Hans van Willigen

Fourier transform‐electron paramagnetic resonance study of the photochemical reaction of acetone with 2‐propanol

Fourier transform‐electron paramagnetic resonance (FT‐EPR) was used to study the pulsed‐laser induced reduction of acetone with 2‐propanol. By monitoring the EPR signal of the acetone ketyl radical as function of delay time (τd) between laser pulse and microwave pulse, with τd ranging from nanoseconds to 100 μs, information was obtained on the kinetics of free radical formation and decay. The time evolution of the signal also gave an insight into the chemically induced dynamic electron polarization (CIDEP) mechanisms that affect signal amplitudes. It was found that the spectra obtained with τd settings ranging from 0 to 400 ns contain a dispersive signal contribution that is due to the presence of spin‐correlated radical pairs (SCRP) at the time of the microwave pulse. For acetone(D6) in 2‐propanol(D8) the rate constants of formation and decay of the SCRP are found to be 7.5±3.7×106 and ∼5×107 s−1, respectively. The SCRP lifetime in 2‐propanol(D8) at room temperature corresponds to what would be expected ...

Proton ENDOR on VO(H2O)52+ in solid solution

Abstract A proton ENDOR study has been made of VO2+ in frozen aqueous solution, and in Zn(NH4)2(SO4)2·6H2O powder. The study establishes that the vanadyl ion is particularly well suited for ENDOR investigations of binding site structure. Powder and frozen solution samples give ENDOR signals at temperatures well above 77 K, and using moderate microwave and rf power levels. ENDOR signals of VO2+ in frozen solution and in powder are virtually identical, establishing the structure of the aquo complex in frozen solution. Information on ligand hyperfine splitting components derived from the spectra given an insight into the electronic as well as the geometric structure.

Photoinduced electron transfer from porphyrins to quinones in micellar systems: an FT-EPR study

Abstract The photoinduced electron transfer from zinc tetrakis (4-sulfonatophenyl) porphyrin (ZnTPPS) to duroquinone (DQ) in several micellar solutions was investigated with Fourier transform-electron paramagnetic resonance (FT-EPR). The measurements give information on the formation and decay of free radicals, following pulsed-laser excitation of ZnTPPS, with nanosecond time resolution. It was found that the characteristics and time evolution of the EPR spectra depend strongly on surfactant properties. In particular, long-lived [ZnTPPS 3− … DQ − ] spin-correlated radical pairs (lifetime > 1 μs) were observed in solutions of the cationic surfactant cetyltrimethylammonium chloride (CTAC). By contrast, radical pair lifetimes in anionic and neutral micelles are shorter than 100 ns. The experimental data can be rationalized in terms of the Coulombic and hydrophobic interactions that determine the spatial organization and stability of the different species (reactants and products) in the microheterogenous systems.

FT-EPR study of triplet state C60. Spin dynamics and electron transfer quenching

Abstract A FT-EPR study was made of paramagnetic species formed by pulsed-laser excitation of C 60 in fluid solution. Earlier findings that photoexcitation of C 60 in fluid solution gives rise to an EPR signal with narrow linewidth were confirmed. The lifetime of the signal corresponds to that of the C 60 triplet as measured by flash photolysis. In the presence of donors, the rate of signal decay is increased and matches the growing in of EPR signals from oxidized donors. The time dependence of the FT-EPR spectra gives values for electron transfer rate constants which agree with those derived from flash photolysis measurements on the same systems. Based on these findings, we assign the narrow line signal to the triplet of C 60 . The time evolution of the FT-EPR signal establishes that the triplets are born with less spin polarization that the thermal equilibrium value. As a result, signal growth is controlled by spin-lattice relaxation.

