David C. Magri

The Anthracen-9-ylmethyloxy Unit: An Underperforming Motif Within the Fluorescent PET (Photoinduced Electron Transfer) Sensing Framework

Compound 2, which was designed to act as a fluorescent sensor for calcium according to the PET (Photoinduced Electron Transfer) principle, shows a relatively small Ca2+-induced fluorescence enhancement factor (FE) of 1.8 whereas its close relative 1 is known to display a far higher FE value of 16. Though designed as fluorescent PET sensors for solvent polarity, compounds 5 and 6 also show negligible fluorescence enhancement as their environments are made progressively less polar even though their relatives 3 and 4 show limiting FE values of 53 and 3, respectively. Indeed, 3 and 4 are useful since they are fluorescent sensors for solvent polarity without being affected by Bronsted acidity. The poor sensory performance of 2, 5, and 6 relative to their cousins is attributed to the presence of an oxygen proximal to the 9-position of an anthracene unit, which opens up a CT (charge transfer) channel. Normal PET sensing service is resumed when the offending oxygen is deleted.

A fluorescent AND logic gate driven by electrons and protons

A molecular logic gate displays a fluorescence output after oxidation in acidic media according to AND logic.

From PASS 1 to YES to AND logic: building parallel processing into molecular logic gates by sequential addition of receptors

The synthesis and photophysical characterization of a novel molecular logic gate 4, operating in water, is demonstrated based on the competition between fluorescence and photoinduced electron transfer (PET). It is constructed according to a ‘fluorophore–spacer–receptor1–spacer–receptor2’ format where anthracene is the fluorophore, receptor1 is a tertiary amine and receptor2 is a phenyliminodiacetate ligand. Using only protons and zinc cations as the chemical inputs and fluorescence as the output, 4 is demonstrated to be both a two-input AND and INH logic gate. When 4 is examined in context to the YES logic gates 1 and 2, and the two-input AND logic gate 3 and three-input AND logic gate 5, each with one or more of the following receptors including a tertiary amine, phenyliminodiacetate or benzo-15-crown-5 ether, logic gate 4 is the missing link in the homologous series. Collectively, the molecular logic gates 1–5 corroborate the PET ‘fluorophore–spacer–receptor’ model using chemical inputs and a light-signal output and provide insight into controlling the fluorescence quantum yield of future PET-based molecular logic gates.

A naphthalimide-based ‘Pourbaix sensor’: a redox and pH driven AND logic gate with photoinduced electron transfer and internal charge transfer mechanisms

A naphthalimide-based AND logic gate displays a fluorescence output in aqueous methanol upon oxidation of the ferrocene moiety and protonation of the tertiary amine. The molecule introduces a ‘receptor–spacer–fluorophore–spacer–redox-unit’ format for simultaneous monitoring of redox potential and pH.

‘Pourbaix sensors’: a new class of fluorescent pE–pH molecular AND logic gates based on photoinduced electron transfer

A novel anthracene-based molecular AND logic gate displays a fluorescence output in methanol after chemical oxidation of the ferrocene moiety with Fe(III) and protonation of the tertiary amine. This new class of probes could have practical applications in corrosion science and living cell imaging.

Water-soluble amino(ethanesulfonate) and [bis(ethanesulfonate)] anthracenes as fluorescent photoinduced electron transfer (PET) pH indicators and Fe3+ chemosensors

Two novel water-soluble anthracene-based fluorescent indicators appended with amino(ethanesulfonate) groups were designed and synthesised. A monoethanesulfonated or diethanesulfonate ligand is located in the proximity of a tertiary amino moiety separated by a methylene spacer at the 9-position of an anthracene fluorophore. The molecular structure of the monoethanesulfonated species was determined by single crystal X-ray diffraction. The molecules were studied by UV-visible absorption and fluorescence spectroscopy in water as molecular probes for protons and cations. The anthracene probes function according to a photoinduced electron transfer (PET) mechanism based on a ‘fluorophore–spacer–receptor’ format resulting in blue fluorescence on protonation. The excited state pK*a values were evaluated to be 5.7 and 7.4, respectively, for the di- and monoethanesulfonated anthracenes at a constant ionic strength of 0.1 M NaCl. The monoethanesulfonated indicator exhibits a high fluorescence quantum yield of 0.62 in acidic solution, and an enhancement factor (EF) of 9, while the diethanesulfonated indicator has a more modest fluorescence quantum yield of 0.17 and an EF of 2.4. Under acidic conditions both indicators are susceptible to selective quenching of the fluorescence by Fe3+ with linear responses between 0.6–8.9 mM and 0.3–5.0 mM Fe3+ for the diethanesulfonated and monoethanesulfonated anthracenes, respectively. The lack of a vertex in the Jobs plots indicates no metal–ligand complexation suggesting the fluorescence quenching may be due to an inner filter effect from Fe3+ absorbance.

