Maksudul M. Alam

Efficient photoinduced electron transfer between C60 and tetrathiafulvalenes studied by nanosecond laser photolysis

Abstract Photoinduced electron transfer between C 60 and tetrathiafulvalence (TTF) or bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) in polar and non-polar solvents and their mixtures was investigated by nanosecond laser photolysis/transient absorption spectroscopy in the visible and near-IR regions. The transient absorption bands of the C 60 triplet ( T C 60 ∗ ) observed in polar solvents decay on addition of TTF (or BEDT-TTF), accompanied by the appearance of the transient absorption bands of C 60 − . The yields of C 60 − are decreased on addition of O 2 , revealing that about 10% of C 60 − is produced via the singlet state at donor concentrations of less than 5 × 10 −3 M. In benzene, the quenching of T C 60 ∗ without the appearance of C 60 − within the nanosecond laser pulse is attributed to collisional quenching. The quantum efficiencies of C 60 − formation via T C 60 ∗ , which were evaluated from the initial [ T C 60 ∗ ] and maximal [C 60 − ] values, vary with the donor ability and solvent polarity. The forward electron transfer rate constant were evaluated from the decay rates of T C 60 ∗ (or the rise rates of C 60 − ) after multiplying by the quantum efficiencies. The back electron transfer rates are slowed down in polar solvents.

Laser Flash Photolysis of 2,2′‐Dithiobls(pyridine N‐oxide): Reactivity of N‐Oxypyridyl‐2‐thio Radical

Abstract— Reaction kinetics of radicals produced by the nanosecond laser flash photolysis of 2,2′‐dithiobis(pyridine N‐oxide) and related compounds have been studied. The transient absorption band at 360 nm was attributed to the radical in which the unpaired electron mainly localizes on the S atom (N‐oxypyridyl‐2‐thio radical). The reactivities of the radical for conjugated dienes are lower than those of the pyridyl‐2‐thio radical, suggesting that a considerable unpaired electron density on the S atom delocalizes onto the N‐oxypyridine moiety. The addition reaction rate of the radical to the conjugating diene was accelerated with hydrogen‐bonding solvents and with addition of the cation, which may stabilize the N+‐O‐ canonical structure, increasing the unpaired electron density on the S atom. By the photolysis of N‐hydroxypyridine‐2‐thione, the N‐O bond was predominantly dissociated producing a pyr‐idyl‐2‐thio radical. By the photolysis of its anion, photoejection took place followed by the N‐O bond fission, yielding pyridine‐2‐thione.

PHOTOINDUCED ELECTRON TRANSFER BETWEEN FULLERENES (C60/C70) AND ETHYLENEDITHIOTETRATHIAFULVALENE

Electron transfer from ethylenedithiotetrathiafulvalene (EDT-TTF) to photo-excited C 60 /C 70 in polar and in nonpolar solvents has been investigated by laser flash photolysis. The transient absorption band of the triplet state of C60 /C 70 ( 3 C 60 */ 3 C 70 *) was effectively quenched by EDT-TTF with the appearance of the absorption bands of C 60 .- /C 70 .- , showing that the electron transfer takes place from EDT-TTF to 3 C 60 */ 3 C 70 * in polar solvents. Even in non-polar solvents, a small amount of C 61 .- /C 70 .- disappeared by back electron transfer to (EDT - TTF) .+ with second-order kinetics, suggesting the formation of free radical ions. The radical ions disappeared with first-order kinetics in nonpolar solvent, indicating that the radical ions are present as contact ion-pair. In the presence of O 2 , the competition for the quenching of 3 C 60 */ 3 C 70 * takes place between electron transfer from EDT-TTF and energy transfer to O 2 . Protonation reaction of C 60 .- /C 70 .- was also followed in the presence of appropriate proton source.

