Initial Photoinduced Electron Transfer Research Articles

A lysosomal targeted fluorescent probe based on BODIPY for monitoring NO in living cells and zebrafish imaging

Nitric oxide (NO), as a special biological small molecule, plays an important role in physiology and pathology. Studies have revealed a strong correlation between human illness and the amount of NO in lysosomes. In this research, we synthesized a BODIPY-based fluorescent probe BML that can be localized in the lysosome to detect NO. The secondary amine in the BML structure can be N-nitrosation reaction with NO in the lysosome, preventing the initial photo-induced electron transfer (PET) process and leading to the creation of the highly luminescent BML-NO. The reaction mechanism was proved in the high-resolution mass spectrometry and DFT simulation calculations. Additionally, BML was discovered to have some evident benefits, such as fast detection time (3.5 min), low detection limit (LOD = 10.3 nM), and the resistance to interference and acidity, which are essential for the detection of NO in lysosomes. Furthermore, the BML was effectively employed to detect endogenous and exogenous nitric oxide in living cells and zebrafish.

A turn-on fluorescent probe for selective detection of H2S in environmental samples and bio-imaging in human breast cancer cells.

A triphenylamine-benzothaizole-based turn-on fluorescent probe TPB-NO2 was designed and synthesized for tracking H2S in both environmental and biological samples depending upon the sensing strategy of thiolysis of 2,4-dinitrophenyl (DNP) ether. Due to PET (photoinduced electron transfer), occurring from donor triphenylamine moiety to acceptor DNP moiety, the probe TPB-NO2 itself is very weakly fluorescent and colorless in DMSO/H2O solution (1 : 1, v/v; 10 mM HEPES buffer, pH 7.4). But the addition of H2S leads to thiolysis of 2,4-dinitrophenyl ether to block the initial PET process and hence it exhibits naked eye detectable turn-on response with bright cyan fluorescence and intense brown color. Not only that, the probe exhibits excellent selectivity over other bio-thiols like Cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), fast response time (<2 min), and high sensitivity with a detection limit of 9.81 nM. Moreover, to explore the practical applicability of our probe we employed it to monitor H2S successfully in environmental water samples, solid-state TLC strip study, Quantitative determination of H2S in eggs, and in the bioimaging of human breast cancer cells (MDA-MB 231).

Toward Metal-free Photocatalysis: Photochemical Regeneration of Organic Hydride Donors Using Phenazine-Based Photosensitizers

Earth-abundant chromophores and catalysts are important molecular building blocks for artificial photosynthesis applications. Our team previously reported that metal-free hydride donors, such as biomimetic benzoimidazole-based motifs, can reduce CO2 selectively to the formate ion and that they can be electrochemically regenerated using the proton-coupled mechanism. To enable direct utilization of solar energy, we report here the photochemical regeneration of a benzoimidazole-based hydride donor using a phenazine-based metal-free chromophore. The photochemical regeneration was investigated under different experimental conditions involving varying sacrificial donors, proton donors, solvents, and component concentrations. The best hydride regeneration yield of 50% was obtained with phenol as a proton source and thiophenolate as a sacrificial electron donor. The mechanism of photochemical regeneration was studied using steady-state and time-resolved UV/Vis spectroscopies. Based on the results of these studies, we hypothesize that the initial photoinduced electron transfer from photoexcited phenazine chromophores involves the benzoimidazole cation and that this process is likely coupled with proton transfer to generate protonated benzoimidazole-based radical cation. The second photoinduced electron transfer is hypothesized to generate the hydride form. Our findings provide the requisite information for the future development of reductive photocatalysts for solar energy and light-harvesting applications utilizing earth-abundant metal-free materials.

Origin of poor photovoltaic performance of bis(tetracyanoantrathiophene) non-fullerene acceptor

Abstract Light-induced EPR and out-of-phase electron spin echo techniques were applied to identify the origin of low photovoltaic performance of novel non-fullerene antrathiophene-based acceptor 2,2′-[2,2′-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(anthra[2,3-b]thiophene-5,10-diylidene)]tetrapropanedinitrile (AT1), compared to conventional fullerene acceptors. These techniques revealed that average trapping energy of electrons in PCDTBT/AT1 blend is higher than that in the blends of PCDTBT with fullerene acceptors, while the distance of the initial photo-induced electron transfer is decreased. These two effects are suggested to enhance the efficiency of geminate recombination in PCDTBT/AT1 blend and reduce the yield of free charges, which is supposed to be the main reason for poor OPV performance of AT1-containing blends. Low electron mobility μe = 3.5 *10-6 cm2V-1s−1 is estimated for AT1 from photo-CELIV experiment. Strong localization of the electrons on terminal acceptor units of AT1 consistently explains both efficient geminate recombination in blends of antrathiophene-based acceptor with polymer donors and low electron mobility in AT1.

