Competing photochemical reactions of bis-naphthols and their photoinduced antiproliferative activity.

The photophysical properties and photochemical reactivities of a series of bis-naphthols 4a-4e and bis-anthrols 5a and 5e were investigated by preparative irradiation in CH3OH, fluorescence spectroscopy and laser flash photolysis (LFP). Methanolysis taking place via photodehydration (bis-naphthols: ΦR = 0.04-0.05) is in competition with symmetry breaking charge separation (SB-CS). The SB-CS gave rise to radical ions that were detected for 4a and 4e by LFP. Photodehydration gave quinone methides (QMs) that were also detected by LFP (λmax = 350 nm, τ ≈ 1-2 ms). In the aqueous solvent, excited state proton transfer (ESPT) competes with the abovementioned processes, giving rise to naphtholates, but the process is inefficient and can only be observed in the buffered aqueous solution at pH > 7. Since the dehydration of bis-naphthols delivers QMs, their potential antiproliferative activity was investigated by an MTT test on three human cancer cell lines (NCI-H1299, lung carcinoma; MCF-7, breast adenocarcinoma; and SUM159, pleomorphic breast carcinoma). Cells were treated with 4 or 5 with or without irradiation (350 nm). An enhancement of the activity (up to 10-fold) was observed upon irradiation, which may be associated with QM formation. However, these QMs do not cross-link DNA. The activity is most likely associated with the alkylation of proteins present in the cell cytoplasm, as evidenced by photoinduced alkylation of bovine and human serum albumins by 4a.

A Binary Mixture of Ionic Liquids as a New Approach for an Experimental Diffusion Correction in the Study of Activated Processes.

A binary system of two ionic liquids, 1-ethyl-3-methylimidazolium and trioctylmethylammonium bis(trifluoromethylsulfonyl)imide ([EMIM][NTf2 ] and [OMA][NTf2 ], respectively), with varying molar fractions, is introduced. It allows the dynamic viscosity to remain constant over a range of almost 60 K; this means that any activated process can be studied independent of the temperature dependence of viscosity itself. This principle is proven upon reinvestigation of electron self-exchange kinetics of tetrathiafulvalene by continuous-wave ESR line-broadening experiments. From these results, it is also confirmed that this process is totally diffusion controlled.

Photodeamination Reaction Mechanism in Aminomethyl p-Cresol Derivatives: Different Reactivity of Amines and Ammonium Salts.

Derivatives of p-cresol 1-4 were synthesized, and their photochemical reactivity, acid-base, and photophysical properties were investigated. The photoreactivity of amines 1 and 3 is different from that for the corresponding ammonium salts 2 and 4. All compounds have low fluorescence quantum yields because the excited states undergo deamination reactions, and for all cresols the formation of quinone methides (QMs) was observed by laser flash photolysis. The reactivity observed is a consequence of the higher acidity of the S1 states of these p-cresols and the ability for excited-state intramolecular proton transfer (ESIPT) to occur in the case of 1 and 3, but not for salts 2 and 4. In aqueous solvent, deamination depends largely on the prototropic form of the molecule. The most efficient deamination takes place when monoamine is in the zwitterionic form (pH 9-11) or diamine is in the monocationic form (pH 7-9). QM1, QM3, and QM4 react with nucleophiles, and QM1 exhibits a shorter lifetime when formed from 1 (τ in CH3CN = 5 ms) than from 2 (τ in CH3CN = 200 ms) due to the reaction with eliminated dimethylamine, which acts as a nucleophile in the case of QM1. Bifunctional QM4 undergoes two types of reactions with nucleophiles, giving adducts or new QM species. The mechanistic diversity uncovered is of significance to biological systems, such as for the use of bifunctional QMs to achieve DNA cross-linking.

Thioxanthone Dioxide Triplet States Have Low Oxygen Quenching Rate Constants.

