CHEMILUMINESCENCE and EPR STUDIES ON THE EXCITATION SITE OF FERRIC-HEME-OXO COMPLEXES OF NATURAL and MODEL HEME SYSTEMS

Chemiluminescence was detected both in the reaction system of H₂O₂ plus heme proteins such as methemo- and metmyoglobin and ferric-protoheme complexes used as a model system. The intensity of chemiluminescence was found to be mediated by ligand binding to the sixth coordination site of the ferric-protoheme compounds, e.g. chemiluminescence was not observed with the bisimidazole ferric-protoheme complex. On the other hand the pentacoordinated histidine ferric-protoheme complex exhibited strong light emission. Comparative studies with various ligand-heme compounds elucidated that light emission was inversely correlated with the binding strength of the respective ligand at the sixth coordination site. The basic reaction mechanism causing the establishment of an excited state was studied by monitoring chemiluminescence and EPR signal formation of ligand-modified heme proteins in the presence of different electron donors. External electron donors such as Trolox C, TMPD and ascorbic acid affected a strong reduction in the development of chemiluminescence suggesting the essential involvement of an inner-molecular electron transfer process. Our results allow the conclusion that chemiluminescence is generated from the decay of an excited state of oxo-heme compounds established as a result of a one electron transfer step from a ligand group to heme iron.     

Redox photochemistry of thiouredopyrenetrisulfonate

1-Thiouredopyrene-3,6,8-trisulfonate (TUPS) has recently been used as a photoinduced covalent redox label capable of reducing various cofactors of proteins. A new reaction of this dye, whereby its excited triplet state oxidizes suitable electron donors, is now reported. The characteristic difference spectrum of the reduced radical of TUPS is determined. We also observe the self-exchange electron transfer between two TUPS molecules in their triplet excited states and determine the reaction scheme and the rate constants of the various pathways in the process of triplet depletion. The ability of photoexcited TUPS to withdraw an electron from reduced cytochrome-c is also observed. It is thus demonstrated that TUPS is an appropriate photoinduced covalent redox label for initiating both the oxidative and reductive phases of electron transfer processes in biological macromolecules.     

The Witkop Cyclization: A Photoinduced CH Activation of the Indole System

Investigations of excited‐state redox processes have an exceptional impact on the field of organic photochemistry and its application to the synthesis of complex target molecules. In such a photochemical process a single‐electron transfer takes place to produce ion‐radical intermediates, if the reduction and oxidation potentials, as well as excited‐state energies of electron donors and acceptors are chosen appropriately. The Witkop cyclization constitutes an intramolecular variant of such a process, typically with an indole heterocycle as an electron donor. The specific synthetic value of this reaction lies in a C? C bond formation without requiring any prefunctionalization of the indole system. Although this photoreaction has been applied to the total synthesis of natural products, it has still not been used to its full capacity. The following review details synthetic efforts using the Witkop cyclization, and aims to incite further applications of reaction in the synthesis of complex molecular architectures.     

Experimental investigation of excited state electron transfer reactions between some bicyclic molecules and tetracyanoquinodimethane (TCNQ)

Investigations on photoinduced electron transfer (ET) reactions between excited (ground) bicyclic electron donors 5,6,7,8-tetrahydro-2-naphthol (TH2N), 2-methoxy-5,6,7,8-tetrahydro naphthalene (2MTHN) and ground state (excited) acceptor tetracyanoquinodimethane (TCNQ) in fluid solutions of different polarity at the ambient temperature (300 K) by electronic absorption, steady state fluorescence and time-resolved spectroscopic methods in the time domain of nanosecond order have been carried out. It is suggested that in highly polar solvent acetonitrile (ACN), a loosely-structured transient geminate ion-pair complex (GIP) in the excited singlet state (S₁) is formed due to the ET encounter between the present donor TH2N or 2MTHN and TCNQ and this GIP complex rapidly dissociates into stable excited radical ions, as evidenced from steady state spectra. In polar DMF solvents, TCNQ exhibits an electronic absorption band of its anion without the presence of donor molecules. Both steady state and time-resolved data indicate that ET reactions between the present donors and acceptor TCNQ are largely impeded in the less polar solvent tetrahydrofuran (THF). In the highly polar solvent ACN, ET reactions between the donors and acceptor TCNQ have been suggested to be of adiabatic or intermediate between adiabatic and non-adiabatic types, from the observation of radical ion species in the electronic excited state. For some bicyclic donors and TCNQ acceptor systems, large negative ΔG, which is a measure of the gap between locally excited and radical ion-pair states, shows reaction occurs in highly exothermic regions. Further observations of −ΔG>λ, nuclear reorganization energy parameters and the decrement of ET rate (k_ET) with increasing exothermicity (more negative ΔG values) suggest the ET reaction for the bicyclic donor—TCNQ acceptor systems studied in the present investigation might occur in the Marcus inverted region. The possibility of building up efficient photoconducting materials with the present donor acceptor systems is suggested.     

