One-electron oxidation of alcohols by the 1,3,5-trimethoxybenzene radical cation in the excited state during two-color two-laser flash photolysis

One-electron oxidation of alcohols such as methanol, ethanol, and 2-propanol by 1,3,5-trimethoxybenzene radical cation (TMB*+) in the excited state (TMB*+*) was observed during the two-color two-laser flash photolysis. TMB*+ was formed by the photoinduced bimolecular electron-transfer reaction from TMB to 2,3,5,6-tetrachlorobenzoquinone (TCQ) in the triplet excited-state during the first 355-nm laser flash photolysis. Then, TMB*+* was generated from the selective excitation of TMB*+ during the second 532 nm laser flash photolysis. Hole transfer rate constants from TMB*+* to methanol, ethanol, and 2-propanol were calculated to be (5.2 +/- 0.5) x 10(10), (1.4 +/- 0.3) x 10(11), and (3.2 +/- 0.6) x 10(11) M-1 s-1, respectively. The order of the hole transfer rate constants is consistent with oxidation potentials of alcohol. Formation of TCQH radical (TCQH*) with a characteristic absorption peak at 435 nm was observed in the microsecond time scale, suggesting that deprotonation of the alcohol radical cation occurs after the hole transfer and that TCQ radical anion (TCQ*-), generated together with TMB*+ by the photoinduced electron-transfer reaction, reacts with H+ to give TCQH*.     

Photodissociation of naphthalene dimer radical cation during the two-color two-laser flash photolysis and pulse radiolysis-laser flash photolysis

Photodissociation of naphthalene (Np) dimer radical cation (Np2*+) to give naphthalene radical cation (Np*+) and Np and the subsequent regeneration of Np2*+ by the dimerization of Np*+ and Np were directly observed during the two-color two-laser flash photolysis in solution at room temperature. When Np2*+ was excited at the charge-resonance (CR) band with the 1064-nm laser, the bleaching and recovery of the transient absorption at 570 and 1000 nm, assigned to the local excitation (LE) and CR bands of Np2*+, respectively, were observed together with the growth and decay of the transient absorption at 685 nm, assigned to Np*+. The dissociation of Np2*+ proceeds via a one-photon process within the 5-ns laser flash to give Np*+ and Np in the quantum yield of 3.2 x 10(-3) and in the chemical yield of 100%. The recovery time profiles of Np2*+ at 570 and 1000 nm were equivalent to the decay time profile of Np*+ at 685 nm, suggesting that the dimerization of Np*+ and Np occurs to regenerate Np2*+ in 100% yield. Similar experimental results of the photodissociation and regeneration of Np2*+ were observed during the pulse radiolysis-laser flash photolysis of Np in 1,2-dichloroethane. The photodissociation mechanism can be explained based on the crossing between two potential surfaces of the excited-state Np2*+ and ground-state Np*+.     

Inhibition of the formation and decay of stilbene core radical cations by the dendron during the photoinduced electron transfer

Formation and decay processes of stilbene core radical cation (ST*+) during the photoinduced electron transfer have been studied for a series of stilbene bearing benzyl ether-type dendrons (D). ST*+ and the radical cation of peripheral dendron (D*+) were generated by intermolecular hole transfer from biphenyl radical cation, which was generated from photoinduced electron transfer from biphenyl to the singlet-excited 9,10-dicyanoanthracene in a mixture of acetonitrile and 1,2-dichloroethane (3:1). An intramolecular dimer radical cation of benzyl groups at the terminal of stilbene dendrimer was indicated as a hole trapping site. Subsequent hole transfer from the trapping site to the core ST generated ST*+. The shielding effects of D depending on the dendrimer generation on the growth and decay of ST*+ were observed. It was revealed for the first time that D acts as the hole trapping site and the hole conductor on the way of the exothermic hole transfer from the terminal of D to the central core ST. We also found that D inhibits the charge recombination with 9,10-dicyanoanthracene radical anion because of the steric hindrance.     