With few exceptions, triplet excited states of organic molecules, 3M1, are quenched by ground state molecular oxygen, O2(X3Σg-), with rate constants kq greater than ∼109 M-1 s-1 in fluid solutions. If the energy of the triplet state is above 94 kJ/mol, then such quenching can result in the sensitized production of singlet oxygen, O2(a1Δg). In the interaction between 3M1 and O2(X3Σg-), the magnitudes of both kq and the yield of the O2(a1Δg) depend appreciably on mixing with the M-O2 charge-transfer state. Here, we report that triplet states of several thioxanthen-9-one-10,10-dioxide derivatives have unusually low kq values (as low as ∼1 × 108 M-1 s-1) but have quantum yields for the photosensitized production of O2(a1Δg) that approach unity. Because these molecules possess high oxidation potentials (∼3.5 V vs SCE), we suggest that charge transfer character in the 3M1-O2(X3Σg-) encounter complex is reduced, thereby lowering kq while maintaining high O2(a1Δg) yields. These results provide important experimental support for existing models for the quenching of organic molecule excited states by O2(X3Σg-).

Time-resolved magnetic field effects distinguish loose ion pairs from exciplexes.

We describe the experimental investigation of time-resolved magnetic field effects in exciplex-forming organic donor-acceptor systems. In these systems, the photoexcited acceptor state is predominantly deactivated by bimolecular electron transfer reactions (yielding radical ion pairs) or by direct exciplex formation. The delayed fluorescence emitted by the exciplex is magnetosensitive if the reaction pathway involves loose radical ion pair states. This magnetic field effect results from the coherent interconversion between the electronic singlet and triplet radical ion pair states as described by the radical pair mechanism. By monitoring the changes in the exciplex luminescence intensity when applying external magnetic fields, details of the reaction mechanism can be elucidated. In this work we present results obtained with the fluorophore-quencher pair 9,10-dimethylanthracene/N,N-dimethylaniline (DMA) in solvents of systematically varied permittivity. A simple theoretical model is introduced that allows discriminating the initial state of quenching, viz., the loose ion pair and the exciplex, based on the time-resolved magnetic field effect. The approach is validated by applying it to the isotopologous fluorophore-quencher pairs pyrene/DMA and pyrene-d10/DMA. We detect that both the exciplex and the radical ion pair are formed during the initial quenching stage. Upon increasing the solvent polarity, the relative importance of the distant electron transfer quenching increases. However, even in comparably polar media, the exciplex pathway remains remarkably significant. We discuss our results in relation to recent findings on the involvement of exciplexes in photoinduced electron transfer reactions.

Experimental observation of preferential solvation on a radical ion pair using MARY spectroscopy.

The effect of preferential solvation on the exciplex luminescence detected magnetic field effect has been studied using magnetic-field-effect-on-reaction-yield (MARY) spectroscopy. By designing solvent mixtures which can provide a micro-environment around the magneto-sensitive radical ion pair (RIP) from highly heterogeneous to quasi-homogenous, the effect of the polarity scan on an absolute magnetic field effect (χ(E)) and B(1/2) (the field value marking half saturation) has been studied on the system 9,10-dimethylanthracene (fluorophore)/N,N'-dimethylaniline (quencher). While the trend in χ(E) (although with subtle differences) follows the usual norm of passing through maxima with increasing polarity, the B(1/2) values show either a large monotonic decrease (for heterogeneous solvents) or remain constant (for quasi-homogenous systems) with increasing polarity. The observations have been interpreted invoking the concept of amplification of the "cage-effect" as a result of preferential solvation in binary solvents and its influence on the decaying exciplex. The use of ternary solvents further confirms the proposed mechanism. Additionally electron hopping from the radical ion pair to the surrounding neutral donor molecules could also possibly contribute to the observed trend.

High pressure ESR studies of electron self-exchange reactions of organic radicals in solution.

Simple electron self-exchange reactions are often used to study the role of the reaction medium on a chemical process, commonly implying the use of various solvents with different physical properties. In principle, similar studies may be conducted using a single solvent, changing its physical properties by application of elevated pressures, but so far only little information is available on pressure dependent exchange reactions. In this work, we have used a recently constructed high pressure apparatus for use with electron spin resonance (ESR) spectroscopy to investigate simple electron self-exchange reactions involving 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) and tetracyanoethylene (TCNE) and their respective radical anions as well as TMPPD and its radical cation in three different solvents. The self-exchange was observed by ESR line broadening experiments, yielding rate constants and volumes of activation. The experimental results were compared to theoretical calculations based on Marcus theory and taking into account solvent dynamic effects. The use of elevated pressures has enabled the study of solvent effects without commonly encountered problems like solubility issues or chemical reactions between solvent and solute which sometimes limit the range of useable solvents.

