were estimated by means of numerical calculations with the modified Bloch equations as 1.2·108 and 6·107 s−1 for the ZnTPP-3-NOPy complex at the X- and W-bands, respectively. The lower limit of the ISC rate was obtained as 109s−1 for the ZnPc-3-NOPy complex and the higher limit was found to be 3.1·108 s−1 for the ZnPc-4-NOPy complex.

On the g-value shift and intersystem crossing in photo-excited radical-triplet systems

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On the g-value shift and intersystem crossing in photo-excited radical-triplet systems