The rearrangement of the cubane radical cation in solution

The rearrangement of the cubane radical cation (1*+) was examined both experimentally (anodic as well as (photo)chemical oxidation of cubane 1 in acetonitrile) and computationally at coupled cluster, DFT, and MP2 [BCCD(T)/cc-pVDZ//B3LYP/6-31G* ZPVE as well as BCCD(T)/cc-pVDZ//MP2/6-31G* + ZPVE] levels of theory. The interconversion of the twelve C2v degenerate structures of 1*+ is associated with a sizable activation energy of 1.6 kcalmol(-1). The barriers for the isomerization of 1*- to the cuneane radical cation (2*+) and for the C-C bond fragmentation to the secocubane-4,7-diyl radical cation (10*+) are virtually identical (deltaH0++ = 7.8 and 7.9 kcalmol(-1), respectively). The low-barrier rearrangement of 10*+ to the more stable syn-tricyclooctadiene radical cation 3*+ favors the fragmentation pathway that terminates with the cyclooctatetraene radical cation 6*+. Experimental single-electron transfer (SET) oxidation of cubane in acetonitrile with photoexcited 1,2,4,5-tetracyanobenzene, in combination with back electron transfer to the transient radical cation, also shows that 1*+ preferentially follows a multistep rearrangement to 6*+ through 10*+ and 3*+ rather than through 2*+. This was confirmed by the oxidation of syn-tricyclooctadiene (3), which, like 1, also forms 6 in the SET oxidation/rearrangement/electron-recapture process. In contrast, cuneane (2) is oxidized exclusively to semibullvalene (9) under analogous conditions. The rearrangement of 1*+ to 6*+ via 3*+, which was recently observed spectroscopically upon ionization in a hydrocarbon glass matrix, is also favored in solution.     

Stepwise hydration and multibody deprotonation with steep negative temperature dependence in the benzene.+ -water system

We studied the stepwise hydration and solvent-mediated deprotonation of the benzene*+ cation (Bz*+) and found several unusual features. The solvent binding energies DeltaH on-1,n for the reactions Bz*+(H2O)n-1 + H2O --> Bz*+(H2O)n are nearly constant at 9 +/- 1 kcal mol-1 for n = 1 to 8. We observed a remarkable sudden decrease in the entropy of association accompanying the formation of Bz*+(H2O)7 and Bz*+(H2O)8, indicating strong orientational restraint in the hydration shells of these clusters consistent with the formation of cagelike structures. We observed the size-dependent deprotonation of Bz*+ in a cooperative multibody process, where n H2O molecules (n >/= 4) can remove a proton from Bz*+ to form protonated water clusters. We measured, for the first time, the temperature dependence of such a process and found a negative temperature coefficient of a magnitude unprecedented in any chemical reaction, of the form k = AT-67+/- 4, or in an Arrhenius form having an activation energy of -34 +/- 1 kcal mol-1. The temperature effect may be explained by Bz*+ and four H2O molecules needing to be assembled from gas-phase components to form the reactive species. Such large temperature effects may be therefore general in solvent cluster-mediated reactions.     

Kinetics of weak distance-dependent hole transfer in DNA by adenine-hopping mechanism

The kinetics of hole transfer in DNA by adenine-hopping mechanism was investigated by the combined pulse radiolysis-laser flash photolysis method. The hole transfer from Ptz*+* to oxG across the (A)n-bridge preceded by the A-hopping mechanism and the weak distance-dependent hole transfer with the rates faster than 108 s-1 over the distance range of 7-22 A was demonstrated. In contrast, hole transfer from oxG*+ to Ptz followed the single-step super exchange mechanism. Thus, two different processes for the hole transfer across the identical (A)n-bridge in DNA have been demonstrated. The results clearly show that the mechanism of hole transfer in DNA strongly depends on the redox nature of the oxidant, whether it produces only G*+ or both A*+ and G*+.     