Photochemical electron transfer mediated addition of naphthylamine derivatives to electron-deficient alkenes.

Using photochemical electron transfer, N,N-dimethylnaphthylamine derivatives are added to α,ß-unsaturated carboxylates. The addition takes place exclusively in the α-position of electron-deficient alkenes and mainly in the 4-position of N,N-dimethylnaphthalen-1-amine. A minor regioisomer results from the addition in the 5-position of this naphthylamine. A physicochemical study reveals that the fluorescence quenching of N,N-dimethylnaphthalen-1-amine is diffusion-controlled and that the back electron transfer is highly efficient. Therefore no transformation is observed at lower concentrations. To overcome this limitation and to induce an efficient transformation, minor amounts of water or another proton donor as well as an excess of the naphthylamine derivative are necessary. A mechanism involving a contact radical ion pair is discussed. Isotopic labeling experiments reveal that no hydrogen is directly transferred between the substrates. The hydrogen transfer to the furanone moiety observed in the overall reaction therefore results from an exchange with the reaction medium. An electrophilic oxoallyl radical generated from the furanone reacts with the naphthylamine used in excess. Concerning some mechanistic details, the reaction is compared with radical and electrophilic aromatic substitutions. The transformation was carried out with a variety of electron-deficient alkenes. Sterically hindered furanone derivatives are less reactive under standard conditions. In a first experiment, such a compound was transformed using heterogeneous electron transfer photocatalysis with TiO(2).

Viscosity dependence of rubrene fluorescence quenching by organic radicals via energy transfer.

The kinetics of rubrene (5,6,11,12-tetraphenylnaphthacene) fluorescence quenching by energy transfer to the radicals of N,N,N',N'-tetramethyl-para-phenylenediamine (TMPD+*), tetracyanoquinonedimethane (TCNQ-*) and galvinoxyl (2,6-di-tert-butyl-alpha-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy; GAL*), have been measured in solvents of different viscosities. At room temperature the observed quenching rate constants vary between 0.12 and 7.8 x 10(10) M(-1) s(-1), always exceeding the diffusion rate constants given by the simplest version of Smoluchowski theory combined with the rate constants for dipole-dipole energy transfer. This discrepancy is explained by accounting for the non-stationary stage in the Förster kinetics, even though the experimental fluorescence decays are apparently mono-exponential.

Electron-self-exchange kinetics of the cyclooctatetraene/cyclooctatetraene radical-anion couple.

Rate constants k(hom) and k(het) are reported for the homogeneous electron-self-exchange and the heterogeneous electrochemical electron-transfer reactions, respectively, of the cyclooctatetraene/cyclooctatetraene(-) (COT/COT(.-)) redox couple. In acetonitrile, the values k(hom) (298 K)=(5+/-3) x 10(5) M(-1) s(-1) and k(het) (295 K)=8 x 10(-3) cm s(-1) are found, whereas slightly faster rates are obtained in dimethylformamide, namely, k(hom) (298 K)=(1.6+/-0.6) x 10(6) M(-1) s(-1) and k(het) (295 K)=2 x 10(-2) cm s(-1). The k(hom) rates are obtained from electron spin resonance (ESR) line broadening whereas the k(het) rates are measured at a mercurized Pt electrode by using Nicolson's method. The slowness of both electron-transfer reactions is caused by the high inner-sphere reorganization energy that results from the inevitable conformational change that takes place upon going from the tub-like COT molecule to the planar COT(.-) anion. The rates are well-understood in terms of Marcus theory, including an additional medium inner-sphere mode which is responsible for the flattening of COT.

Kinetics of photoinduced electron transfer reactions of ruthenium(II) complexes and phenols, tyrosine, N-acetyl-tyrosine and tryptophan in aqueous solutions measured with modulated fluorescence spectroscopy.