Stepwise Photoinduced Electron Transfer in a Tetrathiafulvalene‐Phenothiazine‐Ruthenium Triad

A molecular triad comprising a [Ru(bpy)3]2+ (bpy = 2,2′‐bipyridine) photosensitizer, a primary phenothiazine (PTZ) donor and a secondary (extended) tetrathiafulvalene (exTTF) donor was synthesized and explored by UV/Vis transient absorption spectroscopy. Initial photoinduced electron transfer from PTZ to the 3MLCT‐excited [Ru(bpy)3]2+ occurs within less than 60 ps, and subsequently PTZ is regenerated by electron transfer from exTTF with a time constant of 300 ps. The resulting photoproduct comprising exTTF·+ and [Ru(bpy)3]+ has a lifetime of 6100 ps in de‐aerated CH3CN at room temperature. Additional one‐ and two‐pulse laser flash photolysis studies of the triad were performed in the presence of excess methyl viologen (MV2+), to explore the possibility of light‐driven charge accumulation on exTTF. MV2+ clearly oxidized [Ru(bpy)3]+ and thereby re‐instated ground‐state [Ru(bpy)3]2+ in triads in which exTTF had been oxidized to exTTF·+, but further excitation of the solution containing the exTTF·+‐PTZ‐[Ru(bpy)3]2+ photoproduct did not provide evidence for exTTF2+. Nevertheless, it seems that the design principle of a covalent donor‐donor‐sensitizer triad (as opposed to simpler donor‐sensitizer dyads) is beneficial for light‐driven accumulation of oxidation equivalents. These investigations are relevant in the greater context of multi‐electron photoredox chemistry and artificial photosynthesis.

Long-range proton-coupled electron transfer in phenol–Ru(2,2′-bipyrazine)32+ dyads

Two dyads in which either 4-cyanophenol or un-substituted phenol is connected via a p-xylene spacer to a Ru(bpz)3(2+) (bpz = 2,2'-bipyrazine) complex were synthesized and investigated. Selective photo-excitation of Ru(bpz)3(2+) at 532 nm in a CH3CN-H2O mixture leads to the formation of 4-cyanophenolate or phenolate along with Ru(bpz)3(2+) in its electronic ground state. This apparent photoacid behavior can be understood on the basis of a reaction sequence comprised of an initial photoinduced proton-coupled electron transfer (PCET) during which 4-cyanophenol or phenol is oxidized and deprotonated, followed by a thermal electron transfer event in the course of which 4-cyanophenoxyl or phenoxyl is reduced by Ru(bpz)3(+) to 4-cyanophenolate or phenolate. Conceptually, this reaction sequence is identical to a sequence of photoinduced charge-separation and thermal charge-recombination events as observed previously for many electron transfer dyads, with the important difference that the initial photoinduced electron transfer process is proton-coupled. The dyad containing 4-cyanophenol reacts via concerted-proton electron transfer (CPET) whereas the dyad containing un-substituted phenol appears to react predominantly via a stepwise PCET mechanism. Long-range PCET is a key reaction in photosystem II. Understanding the factors that govern the kinetics of long-range PCET is desirable in the broader context of light-to-energy conversion by means of proton-electron separation across natural or artificial membranes.