Photochemical reactions of electron-deficient olefins with N,N,N′,N′-tetramethylbenzidine via photoinduced electron-transfer

Photoinduced electron transfer reactions of several electron-deficient olefins with N,N,N′,N′-tetramethylbenzidine (TMB) in acetonitrile solution have been studied by using laser flash photolysis technique and steady-state fluorescence quenching method. Laser pulse excitation of TMB yields ³TMB* after rapid intersystem crossing from ¹TMB*. The triplet which located at 480 nm is found to undergo fast quenching with the electron acceptors fumaronitrile (FN), dimethyl fumarate (DMF), diethyl fumarate (DEF), cinnamonitrile (CN), -acetoxyacrylonitrile (AAN), crotononitrile (CrN) and 3-methoxyacrylonitrile (MAN). Substituents binding to olefin molecule own different electron-donating/withdrawing powers, which determine the electron-deficient property (π-cloud density) of olefin molecule as well as control the electron transfer rate constant directly. The detection of ion radical intermediates in the photolysis reactions confirms the proposed electron transfer mechanism, as expected from thermodynamics. The quenching rate constants of triplet TMB by these olefins have been determined at 510 nm to avoid the disturbance of formed TMB cation radical around 475 nm. All the values approach or reach to the diffusion-controlled limit. In addition, fluorescence quenching rate constants have been also obtained by calculating with Stern–Volmer equation. A correlation between experimental electron transfer rate constants and free energy changes has been explained by Marcus theory of adiabatic outer-sphere electron transfer. Disharmonic k_q values for CN and CrN in endergonic region may be the disturbance of exciplexs formation.     

Structural and environmental requirements for quenching of singlet oxygen by cyanine dyes

Singlet-oxygen quenching constants were measured for 19 cyanine dyes in acetonitrile. The most efficient quenchers were 1-butyl-2-[2-[3-[(1-butyl-6-chlorobenz-[cd]indol-2(1H)- ylidene)ethylidene]-2-chloro-1-cyclohexen-1-yl]ethenyl]-6-chlorobenz[cd] indolium and 6-chloro-2-[2-[3-(6-chloro-1-ethylbenz[cd]indol-2(1H)-ylidene) ethylidene]-2-phenyl-1-cyclopenten-1-yl]ethenyl]-1-ethyl-benz[cd]indolium, having quenching constants with diffusion-controlled values of 2.0 +/- 0.1 x 10(10) and 1.5 +/- 0.1 x 10(10) M-1 s-1, respectively. There was a trend toward increased quenching constants for cyanine dyes with the absorption band maxima at longer wavelengths. However, the quenching constants correlated better with the oxidation potentials of the cyanine dyes, suggesting that quenching proceeds by charge transfer rather than energy transfer. The quenching constants for 1,1',3,3,3',3'-hexamethylindotricarbocyanine perchlorate and 1,1'-diethyl-4,4'-carbocyanine iodide were measured in several solvents as well as in aqueous solutions of detergent micelles. In different solvents, the quenching constants varied by as much as a factor of 50. The quenching constants were largest in solvents with the highest values on the pi* scale of Kamlet, Abboud, Abraham and Taft. This was consistent with quenching occurring by charge transfer. Within cells, cyanine dyes concentrate in membrane-bound organelles. The quenching constants were substantial within detergent micelles. To the extent that micelles are models for biological membranes, cyanine dyes may be effective biological singlet-oxygen quenchers.     

Properties of excited radical cations of substituted oligothiophenes

Excited-state properties of radical cations of substituted oligothiophenes ( nT (*+), n denotes the number of thiophene rings, n = 3, 4, 5) in solution were investigated by using various laser flash photolysis techniques including two-color two-laser flash photolysis. nT (*+) generated by photoinduced electron transfer to p-chloranil or resonant two-photon ionization (RTPI) by using the first 355-nm ns laser irradiation was selectively excited with the second picosecond laser (532 nm). Bleaching of the absorption of nT (*+) together with growth of a new absorption was observed during the second laser irradiation, indicating the generation of nT (*+) in the excited state ( nT (*+)*). The D 1 state lifetime was estimated to be 34 +/- 4, 24 +/- 2, and 18 +/- 1 ps for 3T (*+), 4T (*+), and 5T (*+), respectively. In the presence of hole acceptor (Q), bleaching of nT (*+) and growth of Q (*+) were observed upon selective excitation of nT (*+) during the nanosecond-nanosecond two-color two-laser flash photolysis, indicating the hole transfer from nT (*+)(D 1) to Q. Recovery of nT (*+) was also observed together with decay of Q (*+) because of regeneration of nT (*+) by hole transfer from Q (*+) to nT at the diffusion-limiting rate. It was suggested that the hole transfer rate ( k HT) from nT (*+)(D 1) to Q depended on the free-energy change for hole transfer (-Delta G = 1.41-0.46 eV). The estimated k HT faster than the diffusion-limiting rate can be explained by the contribution of the static quenching for the excited species in the presence of high concentration of Q (0.1-1.0 M).     

