Photoinduced Intermolecular Electron Transfer Reaction Research Articles

Photoinduced electron transfer reaction in mitochondria for spatiotemporal selective photo-oxidation of lipids by donor/acceptor linked molecules.

Donor-acceptor-linked molecules have been synthesized and utilized to induce the rapid and site-selective lipid-oxidation in mitochondria by utilizing a photoinduced intermolecular electron transfer reaction. Two water-soluble donor-acceptor molecules (1 and 2) were designed and synthesized for this purpose. 2 was prepared to modulate its affinity to cell membrane in mitochondria. Confocal laser microscopy experiments revealed that 1 and 2 possess high localization abilities in mitochondria. By the photoinduced electron transfer, 2 exhibited the remarkable oxidation ability of lipids, mainly cardiolipin. In HeLa cells, 2 triggered mitochondrial lipid oxidation, which was followed by apoptotic cell death, under illumination within a few seconds. These results show that the present molecular system is highly promising to utilize the photoinduced intermolecular electron transfer reaction in a precise spatiotemporal manner in a cell by using light.

Optical control of mitochondrial reductive reactions in living cells using an electron donor-acceptor linked molecule.

It has been known for decades that intracellular redox reactions control various vital functions in living systems, which include the synthesis of biomolecules, the modulation of protein functions, and cell signaling. Although there have been several reports on the control of such functions using DNA and RNA, the non-invasive optical control of biological functions is an important ongoing challenge. In this study, a hybrid of an electron donor-acceptor linked molecule based on a ferrocene(Fc)-porphyrin(ZnP)-fullerene(C60) analogue and an elaborately designed nano-carrier, referred to herein as a MITO-Porter, resulted in a successful photoinduced intermolecular electron transfer reaction via the long-lived intramolecular charge separation, leading to site-specific reductive reactions in the mitochondria of living HeLa cells. A Fc-ZnP-C60 linked molecule, 1-Oct, was designed and prepared for taking advantage of the unique photophysical properties with excellent efficiency (i.e. a long lifetime and a high quantum yield) for photoinduced charge separation. The targeted delivery of 1-Oct to mitochondria was accomplished by using a combination of the Fc-ZnP-C60 molecule and a drug delivery nano-carrier, MITO-Porter, that was recently established by our group for intracellular cargo delivery. The successful delivery of 1-Oct by the MITO-Porter permitted the optically-controlled generation of O2- in the mitochondria of HeLa cells and the following induction of apoptosis as a cell signalling response was observed in confocal laser microscopy experiments. The obtained results indicate the use of an electron donor-acceptor system such as this can be a promising tool for the non-invasive triggering of redox-coupled cellular activities in living systems.

Photoinduced intermolecular electron transfer in a room temperature imidazolium ionic liquid: An excitation wavelength dependence study

In this work we have reported the photoinduced intermolecular electron transfer reaction from N, N-dimethylaniline to different Coumarin dyes in a neat room temperature imidazolium ionic liquid, 1-ethyl-3-methylimidazolium-bis (trifluoromethylsulfonyl) imide ([Emim][Tf 2N]) and observed a difference in electron transfer rate with the variation of excitation wavelengths. We have also observed an excitation wavelength dependent solvation dynamics in this ionic liquid media. The excitation wavelength dependence is attributed to the presence of structural heterogeneity in this neat ionic liquid media. The electron transfer rate vs free energy correlation curve show a retardation in the electron transfer rate at higher free energy region.

Time-resolved Raman study on the photoinduced intermolecular electron–transfer reaction of biphenyl in alcohols

Time-resolved Raman spectroscopy has uncovered a new type of photoinduced intermolecular electron–transfer reaction of biphenyl in alcohols. The Raman bands of the first excited singlet (S1) state, the cation radical and the anion radical of biphenyl were simultaneously observed in the time range 0–200 ns after photoexcitation. The two ion radicals exhibit distinct rise and decay behaviors from each other. A kinetic analysis shows that the electron–transfer reaction consists of the following three elementary processes; (i) monophotonic formation of a biphenyl cation radical and a solvated electron via the S1 state, (ii) migration of the electron in the solvent medium, and (iii) anion radical formation resulting from the capture of the electron by a biphenyl molecule in the ground state. The formation of the cation radical from the S1 state is a slow process with a rate not greater than 10−7 s−1.