Conformationally constrained macrocyclic diporphyrin-fullerene artificial photosynthetic reaction center

Photosynthetic reaction centers convert excitation energy from absorbed sunlight into chemical potential energy in the form of a charge-separated state. The rates of the electron transfer reactions necessary to achieve long-lived, high-energy charge-separated states with high quantum yields are determined in part by precise control of the electronic coupling among the chromophores, donors, and acceptors and of the reaction energetics. Successful artificial photosynthetic reaction centers for solar energy conversion have similar requirements. Control of electronic coupling in particular necessitates chemical linkages between active component moieties that both mediate coupling and restrict conformational mobility so that only spatial arrangements that promote favorable coupling are populated. Toward this end, we report the synthesis, structure, and photochemical properties of an artificial reaction center containing two porphyrin electron donor moieties and a fullerene electron acceptor in a macrocyclic arrangement involving a ring of 42 atoms. The two porphyrins are closely spaced, in an arrangement reminiscent of that of the special pair in bacterial reaction centers. The molecule is produced by an unusual cyclization reaction that yields mainly a product with C(2) symmetry and trans-2 disubstitution at the fullerene. The macrocycle maintains a rigid, highly constrained structure that was determined by UV-vis spectroscopy, NMR, mass spectrometry, and molecular modeling at the semiempirical PM6 and DFT (B3LYP/6-31G**) levels. Transient absorption results for the macrocycle in 2-methyltetrahydrofuran reveal photoinduced electron transfer from the porphyrin first excited singlet state to the fullerene to form a P(?+)-C(60)(?-)-P charge separated state with a time constant of 1.1 ps. Photoinduced electron transfer to the fullerene excited singlet state to form the same charge-separated state has a time constant of 15 ps. The charge-separated state is formed with a quantum yield of essentially unity and has a lifetime of 2.7 ns. The ultrafast charge separation coupled with charge recombination that is over 2000 times slower is consistent with a very rigid molecular structure having a small reorganization energy for electron transfer, relative to related porphyrin-fullerene molecules.     

Photoalkylation of 10-alkylacridinium ion via a charge-shift type of photoinduced electron transfer controlled by solvent polarity

A charge-shift type of photoinduced electron-transfer reactions from various electron donors to the singlet excited state of 10-decylacridinium cation (DeAcrH+) in a nonpolar solvent (benzene) is found to be as efficient as those of 10-methylacridinium cation (MeAcrH+) and DeAcrH+ in a polar solvent (acetonitrile). Irradiation of the absorption bands of MeAcrH+ in acetonitrile solution containing tetraalkyltin compounds (R(4)Sn) results in the efficient and selective reduction of MeAcrH+ to yield the 10-methyl-9-alkyl-9,10-dihydroacridine (AcrHR). The same type of reaction proceeds in benzene when MeAcrH+ is replaced by DeAcrH+ which is soluble in benzene. The photoalkylation of R'AcrH+ (R' = Me and De) also proceeds in acetonitrile and benzene using 4-tert-butyl-1-benzyl-1,4-dihydronicotinamide (Bu(t)BNAH) instead of R(4)Sn, yielding MeAcrHBu(t). The quantum yield determinations, the fluorescence quenching of R'AcrH+ by electron donors, and direct detection of the reaction intermediates by means of laser flash photolysis experiments indicate that the photoalkylation of R'AcrH+ in benzene as well as in acetonitrile proceeds via photoinduced electron transfer from the alkylating agents (R(4)Sn and Bu(t)BNAH) to the singlet excited states of R'AcrH+. The limiting quantum yields are determined by the competition between the back electron-transfer process and the bond-cleavage process in the radical pair produced by the photoinduced electron transfer. The rates of back electron transfer have been shown to be controlled by the solvent polarity which affects the solvent reorganization energy of the back electron transfer. When the free energy change of the back electron transfer (DeltaG(0)(bet)) in a polar solvent is in the Marcus inverted region, the rate of back electron transfer decreases with decreasing the solvent polarity, leading to the larger limiting quantum yield for the photoalkylation reaction. In contrast, the opposite trend is obtained when the DeltaG(0)(bet) value is in the normal region: the limiting quantum yield decreases with decreasing the solvent polarity.     

