Oxidative Photoinduced Electron Transfer Research Articles

An investigation of the Excitation Wavelength-Dependent Dynamic Changes in the Mechanism of Detection of Picric Acid using Pyrene-Based Donor-Acceptor Systems.

Picric acid (PA) is an important industrial feedstock and hence the release of industrial effluents without proper remediation results in its buildup in soil and water bodies. The adverse effects of PA accumulation in living beings necessitate the development of efficient methods for its detection and quantification. Herein, we describe pyrene-based fluorescent sensors for PA, where pyrene is appended with electron-withdrawing groups, malononitrile, and 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene) malononitrile (DCDHF). These molecules displayed the typical emission of pyrene monomers, as well as a broad red-shifted emission resulting from an intramolecular charge transfer (ICT) in the excited state. Both the emissions displayed a turn-off response to PA with high selectivity and sensitivity and the lowest limit of detection was estimated as 27 nM. To prove the feasibility of on-site detection, test paper strips were prepared, which could detect PA up to 4.58 picograms. Using a combination of experimental and theoretical studies the mechanism of the detection was identified as primary/secondary inner filter effect, oxidative photoinduced electron transfer, or a combination of both depending on the excitation wavelength. Interestingly, the contribution of each of these mechanisms to the total quenching process varied with a change in the excitation wavelength.

A New Biphenol-bis(polyazacyclophane) Receptor with Unusual Photophysical Properties Towards Zn2.

The new ligand 3,3'-bis(((2-(3,6,9-triaza-1(2,6)-pyridinacyclodecaphane-6-yl)ethyl)amino)methyl)-[1,1'-biphenyl]-2,2'-diol (L) has been synthesized and characterized. It contains two pyridinacyclophane macrocycles spaced by a 2,2'-biphenol moiety. The acid-base behaviour of L as well as its binding properties towards Zn2+ ion have been investigated. This work is inserted in the field of fluorescent ditopic receptors, formed by two polyamines spaced by a aromatic fragments. This ligand represents a new example of a peculiar case of polyamine fluorescent receptor in which the interaction with Zn2+ is translated into a deactivation of the emission. Enough data to describe and explain this unusual behaviour was obtained through potentiometric, UV-Vis, fluorescence and NMR titrations as well as theoretical calculations. This studies have shown that the metal cation is indirectly affecting the emission favouring a conformation in which the fluorophore is at stacking distance from the electron poor pyridine moieties. This gives rise to an oxidative photoinduced electron transfer from the excited state of the fluorophore to the electron-poor Zn2+ coordined pyridine.

Formation of vesicles between negatively charged carbon quantum dots and cationic surfactant cetylpyridinium chloride (CPC) due to oxidative photo induced electron transfer

The tuning of photo physical properties of highly emissive carbon quantum dots which was synthesized from citric acid in DMF medium through solvothermal method is investigated inside micelles formed by different types of surfactants like cationic surfactant cetylpyridinium chloride (CPC), cetrimonium bromide (CTAB), anionic surfactant sodium dodecyl sulfate (SDS) and neutral surfactant polyethylene glycol tert-octyl phenyl ether (TX100), polyoxoethylene lauryl ether (BRIJ-35). Interestingly, only CPC interacts with greater extent and formed co-assembly with negatively-charged carbon quantum dots (CQDs) whereas other surfactants like CTAB, SDS, TX100 and BRIJ-35 do not alter the fluorescence intensity extensively. This results in the formation of a non fluorescent vesicle. No phase separation was observed over the large concentration barrier of the CPC surfactant. However, no such vesicle formation takes place for another cationic surfactant CTAB excluding the role of only electrostatic interaction. The synthesized vesicle is characterized by various techniques including HRTEM, DLS. For the aqueous solution of CQDs having concentration of 0.5 mg/ml, addition of CPC leads to formation of different sized vesicles of size 40 nm to 100 nm. The driving force behind this formation of non-fluorescent vesicles is oxidative photo induced electron transfer which is revealed by the highest occupied molecular orbital (HOMO) –lowest unoccupied molecular orbital (LUMO) interaction between the donor (CQDs) and the acceptor (CPC). We trust such type vesicle can be a suitable alternative for drug carriers, biomarkers and other various applications also.

Influence of Regioisomerism in 9‐Anthracenyl‐Substituted Dithiodicyanoethene Derivatives on Photoinduced Electron Transfer Controlled By Intramolecular Charge Transfer

AbstractIn this paper, we report on the fluorescence behaviour of three regioisomers which consist of two 9‐anthracenyl fluorophores and of differently substituted dithiodicyanoethene moieties. These isomeric fluorescent probes show different quantum yields (ϕf). In these probes, an oxidative photoinduced electron transfer (PET) from the excited 9‐anthracenyl fluorophore to the dithiodicyanoethene unit quenches the fluorescence. This quenching process is accelerated by an intramolecular charge transfer (ICT) of the push‐pull π‐electron system of the dithiodicyanoethene group. The acceleration of the PET depends on the strength of the ICT unit. The higher the dipole moment of the ICT unit, the stronger the observed fluorescence quenching. To the best of our knowledge, this is the first report of a regioisomeric influence on an oxidative PET by an ICT.