Effect of laser intensity on the determination of intermolecular electron transfer rate constants—Observation of Marcus inverted region in photoinduced back electron transfer reactions

The light intensity and concentration dependence of the photoproduct yield are investigated in a monophotonic process. The relationship of the photoproduct yield with the laser intensity and the complex concentration for a monophotonic process is derived under laser flash photolysis. The relationship is confirmed experimentally in a monophotonic process, i.e., triplet–triplet transition for a Cu(I) complex Cu6(DMNSN′)6 (DMNSN′=4,6-dimethylpyrimidine-2-thiolate). At low light intensity, the relationship can be approximated by a linear inverse square root dependence on the light intensity. Based on this equation, a method is proposed to determine the intrinsic back electron transfer rate constant kETb in photoinduced intermolecular electron transfer reactions, precluding the effect from the diffusional encounter pairs. The Marcus “inverted region” is observed by using the method in photoinduced back electron transfer reactions of [Au2(dppm)2](ClO4)2 (dppm=bis(diphenylphosphino)methane) with a series of substituted pyridinium acceptors.

Investigation of the Excited-State Dynamics of Radical Ions in the Condensed Phase Using the Picosecond Transient Grating Technique

A study of the dynamics of ground-state recovery of the perylene radical cation (Pe•+), of perylene radical anion (Pe•-), and of anthraquinone radical anion (AQ•-) is reported. In boric acid glass, the excited-state lifetime of Pe•+ is 35 ± 3 ps, while in concentrated sulfuric acid, it is smaller than 15 ps, the time resolution of the experimental setup. The excited-state lifetime of Pe•+, Pe•-, and AQ•- generated by photoinduced intermolecular electron-transfer reaction in MeCN is shorter than 15 ps. In the case of Pe•-, the uncomplete ground-state recovery is ascribed to the occurrence of electron photoejection. The free ion yield in the intermolecular electron-transfer reaction between 9,10-dicyanoanthracene (DCA) and two electron acceptors was measured in a two-pulse experiment, where the second pulse excited the ensuing DCA•-. This excitation has no influence on the magnitude of the free ion yield, indicating a short excited-state lifetime of DCA•-* relative to the time scale of back electron transfer and ionic dissociation. A red emission, ascribed to the fluorescence of protonated Pe, was detected in boric acid glass and sulfuric acid. No fluorescence that could be clearly ascribed to Pe•+* could be observed.

The influence of molecular oxygen on transient absorption and transient photocurrent kinetics

The transient absorption and transient photocurrent kinetics of photoinduced intermolecular electron transfer reactions in solution can show considerable discrepancies at different molecular oxygen concentrations. The lifetimes of the radical anions observed by transient absorption are decreased in the presence of oxygen, and this is explained by a secondary charge shift reaction of the primary radical anion to oxygen, generating the oxygen anion. Transient photocurrent kinetics are only slightly affected, because no net charge loss results from this mechanism and only the mobility of the corresponding charge carriers changes.

Temperature and specific anion effects in photo-induced intermolecular electron-transfer reactions

The rate constants for the bimolecular oxidative quenching of *Ru(bpy)32+ by methylviologen (kq) and the electron-transfer recombination (krec) reaction of the redox products were determined as a function of the ionic strength and temperature in aqueous solution. At 25°C for Na+ salts at constantμ, kq andkrec are in the order H2PO4−<CH3CO2−∼HPO42−∼SO42−≪ClO4−; activation energies are independent of μ and the anion (∼16kJ/mol), except for ClO4−, whereEa is ∼30% lower. The trends inkq andkrec are attributed to the existence of ion-paired aggregates of the cationic reactants and anions; correlations with the sizes and hydration enthalpies of the anions can be made. It is suggested that ClO4−, with the smallest hydration radius and the least negative hydration enthalpy, is more closely ion-paired within the Ru(bpy)32+ (or Ru(bpy)33+) interligand pockets, “tethering” the MV2+ (or MV.+) into closer proximity, thereby lowering the activation barrier and increasing the rate of electron transfer.