PHOTO-INDUCED INTERMOLECULAR AND INTRAMOLECULAR ELECTRON-TRANSFER REACTIONS BETWEEN XANTHENE DYES AND DONORS/ACCEPTORS

A number of electron donors, acceptors and diads containing xanthene dyes were sythesized. When the dyes were excited, the rate constants and the efficiencies of the intermolecular and intramolecular photo-induced electron transfer reactions were determined and calculated. It is found that the photo-induced electron transfer reactions occurred between xanthene dyes and many, including very weak donors or acceptors. The rate constants of intermolecular reactions were controlled by diffusion, and influenced by the reactant concentrations. The laser flash experiments showed that for low reactant concentrations, this kind of reactions took place mainly via the triplet excited state of the dyes. If different electric charges exist with dyes and donors/acceptors, there will be static quenching of the dyes' fluorescence. The intramolecular electron transfer reactions are independent of the solution concentrations, and they may directly proceed via the singlet excited state of the dyes effectively.     

Photogeneration of 2-deoxyribonolactone in benzophenone-purine dyads. Formation of ketyl-C1' biradicals

Photolysis of the title dyads under aerobic conditions leads to a 2-deoxyribonolactone derivative. Laser flash photolysis reveals that the process occurs from the short-lived benzophenone-like triplet excited state. A mechanism involving intramolecular electron transfer with the purine bases (adenine, guanine, or 8-oxoadenine) as donors is proposed.     

Electrocatalytic reduction of Molecular Oxygen with a Copper (II) Coordination Polymer

Oxygen reduction at the polarized water/1,2-dichloroethane (DCE) interface catalyzed by a Cu (II) coordination polymer (Cu–pol) was studied with two lipophilic electron donors ferrocene (Fc) and tetrathiafulvalene (TTF). The results of the ion transfer voltammetry and two-phase shake flask experiments suggest proceeding of the catalytic reaction as proton-coupled electron transfer reduction of oxygen to hydrogen peroxide and water. In this process, while the protons supplied from the aqueous phase, the electrons provided from the organic phase by the weak electron donor, Fc. The O₂ molecule takes a superoxide structure with Cu–pol which resulted to hydrogen peroxide or water on reduction. Furthermore, the results revealed that the apparent rate constant of TTF + Cu-pol is higher than that of Fc + Cu-pol system due to the faster kinetic reaction of TTF with respect to Fc.     

Metal phthalocyanine-sensitized photoreduction of nitro-compound

Metal phthalocyanine-sensitized photoreduction of dimethyl 4-nitrophthalate with ascorbic acid has been investigated. The primary photoreaction products are the corresponding amino-and hydroxylamino-compounds. The azoxy-compound is formed by coupling of the nitrosocompound with hydroxylamino-compound in the presence of air through secondary dark reaction. The redox potential and fluorescence quantum yield are also determined. The variation of the quantum yield of the sensitized photoreduction, the relative fluorescence quantum yield and their product with the concentration of nitro-compound has been examined. The efficiency of photoreduction sensitized by the excited singlet and triplet state of metal phthalocyanine has been also calculated. It is believed that electron transfer from the excited metal phthalocyanine to the nitro-compound is the initial process in the sensitized photoreduction. Quenching by electron transfer involves creation of an ion pair. Charge separation and back electron transfer is then a competitive process. Due to the spin selection rules, the efficiency of photoreduction sensitized by excited triplet state of metal phthalocyanine is higher than excited singlet state. Thus, a necessary requirement for a good sensitizer is that the triplet state is populated in high yield. An alternative way and also the aim of our work is to design a suitable phthalocyanine skeleton to overcome geminate recombination of the ion pair, in order to increase the efficiency of photoreduction sensitized by sir glet excited state of the sensitizer, so as to increase the quantum yield of the total sensitized photoreduction.     

