Akio Yoshimura

LUMIFLAVIN‐SENSITIZED PHOTOOXYGENATION OF INDOLE

Abstract— The lumiflavin‐sensitized photooxygenation of indole in aqueous solutions has been investigated by means of steady light photolysis and flash photolysis. The semiquinone of lumiflavin and the half‐oxidized radical of indole were formed by the reaction between triplet lumiflavin and indole (3.7 times 109M‐1 s‐1). The semiquinone anion radical of lumiflavin reacted with oxygen to form superoxide radical. The triplet state of lumiflavin also reacted with oxygen forming singlet oxygen, 1O2. But the reaction between 1O2 and indole (7 times 107 M_l s_1; estimated from steady light photolysis using Rose Bengal as a sensitizer) was far less efficient than the reaction between indole and triplet lumiflavin. The quantum yield of the lumiflavin‐sensitized photooxygenation of dilute indole via radical processes was much higher than that via1O2 processes, though appreciable 1O2 was formed.

TRIPLET LIFETIMES OF SOME FLAVINS

Abstract— Using flash photolysis the rate constants of the triplet decay at pH 7.0 (26°C) of 10‐hydroxyethyl‐isoalloxazine (2700 s−1), 2′‐deoxyRF (2300 s−1), 5′‐deoxyRF (3200 s−1), 8‐hydroxyRF (18 000 s−1) and 8‐aminoRF (28 000 s−1) have been measured. The results agree with other evidence that photochemical properties of flavins are influenced by the interaction of the sidechain with the isoalloxazine nucleus. In addition, our data on RF and FMN indicate that the triplet decay rates given in the literature for these compounds have to be corrected to 3200 s−1 (RF) and 4900 s−1 (FMN), respectively. The rate constants for the quenching of the triplet by ground state molecules for all above compounds are given.

Non-radiative processes of excited CT states of Ru(II) and Pt(II) compounds and excited d-d states of Rh(III) compounds in the solid state and at higher temperatures

Abstract Radiative and non-radiative processes of excited CT states of Ru(II) and Pt(II) compounds and excited d-d states of Rh(III) compounds in the solid state and at higher temperatures are examined. A rapid non-radiative transition of a metal-to-ligand charge transfer excited state to the ground state does not occur over a wide temperature range (77–300 K) for [Ru(bpy) 3 ](PF 6 ) 2 and [Pt 2 (tpy) 2 (guanidinato)](ClO 4 ) 3 . Spectral simulations of the d-d emissions of Rh(III) compounds in the range 77–536 K are carried out by assuming a simple configuration coordinate. The large Huang-Rhys factors indicate that the lowest and second lowest d-d excited states of Rh(III) compounds are strongly coupled to the excited vibrational state of the ground state. The frequency factors of non-radiative decay are 10 11 –10 12 s −1 for the excited triplet state.

Synthesis, electrochemistry and photoexcited-state properties of dinuclear ruthenium complexes bridged by 2,6′-bis(2-pyridyl)-2,2′:6,2″-thiazolo[4,5-d]-benzothiazole

Abstract The new bridging ligand, 2,6′-bis(2-pyridyl)-2,2′:6,2″-thiazolo[4,5-d]-benzothiazole (bptb) and its dinuclear Ru complexes have been synthesized. The combination of the transient absorption (TA) spectra and spectroelectrochemical difference spectra has led to the assignment of the lowest excited state for the dinuclear Ru(II) complexes as a Ru-to-bptb charge transfer state. Although the ab initio molecular calculations of bridging ligands have provided lower HOMO/LUMO energies for bptb compared to those of 2,6-bis(2-pyridyl)benzodiimidazole (dpimbH2), the degree of electronic coupling for the dinuclear bptb complex is smaller than that for the dpimbH2 complex from the analysis of intervalence charge transfer band. Since both dinuclear Ru complexes have a similar coordination environment, the Ru dπ-HOMO Lπ mixing is dominant for the metal-metal interaction through the hole transfer mechanism.

QUENCHING OF EXCITED TRIPLET LUMIFLAVIN BY LUMIFLAVIN RADICALS FORMED IN ITS T‐T REACTION

Abstract— Reactions of the triplet state of lumiflavin (3LF) in water adjusted at pH 7.2 were reexamined by means of a Xe‐flash photolysis and a laser photolysis. Measurements of the decay of 3LF were made on solutions of LF ranging in the concentration from 4 to 61 times 10‐6 mol/dm3. A one‐electron reduced and a one‐electron oxidized species of lumiflavin (LF‐ and LF+) were produced in the first decay stage of 3LF with a high efficiency (0.6 ± 0.1) in a bimolecular triplet‐triplet reaction. The product radicals (LFH‐ and LF+) quench 3LF very efficiently (3 ± 0.8 × 109 mol‐1dm3 s‐1) compared with LF in the ground state (> 2 × 107 dm3 mol‐1).