Laser Photolysis Study on Mechanism and Efficiency of Electron Transfer Between Fullerenes (C60 and C70) and Tetraselenafulvalenes

Abstract Efficient electron-transfer reactions from three kind of tetraselenafulvalenes (TSeFs) to photoexcited triplet state of C60 or C70 in polar solvents have been confirmed by transient absorption spectroscopy observing the decay of 3C60*/3C70* and rise of C60 −•/C70 −•. Growth of single crystal seems to be stimulated by laser irradiation of the solution containing C60 and bis(ethylenedithio)tetraselena-fulvalene (BEDT-TSeF), in which C60 −• was effectively formed.

Photochemical reactions of triplet state of N-hydroxyacridine-9-thione studied by laser-flash photolysis

By laser-flash photolysis of N-hydroxyacridine-9-thione (HOAT) in organic solvents, the transient absorption bands observed at 410 and 530 nm, are assigned to the triplet–triplet absorption of 3(HOAT)*. The lowest triplet energy (ET1), the intrinsic triplet lifetime (τT°) and the quantum yield (ΦT) of intersystem crossing have been determined. A rather large self-quenching rate constant (ksq=1.6±0.1×109 mol-1 d s-1 in THF) was observed. In the photoinduced electron-transfer reactions, 3(HOAT)* acts as electron acceptor for tetramethylbenzidine and triphenylamine, while for dinitrobenzene and the methylviologen dication it acts as electron donor in polar solvents. In the addition reactions of 3(HOAT)* with various alkenes, the electrophilic character of 3(HOAT)* has been established. By comparison of the experimental results with MO calculations, the lowest triplet electronic-configuration of 3(HOAT)* is revealed.

Laser flash photolysis of 1,8-bis (halomethyl) naphthalenes

Abstract Transient absorption spectra were observed by the laser flash photolysis of 1,8-bis(halomethyl)naphthalenes for Cl and Br substituents. In the case of 1,8-bis(chloromethyl)naphthalene, the main transient absorption bands at 340 and 365 nm were attributed to the carbon-centred radical which was formed by bond fission of one of two CCl bonds. In the case of the dibromo derivative, the transient bands appeared at 435 and 350 nm. The 350 nm band was assigned to the carbon-centred radical. Because of the low reactivity to O 2 and triplet quenchers, the main transient absorption band appearing at 435 nm was attributed to the rearranged radicals. The radical structures were based on molecular orbital calculation.

Laser photolysis study of photochemical reactions of triplet states of pyridinethiones

Photochemical reactions of pyridine-4(1H)-thione (4PyT) and pyridine-2(1H)-thione (2PyT) have been studied by nanosecond laser photolysis and steady photolysis methods. The transient absorption bands at 430 and 460 nm are assigned to 3(4PyT)* and 3(2PyT)*, respectively. The lowest triplet energies, triplet lifetimes and quantum yields of intersystem crossing have been determined. Photoinduced electron-transfer reaction from tetramethylbenzidine (TMB) to 3(4PyT)* occurs with a similar rate to that of 3(2PyT)* in a polar solvent. For dinitrobenzene (DNB), an electron-transfer reaction occurred from 3(4PyT)* or 3(2PyT)* to DNB with a diffusion controlled limit. The reactivity of 3(4PyT)* with H-atom donors is higher than that of 3(2PyT)*. The negative ρ-values of the Hammett plots of hydrogen-abstraction rate constants (kh) from p-XC6H4SH indicate that 3(2PyT)* is more electrophilic than 3(4PyT)*. From the addition reaction of 3(2PyT)* to various alkenes, the more electrophilic character of 3(2PyT)* than 3(4PyT)* is also confirmed. By comparison of these experimental results with MO calculations, the lowest electronic configurations of 3(4PyT)* and 3(2PyT)* are attributed to 3(n,π*) and 3(π,π*), respectively.