PHOTOSENSITIZATION OF PYRIMIDINE DIMER SPLITTING BY A COVALENTLY BOUND INDOLE

Abstract— Photosensitized pyrimidine dimer splitting characterizes the enzymatic process of DNA repair by the DNA photolyases. Possible pathways for the enzymatic reaction include photoinduced electron transfer to or from the dimer. To study the mechanistic photochemistry of splitting by a sensitizer representative of excited state electron donors, a compound in which an indole is covalently linked to a pyrimidine dimer has been synthesized. This compound allowed the quantitative measurement of the quantum efficiency of dimer splitting to be made without uncertainties resulting from lack of extensive preassociation of the unlinked dimer and sensitizer free in solution. Irradiation of the compound with light at wavelengths absorbed only by the indolyl group (approximately 280 nm) resulted in splitting of the attached dimer. The quantum yield of splitting of the linked system dissolved in N₂0-saturated aqueous solution was found to be 0.04 ± 0.01. The fluorescence typical of indoles was almost totally quenched by the attached dimer. A splitting mechanism in which an electron is efficiently transferred intramolecularly from photoexcited indole to ground state dimer has been formulated. The surprisingly low quantum yield of splitting has been attributed to inefficient splitting of the resulting dimer radical anion. Insights gained from this study have important mechanistic implications for the analogous reaction effected by the DNA photolyases.     

Covalently linked acceptor-donor systems based on isoquinoline N-oxide acceptor: photoinduced electron transfer produces dual-channel luminescent systems that evolve chemically to photohydroxylation of the aromatic donor

Acceptor-donor compounds containing the isoquinoline N-oxide acceptor and (methoxy)(n)benzene (n = 0, 1, 2, 3) electron donors were studied. The two chromophores are connected by a CH(2) bridging unit. All acceptor-donor compounds exhibit photoinduced electron transfer in acid medium that results in the formation of a charge-transfer (CT) state. Measurements of the corresponding electronic emission spectra revealed that these bichromophoric systems exhibit a dual fluorescence that is strongly dependent on the protonation of the N-oxide function and the donor ability. The CT state responsible for the red-shifted luminescence in the studied compounds is directly connected with the initial excited state S(1). On the basis of the spectroscopic and photochemical evidence, N[bond]O scission is the dominant primary photochemical process involving the CT state, the subsequent radical coupling resulting in efficient aromatic hydroxylation. The outcome of both quenching and sensitization experiments confirms this assertion. The results strongly suggest that the ensuing photohydroxylation reaction is not a concerted process, but rather a two-step N[bond]O scission followed by C[bond]O formation, which is regioselectively guided by the electronic distribution of the resulting donor cation-radical.     

Time-resolved EPR studies of the benzophenone—diphenyl kethyl radical system. Possible evidence for quartet—doublet intersystem crossing

The photoreduction of benzophenone by several substrates has been studied by time-resolved EPR. When cyclohexadiene, MTHF, and decane are employed as hydrogen donors to the excited triplet state of benzophenone the diphenyl ketyl radical which is produced exhibits emissive electron spin polarization with rise times on the order of a few microseconds. It is suggested that the experimental observations can be explained by intersystem crossing to a quartet state in the excited ketyl radical.     

