Resulting Radical Ion Pair Research Articles

Effects of magnesium salts on photoinduced electron transfer reaction between ammonia, 2,5-dimethylhexa-2,4-diene, and 9-cyanophenanthrene

Mechanistic studies and product analyses were performed on the single electron transfer (SET) promoted photoamination reaction between 2,5-dimethylhexa-2,4-diene ( 1) and 9-cyanophenanthrene in order to gain a comprehensive understanding of the effects of additive metal salts. The product selectivity of this process was found to be dramatically increased by the addition of Mg 2+ salts. An investigation involving the analyses of fluorescence and time-resolved absorption spectroscopic properties reveals that Mg 2+ enhances the rate of the initial SET step and stabilizes the resulting radical ion pair in this photoreaction.

Self-Association and Electron Transfer in Donor−Acceptor Dyads Connected by meta-Substituted Oligomers

The synthesis of a new series of electron donor-acceptor conjugates (5, 10, 13, and 16) in which the electron acceptor--C(60)--and the electron donor--pi-extended tetrathiafulvalene (exTTF)--are bridged by means of m-phenyleneethynylene spacers of variable length is reported. The unexpected self-association of these hybrids was first detected to occur in the gas phase by means of MALDI-TOF spectrometry and subsequently corroborated in solution by utilizing concentration-dependent and variable-temperature (1)H NMR experiments. Furthermore, the ability of these new conjugates to form wirelike structures upon deposition onto a mica surface has been demonstrated by AFM spectroscopy. In light of their photoactivity and redoxactivity, 5, 10, 13, and 16 were probed in concentration-dependent photophysical experiments. Importantly, absorption and fluorescence revealed subtle dissimilarities for the association constants, that is, a dependence on the length of the m-phenylene spacers. The binding strength is in 5 greatly reduced when compared with those in 10, 13, and 16. Not only that, the spacer length also plays a decisive role in governing excited-state interactions in the corresponding electron donor-acceptor conjugates (5, 10, 13, and 16). To this end, 5, in which the photo- and electroactive constituents are bridged by just one aromatic ring, displays--exclusively and independent of the concentration (10(-6) to 10(-4) M)--efficient intramolecular electron transfer events on the basis of a "through-bond" mechanism. On the contrary, the lack of conjugation throughout the bridges in 10 (two m-phenyleneethynylene rings), 13 (three m-phenyleneethynylene rings), and 16 (four m-phenyleneethynylene rings) favors at low concentration (10(-6) M) "through space" intramolecular electron transfer events. These are, however, quite ineffective and, in turn, lead to excited-state deactivations that are at high concentrations (10(-4) M) dominated by intracomplex electron transfer events, namely, between exTTF of one molecule and C(60) of another molecule, and that stabilize the resulting radical ion pair state with lifetimes reaching 4.0 micros.

The factor which determines whether excitation of charge-transfer complexes leads to final net products in comparison with the reactivity on excitation of one of the components.

Selective excitation of charge-transfer complexes of indene or acenaphthylene with various electron acceptors does or does not afford final net reaction products, depending on the free energy of the resulting radical ion pairs over the ground state, -deltaGBET, with threshold values. A similar factor governs the efficiency of the reaction on direct excitation of either the donor or the acceptor of their components, except that it does not fall to nil below the threshold and the reaction affords higher quantum yields than the selective excitation of the charge-transfer complex.

Femtosecond Dynamics of Interfacial and Intermolecular Electron Transfer at Eosin-Sensitized Metal Oxide Nanoparticles

Photoinduced electron injection from eosin Y into the conduction band of titanium dioxide was further scrutinized, as previous studies on the charge injection from xanthene dyes have led to diverging conclusions. Eosin-sensitized TiO2 constitutes in many aspects a model system for studying the dynamics of charge injection: Adsorption of the sensitizer onto the oxide surface through electrostatic interaction and hydrogen bonding make this system exemplary of the weak electronic coupling case. The formation of dimeric eosin on the surface of metal oxide nanoparticles in an aqueous suspension was inferred from the study of the deactivation of the dye's singlet excited state on insulating particles, such as ZrO2 and Al2O3, and of the formation of the resulting radical ion pair, using femtosecond stimulated emission and transient absorption. It was found that dimers undergo ultrafast dismutation with a time constant of 500 fs. The same process occurs also on TiO2 particles and results in a competition between...

Hydrogen-bonding dynamics in photoinduced electron transfer in a ferrocene-quinone linked dyad with a rigid amide spacer.

A newly designed ferrocene-quinone dyad with an amide space (Fc-Q) is employed to examine formation of the hydrogen bonding in the one-electron reduced form (Q*-) and the dynamics in the photoinduced electron-transfer reaction from the ferrocene to the quinone moiety. Photoexcitation of the Q moiety in Fc-Q in deaerated PhCN with 388 nm results in intramolecular electron transfer from Fc to the singlet excited state of Q to produce Fc+-Q*- without changing the conformation (<1 ps), followed by hydrogen bond formation with the amide proton of the spacer (tau = approximately 5 ps). The resulting radical ion pair decays via a back electron transfer to the ground state at a longer time scale with a rate constant of 2.6 x 108 s-1.

Photocycloaddition of acetylacetonatoboron derivatives with simple olefins

Acetylacetonatoboron difluoride (AABF2) and oxalate (AABO) undergo cycloaddition from their singlet excited state with cyclic and terminal olefins to give stereospecific but non-regiospecific products; the latter oxalate reacted slightly faster than the former difluoride. A conjugated diene, 1,3-cyclooctadiene, and methyl substituted benzenes also reacted similarly but with slower rates owing to electron transfer at the encounter stage. The resulting radical ion pairs may undergo radical ion type reactions and/or reverse electron transfer, which leads to the partial waste of the photoenergy. For the same reasons, the reaction occurs more efficiently in non-polar solvents, such as ether, dioxane and methylene chloride than in acetonitrile.