Dynamics and mechanism of repair of ultraviolet-induced (6–4) photoproduct by photolyase

One of the detrimental effects of UV radiation on DNA is the formation of the (6-4) photoproduct (6-4PP) between two adjacent pyrimidines1. This lesion interferes with replication and transcription and may result in mutation and cell death2. In many organisms a flavoenzyme called photolyase uses blue light energy to repair the 6-4PP3. The molecular mechanism of the repair reaction is poorly understood. Here, we use ultrafast spectroscopy to show that the key step in the repair photocycle is a cyclic proton transfer between the enzyme and the substrate. By femtosecond synchronization of the enzymatic dynamics with the repair function, we followed the function evolution and observed direct electron transfer from the excited flavin cofactor to the 6-4PP in 225 ps but surprisingly fast back electron transfer in 50 ps without repair. Strikingly, we found that the catalytic proton transfer between a histidine residue in the active site and the 6-4PP, induced by the initial photoinduced electron transfer from the excited flavin cofactor to 6-4PP, occurs in 425 ps and leads to 6-4PP repair in tens of nanoseconds. These key dynamics define the repair photocycle and explain the underlying molecular mechanism of the enzyme’s modest efficiency.

Competitive Reaction Pathways foro-Anilide Aryl Radicals: 1,5- or 1,6-Hydrogen Transfer versus Nucleophilic Coupling Reactions. A Novel Rearrangement to Afford an Amidyl Radical

The photoinduced reactions of o-iodoanilides (o-IC6H4N(Me)COR, 4a-d) with sulfur nucleophiles such as thiourea anion (1, -SCNH(NH2)), thioacetate anion (2, MeCOS-), and sulfide anion (3, S(2-)) follow different reaction channels, giving the sulfides by a radical nucleophilic substitution or the dehalogenated products by hydrogen atom transfer pathways. After an initial photoinduced electron transfer (PET) from 1 to iodide 4, the o-amide aryl radicals 12 are generated. These aryl radicals 12 afford alternative reaction pathways depending on the structure of the alpha-carbonyl moiety: (a) 12b (R = Me) adds to 1 to render the methylthio-substituted compounds by quenching the thiolate anion intermediate with MeI after irradiation; (b) 12c (R = -CH2Ph) follows a 1,5-hydrogen transfer to give a stabilized alpha-carbonyl radical (17); and (c) 12d (R = t-Bu) affords 1,6-hydrogen transfer, followed by a 1,4-aryl migration to render an amidyl radical (20), which is reduced to the N-benzyl-N,2-dimethylpropanamide (10). Together with this last rearranged product, the ipso substitution derivative was also observed. Similar results were obtained in the PET reactions of 4d (R = t-Bu) with anions 2 and 3 under entrainment conditions with the enolate anion from cyclohexenone (5) or the tert-butoxide anion (6). From this novel rearrangement, and only under reductive conditions by PET reaction with anion 5, iodide 4d (R = t-Bu) affords quantitatively the propanamide 10. The energetic of the intramolecular rearrangements followed by radicals 12b-d were rationalized by B3LYP/6-31+G* calculations.

Solvent dependence of the charge-transfer properties of a quaterthiophene–anthraquinone dyad

An electron donor–acceptor dyad (quaterthiophene–anthraquinone) mediates ultrafast intramolecular photoinduced charge separation and consequent charge recombination when in polar or moderately polar solvents. Alternatively, non-polar media completely impedes the initial photoinduced electron transfer by causing enough destabilization of the charge-transfer state and shifting its energy above the energy of the lowest locally excited singlet state. Furthermore, femtosecond transient-absorption spectroscopy reveals that for the solvents mediating the initial photoinduced electron-transfer process, the charge recombination rates were slower than the rates of charge separation. This behavior of donor–acceptor systems is essential for solar-energy-conversion applications. For the donor–acceptor dyad described in this study, the electron-transfer driving force and reorganization energy place the charge-recombination processes in the Marcus inverted region.

Substituent effects in the intramolecular photoredox reactions of benzophenones in aqueous solution

A number of α-hydroxy-3-benzylbenzophenones 7–11 have been synthesized for the purpose of studying the effect of a phenyl substituent on the intramolecular photoredox reaction of 3-(hydroxymethyl)benzophenone (5) discovered in our laboratory. This latter compound was found to undergo a unimolecular (formal) intramolecular redox reaction upon photolysis in aqueous acid that results in clean reduction of the benzophenone ketone (to secondary alcohol) and oxidation of the alcohol to aldehyde. Three of the phenyl-substituted compounds with simple phenyl (7), p-methylphenyl (8), and p-methoxyphenyl (9) were found to undergo the acid-catalyzed intramolecular photoredox reaction with the observation that 9 also undergoes a residual photoredox reaction that is not acid-mediated and may involve initial photoinduced electron transfer, which is supported by LFP data. The m-methoxyphenyl (10) compound did not undergo the reaction. The trend in observed relative reactivity may be partially rationalized by examining changes in molecular orbital coefficients observed in the calculated HOMOs and LUMOs. The photoredox reaction has also been applied twice in succession in a single compound 11, demonstrating that the photoredox reaction may be useful for sequential photoredox reactions in a multifunctional compound.Key words: intramolecular photoredox, acid catalysis, meta effect, benzophenone photochemistry.

