Elena E. Batova

Magnetic field effect on the recombination of the triplet radical ion pair state of the semirigid Zn-porphyrin-viologen dyad

Abstract Laser flash photolysis technique was used to record the recombination kinetics of the triplet radical pair state of the new semi-rigid Zn-porphyrin-viologen (P-Ph-Vi 2+ ) dyad in which Zn-meso-4-methoxyphenyl-tris (4-tolyl) porphine (P) and methylbenzylviologen (Vi 2+ ) are linked by the 1,4′-phenylene spacer (Ph). In polar solvents the 3 [P +• -Ph-Vi +• ] decay kinetics fits fairly into the first-order law and the corresponding rate constants ( k 2 ) equal (1.8–3.3) x 10 6 s −1 and do not correlate to a solvent polarity. Application of an external magnetic field ( B ≤0.24 T) decreases the k 2 value to (0.3–0.7) x 10 6 s −1 . The B 1 2 values (defined as the B values at which the magnetic field effect reaches half of its saturated value) are 15–30 mT. The largest magnetic field effect on the k 2 value is observed in glycerol, k 2 ( B =0)/ k 2 ( B =0.24 T)= 11. In ethanol solution the quenching rate constant of 3 [P +• -Ph-Vi +• ] by 3 O 2 equals (8±0.8) × 10 9 M −1 s −1 and is close to 1 3 of the diffusion limit. The mechanism of the triplet-singlet (T-S) evolution of RP and the magnetic field effect as well as high efficiency of the RP quenching by molecular oxygen ( 3 O 2 ) are discussed with respect to the different behaviour of the RP in several conformers. In opened conformer of RP (with low value of exchange interaction, J , between radical centers) the hyperfine coupling (hfc) and the relaxation mechanism of T-S evolution of the RP are active; in closed conformer of RP where 2J is large enough the spin-orbit coupling dominates. The stimulation of the T-S transitions in closed RP by the exchange interaction of the radicals with 3 O 2 is very efficient.

Solvent effects on the magnetic field dependent recombination kinetics of the Zn-porphyrin—viologen dyad radical ion pair state

Abstract Magnetic field dependent recombination kinetics of the triplet radical pair state (RIPS) of the semirigid Zn-porphyrin—viologen (PPhVi2+) dyad in which Zn-porphyrin (P) and viologen (Vi2+) linked by the semirigid spacer (Ph) containing a 1,4-phenylene moiety has been studied by nanosecond laser flash photolysis technique in sixteen solvents of widely varying properties. The influence of solvent viscosity, the size of solvent molecules and the specific solvation on the triplet RIPS recombination kinetics in an external magnetic field has been found and discussed in terms of the hyperfine coupling mechanism including an exchange interaction modulated by the stochastic spacer motion in low magnetic fields and the spin—orbit coupling induced intersystem electron transfer in high magnetic fields.

Magnetic field effect on the recombination of the triple radical ion pair state of the Zn-porphyrin—violegen dyad: the influence of the supramolecule microenvironment

Abstract Photochemical behaviour of the semirigid Zn-porphyrin—violegen dyad depends on its microenvironment which affects the rate of the dyad conformational transitions and the radical ion pair (RIP) triple—singlet splitting in zero magnetic field. The heavy-atom environment of RIP enhances the RIP recombination and quenches the magnetic field effect.

Cyclodextrin complexes of zinc porphyrin-tetraviologen supramolecule. Magnetic field effect on the recombination kinetics of their triplet radical ion pair states

Abstract In aqueous solutions α- and γ-cyclodextrins (CDs) separate the ground state complexes of the porphyrin (P) and viologen (Vi 2+ moieties in the zinc porphyrin-tetraviologen supramolecule and modulate dynamics of the connecting polymethylene chains between P and Vi 2+ . Effects of the chain dynamics being strongly dependent on the CD pore size appear in the quite different rates of the intramolecular quenching of 3 P by Vi 2+ and the recombination rates of the supramolecule triplet radical ion pair states in an external magnetic field.

Effects of spacer structure and external magnetic fields on the recombination kinetics of radical ion pair states of zinc porphyrin–viologen dyads

Magnetic field effects on the recombination kinetics of the triplet radical ion pair state (RIPS) of the Zn porphyrin viologen dyad (P–Sp–Vi2+) with Zn porphyrin (P) and viologen (Vi2+) linked by a flexible [(CH2)n, n= 3, 6, 10] or semi-rigid (with the 1,4-phenylene and 4,4′-biphenylene moieties) spacer (Sp) have been studied by the nanosecond laser flash photolysis technique in acetonitrile. As the Sp lengthens the rate constant (kr) of the RIPS recombination in zero magnetic field (B= 0) increases from 0.80 × 106 to 5.9 × 106 s–1, while the kr value in a strong magnetic field (B= 0.24 T) is almost invariant at (0.3–0.7)× 106 s–1. The experimental results are discussed in terms of a simple kinetic scheme including only two subensembles of the RIPS conformers (‘closed’ and ‘open’). The rate-determining processes of the RIPS recombination are concluded to be the singlet–triplet conversion stimulated by the hyperfine coupling modulated by the exchange interaction in zero magnetic field and spin–orbit coupling induced intersystem electron transfer in a strong magnetic field. The considerations of the spin dynamics based on the relative yields and the formation kinetics of the triplet RIPS are involved.