Asymmetric induction via intramolecular T2-sensitization: Photoisomerization of α-(9″-anthryl)ethyl spiro[cyclopropane-1,9′-fluorene]-2-carboxylates

Abstract The photoisomerization of α-(9″-anthryl)ethyl spiro[cyclopropane-1,9′-fluorene]-2-carboxylates (1c⇄2c) proceeds diastereoselectively through the selective intramolecular energy transfer from the second triplet state of anthryl group to the fluorenyl moiety. Asymmetric induction using optically active α-(9″-anthryl)ethyl group is achieved in high optical and chemical yields.

Inner-sphere reorganization of photo-induced electron transfers on the laser-excitation of ligand-bridged Ru(II)Rh(III) and Ru(II)Co(III) compounds

Intramolecular reorganization for photo-induced electron transfers of Ru(II)-(L)M(III)5+ (L=a bridging tetradentate ligand, and MCo or (L = a bridging tetradentate ligand, and MCo or Rh) and for formation of ligand-field (LF) excited states are investigated. Inner-sphere reorganization energies (λin) around the metal ions are estimated from the dependence of electron transfer rates on temperature and ΔG°. For back electron transfer processes, the magnitude of λin around rhodium ion (0.9 eV) is close to that around the cobalt ion. Inner-sphere reorganization after LF emission of rhodium(III) ion is estimated (0.9 eV) from the Stokes shift of the emission. The magnitude of λin or the electron transfer and the optical transition are interpreted in terms of gain or loss of the dσ∗ electron in the metal orbital.

FLAVIN-PHOTOSENSITIZED MONOMERIZATION OF DIMETHYLTHYMINE CYCLOBUTANE DIMER IN THE PRESENCE OF MAGNESIUM PERCHLORATE

We have investigated the photosensitized monomerization of the cis,syn‐cyclobutane dimer of 1,3‐di‐methylthymine using riboflavin tetraacetate and a 5‐deazaflavin derivative as photosensitizer. Although little monomerization of the dimer is induced by photoexcitation of the flavins in the absence of any additives, the flavins can function as an efficient photosensitizer in the presence of magnesium perchlorate. Mechanistic studies involving spectroscopic, quantum‐yield and flash‐photolysis measurements demonstrated that the photosensitized monomerization exclusively proceeds through electron transfer from the dimer to the triplet flavins complexed with Mg2+. The effects of magnesium perchlorate are compared with those on the chloranil‐photosensitized monomerization and also with the effects of HClO4 on the flavin‐photosensitized reaction.

Solid-state photochemistry: energy-transfer and electron-transfer of 3CT in crystals of [OsxRu1−x(bpy)3]X2 (x=0–0.23)

Abstract Time-correlated single-photon counting method and time-resolved absorption-spectroscopy were applied to bimolecular reactions of energy-transfer and electron-transfer in the crystals of [OsxRu1−x(bpy)3]X2 (bpy=2,2′-bipyridine). Migration of the triplet excited charge-transfer state (3CT) of [Ru(bpy)3](PF6)2 gave rise to either energy-transfer to a small amount of [Os(bpy)3]2+ or bimolecular annihilation with the 3CT. The rate of hopping in the migration of 3CT ((2–24)×108 s−1) was determined from the bimolecular rate-constants of the energy-transfer and 3CT–3CT annihilation in the single crystal of [Ru(bpy)3]X2 (X=Cl−, ClO4− and PF6−) or the transparent crystal of [Ru(bpy)3]X2 ground together with KCl. Rate of the energy-transfer from 3CT(Ru) to an Os(II) without energy-migration was determined to be 1.7×1011 s−1 for the crystal heavily doped with Os2+. Time-resolved absorption spectroscopy revealed that the annihilation of the 3CT in the transparent crystal of [Ru(bpy)3]X2 ground together with KCl formed the one-electron-transferred products in 40 ps.

Photoinduced ET and back-ET in bimetallated compounds of Ru(II)-Rh(III) and Ru(II)-Co(III)

Intramolecular electron transfer processes in bimetallated donor-acceptor compounds have been investigated by means of laser photolysis kinetic spectroscopy. An excited Ru(II)-moiety of donor-acceptor compounds undergoes intramolecular electrontransfer to either a rhodium(III) ion or a cobalt(III) ion, followed by back-electron transfer, an Arrhenius plot of the electron-transfer-rate gave a straight line of intercept (frequency factor) and slope (activation energy) for the photoinduced electron transfers and the back electron transfers. A common and large frequency factor observed for Ru(H)-Rh(III) compounds is accounted for in terms of solvent-relaxation dynamics. The activation energy observed consists of outersphere rearrangement energy depending on the metal ion-metal ion distance. For the photoinduced electron transfers and subsequent back-electron transfers in the Ru(II)-Co(III) compounds, the electron-transfer-rates are reduced because of weak electronic coupling, large rearrangement energy and negative entropy change.