Identification of an additional low-lying excited state of carotenoid radical cations

Carotenoids are the crucial pigments involved in photoprotection and in scavenging harmful free radicals in all living organisms. The underlying chemical processes are charge transfer and free radical reactions, both of them leading to carotenoid radical cation (Car*+) formation. Accurate knowledge of the molecular properties of Car*+ is thus a prerequisite for understanding of their function as photoprotective and antioxidant agents. Despite their fundamental importance in nonphotochemical quenching in green plants, only little is known about the Car*+ excited states and their dynamics. Our combined experimental and theoretical investigation employing femtosecond time-resolved pump-probe spectroscopy and quantum chemical calculations proves the existence of a second low-lying pipi* excited-state energetically below the well-known strongly allowed excited-state responsible for the intense absorption of Car*+ in the near-IR region. Hence, we suggest denoting the latter state as D3 state in the future. Our findings have also implications for nonphotochemical quenching in green plants, since direct quenching of chlorophyll excited states by Forster energy transfer to Car*+ is possible and efficient.     

EPR studies of amine radical cations. Part 2. Thermal and photo-induced rearrangements of propargylamine and allylamine radical cations in low-temperature freon matrices

Matrix EPR studies and quantum chemical calculations have been used to characterize the consecutive H-atom shifts undergone by the nitrogen-centered parent radical cations of propargylamine (1b*+) and allylamine (5*+) on thermal or photoinduced activation. The radical cation rearrangements of these unsaturated parent amines occur initially by a 1,2 H-atom shift from C1 to C2 with pi-bond formation at the positively charged nitrogen; this is followed by a consecutive reaction involving a second H-atom shift from C2 to C3. Thus, exposure to red light (lambda > 650 nm) converts 1b*+ to the vinyl-type distonic radical cation 2*+ which in turn is transformed on further photolysis with blue-green light (lambda approximately 400-600 nm) to the allene-type heteroallylic radical cation 3*+. Calculations show that the energy ordering is 1b*+ > 2*+ > 3*+, so that the consecutive H-atom shifts are driven by the formation of more stable isomers. Similarly, the parent radical cation of allylamine 5*+ undergoes a spontaneous 1,2-hydrogen atom shift from C1 to C2 at 77 K with a t1/2 of approximately 1 h to yield the distonic alkyl-type iminopropyl radical cation 6*+; this thermal reaction is attributed largely to quantum tunneling, and the rate is enhanced on concomitant photobleaching with visible light. Subsequent exposure to UV light (lambda approximately 350-400 nm) converts 6*+ by a 2,3 H-shift to the 1-aminopropene radical cation 7*+, which is confirmed to be the lowest-energy isomer derived from the ionization of either allylamine or cyclopropylamine. Although the parent radical cations of N, N-dimethylallylamine (9*+) and N-methylallylamine (11*+) are both stabilized by the electron-donating character of the methyl group(s), the photobleaching of 9*+ leads to the remarkable formation of the cyclic 1-methylpyrrolidine radical cation 10*+. The first step of this transformation now involves the migration of a hydrogen atom to C2 of the allyl group from one of the methyl groups (rather than from C1); the reaction is then completed by the cyclization of the generated MeN + (=CH2) CH2CH2CH2* distonic radical cation, possibly in a concerted overall process. In contrast to the ubiquitous H-atom transfer from carbon to nitrogen that occurs in the parent radical cations of saturated amines, the alternate rearrangements of either 1b*+ or 5*+ to an ammonium-type radical cation by a hypothetical H-atom shift from C1 to the ionized NH2 group are not observed. This is in line with calculations showing that the thermal barrier for this transformation is much higher (approximately 120 kJ mol-1) than those for the conversion of 1b*+ --> 2*+ and 5*+--> 6*+ (approximately 40-60 kJ mol-1).     