Magnetic field and magnetic isotope effects on the recombination kinetics of chain-linked ketyl-phenoxyl triplet radical pairs

The recombination kinetics of three photogenerated covalently-linked ketyl-phenoxyl triplet radical pairs,3[PhC.(OD)C6H4O(CH2) n OC6H4C6H4O.] (n=3, 6, and 10), and of the corresponding deuterated derivatives were examined by the laser flash technique under an external magnetic field (up to 0.2 T) in a CDCl3/CD3OD (2∶1) mixture. In zero magnetic field, radical pairs (RPs) with small exchange interactions (n=6 and 10) are characterized by high values of the magnetic isotope effect (MIE), which reach 3 for pairs withn=10. Under strong magnetic fields (up to 0.2 T), the values of MIE decrease to 1.2 to 1.1. The photochemical behavior of covalently-linked RPs is compared with that of similar unlinked RPs in micelles.

RESEARCH NOTE: TRIPLET RADICAL ION PAIR STATE OF THE Zn‐PORPHYRIN‐VIOLOGEN DYAD AS A MAGNETIC FIELD SENSITIVE PROBE OF PHASE TRANSITIONS IN SMALL UNILAMELLAR VESICLES

Abstract— The magnetic field effect on the recombination kinetics of the triplet radical ion pair state (RIPS) of the Zn‐porphyrin‐viologen dyad (P‐Ph‐Vi2+) in the small unilamellar vesicles (SUV) of d, l‐dipalmitoyl‐α‐phosphatidylcholine has been studied by the nanosecond laser flash photolysis technique at 5–60°C. The increase in temperature from 25 to 40°C enhances the rate constant (kr) of the RIPS recombination in zero magnetic field from 0.9 × 106 to 1.6 × 106 s‐1 while k, is temperature insensitive at 5–25 and 40–60°C. The typical break in the kr temperature dependence is observed in the temperature range of the phase transition of the SUV bilayers from the solid to the fluid state. The kr value in a strong magnetic field (B= 0.24 T) is equal to 2.7 × 105 s‐1 it is independent of temperature at 5–60°C. The shape of the magnetic field dependence of kr is unaffected by the phase transition of the SUV bilayers and is characterized by the existence of an initial plateau of kr at B= 0 to 0.5 mT.

Photoinduced electron transfer in zinc porphyrin-crown ether supramolecule with Eu3+ electron acceptor in crown void

Abstract Photoinduced electron transfer (ET) reactions in the zinc porphyrin-crown ether (ZnPCE) supramolecule, in which one crown ether moiety containing Eu 3+ as electron acceptor is convalently linked to zinc porphyrin (ZnP), were studied by flash photolysis. In methanol solutions, highly efficient charge separation occurs via intramolecular ET from 3 ZnP to Eu 3+ encapsulated in the crown ether void k 1 =(3±1)×10 3 s −1 ) and intramolecular ET from 3 ZnP to uncomplexed Eu 3+ ( k 2 =(2.5±0.5)×10 3 s -1 ). Intermolecular ET from Eu 2+ escaped from the crown ether void to ZnP .+ ( k r =(4.3±0.7)×10 8 M −1 s −1 ) seems to be the main pathway of charge recombination.

Unidirectional Electron Transfer in the Supramolecules: The Control by an External Magnetic Field

The design of the supramolecules: Zn-porphyrin-viologen dyads and Zn-porphyrin-viologen-quinone triads, which give rise to charge separation and stabilization after absorption of visible light in polar media has been developed. The photoinduced charge separation is achieved by the competition of the fast spin-allowed forward and backward spin-forbidden electron transfers (ETs). An external magnetic field is used to control the recombination of the triplet radical ion pair (RIP) state formed in a primary photochemical reaction. The triplet RIP state is stabilized by the subsequent ET to the second electron acceptor (quinone) which is linked to the first one. The mechanism of the triplet-singlet (T-S) evolution of RIPs and the magnetic field effect on their recombination is discussed in respect of the different behaviour of RIPs in several conformers. In the opened conformer of the RIP (with a small value of the exchange interaction (J) between radical centers) the hyperfine coupling (hfc) and the relaxation mechanisms of T-S evolution of the RIP are active; in the closed conformer of the RIP, where J is large enough (2J >> Ahfc), the spin-orbit coupling (SOC) dominates.

MAGNETIC FIELD AND MAGNETIC ISOTOPE EFFECTS ON THE RECOMBINATION KINETICS OF COVALENTLY-LINKED KETYL-PHENOXYL TRIPLET RADICAL PAIRS

The recombination kinetics of three photogenerated covalently-linked ketyl-phenoxyl triplet radical pairs,3[PhC.(OD)C6H4O(CH2)nOC6H4C6H4O.] (n=3, 6, and 10), and of the corresponding deuterated derivatives were examined by the laser flash technique under an external magnetic field (up to 0.2 T) in a CDCl3/CD3OD (2∶1) mixture. In zero magnetic field, radical pairs (RPs) with small exchange interactions (n=6 and 10) are characterized by high values of the magnetic isotope effect (MIE), which reach 3 for pairs withn=10. Under strong magnetic fields (up to 0.2 T), the values of MIE decrease to 1.2 to 1.1. The photochemical behavior of covalently-linked RPs is compared with that of similar unlinked RPs in micelles.