Photooxidation kinetics of phenol, 1-naphthol, 2-naphthol, tyrosine (TyrOH) and N-acetyl-tyrosine (AcTyrOH), tryptophan (TrpH) by ruthenium(II) polypyridyl complexes: [Ru(bpy)3]Cl2 (1), [Ru(phen)3]Cl2 (2), [Ru(bpy)(phen)(bpg)]Cl2 (3), and [Ru(dpq)2(bxbg)]Cl2 (4) where bpy is 2,2'-bipyridine, phen - 1,10-phenanthroline, bpg - bipyridine-glycoluril, dpq - dipyrido[3,2-d:2',3'-f]quinoxaline, and bxbg - bis(o-xylene)bipyridine-glycoluril are investigated. Rate constants have been measured by steady-state luminescence and phase-modulation fluorometry in aqueous solutions at different pH's. The rates for the oxidation of the phenols and phenolic aromatic amino acids spreads over a wide range from 4.2×106 to 6.8×109M-1s-1, depending on pH and the nature of solutes. At pH>pKa of the quenchers, the presence of reactive species (PhO-) in the alkaline solutions is accounted for the rapid ET rates. In the pH range between 4 and 10 (pH

Spectral and thermodynamic properties for the exciplexes of N-alkyl carbazoles with dicyanobenzenes in THF.

The exciplex emission spectra of N-ethylcarbazole with 1,2-dicyanobenzene (NEC/1,2-DCB), N-methylcarbazole with 1,2-dicyanobenzene (NMC/1,2-DCB), 1,3-dicyanobenzene (NMC/1,3-DCB), and 1,4-dicyanobenzene (NMC/1,4-DCB) are studied in tetrahydrofuran (THF) for the temperature range starting from 253 K to 334 K. Thermochromic shifts along with the spectral properties including change in peak intensities and the ratio of exciplex peak intensity to fluorophore peak intensity are studied. Effect of temperature on the energy of zero-zero transitions hνo('), Huang-Rhys factor (S), Gauss broadening of vibronic level (σ) and the dominant high-frequency vibration (hνν) are also part of investigation. Enthalpy of exciplex formation (ΔH(EX)(∗)) calculated by the model proposed by A. Weller and the Gibb's energy of electron transfer (ΔG(et)(∗)) for all exciplex systems are also discussed in the present paper. All the exciplexes under study were observed to be dipolar in nature. The exciplex of the N-methylcarbazole/1,4-dicyanobenzene was found to be the most stable and the N-methylcarbazole/1,3-dicyanobenzene was the weakest exciplex system.

In situ monitoring magnetism and resistance of nanophase platinum upon electrochemical oxidation.

Controlled tuning of material properties by external stimuli represents one of the major topics of current research in the field of functional materials. Electrochemically induced property tuning has recently emerged as a promising pathway in this direction making use of nanophase materials with a high fraction of electrode-electrolyte interfaces. The present letter reports on electrochemical property tuning of porous nanocrystalline Pt. Deeper insight into the underlying processes could be gained by means of a direct comparison of the charge-induced response of two different properties, namely electrical resistance and magnetic moment. For this purpose, four-point resistance measurements and SQUID magnetometry were performed under identical in situ electrochemical control focussing on the regime of electrooxidation. Fully reversible variations of the electrical resistance and the magnetic moment of 6% and 1% were observed upon the formation or dissolution of a subatomic chemisorbed oxygen surface layer, respectively. The increase of the resistance, which is directly correlated to the amount of deposited oxygen, is considered to be primarily caused by charge-carrier scattering processes at the metal-electrolyte interfaces. In comparison, the decrease of the magnetic moment upon positive charging appears to be governed by the electric field at the nanocrystallite-electrolyte interfaces due to spin-orbit coupling.

On the coherent description of diffusion-influenced fluorescence quenching experiments.

The fluorescence quenching by electron transfer of a fluorophore, 2,5-bis(dimethylamino)-1,3-benzenedicarbonitrile, to 1,3-dimethyl-2-nitrobenzene, has been studied by means of time-resolved and steady-state experiments at different viscosities and up to large quencher concentrations. Differential Encounter Theory (DET) has been used to rationalize the results, in combination with electron transfer modelled by the Marcus theory. Additionally, the solvent structure and the hydrodynamic effect on the diffusion coefficient have been taken into account. Any simpler model failed to simultaneously fit all the results. The large number of quencher concentrations used is crucial to unambiguously extract the electron transfer parameters.