Photoreduction of 9,10-anthraquinone derivatives: transient spectroscopy and effects of alcohols and amines on reactivity in solution

The photoreduction of 9,10-anthraquinone (AQ), the 2-methyl, 2-ethyl, 2,3-dimethyl, 1,4-difluoro, 1-chloro and 1,8-dichloro derivatives as well as 1,4,4a,9a-tetrahydroanthraquinone, 1,2-benzanthraquinone and 6,13-pentacenequinone in nonaqueous solution at room temperature was studied by time-resolved UV-visible spectroscopy. Upon 308 nm excitation of AQ the triplet state reacts with alcohols and triethylamine (TEA). The rate constant of triplet quenching by amines is close to the diffusion-controlled limit. The semiquinone radical *QH/ Q*- is the main intermediate, and the half-life of the second-order decay kinetics depends significantly on the donor and the medium. Photoinduced charge separation after electron transfer from amines to the triplet state of AQ in acetonitrile and the subsequent charge recombination or neutralization also were measured by transient conductivity. The maximum quantum yield, lambdairr = 254 nm, of photoconversion into the strongly fluorescing 9,10-dihydroxyanthracenes is close to unity. The fluorescence with maximum at 460-480 nm and a lifetime of 20-30 ns disappears as a result of a complete recovery into AQ, when the dihydroxyanthracenes are exposed to oxygen. The mechanisms of photoreduction of parent AQ in acetonitrile by 2-propanol and in benzene and acetonitrile by TEA are discussed. The effects of AQ follow essentially the same pattern. The various functions of oxygen, e.g. (1) quenching of the triplet state; (2) quenching of the semiquinone radical, thereby forming HO2*/O2*- radicals; and (3) trapping of the dihydroxyanthracenes are outlined.     

Carotenoid radical cations as a probe for the molecular mechanism of nonphotochemical quenching in oxygenic photosynthesis

Nonphotochemical quenching (NPQ) is a fundamental mechanism in photosynthesis which protects plants against excess excitation energy and is of crucial importance for their survival and fitness. Recently, carotenoid radical cation (Car*+) formation has been discovered to be a key step for the feedback deexcitation quenching mechanism (qE), a component of NPQ, of which the molecular mechanism and location is still unknown. We have generated and characterized carotenoid radical cations by means of resonant two color, two photon ionization (R2C2PI) spectroscopy. The Car*+ bands have maxima located at 830 nm (violaxanthin), 880 nm (lutein), 900 nm (zeaxanthin), and 920 nm (beta-carotene). The positions of these maxima depend strongly on solution conditions, the number of conjugated C=C bonds, and molecular structure. Furthermore, R2C2PI measurements on the light-harvesting complex of photosystem II (LHC II) samples with or without zeaxanthin (Zea) reveal the violaxanthin (Vio) radical cation (Vio*+) band at 909 nm and the Zea*+ band at 983 nm. The replacement of Vio by Zea in the light-harvesting complex II (LHC II) has no influence on the Chl excitation lifetime, and by exciting the Chls lowest excited state, no additional rise and decay corresponding to the Car*+ signal observed previously during qE was detected in the spectral range investigated (800-1050 nm). On the basis of our findings, the mechanism of qE involving the simple replacement of Vio with Zea in LHC II needs to be reconsidered.     

Higher triplet excited states of benzophenones and bimolecular triplet energy transfer measured by using nanosecond-picosecond two-color/two-laser flash photolysis

The lifetimes of benzophenone in the higher triplet excited state (BP(T(n))) and several BP derivatives in the T(n) states were measured directly to be tau(T(n))=37+/-7 ps and 20-33 ps, respectively, by using the nanosecond-picosecond (ns-ps) two-color/two-laser flash photolysis method. Based on the direct measurements of tau(T(n)) of BP(T(n)), the triplet energy transfer (TET) from BP(T(n)) to quenchers (Q), such as carbon tetrachloride (CCl4), benzene (Bz), and p-dichlorbenzene (DCB), was investigated. The fast TET from BP(T(n)) to Q can be attributed to the lifetime-dependent quenching process, according to the Ware theoretical model of the bimolecular energy transfer reaction. The contribution of the lifetime-dependent term on k(TET) was 27, 60, and 86% for CCl4, Bz, and DCB as the Q of BP(T(n)), respectively, indicating that the TET from BP(T(n)) to Q is influenced not only by tau(T(n)), but also by the size of Q.     

