Spin Chemical Control of Photoinduced Electron-Transfer Processes in Ruthenium(II)-Trisbipyridine-Based Supramolecular Triads

Abstract

Nanosecond time-resolved absorption studies in a magnetic field ranging from zero to 3.0 T have been performed on a series of covalently linked donor-Ru(bipyridine) 3 -acceptor complexes (D-C 2+ -A 2+ ). In these complexes the electron donor is a phenothiazine moiety linked to a bipyridine by a (-CH 2 -) p (p ) 1, 4, 5, 7) chain, and the electron acceptor is an N,N′-diquaternary-2,2′-bipyridinium moiety, linked to a bipyridine by a (-CH 2 -) 2‚ chain. On the nanosecond time scale the first detectable photoinduced electron-transfer product after exciting the complex C 2+ is the charge-separated (CS) state, D + -C 2+ -A + , where an electron of the phenothiazine moiety, D, has been transferred to the diquat moiety, A 2+ . In zero field the lifetime of the CS state is about 150 ns. At low fields (B 0 < 0.5 T) the magnetic field strongly affects the decay kinetics, splitting it up into a major component, the rate constant of which decreases by a factor of about 10 at fields of several 100 mT, and a minor component with an approximately field independent rate constant. At high fields (B 0 > 0.5 T) the total amplitude of the CS absorption signal decreases and the relative contribution of the fast decaying component increases. The magnetic field effects can be consistently interpreted and quantitatively modeled by taking into account the mechanisms and kinetics of the spin multiplicity changes in the CS state and its precursor, a short-lived CT state (D-C 3+ -A + ) formed upon primary electron transfer from the triplet excited complex to the diquat moiety. Exploiting the magnetic field dependent kinetics, the rate constants of the triplet-singlet transitions in the two types of linked radical pairs and of all the electron-transfer processes following the primary one can be assessed. Magnetic-field-dependent investigations thus can be essential for the understanding of the complex kinetics in supramolecular systems with sequential cyclic electron transfer.

Spin Chemical Control of Photoinduced Electron-Transfer Processes in Ruthenium(II)-Trisbipyridine-Based Supramolecular Triads: 2. The Effect of Oxygen, Sulfur, and Selenium as Heteroatom in the Azine Donor

“…[1][2][3][4][5][6][7] Our group studies complexes exemplified by the triad in Figure 1. [8][9][10][11] These supramolecular assemblies consist of a central trisbipyridine ruthenium chromophore covalently linked through variable-length polymethylene chains to a "diquat" electron acceptor and a pair of azine-type electron donors. 12 These triads have proven to be a very fruitful platform for the study of photoinduced electron-transfer processes and charge separation.…”

“…Interestingly, in the case where donors are phenothiazine (i.e., XdS; PTZ), the radical cation pair that constitutes the CS is formed with almost pure triplet spin correlation. 8 Depending on the structural details of the particular triad complex, this 3 CS recombines to the ground state with a lifetime in the range of 100-300 ns. 8 Spin chemical studies of these triads have proven to be a valuable tool for developing a more sophisticated understanding of the complexities of the CS formation and recombination to the ground state.…”

“…8 Depending on the structural details of the particular triad complex, this 3 CS recombines to the ground state with a lifetime in the range of 100-300 ns. 8 Spin chemical studies of these triads have proven to be a valuable tool for developing a more sophisticated understanding of the complexities of the CS formation and recombination to the ground state. 8 Because direct recombination of the 3 CS to the singlet ground state is formally a spin-forbidden process, magnetic-field effect (MFE) studies of CS formation and recombination have proven to be of critical importance to the understanding of this spin chemistry.…”

“…8 Spin chemical studies of these triads have proven to be a valuable tool for developing a more sophisticated understanding of the complexities of the CS formation and recombination to the ground state. 8 Because direct recombination of the 3 CS to the singlet ground state is formally a spin-forbidden process, magnetic-field effect (MFE) studies of CS formation and recombination have proven to be of critical importance to the understanding of this spin chemistry. In previous studies (again, involving DCA triads incorporating PTZ donors), the model we have successfully employed to explain the spin chemical behavior of the triad CS is the relaxation mechanism of Hayashi and Nagakura.…”

“…In the absence of any applied field, 3 CS/ 1 CS spin equilibration is rapid relative to recombination via the spin-allowed path k s (which is the only kinetically important route to recombination in zero applied field), resulting in a nearly monoexponential decay of 3 CS back to the ground state. 8 Upon application of a magnetic field, the triplet state experiences Zeeman splitting, as shown in Scheme 1, wherein the T 0 state energy is unchanged and the energies of the T + and T -states move as indicated. At relatively small fields, the difference in energies of the T -and T + states and those of the 1 CS and T 0 states becomes significant relative to the hyperfine coupling energy, thus resulting in a slowing of the rate of their conversion to the 1 CS state.…”