Photoinduced Intercomponent Processes in Selectively Addressable Bichromophoric Dyads Made of Linearly Arranged Ru(II) Terpyridine and Expanded Pyridinium Components.

Three new linearly arranged bichromophoric systems 1-3 have been prepared, and their photophysical properties have been studied, taking also advantage of femtosecond pump-probe transient absorption spectroscopy. The three compounds contain the same chromophores, that is a Ru(II)-terpy-like species and a fused expanded bipyridinium (FEBP) unit, separated by three different, variously methylated biphenylene-type bridges. The chromophores have been selected to be selectively addressable, and excitation involving the Ru-based or the FEBP-based dyes results in different excited-state decays. Upon Ru-based excitation at 570 nm, oxidative photoinduced electron transfer (OPET) takes place in 1-3 from the 3MLCT state; however, the charge-separated species does not accumulate, indicating that the charge recombination rate constant exceeds the OPET rate constant. Upon excitation of the organic dye at 400 nm, the FEBP-based 1π-π* level is prepared, which undergoes a series of intercomponent decay events, including (i) electron-exchange energy transfer leading to the MLCT manifold (SS-EnT), which successively decays according to 570 nm excitation, and (ii) reductive photoinduced electron transfer (RPET), leading to the preparation of the charge-separated (CS) state. Reductive PET, involving the FEBP-based singlet state, is much faster than oxidative PET, involving the MLCT triplet state, essentially because of driving force reasons. The rate constant of CR is intermediate between the rate constants of OPET and RPET, and this makes 1-3 capable to selectively read the 400 nm excitation as an active input to prepare the CS state, whereas excitation at wavelengths longer than 480 nm is inefficient to accumulate the CS state. Moreover, intriguing differences between the rate constants of the various processes in 1-3 have been analyzed and interpreted according to the superexchange theory for electron transfer. This allowed us to uncover the role of the electron-transfer and hole-transfer superexchange pathways in promoting the various intercomponent photoinduced decay processes occurring in 1-3.

A novel dual channel responsive zinc(II) probe

A novel compound, 1, which is based on a 1,10-phenanthroline scaffold with cofacial BODIPY units, is synthesized via a three-step reaction sequence. It is noteworthy that 1 can be utilized for both visual and turn-off fluorometric detection of Zn2+ ions in aqueous acetonitrile solution. The fluorescence response is based on cation-mediated oxidative photoinduced electron transfer (PET). The digital action of a two-input NOR logic gate is also demonstrated.

Tuning Reductive and Oxidative Photoinduced Electron Transfer in Amide‐Linked Anthraquinone–Porphyrin–Ferrocene Architectures

AbstractPorphyrin amino acids 3a–3h with meso substituents Ar of tunable electron‐donating power (Ar = 4‐C6H4OnBu, 4‐C6H4OMe, 4‐C6H4Me, Mes, C6H5, 4‐C6H4F, 4‐C6H4CF3, C6F5) have been linked at the N terminus to anthraquinone Q as electron acceptor through amide bonds to give Q‐PAr dyads 4a–4h. These were conjugated to ferrocene Fc at the C terminus as electron donor to give the acceptor‐chromophore‐donor Q‐PAr‐Fc triads 6a–6h. To further modify the energies of the electronically excited and charge‐separated states, the triads 6a–6h were metallated with zinc(II) to give the corresponding Q‐(Zn)PAr‐Fc triads Zn‐6a–Zn‐6h. The Q‐PAr1 dyad (Ar1 = C6H5) was further extended with a second porphyrin PAr2 (Ar2 = 4‐C6H4Me) as well as appended to a ferrocene to give the tetrad Q‐PAr1‐PAr2‐Fc 9. Almost all the conjugates show strongly reduced fluorescence quantum yields and excited‐state lifetimes, which has been interpreted as photoinduced electron transfer (PET) either from the excited porphyrin to the quinone (oxidative PET) or from the ferrocene to the excited porphyrin (reductive PET). Electrochemical data, absorption spectroscopy, steady‐state emission, time‐resolved fluorescence, transient absorption pump‐probe spectroscopy as well as DFT calculations have been used to elaborate the preferred PET pathway (reductive vs. oxidative PET) in these architectures with systematically varied electron‐donating substituents at the central chromophore.

