Photoinduced Electron Transfer Competitive with Energy Transfer of the Excited Triplet State of [60]Fullerene to Ferrocene Derivatives Revealed by Combination of Transient Absorption and Thermal Lens Measurements

The quenching processes of the exited triplet state of fullerene (3C60*) by ferrocene (Fc) derivatives have been observed by the transient absorption spectroscopy and thermal lens methods. Although 3C60* was efficiently quenched by Fc in the rate close to the diffusion controlled limit, the quantum yields (Φet) for the generation of the radical anion of C60 (C60•-) via 3C60* were quite low even in polar solvents; nevertheless, the free-energy changes (ΔGet) of electron transfer from Fc to 3C60* are sufficiently negative. In benzonitrile (BN), the Φet value for unsubstitued Fc was less than 0.1. The thermal lens method indicates that energy transfer from 3C60* to Fc takes place efficiently, suggesting that the excited triplet energy level of Fc was lower than that of 3C60*. Therefore, energy transfer from 3C60* to ferrocene decreases the electron-transfer process from ferrocene to 3C60*. To increase the participation of electron transfer, introduction of electron-donor substituents to Fc (Φet = 0.46 for decamethylferrocene in BN) and an increase in solvent polarity (Φet = 0.58 in BN:DMF (1:2) for decamethylferrocene) were effective

A Dramatic Elongation of the Lifetime of Charge-Separated State by Complexation with Yttrium Triflate in Ferrocene−Anthraquinone Linked Dyad

Seven million times elongation of the lifetime of charge-separated state is attained in the presence of yttrium triflate [Y(OTf)3] in the photoinduced electron-transfer reaction of a ferrocene−anthraquinone dyad (Fc−AQ) with a rigid amide spacer in benzonitrile at 298 K as compared with the lifetime in its absence. Such remarkable elongation of the CS lifetime in the presence of Y(OTf)3 results from the strong binding of Y(OTf)3 with the AQ•- moiety of Fc+−AQ•-

Photoinduced Microsecond-Charge-Separation in Retinyl-C₆₀ Dyad

Intramolecular photoinduced charge separation and recombination processes in a retinyl-C60 dyad molecule (Ret-C60) have been investigated in various solvents by time-resolved absorption and fluorescence techniques. Upon laser excitation of the C60-moiety in nonpolar toluene, the intersystem crossing proceeded from the excited singlet state of the C60-moiety (Ret-1C60*) to the excited triplet state (Ret-3C60*), followed by energy transfer yielding the excited triplet state of the retinyl-moiety (3Ret*-C60) without charge separation. On the other hand, in polar solvents such as N,N-dimethylformamide and benzonitrile, the charge separation occurred from Ret-1C60* at rate on the order of 1010 s-1. The quantum yield was close to unity in these polar solvents. Most parts of the ion pair (Ret•+-C60•-) changed to Ret-3C60* and 3Ret*-C60 by the charge recombination which took place at rate on the order of 109 s-1. However, some parts of the charge-separated state were kept in microsecond time-region:  The lifetimes of Ret•+-C60•- were 16 μs and 19 μs in DMF and benzonitrile, respectively, which were as long as those of Ret-3C60* and 3Ret*-C60, suggesting an equilibrium between the charge-separated state and the excited triplet states

Comprehensive Structural and Electronic Properties of 2‑Azaadamantane <i>N</i>‑Oxyl Derivatives Correlated with Their Catalytic Ability

Herein, a comprehensive kinetic study is performed to compare the catalytic efficiency of 2-azaadamantane N-oxyl (AZADO) derivatives with that of 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) used as radical catalysts in the aerobic oxidation of l-menthol. Furthermore, the correlation between the catalytic activity and structural/electronic parameters of AZADOs and TEMPO is elucidated. The reaction rate constants achieved with several AZADO derivatives exhibit moderate relationships with spectroscopic parameters, such as the hyperfine coupling constant of the N atom (AN) and NO stretching vibration frequency (νNO) observed in electron spin resonance and infrared spectra, respectively. The planarity C–(NO)–C angle (φ) at the N atom, determined by density functional theory (DFT) calculations, also strongly correlates with the AN and νNO. Moreover, the bond order of NO, which strongly depends on the structural and electronic properties of NO radicals, correlates with radical activity; thus, the radical activity can be predicted by DFT calculations, thereby accelerating the synthesis of new AZADO derivatives without requiring alcohol oxidation experiments