LASER FLASH PHOTOLYSIS STUDY OF BIS(1,3-BENZOXAZOL-2-YL) DISULFIDE AND BIS(1,3-BENZOTHIAZOL-2-YL) DISULFIDE ; REACTIVITIES OF BENZOXAZOL-2-YLSULFANYL AND BENZOTHIAZOL-2-YLSULFANYL RADICALS

Photo-cleavage of the S–S bond of bis(1,3-benzoxazol-2-yl) disulfide and bis(1,3-benzothiazol-2-yl) disulfide have been studied by nanosecond laser flash photolysis. The transient absorption bands at ca. 590 nm were attributed to the radical species formed by S–S bond fission. Radical addition to conjugated dienes takes place forming the S–C bond, but not the N–C bond, as evidenced by the absence of the thione group in the product, suggesting that unpaired electron density is greater on the S atom of the primary radicals formed. From the decay rates of the radicals, the addition reaction rate constants for 2-methylbuta-1,3-diene are evaluated to be 3.0 × 108 and 5.0 × 108 dm3 mol–1 s–1 in THF at 23 °C, respectively, for the benzoxazol-2-ylsulfanyl radical and the benzothiazol-2-ylsulfanyl radical. The reactivity of the latter radical is slightly higher than that of the former radical for all dienes studied. The low reactivity of these sulfanyl radicals with O2, which is one of the characteristics of the S-centred radical, was confirmed. The rate constants for hydrogen abstraction from cyclohexa-1,4-diene are ca. 106 dm3 mol–1 s–1, which are ca. 1/100 compared with the addition reaction to conjugated dienes. MO calculations have been performed for these radicals to reveal the factors controlling the reactivity of the radicals.

Laser-flash photolysis study of dithiobis (tetrazole); reactivities of tetrazole-thio radical

The photo-cleavage of SS bond of 5,5′-dithiobis (1-phenyl-1H-tetrazole) has been studied by the nanosecond-laser flash photolysis method. The transient absorption band at ca. 430 nm was attributed to 1-phenyl-1H-tetrazole-5-thio radical forming by the SS bond fission. For the reaction with conjugated dienes, an addition reaction takes place forming the SC bond, suggesting that unpaired electron of the radical localizes mainly on the S-atom. From the decay rates of the radical, the addition reaction rate constant for 2-methyl-1,3-butadiene is evaluated to be 5.5 × 109 M−1 s−1 in THF at 23°C, which is as fast as diffusion controlled limit. The reactivity of the radical is ca. 100 times higher than that of the PhS˙. The reactivity of the thio radical to O2 was too low to evaluate, which is one of the characteristics of a S-centered radical. The rate constant for 1,4-cyclohexadienene (1.4 × 108 M−1 s−1) is larger than that of cyclohexene (2.8 × 107 M−1 s−1) suggesting the hydrogen abstraction is a main reaction. The MO calculations have been performed for these radicals to reveal the reason of the high reactivity of the radical.

ULTRASENSITIVE AND ACCURATE ALZHEIMER’S DISEASE DIAGNOSTICS: FROM BENCH TO BEDSIDE

Dementia, Institute Born-Bunge, Antwerp, Belgium; Neurodegenerative Brain Diseases Group, Center for Molecular Neurology, VIB, Antwerp, Belgium; Reference Center for Biological Markers of Dementia, Laboratory of Neurochemistry and Behavior, Institute Born-Bunge, University of Antwerp, Antwerp, Belgium; Institute Born-Bunge, Wilrijk, Belgium; Neurodegenerative Brain Diseases Group, VIB-UAntwerp Center for Molecular Neurology, Antwerp, Belgium; Laboratory of Neurobiology, Institute Born-Bunge, University of Antwerp, Antwerp, Belgium; Institute Born-Bunge, University of Antwerp, Antwerp, Belgium; VU University Medical Center, Amsterdam, Netherlands; Reference Center for Biological Markers of Dementia, Institute Born-Bunge, University of Antwerp, Antwerp, Belgium. Contact e-mail: e.willemse@vumc.nl