Photoinduced intermolecular electron transfer reaction between N,N-dimethylaniline and anthracene in acetonitrile solution: A theoretical study

The photoinduced intermolecular electron transfer (ET) reaction between N,N-dimethylaniline and excited state anthracene in acetonitrile solution is studied theoretically. A solvation coordinate s which represents stochastic one-dimensional dynamics of the solution phase reaction is defined and a Hamiltonian in terms of s and perpendicular bath modes is derived from the spin-boson Hamiltonian. This has an advantage that the dynamics of the transferring electron is influenced by the bath only through coupling with the coordinate s. Intra- and intermolecular potentials are constructed by using ab initio molecular orbital methods, and a series of molecular dynamics simulation analysis is performed. Mean force potentials as a function of the donor–acceptor distance R are computed and the bimolecular encounter dynamics is investigated. Diabatic free energy curves for the coordinate s are computed and shown to be well approximated by parabolas, indicating that the dielectric saturation effect is negligible. The dependence of the free energy relationships on R is examined. It is shown that the present system corresponds to the increasing region of the rate constant, in contrast with the conventional picture. The electronic coupling of the ET is evaluated by the method of corresponding orbitals. The R dependence of the ET rate is evaluated and the reaction adiabaticity and mechanism are discussed. Dynamical solvent effects are taken into consideration in terms of the generalized Langevin equation formalism.

Charge transfer emissive singlet excited states and photoinduced electron transfer properties in the diarylideneacetone compounds (RCHCH)2CO; R = phenyl, 1- and 2-naphthyl, 3-(N-ethylcarbazoyl), and 4-(C5H5)Fe(C5H4C6H4CHCH(CO)CHCHC6H5)

Four diarylideneacetone compounds ((RCHCH)2CO, where the aryl groups are phenyl (dba), 1-naphthyl (1-dNapha), 2-naphthyl (2-dNapha), and 3-(N-ethylcarbazoyl) (dNECa)), and 4-(C5H5)Fe(C5H4C6H4CHCH(CO)CHCH(C6H5) (dba-Fc) have been prepared and characterized. The compounds are found to be fluorescent and photochemically and reversibly electrochemically active. The lowest-energy absorption bands for the diarylideneacetones are assigned to a charge transfer (CT) electronic transition, except for dba-Fc, in which a ferrocenyl ligand field transition assignment is made. The 77 K CT absorption and fluorescence bands are vibrationally structured (vibrational spacings = 1260–1360 cm−1). While the fluorescence decay at 293 K is monoexponential, the excited state fluorescence lifetimes (τF) for the 77 K samples exhibit double exponential decays, the short component being 0.38–0.64 ns and the long one 3.5–10.9 ns. The photophysical results are interpreted in terms of excited state deactivation processes dominated by radiationless pathways that are associated with the presence of fluorescent species with different geometries. Only the dNECa compound is found to be fluorescent in solution at 298 K [Formula: see text]. Cyclic voltammetry and coulometry measurements suggest that a reversible one-electron reduction process and an irreversible higher potential one-electron reduction process take place in the −1 to −2 V vs. SSCE range. In addition, dba-Fc also exhibits an electrochemically reversible one-electron oxidation wave at 0.52 V vs. SSCE centered at the ferrocenyl group. These results together with the spectroscopic electronic data have permitted evaluation of the reduction potentials of the lowest singlet (CT excited states (E1−/*);they range from 1.4 to 2.2 V vs. SSCE, with dba being the strongest photooxidizing agent and dNECa the weakest. Photoinduced intermolecular electron transfer reactions have been investigated by steady state fluorescence techniques and picosecond flash photolysis spectroscopy for dNECa and dba, respectively. The bimolecular deactivation rate constants, kq, for the reductive photoinduced electron transfer reactions of dNECa with diphenylamine (DPA) (kq = (2.65 ± 0.25) × 107 M−1 s−1) and N, N, N′, N′-tetramethylphenylenediamine (TMPM) (kq = (1.38 ± 0.03) × 108 M−1 s−1) have been obtained in THF solutions at 293 K. No fluorescence quenching is observed when oxidative and energy transfer quenchers are used with dNECa. For the non-emissive dba compound at room temperature, picosecond flash photolysis experiments show that quenching of the broad dba transient band (~500 nm) does indeed occur between 5 and 10 ns. Keywords: dibenzylideneacetone, charge transfer, photoelectron transfer.