Fluorescence Up-Conversion Study of Excitation Energy Transport Dynamics in Oligothiophene−Fullerene Linked Dyads

Photoinduced excitation energy transport dynamics in oligothiophene−fullerene linked dyads, nT-C60 (n = 4, 8, and 12), have been investigated by femtosecond fluorescence up-conversion. In 8T-C60 and 12T-C60, each time profile of the fluorescence due to the 1nT* moiety consists of two components. The sub-picosecond component and a few picosecond components were experimentally evaluated depending on the lengths of oligothiophenes (n =8 and 12) and on the analyzing wavelength of the fluorescence. However, the time trace of the fluorescence due to 14T*-C60 decayed with a single short component in ∼300 fs due to direct excited energy transfer (EET) from the 14T* moiety to the C60 moiety. On the basis of the kinetic models considering the short and long locally π-conjugative thiophene segments in 8T-C60 and 12T-C60, the rate parameters of the elemental processes were evaluated. Sub-picosecond time constants of nT-C60 were found to be EET from the thiophene segment vicinal to the C60 moiety and intrachain energy transfer. Slower picosecond dynamics mainly corresponds to EET from the thiophene segments apart from the C60 moiety

Structural and Photophysical Properties of Self-Assembled Porphyrin Nanoassemblies Organized by Ethylene Glycol Derivatives

We demonstrate an ability to control the structures and photodynamics of porphyrin-based nanoassemblies via solvent mixture technique. Surfactants such as ethylene glycol (EG) derivatives with different chain lengths are employed to control the sizes and shapes of nanoparticles composed of meso-substituted tetracarboxyphenyl porphyrin [H2P(CO2H)4] in mixed H2O/THF solvent. With increasing the chain lengths, the diameters of H2P(CO2H)4/EG composite nanoparticles systematically increase in the range of 90−350 nm. In contrast with H2P(CO2H)4/EG composite nanoparticles, pristine H2P(CO2H)4 assemblies show long rod-shaped assemblies with micrometer scales. The porphyrin nanoparticles are stable in solution without precipitation for several days. The nanoparticles exhibit the following optical properties: a large bathochromic shift in the absorption spectra and an increase of the fluorescence quenching properties relative to those for the monomer porphyrin solution. The hierarchical clustering of H2P(CO2H)4 molecules within nanoparticles is caused by the hydrogen bonding and π-stacking effects. The efficient fluorescence quenching of H2P(CO2H)4 is mainly due to the effect of singlet−singlet annihilation of H2P(CO2H)4 moieties within nanoassemblies. We further report the quenching processes of the excited triplet states of H2P(CO2H)4 nanoassemblies. Efficient quenching properties of the excited triplet states of H2P(CO2H)4 moieties are observed in the range of lifetime 26−100 ns, which is largely dependent on the sizes of nanoparticles. These quenching processes can be also analyzed by triplet−triplet annihilation theory

Synthesis of the Axially Substituted Titanium Pc-C₆₀ Dyad with a Convenient Method

We successfully synthesized the axially substituted titanium Pc-C60 dyad with a convenient method that improves on the traditional asymmetrical phthalocyanine routine to covalently linked phthalocyanines with other functional molecules. The intramolecular photoinduced process between phthalocyanine donor and fullerene acceptor was preliminarily studied

Control of Electron Acceptor Ability with Ligands (L) in Photoinduced Electron Transfer from Zinc Porphyrin or Zinc Phthalocyanine to [Ru₃(μ₃-O)(μ-CH₃COO)₆L₃]⁺

Photoinduced electron-transfer processes from the excited triplet states of zinc tetraphenylporphyrin (3ZnTPP*) or zinc tetra-tert-butylphthalocyanine (3ZnTBPc*) to oxo-acetato-bridged triruthenium clusters [Ru3(μ3-O)(μ-CH3CO2)6(L)3]+ have been confirmed by nanosecond laser flash photolysis in the visible and near-IR regions. The rise of the transient absorption spectra of the radical cations of ZnTPP and ZnTBPc and the reduced form of the oxo-acetato-bridged triruthenium cluster ([Ru3(μ3-O)(μ-CH3CO2)6(L)3]0) were observed with the concomitant decays of 3ZnTPP* or 3ZnTBPc*. The evaluated rate constants (kET) and quantum yields (ΦET) for electron-transfer were increased with the order of electron-withdrawing ability of the ligands (L) coordinated to the Ru atoms, 4-cyanopyridine > triphenylphosphine > pyridine > 4-(dimethylamino)pyridine, which is the order of promoting the electron-accepting ability of [Ru3(μ3-O)(μ-CH3CO2)6(L)3]+. The ΦET values for 3ZnTPP* were lower than those for 3ZnTBPc*, suggesting the presence of competitive processes such as energy transfer process from 3ZnTPP* to the triplet states of [Ru3(μ3-O)(μ-CH3COO)6(L)3]+. For the back electron-transfer process, second-order kinetics indicates that the radical cations of ZnTPP or ZnTBPc and [Ru3(μ3-O)(μ-CH3COO)6(L)3]0 return to the original system after solvation in polar solvents at a diffusion controlled limit without side reactions, providing reversible photosensitizing intermolecular electron-transfer systems