Photoreduction of Ru(bpy)(3)2+ by amines in aqueous solution. Kinetics characterization of a long-lived nonemitting excited state

The photochemistry of Ru(bpy)(3)+2 in the presence of amines was investigated in water by laser flash photolysis. N,N'-Dimethylaniline and p-phenylenediamine quench the luminescent metal to ligand charge transfer (MLCT) excited state of the complex by an electron transfer reaction that produces the semireduced form Ru(bpy)3+ in relatively high yields. On the other hand, triethylamine (TEA) and aniline do not quench the MLCT. Nevertheless, when laser flash irradiation at 532 nm is carried out in the presence of these amines, the formation of Ru(bpy)3+ is clearly detected by its transient absorption at 510 nm. These results are interpreted by an electron transfer reaction with the participation of a nonemitting excited state of the complex, formed independently of the MLCT from the Franck-Condon or the relaxed singlet excited state. The rate constants for the quenching of this state by TEA and aniline and the quantum yields for Ru(bpy)(3)+ were determined. The new state is formed in a very fast process and has a lifetime of ca 4 micros in water.     

Comparison of photoinduced electron transfer reactions of C60 and aromatic carbonyl compounds

The one-electron reduction potential of the triplet excited state of C₆₀ is similar to those of some aromatic carbonyl compounds. Thus, photoinduced electron transfer is expected to occur from the common electron donors to both C₆₀ and aromatic carbonyl compounds. In this paper comparison is made between photoinduced electron transfer from organosilanes and organostannanes used as the electron donors to the triplet excited states of C₆₀ and aromatic carbonyl compounds, providing valuable insight into their common mechanistic features for the C-C bond formation via photoinduced electron transfer as well as the new functionalization method of C₆₀.     

Reactivities of carboxyalkyl-substituted 1,4,5,8-naphthalene diimides in aqueous solution

A series of water-soluble 1,4,5,8-naphthalene diimide derivatives has been prepared and their redox and photophysical properties characterized. From laser flash photolysis studies, the triplet excited state of N,N'-bis[2-(N-pyridinium)ethyl]-1,4,5,8-naphthalene diimide (NDI-pyr) was found to undergo oxidative quenching with the electron donors DABCO, tyrosine, and tryptophan as expected from thermodynamics. Interestingly, the reactivities of naphthalene diimides (NDI) possessing alpha- and beta-carboxylic acid substituents (R = -CH2COO-, -C(CH3)2COO-, and -CH2CH2COO-) were strikingly different. In these compounds, the transient produced upon 355 nm excitation did not react with the electron donors. Instead, this transient reacted rapidly (k > 10(8)-10(9) M-1 s-1) with known electron acceptors, benzyl viologen and ferricyanide. The transient spectrum of the carboxyalkyl-substituted naphthalimides observed immediately after the laser pulse was nearly identical to the one-electron-reduced form of 1,4,5,8-naphthalene diimide (produced independently using the bis-pyridinium-substituted naphthaldiimide). From our studies, we conclude that the transient produced upon nanosecond laser flash photolysis of NDI-(CH2)nCOO- is the species produced upon intramolecular electron transfer from the carboxylate moiety to the singlet excited state of NDI. In separate experiments, we verified that the singlet excited state of NDI-pyr does, indeed, react intermolecularly with acetate, alanine, and glycine. The process is further substantiated using thermodynamic driving force calculations. The results offer new prospects of the efficient photochemical production of reactive carbon-centered radicals.     

Radiationless decay in exciplexes with variable charge transfer

Rate constants for radiative decay, radiationless decay, and intersystem crossing are reported for a series of excited states formed by reaction of cyanoanthracene acceptors with alkylbenzenes as donors in several solvents of moderate to low polarity. The excited states have widely varying degrees of charge transfer, from essentially pure electron transfer states to pure locally excited states. The data illustrate the fundamental factors that control the contrasting relative efficiencies of radiative and radiationless processes in electron transfer compared to locally excited states. The radiationless decay rate constants can be described quantitatively as a function of the extent of charge transfer using weighted contributions from a locally excited decay mechanism and a pure electron-transfer type mechanism. The factors that control the rate constants for radiationless decay in excited states with intermediate charge-transfer character are discussed.     