Highly efficient cycloreversion of photochromic dithienylethene compounds using visible light-driven photoredox catalysis

Photochromic cis-1,2-dithienylethene (DTE) compounds are the most suitable to the application in reversible molecular memories and switches, but imbalance in the quantum yields for the chromic interconversion limits the full potentials. We have demonstrated and investigated photoelectrocatalytic cycloreversion of DTE compounds. A series of cyclometalated Ir(III) complexes served as photoredox catalysts to achieve one order of magnitude enhancement in the cycloreversion quantum yields. The mechanism, involving photoinduced oxidation of DTE, electrocatalytic ring opening and reductive termination, has been thoroughly investigated. Nanosecond transient spectroscopic techniques were employed to directly monitor bidirectional electron transfer between DTE and the photoredox catalyst. It was found that the oxidative photoinduced electron transfer was diffusion-controlled and located in the Marcus-normal region, whereas the competing back electron transfer occurred in the Marcus-inverted region. This novel discovery establishes that synthetic control over back electron transfer, rather than photoinduced electron transfer, can improve the performance of the photoelectrocatalysis. Combined studies, including the kinetic investigations with the use of variable-temperature stopped-flow UV-vis absorption spectroscopy and quantum chemical calculations based on time-dependent density functional theory, further enabled identification of the radical intermediate that underwent thermal, electrocatalytic cycloreversion. Finally, analyses based on the Marcus theory of electron transfer suggested regeneration of the excited-state catalyst in the termination step to initiate dark-state electrocatalytic cycloreversion. The results obtained in this work established novel principles to maximizing quantum yields for photoinduced cycloreversion of DTEs. It is envisioned that our findings will provide novel guidance to the future application of the truly reversible photochromism to a broad range of molecular photonic systems.

On the effect of the nature of the bridge on oxidative or reductive photoinduced electron transfer in donor–bridge–acceptor systems

Photoinduced electron transfer is a topical issue in chemistry. In multicomponent donor-bridge-acceptor systems, electron transfer is usually discussed within the frame of superexchange theory, which takes into account electronic coupling mediated by virtual states involving bridge orbitals. However, the schematization used for superexchange in thermal electron transfer processes is not suitable to immediately understand some intriguing aspects of photoinduced charge separation and recombination processes, which are only uncovered by analyzing the virtual states involved in forward and backward excited-state electron transfer. In particular, for oxidative photoinduced electron transfer, a low-energy virtual state which cannot mediate the forward charge separation can efficiently mediate charge recombination via the hole-transfer superexchange route, whereas for reductive photoinduced electron transfer, a low-energy virtual state which cannot mediate the forward process can efficiently mediate charge recombination via electron-transfer superexchange. As a consequence, to obtain long-lived charge-separated states upon oxidative photoinduced electron transfer in donor-bridge-acceptor systems it is preferable to avoid easy-to-oxidize bridges, whereas easy-to-reduce bridges should better be avoided in reductive photoinduced charge separation. These considerations, exemplified by the analysis of some literature cases, can be useful hints for the design of long-lived charge-separated states.

Magnetite Nanoparticle-Induced Fluorescence Quenching of Adenosine Triphosphate–BODIPY Conjugates: Application to Adenosine Triphosphate and Pyrophosphate Sensing

We report that magnetite nanoparticles (Fe3O4 NPs) act as an efficient quencher for boron dipyrromethene-conjugated adenosine 5'-triphosphate (BODIPY-ATP) that is highly fluorescent in bulk solution. BODIPY-ATP molecules attached to the surface of Fe3O4 NPs through the coordination between the triphosphate group of BODIPY-ATP and Fe(3+)/Fe(2+) on the NP surface. The formed complexes induced an apparent reduction in the BODIPY-ATP fluorescence resulting from an oxidative-photoinduced electron transfer (PET) from the BODIPY-ATP excited state to an unfilled d shell of Fe(3+)/Fe(2+) on the NP surface. A comparison of the Stern-Volmer quenching constant between Fe(3+) and Fe(2+) suggests that Fe(3+) on the NP surface dominantly controls this quenching process. The efficiency for Fe3O4 NP-induced fluorescence quenching of the BODIPY-ATP was enhanced by increasing the concentration of Fe3O4 NPs and lowering the pH of the solution to below 6.0. We found that pyrophosphate and ATP compete with BODIPY-ATP for binding to Fe3O4 NPs. Thus, we amplified BODIPY-ATP fluorescence in the presence of increasing the pyrophosphate and ATP concentration; the detection limits at a signal-to-noise ratio of 3 for pyrophosphate and ATP were determined to be 7 and 30 nM, respectively. The Fe3O4 NP-based competitive binding assay detected ATP and pyrophosphate in only 5 min. The selectivity of this assay for ATP over metal ions, amino acids, and adenosine analogues is particularly high. The practicality of using the developed method to determine ATP in a single drop of blood is also validated.