An ENDOR study of alkali metal–fluorenone biradicals randomly oriented in a rigid matrix

An ENDOR study has been made of alkali metal–fluorenone biradicals (M+F−)2 randomly oriented in a rigid matrix. By an appropriate selection of the magnetic field settings in the ESR spectrum, ENDOR signals are obtained that convey information on signs and magnitudes of hyperfine splittings along specific axes in the dimer. The ENDOR spectra also give information on the 23Na quadrupole tensor components in (Na+F−)2. The information gives an insight into the structure of the dimers.

Endor on photoexcited triplets randomly oriented in solid solution

Abstract It is shown that hyperfine interactions in photoexcited triplets randomly oriented in solid solution can be measured with the aid of ENDOR. Results of measurements on naphtalene and zinc tetraphenylporphyrin triplets in polymethyl-methacrylate are presented. Preliminary results obtained with Zn(CN)4TPP show that ENDOR spectra of triplets with short lifetimes (less than 5 ms) can also be obtained due to the signal enhancement resulting from electron spin alignment.

ESR study of photoexcited triplets of some tetra(4-sulfonatophenyl)porphyrin dimers

Abstract Tetra (4-sulfonatophenyl)porphyrin (H 2 TPPS), and the metalloporphyrins ZnTPPS and PdTPPS dimerize in aqueous solution. An ESR study has been made of the photoexcited triplets of the [ZnTPPS] 2 . [H 2 TPPS/ZnTPPS] and [H 2 TPPS/PdTPPS] dimers. It is found that there is a strong dimerization effect on the values of the zero field splitting parameters of the triplets. This contrasts with what has been reported for photoexcited triplets of a series of covalently-linked tetrapyrroles. The experimental data indicate that the dimerization effect on the TPPS triplet parameters must be attributed to charge transfer contributions to the triplet electronic state.

ESR and ENDOR study of photoexcited triplets of cyano-substituted porphyrins

An ENDOR study has been made of the photoexcited triplets of some cyano-substituted zinc tetraphenylporphyrins randomly oriented in a rigid matrix. The results demonstrate that the ENDOR method can be applied in studies of short-lived triplets (triplet lifetime < 1 ms) in amorphous solids. The application is possible because of the fact that the ESR signal intensity is strongly enhanced by electron spin alignment. As a consequence of the spin alignment, ENDOR spectra can appear in absorption or emission depending on the field setting employed. Hyperfine data derived from the spectra reflect the effect of CN substitution on the electron spin distribution.

Photoexcited triplet ESR studies on thiaporphyrins and their protonated derivatives

Abstract The photoexcited triplet states of mono- and di-thiaporphyrins and their protonated derivatives are investigated by ESR spectroscopy. A comparison of the magnitude of the zero-field splitting parameters ( D and E ) of neutral thiaporphyrins with H 2 TPP indicates no significant alterations in the triplet spin density. However, the protonated derivatives show large decreases in D (30%–40%) and E (40%–50%) values. The electron spin polarization pattern (ESP) indicates dominance of an in-plane spin sub-level with respect to the population and decay process for neutral species, while a redirection of spin sub-level activity is observed for protonated species. The structural change accompanying the protonation process accounts for most of these observations.

Short-chain basket handle porphyrins: singlet and triplet excited state properties

Abstract The excited state properties of the title compounds and their Zn 2+ derivatives, determined by fluorescence and photoexcited tripled ESR methods, are reported. Fluorescence spectral studies indicate that the basket handle porphyrins are less fluorescent than the parent meso-tetraphenyl porphyrin (H 2 TPP). The combined effects of intersystem crossing and internal conversion account for this difference. Estimated excited state potentials suggest that the cross trans linked isomer (PSI) is a better electron donor in the first singlet excited state. Zero-field splitting parameters D and E of the three isomers evaluated from photoexcited triplet ESR show small but significant differences attributed to distortion of the porphyrin plane caused by introduction of a short bridging group. The electron spin polarization (ESP) pattern remains the same as in H 2 TPP, indicating the usual spin—orbit coupling mechanism for intersystem crossing.