A Radical‐Anion Chain Mechanism Initiated by Dissociative Electron Transfer to a Bicyclic Endoperoxide: Insight into the Fragmentation Chemistry of Neutral Biradicals and Distonic Radical Anions

The electron-transfer (ET) reduction of two diphenyl-substituted bicyclic endoperoxides was studied in N,N-dimethylformamide by heterogeneous electrochemical techniques. The study provides insight into the structural parameters that affect the reduction mechanism of the O-O bond and dictate the reactivity of distonic radical anions, in addition to evaluating previously unknown thermochemical parameters. Notably, the standard reduction potentials and the bond dissociation energies (BDEs) were evaluated to be -0.55+/-0.15 V and 20+/-3 kcal mol(-1), respectively, the last representing some of the lowest BDEs ever reported. The endoperoxides react by concerted dissociative electron transfer (DET) reduction of the O-O bond yielding a distonic radical-anion intermediate. The reduction of 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]oct-5-ene (1) results in the quantitative formation of 1,4-diphenylcyclohex-2-ene-cis-1,4-diol by an overall two-electron mechanism. In contrast, ET to 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]octane (2) yields 1,4-diphenylcyclohexane-cis-1,4-diol as the major product; however, in competition with the second ET from the electrode, the distonic radical anion undergoes a beta-scission fragmentation yielding 1,4-diphenyl-1,4-butanedione radical anion and ethylene in a mechanism involving less than one electron. These observations are rationalized by an unprecedented catalytic radical-anion chain mechanism, the first ever reported for a bicyclic endoperoxide. The product ratios and the efficiency of the catalytic mechanism are dependent on the electrode potential and the concentration of weak non-nucleophilic acid. A thermochemical cycle for calculating the driving force for beta-scission fragmentation is presented, and provides insight into why the fragmentation chemistry of distonic radical anions is different from analogous neutral biradicals.

Kinetics of the photoinduced dissociative electron transfer reduction of the antimalarial endoperoxide, Artemisinin

Abstract The rate constants (k) for reactions between a series of excited singlet state donors and the antimalarial agent, Artemisinin (ART), were measured in acetonitrile using fluorescence quenching techniques. A plot of log(k) correlates with the excited state oxidation potential of the donor, E D + / D ∗ , while a similar plot of log(k) versus the singlet energy of the donor, Es, which if linear would indicate an energy transfer reaction process, shows a poor correlation. The results suggest that the determined rate constants are for dissociative electron transfer (ET) from the excited state donor to the O–O bond in ART. Using our recently determined standard dissociative reduction potential for ART, Ediss0, the rate constants are related to the free energy of ET, ΔGET0. Analysis of the kinetic data as a function of ΔGET0 correlates well with theories of ET modified for the non-adiabatic nature of the ET to peroxides. A number of thermochemical parameters are estimated from the analysis, in particular the intrinsic barrier (ΔG0≠) that is comprised of the solvent reorganization energy (λ) and the bond dissociation enthalpy of the O–O bond.

Model dialkyl peroxides of the Fenton mechanistic probe 2-methyl-1-phenyl-2-propyl hydroperoxide (MPPH): kinetic probes for dissociative electron transfer

Two dialkyl peroxides, devised as kinetic probes for the heterogeneous electron transfer (ET), are studied using heterogeneous and homogeneous electrochemical techniques. The peroxides react by concerted dissociative ET reduction of the O-O bond. Under heterogeneous conditions, the only products isolated are the corresponding alcohols from a two-electron reduction as has been observed with other dialkyl peroxides studied to date. However, under homogeneous conditions, a generated alkoxyl radical undergoes a rapid beta-scission fragmentation in competition with the second ET resulting in formation of acetone and a benzyl radical. With knowledge of the rate constant for fragmentation and accounting for the diffuse double layer at the electrode interface, the heterogeneous ET rate constant to the alkoxyl radicals is estimated to be 1500 cm s(-1). The heterogeneous and homogeneous ET kinetics of the O-O bond cleavage have also been measured and examined as a function of the driving force for ET, deltaG(ET), using dissociative electron transfer theory. From both sets of kinetics, besides the evaluation of thermochemical parameters, it is demonstrated that the heterogeneous and homogeneous reduction of the O-O bond appears to be non-adiabatic.

Water-soluble naphthalimide-based ‘Pourbaix sensors’: pH and redox-activated fluorescent AND logic gates based on photoinduced electron transfer

Two novel naphthalimide-based ‘Pourbaix sensors’ for redox potential and pH were designed based on a ‘fluorophore–spacer1–receptor–spacer2–electron-donor’ configuration. The synthesised molecular logic gates consist of an alkylated 1,8-naphthalimide fluorophore connected to a tertiary amine by a flexible ethylene spacer to a ferrocene moiety via a methylene spacer. The UV-visible absorption and steady state fluorescent properties were examined in methanol and 1 : 1 (v/v) methanol/water. The spectroscopic properties are modulated by internal charge transfer (ICT) and photoinduced electron transfer (PET) mechanisms. A log βH+ of 9.2 and 8.7 were determined in 1 : 1 (v/v) methanol/water for the methylated 1 and butylated 2 compounds, respectively. An apparent log βFe3+ of 4.2 was determined in 1 : 1 (v/v) methanol/water at pH 4. Time-resolved spectroscopic studies elucidated the stimulus-modulated photoinduced electron transfer pathways. In the oxidised and protonated state, 1 exhibits a single fluorescence lifetime of 8.5 ns, while an efficient photoinduced electron transfer characterised by a time constant of 20 ps is revealed by femtosecond transient absorption spectroscopy in the absence of a perturbing stimulus.