Systematic Investigation of Photoinduced Electron Transfer Controlled by Internal Charge Transfer and Its Consequences for Selective PdCl2 Coordination

Fluoroionophores of fluorophore-spacer-receptor format were prepared for detection of PdCl(2) by fluorescence enhancement. The fluorescent probes 1-13 consist of a fluorophore group, an alkyl spacer and a dithiomaleonitrile PdCl(2) receptor. First, varying the length of the alkylene spacer (compounds 1-3) revealed a dominant through-space pathway for oxidative photoinduced electron transfer (PET) in CH(2)-bridged dithiomaleonitrile fluoroionophores. Second, fluorescent probes 4-9 containing two anthracene or pyrene fragments connected through CH(2) bridges to the dithiomaleonitrile unit were synthesized. Modulation of the oxidation potential (E(Ox)) through electron-withdrawing or -donating groups on the anthracene moiety regulates the thermodynamic driving force for oxidative PET (DeltaG(PET)) in bis(anthrylmethylthio)maleonitriles and therefore the fluorescence quantum yields (Phi(f)), too. The new concept was confirmed and transferred to pyrenyl ligands, and fluorescence enhancements (FE) greater than 3.2 in the presence of PdCl(2) were achieved by 7 and 8 (FE=5.4 and 5.2). Finally, for comparison, monofluorophore ligands 10-13 were synthesized.

Synthesis of cyclopropyl silyl ethers and their facile ring opening by photoinduced electron transfer as key step in radical/radical cationic cascade reactions

Various ring-fused cyclopropyl silyl ethers with an benzylic, olefinic or acetylenic side chain have been synthesized. Upon oxidative photoinduced electron transfer (PET) the cyclopropane ring opens and forms a reactive β-keto radical, which undergoes intramolecular cyclization. In some cases we observed only formation of ring opened non-cyclized products. With olefinic side chain 5- exo- trig mode of cyclization rather than 6- endo- trig mode of cyclization takes place whereas in case of acetylenic side chain we observed 6- endo cyclization.

Synthetic applications in radical/radical cationic cascade reactions.

Oxidative photoinduced electron transfer (PET) reactions have been performed with various cyclic cyclopropyl(vinyl) silyl ethers bearing an olefinic or acetylenic side chain. The reactions result in bi- to tetracyclic ring systems via a fragmentation-radical/radical cationic addition reaction pathway with well defined ring juncture. The mode of cyclisation (endo/exo) can be partially controlled by addition of nucleophiles due to the suppression of radical cationic reaction pathways. Quantum chemical calculation of the cyclisation transition states underline the experimentally found selectivities. Additional mechanistic studies concerning the saturation step reveal that the final radical is saturated mostly by the solvent and traces of water in the solvent.

Anomalous C-C bond cleavage in sulfur-centered cation radicals containing a vicinal hydroxy group.

1,3-dithianyl cation radicals having alpha-hydroxy-neopentyl or similar groups in position 2, which are generated via oxidative photoinduced electron transfer, undergo anomalous fragmentation necessitating refinement of the accepted mechanism. Experimental and computational data support a rationale in which proton abstraction from the hydroxy group in the initial cation radical does not cause a Grob-like fragmentation, but rather produces a neutral radical species, the alkoxy radical, that undergoes fragmentation in either direction, i.e., cleaving the C-C bond to dithiane or to the tertiary alkyl group.

Modulation of boradiazaindacene emission by cation-mediated oxidative PET.

[reaction: see text] Bright green boradiazaindacene fluorescence is quenched by an oxidative photoinduced electron transfer (PET) from the excited state fluorophore to the bipyridyl unit complexed to metal cations. The closed shell diamagnetic cation Zn(II) is one of the most effective quenchers of fluorescence in this system, demonstrating that the quenching is not simply related to the facilitated intersystem crossing. The molecule also acts as a NOR logic gate with two chemical inputs, TFA and Zn(II).

Regio- and Stereoselective Cyclization Reactions of Unsaturated Silyl Enol Ethers by Photoinduced Electron Transfer – Mechanistic Aspects and Synthetic Approach

Oxidative photoinduced electron transfer (PET) reactions have been performed with various silyl enol ethers and silyloxy-2H-chromones bearing an olefinic or silylacetylenic side chain, The reactions result in regioselective ring closure with the formation of bi- to tetracyclic ring systems with a well-defined ring juncture, e.g. perhydrophenanthrenones 13 or benzo-annellated xanthenones 24. Our investigations have focussed on the optimization of this cyclization method with regard to irradiation time and product yield. The irradiation times could be reduced by using the cosensitized PET method. Modifying the substrate at the silyl group led to enhanced yields. In addition, we found that solvent and pressure dependences are important tools, allowing control of the regiochemistry. Both the synthesis of 6-endo products by radical cationic reaction pathways, as well as 5-exo ring closure by radical intermediates was achieved. Mechanistic details, including findings from deuterium labelling experiments, are discussed.