Horst E. A. Kramer

DETERMINATION OF THE pK VALUES OF THE LUMIFLAVIN TRIPLET STATE BY FLASH PHOTOLYSIS

Abstract— The triplet‐triplet absorption spectra in aqueous solution of the acid (3LfH2+), the neutral (3LfH) and the basic (3Lf‐) forms of lumifiavin (6,7,9‐trimethylisoalloxazine) were measured by flash photolysis. The pKa values of the corresponding protolytic equilibria of the lumifiavin triplet were found to be 4.45±0.1 and 9.8±0.15.

Reactivity of Organic Dye Triplet States in Electron Transfer Processes

— 10 kcal/mol, kqT reaches the diffusion-controlled limit, decreases with increasing AG, and for AG > 4 kcal/mol kqT becomes proportional to exp (— AG/RT). The different reactivities of the acid and basic triplet forms of thionine and lumiflavin can be attributed to the difference in reduction potentials of these species. This difference is available from the pK values of triplet state and semireduced dye radical.

SENSITIZED PHOTOOXYGENATION ACCORDING TO TYPE I MECHANISM (RADICAL MECHANISM) — PART II. FLASH PHOTOLYSIS EXPERIMENTS

Abstract— The photooxygenation of allylthiourea (ATU) sensitized by thionine does not occur according to the singlet oxygen mechanism but rather proceeds via the formation of radicals. In oxygen‐free solution the primary process is a redox reaction between the thionine triplet and ATU where a semithionine‐ and an ATU‐radical are formed. In further reaction steps the leuco form of the dye is finally produced (reductive photobleaching; D

SENSITIZED PHOTOOXYGENATION: CHANGE FROM TYPE I (RADICAL) TO TYPE II (SINGLET OXYGEN) MECHANISM

Abstract— As we have shown in previous papers, thionine‐sensitized photooxygenation reactions follow a Type I (radical) mechanism. We now demonstrate that, by an appropriate choice of the acceptor and its concentration (solvent: pyridine) or by working in a rigid matrix (ethyl cellulose), the reaction can be switched to a Type II (singlet oxygen) mechanism. The system studied in the present investigation, thionine and 9,10‐dimethylanthracene, represents to a certain extent an intermediate type. Photooxygenation at low DMA‐concentration occurs according to a Type II mechanism as verified by the method of competitive photooxygenation, while in oxygen‐free solutions, the sensitizer is partially photoreduced by the acceptor, which is typical for Type I systems. Whereas the photooxygenation of allylthiourea (ATU) with thionine as sensitizer takes place via radicals at high ATU concentrations, a change to the singlet oxygen mechanism could be observed at low ATU concentrations in pyridine solution.

Synthesis, electrochemistry and emission spectroscopy in fluid solution of four isomeric (α-diimine)Re(CO)3Hal complexes

Abstract Complexes (bdz)Re(CO) 3 Hal (Hal = Cl or Br) derived from the four isomeric bidiazine (bdz) chelate ligands 3,3′-bipyridazine, 2,2′-bipyrazine, 2,2′- and 4,4′-bipyrimidine have been synthesized from Re(CO) 5 Hal by thermal substitution. All the complexes were found to show long-wavelength emission at room temperature in chloroform solution after irradiation into the metal-to-ligand charge transfer (MLCT) band. Spectroscopic data and electrochemical reduction potentials confirm the superior polarizing ability of neutral Re(CO) 3 Hal fragments for α-diimine π system, and the electrochemical and photophysical data can be correlated with the established properties of the free ligands, of their anion radicals, and of other d 6 metal (W 0 , Ru II ) complexes.

Substituent effects on triplet yields in aminoanthraquinones : radiationless deactivation via intermolecular and intramolecular hydrogen bonding

Abstract The dependence of the triplet quantum yields of monosubstituted and disubstituted aminoanthraquinones on the number of substituents and on their position was studied. Using fluorescence lifetimes, fluorescence quantum yields and triplet quantum yields, the rate constants of the intersystem crossing into the triplet state and of the internal conversion into the ground state were evaluated. For monosubstituted aminoanthraquinones the individual contributions of various mechanisms to the internal conversion have been estimated. In quenching the excited singlet state, intermolecular hydrogen bonding to the solvent is more efficient by an order of magnitude than intramolecular hydrogen bonding.

REACTIVITY OF EXCITED STATES OF FLAVIN AND 5‐DEAZAFLAVIN IN ELECTRON TRANSFER REACTIONS

Abstract— Quenching of the excited states of lumiflavin and 3‐methyl‐5‐deazalumiflavin by methyl‐and methoxy‐substituted benzenes and naphthalenes in methanol was investigated. The observed difference in the reactivity of acid and neutral lumiflavin triplets is explained thermodynamically by applying the Michaelis cycle, as being due to the higher reduction potential of the acid triplet. In this connection the pK values of lumiflavin triplet (pKM= 6.5) and semiquinone (pKM= 11.3) have also been determined in methanol. The difference in the reactivity between the singlet and triplet states of lumiflavin is found to be greater as predicted by the difference in excitation energy. The reactivities of the excited states of flavin and 5‐deazaflavin differ only slightly in contrast to the marked difference in the ground state reactivities of electron transfer reactions. This is explained in terms of the model of Rehm and Weller. The pH dependence of the electron transfer quenching of 5‐deazaflavin triplet was investigated in water, yielding a triplet pK of 2.5. In contrast to the flavin, this triplet pK does not significantly differ from the pK of the 5‐deazaflavin ground state. From this, different sites of protonation are deduced for the photoexcited triplet states of flavin and 5‐deazaflavin.

PHOTOCHEMICAL INVESTIGATIONS OF OXAZINE, THIAZINE AND SELENAZINE DYES. THE REACTIVITY OF PROTOLYTIC TRIPLET FORMS IN ELECTRON TRANSFER REACTIONS

Abstract— The photoreduction of oxonine, thionine and selenine with the reducing agent allylthiourea was investigated by flash photolysis. The oxonine triplet state was produced by triplet‐triplet energy transfer with 9,10‐dibromoanthracene as donor. For all three dyes the rate constant of the electron transfer is considerably higher for the acid triplet form than that of the corresponding reaction of the basic triplet form. It is shown that the higher reactivity of the acid triplet can be related to its higher reduction potential which is available from the difference of the pK values of triplet and semiquinone of the dye.

One and two electron transfer pathways in the photoreduction of flavin

The stoichiometry of flavin photoreduction was studied by flash photolysis, monitoring the conversion of photoexcited flavin to one—electron and two-electron reduction products and following their fate. The reactivity of the photoexcited states of flavin in one-electron transfer reactions was determined, the reasons for differences between them are discussed and the mode of radical decay was investigated in dependence of the photosubstrate structure. The nature of the two—electron transferring photosubstrate was revealed in order to define the range of the two reduction pathways.

INCREASED QUANTUM YIELD OF PHOTOREDUCTION OF DYES (A CATALYTIC EFFECT)

Abstract— While studying the photoreduction of some dyes (D) by reducing agents (R), it was observed that the quantum yield of the photoreduction increases considerably upon addition of a third substance (C), whereas it is very small when the dye is photoreduced by C alone (catalytic effect), (see Table 1).