Effects of Hydrogen Bonding on Metal Ion-Promoted Intramolecular Electron Transfer and Photoinduced Electron Transfer in a Ferrocene-Quinone Dyad with a Rigid Amide Spacer

A ferrocene-quinone dyad (Fc-Q) with a rigid amide spacer and Fc-(Me)Q dyad, in which the amide proton acting as a hydrogen-bonding acceptor is replaced by the methyl group, are employed to examine the effects of hydrogen bonding on both the thermal and the photoinduced electron-transfer reactions. The hydrogen bonding of the semiquinone radical anion with the amide proton in Fc-Q•- produced by the electron-transfer reduction of Fc-Q is indicated by the significant positive shift of the one-electron reduction potential of Fc-Q. The hyperfine coupling constants of Fc-Q•- also indicate the existence of hydrogen bonding, agreeing with those predicted by the density functional calculation. The hydrogen-bonding dynamics in the photoinduced electron transfer from the ferrocene (Fc) to the quinone moiety (Q) in Fc-Q have been successfully detected in the femtosecond laser flash photolysis experiments. Thermal intramolecular electron transfer from Fc to Q in Fc-Q and Fc-(Me)Q also occurs efficiently in the presence of metal ions in acetonitrile at 298 K. The hydrogen bond formed between the semiquinone radical anion and the amide proton in Fc-Q results in remarkable acceleration of the rate of metal ion-promoted electron transfer as compared to the rate of Fc-(Me)Q in which hydrogen bonding is prohibited. The metal ion-promoted electron-transfer rates are well correlated with the binding energies of superoxide ion-metal ion complexes, which are derived from the gzz values of the ESR spectra

Arg97 at the Heme-Distal Side of the Isolated Heme-Bound PAS Domain of a Heme-Based Oxygen Sensor from <i>Escherichia coli</i> (<i>Ec</i> DOS) Plays Critical Roles in Autoxidation and Binding to Gases, Particularly O₂

The catalytic activity of heme-regulated phosphodiesterase from Escherichia coli (Ec DOS) on cyclic di-GMP is markedly enhanced upon binding of gas molecules, such as O2 and CO, to the heme iron complex in the sensor domain. Arg97 interacts directly with O2 bound to Fe(II) heme in the crystal structure of the isolated heme-bound sensor domain with the PAS structure (Ec DOS-PAS) and may thus be critical in ligand recognition. To establish the specific role of Arg97, we generated Arg97Ala, Arg97Glu, and Arg97Ile mutant Ec DOS-PAS proteins and examined binding to O2, CO, and cyanide, as well as redox potentials. The autoxidation rates of the Arg97Ala and Arg97Glu mutant proteins were up to 2000-fold higher, while the O2 dissociation rate constant for dissociation from the Fe(II)−O2 heme complex of the Arg97Ile mutant was 100-fold higher than that of the wild-type protein. In contrast, the redox potential values of the mutant proteins were only slightly different from that of the wild type (within 10 mV). Accordingly, we propose that Arg97 plays critical roles in recognition of the O2 molecule and redox switching by stabilizing the Fe(II)−O2 complex, thereby anchoring O2 to the heme iron and lowering the autoxidation rate to prevent formation of Fe(III) hemin species not regulated by gas molecules. Arg97 mutations significantly influenced interactions with the internal ligand Met95, during CO binding to the Fe(II) complex. Moreover, the binding behavior of cyanide to the Fe(III) complexes of the Arg mutant proteins was similar to that of O2, which is evident from the Kd values, suggestive of electrostatic interactions between cyanide and Arg97