Energy transfer quenching of a fluorescent excited radical cation by counter radical anion: dissipation of radical ions generated by photoinduced electron transfer

Fluorescence from the excited 1,3,5-trimethoxybenzene radical cation (TMB ⋅+*) coexsisting with 1,4-dicyanonaphthalene radical anion (DCN ⋅−) in acetonitrile was studied by the two-step two-laser excitation technique. The fluorescence intensity observed as a function of the delay time of the excitation pulse relative to generation pulse for TMB ⋅+ indicated the quenching by DCN ⋅− at ⩽220 ns. From the analysis of the fluorescence behavior by the Förster-type energy transfer theory, the distance between TMB ⋅+* and DCN ⋅− during their dissipation after the initial photoinduced electron transfer was estimated as a function of time.

Cleavage Mode of Benzyltributylstannane Radical Cations Produced in Photoinduced Electron Transfer

Abstract Cleavage of metal-carbon bond of benzyltributylstannane radical cation produced in photoinduced electron transfer from benzyltributylstannane to 10-methylacridinium ion occurs to give benzyl radical rather than benzyl cation, resulting in formation of 9-benzyl-10-methyl-9,10-dihydroacridine selectively in dehydrated acetonitrile. In the presence of water, however, 10-methyl-9,10-dihydroacridine is also formed via electron transfer from benzyl radical to the dihydroacridine radical cation produced by protonation of acridinyl radical following the initial photoinduced electron transfer.

Photochemical Activation of Amino Acids: From the Synthesis of Enantiomerically Pure β,γ‐Unsaturated Amino Acids to Macrocyclic Ring Systems

AbstractAmino acids whose amino groups are transformed into phthalimido chromophores are versatile substrates for photochemical transformations such as cyclization, isomerization, and β‐cleavage. These processes are initiated by homolytic or mesolytic CH activation and depend on the spin state of the excited substrate. Derivatives of α‐amino acids without strong electron‐donating substituents [Abu, Nva, Val, Leu, Ile, Tle, Phe, Met(SO)] react by initial homolytic CH activation and solely via their first excited singlet states, whereas substrates with strong electron‐donating substituents (Tyr, DOPA, Met, Glu, Asp) react by initial photoinduced electron transfer (PET) followed by mesolytic CH activation and solely via their first excited triplet states. The S‐alkyl cysteine derivatives are the only ones which show competing singlet and triplet state reactivity with pronounced spin‐determined regioselective CH activation. The amino dicarboxylic acids Glu and Asp could be applied as the carboxylates in PET reactions, a strategy which also opened up a new synthetic approach to medium‐ and large‐ring systems.

Charge transfer across a single lipid-water interface causes ion pumping across the bilayer

The photoformation of magnesium-porphyrin cations (P+) at a single lipid bilayer-water interface can pump lipophilic borate anions completely across the lipid bilayer and causes an actual reversal of the photovoltage. The system consists of a lipid bilayer containing magnesium octaethylporphyrin, an aqueous or interfacial electron acceptor on one side, and chloro- or fluoro-substituted tetraphenylborate in both aqueous electrolyte solutions. With 1-micros pulsed illumination, an immediate positive photovoltage is observed, which decreases on the microsecond and millisecond time scales. On the time scale of seconds, as the P+ cation concentration decays in reverse electron transfer, the voltage swings negative to a value almost equal to its initial value and finally decays with a half-time (approximately 20 s) longer than the time constant of the system (approximately 5 s). Thus, an ion gradient across the membrane is formed, trapped by the nonlinear relation between ion mobility and ion concentration. Continuous light illumination confirms that negative charge moves in the direction opposite that of the initial photoinduced electron transfer. Steady-state measurements indicate an ion pumping efficiency of approximately 30%. This simple mechanism may be a progenitor of photobiological ion pumps.