Significant effects of substituents on substituted naphthalenes in the higher triplet excited state

Substituent effect on the lifetimes of a series of substituted naphthalenes (Np) in the higher triplet excited state (Tn) was studied with transient absorption measurements using the two-color two-laser flash photolysis technique. Lifetimes of Np(Tn) in cyclohexane solution were determined from the triplet energy transfer quenching by carbon tetrachloride to be 0.98-63 ps. The different lifetimes of Np(Tn) were explained by the energy gap law for the internal conversion from Np(Tn) to Np(T1), indicating that Np(Tn) quenched by carbon tetrachloride is assigned to Np(T2) with the longest lifetime among Np(Tn). The lifetime of Np(Tn) was correlative with the Hammett sigmap constant. Electronic characters of substituents showed a more significant influence on the energy of the T2 state than that of the T1 state.     

Solvent dependence of intramolecular electron transfer in a helical oligoproline assembly

The helical oligoproline assembly CH3-CO-Pro-Pro-Pro-Pra(Ptzpn)-Pro-Pro-Pra(RuIIb2m2+ -Pro-Pro-Pra(Anq)-Pro-Pro-Pro-NH2, having a spatially ordered array of functional sites protruding from the proline backbone, has been prepared. The 13-residue assembly formed a linear array containing a phenothiazine electron donor, a tris(bipyridine)ruthenium(II) chromophore, and an anthraquinone electron acceptor with the proline II secondary structure as shown by circular dichroism measurements. Following RuII --> b2m metal-to-ligand charge-transfer (MLCT) excitation at 457 nm, electron-transfer quenching occurs, ultimately to give a redox-separated (RS) state containing a phenothiazine (PTZ) radical cation at the Pra(Ptzpn) site and an anthraquinone (ANQ) radical anion at the Pra(Anq) site. The redox-separated state was formed with 33-96% efficiency depending on the solvent, and the transient stored energy varied from -1.46 to -1.71 eV at 22 +/- 2 degrees C. The dominant quenching mechanism is PTZ reductive quenching of the initial RuIII(b2m*-) MLCT excited state which is followed by m*- --> ANQ electron transfer to give the RS state. Back electron transfer is highly exergonic and occurs in the inverted region. The rate constant for back electron transfer is solvent dependent and varies from 5.2 x 10(6) to 7.7 x 10(6) s-1 at 22 +/- 2 degrees C. It is concluded that back electron transfer occurs by direct ANQ*- --> PTZ*+ electron transfer. Based on independently evaluated kinetic parameters, the electron-transfer matrix element is HDA approximately 0.13 cm-1.     

Photoreduction of p-benzoquinones: effects of alcohols and amines on the intermediates and reactivities in solutions

The photochemistry of 1,4-benzoquinone (BQ) and alkyl-, Cl- and related derivatives, e.g. methyl-, 2,6-dimethyl-, chloro-, 2,5-dichloro-1,4-benzoquinone, duroquinone and chloranil, was studied in nonaqueous solvents by UV-vis spectroscopy using nanosecond laser pulses at 308 nm. The reactivity of the triplet state (3Q*) of the quinones with 2-propanol in the absence of water is largest for BQ and depends mainly on the quinone structure, whereas the rate constant of electron transfer from amines, such as triethylamine (TEA) or 1,4-diazabicyclo[2.2.2]octane, is close to the diffusion-controlled limit for BQ and most derivatives. Photoinduced charge separation after electron transfer from amines to 3Q* and the subsequent charge recombination or neutralization are supported by time-resolved conductivity measurements. The half-life of the decay kinetics of the semiquinone radical (*QH/Q*-) depends significantly on the donor and the medium. The photoconversion into the hydroquinones was measured under various conditions, the quantum yield, lambda(irr) = 254 nm, increases with increasing 2-propanol and TEA concentrations. The effects of quenching of 3Q*, the *QH/Q*- radicals and the photoconversion are outlined. The mechanisms of photoreduction of quinones in acetonitrile by 2-propanol are compared with those by TEA in benzene and acetonitrile, and the specific properties of substitution are discussed.     