THE FORMATION OF HYDRATED ELECTRONS FROM THE EXCITED STATE OF INDOLE DERIVATIVES

Abstract— –Hydrogen atoms can be observed in u.v. irradiated aqueous solutions of indole derivatives. These H' atoms are produced in a reaction between H⁺ and solvated electrons which are formed in the excited state of indole. Protons are also known to be good quenching agents for the fluorescence of indole. However the pH dependence and effect of oxygen on the yield of hydrogen atoms indicates clearly that although both fluorescence and electron ejection originate in the excited singlet state the fluorescence quenching by protons is not caused by a transfer of electronic charge from the excited ring to H⁺. The temperature dependencies of both fluorescence and electron ejection yield an abnormally large "activation energy". It is proposed that this temperature dependence is to a large extent determined by a process characteristic of water as a solvent.     

Theoretical investigation of charge transfer excitation and charge recombination in acenaphthylene–tetracyanoethylene complex

Ab initio calculations were performed to investigate the charge separation and charge recombination processes in the photoinduced electron transfer reaction between tetracyanoethylene and acenaphthylene. The excited states of the charge‐balanced electron donor–acceptor complex and the singlet state of ion pair complex were studied by employing configuration interaction singles method. The equilibrium geometry of electron donor–acceptor complex was obtained by the second‐order Møller–Plesset method, with the interaction energy corrected by the counterpoise method. The theoretical study of ground state and excited states of electron donor–acceptor complex in this work reveals that the S₁ and S₂ states of the electron donor–acceptor complexes are excited charge transfer states, and charge transfer absorptions that corresponds to the S₀ → S₁ and S₀ → S₂ transitions arise from π–π* excitations. The charge recombination in the ion pair complex will produce the charge‐balanced ground state or excited triplet state. According to the generalized Mulliken–Hush model, the electron coupling matrix elements of the charge separation process and the charge recombination process were obtained. Based on the continuum model, charge transfer absorption and charge transfer emission in the polar solvent of 1,2‐dichloroethane were investigated. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 23–35, 2003     

Scandium ion-promoted photoinduced electron transfer from electron donors to acridine and pyrene. Essential role of scandium ion in photocatalytic oxygenation of hexamethylbenzene

Photoinduced electron transfer from a variety of electron donors including alkylbenzenes to the singlet excited state of acridine and pyrene is accelerated significantly by the presence of scandium triflate [Sc(OTf)(3)] in acetonitrile, whereas no photoinduced electron transfer from alkylbenzenes to the singlet excited state of acridine or pyrene takes place in the absence of Sc(OTf)(3). The rate constants of the Sc(OTf)(3)-promoted photoinduced electron-transfer reactions (k(et)) of acridine to afford the complex between acridine radical anion and Sc(OTf)(3) remain constant under the conditions such that all the acridine molecules form the complex with Sc(OTf)(3). In contrast to the case of acridine, the k(et) value of the Sc(OTf)(3)-promoted photoinduced electron transfer of pyrene increases with an increase in concentration of Sc(OTf)(3) to exhibit first-order dependence on [Sc(OTf)(3)] at low concentrations, changing to second-order dependence at high concentrations. The first-order and second-order dependence of k(et) on [Sc(OTf)(3)] is ascribed to the 1:1 and 1:2 complexes formation between pyrene radical anion and Sc(OTf)(3). The positive shifts of the one-electron redox potentials for the couple between the singlet excited state and the ground-state radical anion of acridine and pyrene in the presence of Sc(OTf)(3) as compared to those in the absence of Sc(OTf)(3) have been determined by adapting the free energy relationship for the photoinduced electron-transfer reactions. The Sc(OTf)(3)-promoted photoinduced electron transfer from hexamethylbenzene to the singlet excited state of acridine or pyrene leads to efficient oxygenation of hexamethylbenzene to produce pentamethylbenzyl alcohol which is further oxygenated under prolonged photoirradiation of an O(2)-saturated acetonitrile solution of hexamethylbenzene in the presence of acridine or pyrene which acts as a photocatalyst together with Sc(OTf)(3). The photocatalytic oxygenation mechanism has been proposed based on the studies on the quantum yields, the fluorescence quenching, and direct detection of the reaction intermediates by ESR and laser flash photolysis.