Light‐induced electron spin polarization in the ground state of water‐soluble copper porphyrins

Abstract

Light‐induced spin‐polarized transient EPR spectra are reported for several water‐soluble copper porphyrins. The spectra are assigned to the doublet ground state, with emissive spin polarization resulting from photoexcitation and subsequent electronic relaxation. In contrast to other systems for which polarization of a doublet ground state has been observed, the exchange interactions in the copper porphyrins are strong and the geometry is fixed. It is proposed that intersystem crossing from the photoexcited trip‐doublet to the trip‐quartet state can lead to net polarization of the spin system and that this polarization is maintained during electronic decay, possibly via charge‐transfer and exciplex states. The intensity of the observed spin polarization is essentially independent of the molecular orientation in the external field, but is strongly dependent on the nature of the charged peripheral groups. Possible reasons for this behavior are discussed.

Probing Microenvironment in Ionic Liquids by Time-Resolved EPR of Photoexcited Triplets

“…For all three standard solvents used for comparison, the k value was found to be ∼0.3. Closely the same ratio 1/3 was previously obtained for TR EPR kinetics of another porphyrin CuTPP in toluene, being tentatively assigned either to the two coordination types of CuTPP in solution or to the equilibrium between dimers and monomers . A similar ratio k = 0.31 was found by us for ZnTPP in [C 10 mim]­BF 4 (Table ); however, for [Bmim]­PF 6 and [Bmim]­BF 4 it is noticeably larger ( k = 0.43 and 0.59, respectively).…”

“…However, the situation is more complicated because TR EPR kinetics shows biexponential decay even in common frozen organic solvents. Such biexponential behavior was previously observed for parent compound CuTPP and explained by either two types of coordination or equilibrium between dimers and monomers coexisting in solution (for both components the primary relaxation mechanisms are the modulation of ZFS and coupling with phonon) . In the case of ZnTPP a similar trend is readily observed in frozen toluene, glycerol, and N -methyl-2-pyrrolidone (NMP) (Figure a).…”

“…Such biexponential behavior was previously observed for parent 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 7 compound CuTPP and explained by either two types of coordination or equilibrium between dimers and monomers coexisting in solution (for both components the primary relaxation mechanisms are the modulation of ZFS and coupling with phonon). 50 In case of ZnTPP similar trend is readily observed in frozen toluene, glycerol, N-methyl-2-pyrrolidone (NMP) (Fig.2a). Remarkably, for three standard solvents the TR EPR spectrum shape does not noticeably change depending on τ DAF ( Fig.1e-g) and, in agreement with that, TR EPR kinetics is closely the same across the spectrum.…”

Photoinduced Processes in Some Mechanically Interlocked Supramolecules as Studied by Time-Resolved Electron Paramagnetic Resonance

“…The 45 mT signal also cannot be ascribed to the polarized doublet ground state of the Cu(II) moiety, which would result in a net absorption or emission signal but not the observed e/a signal. This signal is also not attributable to the triplet state of phenanthroline since this state can only be created by using significantly shorter wavelength irradiation, ∼390 vs 460 and 532 nm used in our experiments.…”

Observation of a photoexcited state of a paramagnetic transition metal complex by time-resolved electron paramagnetic resonance spectroscopy

“…Several TREPR studies of organic complexes consisting of a chromophore with one or more attached stable free radicals have shown that the excited doublet, 17,24,25 quartet 14,15,18,[25][26][27] and quintet 19,22,26,27,30 states are observable. Light-induced spin polarization has also been observed in the ground state of some paramagnetic metal complexes 31,32 and in the excited states of a chromophore that is weakly coupled to a metal. [33][34][35][36] In these systems the spinorbit coupling contribution to the zero field splitting and the anisotropy of the g-and hyperfine tensors are both comparatively small.…”