Redox-switchable direction of photoinduced electron transfer in an Ru(bpy)3(2+)-viologen dyad

Quenching of the 3MLCT excited state of [Ru(bpy)3]2+ (bpy=bipyridine) by the reduction products (MV*+ and MV0) of methyl viologen (MV2+) was studied by a combination of electrochemistry with laser flash photolysis or femtosecond pump-probe spectroscopy. Both for the bimolecular reactions and for the reactions in an Ru(bpy)3(2+)-MVn+ dyad, quenching by MV*+ and MV0 is reductive and gives the reduced ruthenium complex [Ru(bpy)3]+, in contrast to the oxidative quenching by MV2+. Rate constants of quenching (kq), and thermal charge recombination (krec) and cage escape yields (phi(ce)) were determined for the bimolecular reactions, and rates of forward (kf) and backward (kb) electron transfer in the dyad were measured for quenching by MV2+, MV*+, and MV0. The reactions in the dyad are very rapid, with values up to kf = 1.3 x 10(12) s(-1) for *Ru(bpy)3(2+)-MV*+. In addition, a long-lived (tau = 15 ps) vibrationally excited state of MV*+ with a characteristically structured absorption spectrum was detected; this was generated by direct excitation of the MV*+ moiety both at 460 and 600 nm. The results show that the direction of photoinduced electron transfer in a Ru(bpy)3-MV molecule can be switched by an externally applied bias.     

Photosensitized generation of singlet oxygen from rhenium(I) and iridium(III) complexes

Photophysical properties in dilute acetonitrile solution are reported for a number of iridium(III) and rhenium(I) complexes. The nature of the lowest excited state of the complexes under investigation is either metal-to-ligand charge transfer ((3)MLCT) or a ligand centred ((3)LC) state. Rate constants, k(q), for quenching of the lowest excited states by molecular oxygen are in the range 1.5 x 10(8) to 1.4 x 10(10) M(-1) s(-1). Efficiency of singlet oxygen production, f(Delta)(T), following oxygen quenching of the lowest excited states of these complexes, are in the range of 0.27-1.00. The rate constants and the efficiency of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition between a non-charge transfer (nCT) and a CT deactivation channel. The balance between CT and nCT deactivation channels, which is described by the relative contribution p(CT) of CT induced deactivation, is discussed. The kinetic model is found to be successfully applied in the case of quenching of the excited triplet states of coordination compounds by oxygen in acetonitrile, as was proposed for the quenching of pi-pi* triplet states by oxygen.     

Donor-acceptor-substituted tetrakis(phenylethynyl)benzenes as emissive molecules during pulse radiolysis in benzene

Emission from charge recombination between radical cations and anions of various tetrakis(phenylethynyl)benzenes (TPEBs) was measured during pulse radiolysis in benzene (Bz). The formation of TPEB in the singlet excited state (1TPEB*) can be attributed to the charge recombination between TPEB*+ and TPEB*-, which are initially generated from the radiolytic reaction in Bz. This mechanism is reasonably explained by the relationship between the annihilation enthalpy change (-DeltaH degrees) for the charge recombination of TPEB*+ and TPEB*- and excitation energy of 1TPEB*. It was found that the charge recombination between TPEB*+ and TPEB*- occurred to give 1TPEB* as the emissive species, but not the excimers because of the large repulsion between substituents caused by the rotation around C-C single bonds of TPEBs. Since donor-acceptor-substituted TPEBs possess three types of charge-transfer pathways (linear-conjugated, cross-conjugated, and "bent" conjugated pathways between the donor and acceptor substituents through the ethynyl linkage), the emission spectra of 1TPEBs* with intramolecular charge transfer (ICT) character depend on the substitution pattern and the various kinds of donor and acceptor groups during pulse radiolysis in Bz.