Photoinduced electron transfer between acenaphthylene and 1,4-benzoquinones. Formation of dimers of acenaphthylene and 1 ∶ 1-adducts and effect of excitation mode on reactivity of the charge-transfer complexes

Photochemical reactions of acenaphthylene (ACN) with 1,4-benzoquinones (BQs) of varying reduction potentials in solution have been investigated in order to determine final products and quantum yields of the reactions and to get an insight into the factors which govern their reactivities. The products were the two isomeric dimers of ACN and three types of 1 ∶ 1-adducts (cyclobutanes, furans, and oxetanes) between ACN and BQs. However, the product distribution varied widely with the substitution on the BQ skeleton. Two modes of excitation, that is, selective excitation of the charge transfer (CT) complex (the CT mode; typical wavelength: 546.1 nm) and direct excitation of ACN or BQs (the direct mode; typical wavelength: 435.8 nm), essentially gave similar product distributions when the reaction took place through both modes. However, the direct mode showed higher quantum yields for the reactions, Φ435.8, than the CT mode, Φ546.1. Moreover, Φ546.1 tended to increase with increase of free energy gap, −ΔGBET, between the ground state CT complexes and the resulting radical ion pair (RIP). These observations can be rationalized by a mechanism involving a distinctive RIP as an intermediate generated by photoinduced electron transfer in each mode. Thus, the solvent-separated radical ion pair (SSIP) produced in the direct mode excitation will undergo dissociation to the free radical ions (FRIs), ACN+˙ and BQ−˙, affording final products competing with backward electron transfer (BET). In contrast, the contact radical ion pair (CIP) produced in the CT mode excitation much more rapidly deactivates to the ground state than dissociates to FRIs via SSIP due to faster BET, whose rate depends on −ΔGBET. The 1 ∶ 1-adducts can be formed from either a cage reaction inside the RIP (CIP or SSIP) or reaction between FRIs, whereas the dimers of ACN should be formed from reaction of ACN+˙ with the ground state ACN.

The Chemistry of Peroxynitrite: Involvement of an ET Process in the Radical Nitration of Unsaturated and Aromatic Systems

Reactions of peroxynitrous acid, HPN, with styrene under acidic conditions lead to the oxime 1, the nitrate 2, benzaldehyde (3), and α-nitroacetophenone (4) in overall yields that depend strongly on the pH value and with a product distribution that depends on the dioxygen concentration. The results are rationalized by assuming that HPN undergoes acid-catalyzed decomposition to give nitrous anhydride, or its synthetic equivalent, which is responsible for the regioselective nitration of the styrene double bond by an ET process. The resulting β-nitrobenzyl radical 6 can, depending on the reaction conditions, undergo reversible coupling with nitric oxide to afford the nitroso derivative 7 and then the tautomeric oxime 1, or trapping by dioxygen, eventually leading to products 2, 3, and 4 through the intermediacy of the peroxynitrite derivative 8. Oxime 1 and nitrate 2 are also obtained by treating styrene with nitrous anhydride under protic conditions, the latter being produced in situ from nitric oxide/dioxygen. Similarly to styrene, 1,4-diphenylbutadiene (14) gives radicals 22 and 21 by competitive trapping at the side chain and at the aromatic ring. In turn, radicals 22 and 21 undergo β-fragmentation reactions or trapping by dioxygen with eventual formation of nitrates 16 and 17, cinnamic aldehyde (18), and the diol 15. Finally, the HPN-promoted reaction of p-cresol (27) leads to the 2-nitro derivative 28 through an initial electron-transfer process followed by in cage recombination of the resulting radical ion pair.

Evidence of high charge mobility in photoirradiated polythiophene–fullerene composites

Interpenetrating networks of polythiophenes and fullerene derivatives give rise to charge transfer after irradiation with visible light. The resulting radical ion pairs have been studied by time-resolved EPR spectroscopy. At low temperatures (20 K), the radical pairs are spin correlated and locked at a distance of 27.1 A, whereas an increase of temperature causes the electron and the hole to move further away from each other.

Control of reaction course of the excited state of charge-transfer complexes by the free energy of backward electron transfer

Selective excitation of charge-transfer (CT) complexes between acenaphthylene and various electron acceptors gives net reaction products when the resulting radical ion pairs lie at sufficiently higher energy than the ground state (large –ΔGBET); however, with decrease of –ΔGBET, these tend to become less reactive and finally non-reactive.

Photooxygenation of polycyclic aromatic hydrocarbons by pyrimido(5,4-g)pteridine N-oxide.

Photooxygenation of naphthalene (4), phenanthrene (5), pyrene (6), and benzo [a] pyrene (7) by pyrimido [5,4-g] pteridine N-oxide (1) was examined in comparison with that of benzene. These polycyclic aromatic hydrocarbons (PAHs) consumed 1 more smoothly than did benzene under irradiation with ultraviolet-visible light, as expected from the fact that their oxidation potentials for conversion to the corresponding phenols and/or further oxidized products are lower. The photooxygenation of the PAHs occurred predominantly at the most reactive position of the corresponding cation-radicals. These and other experimental results led us to conclude that the PAHs are oxygenated via photo-induced single-electron transfer (SET) from the PAHs to 1 followed by oxygen-atom transfer between the resulting radical-ion pairs. The occurrence of the SET in an excited charge-transfer complex formed between the PAHs and 1 is at variance with the case of benzene