Eosin-sensitized photoreduction of benzil with 1-benzyl-1,4-dihydronicotinamide

The mechanism of eosin-sensitized photoreduction of benzil with 1-benzyl-1,4-dihydronicotinamide — a model compound of NAD(P)H and the behavior of the excited states of eosin have been investigated. The effect of anthracene as a diffusion-controlled quencher of the photoreaction indicates that both excited triplet state and an unquenchable excited singlet state of eosin participated in the sensitized photoreaction. From the Stern-Volmer plot of quantum yield vs. anthracene concentration, the triplet reaction rate constant has been calculated to be 0.78 × 10⁸ L M^?1S^?1 while the singlet reaction rate constant determined from quenching of eosin fluorescence by benzil is equal to 7.2 × 10⁹ L M^?1S^?1. The singlet and triplet quantum yields are also determined to be 0.09 and 0.18 respectively. Since both the singlet and triplet energies of eosin are lower than that of benzil, energy transfer sensitization is not feasible. It is proposed that electron transfer from the excited eosin to benzil is responsible for the initiation.     

The Interaction of Ground and Excited States of Lumichrome with Aliphatic and Aromatic Amines in Methanol

The reaction of the ground and excited states of lumichrome (=7,8‐dimethylalloxazine=7,8‐dimethylbenzo[g]pteridine‐2,4(1H,3H)‐dione) with aliphatic and aromatic amines was investigated in MeOH. In the presence of aliphatic amines of high basicity, new bands are observed in the absorption and fluorescence spectra. These bands arise in a proton‐transfer reaction from lumichrome, in the ground and in the singlet excited states, to the amine. On the other hand, amines with lower basicity such as triethanolamine (=2,2′,2″‐nitrilotris[ethanol]) and aromatic amines are not able to deprotonate lumichrome, and hence a quenching of the fluorescent emission takes place without changes in the spectral shape. In this case, bimolecular‐quenching rate constants were determined for the excited singlet and triplet states. Based on laser‐flash‐photolysis experiments, an electron‐transfer mechanism is proposed. Aliphatic amines yield lower rate constants than the aromatic ones for the same driving force. A notable difference arises in the limiting value reached by the singlet and triplet quenching rate constants by aromatic amines. For the singlet quenching, the limit is coincident with a diffusion‐controlled reaction, while those for triplet quenching reach a lower constant value, independent of the driving force. This is explained by an electron‐transfer mechanism, with a lower frequency factor for the triplet‐state process.     

Fluorescence quenching of CdS quantum dots by 4-azetidinyl-7-nitrobenz-2-oxa-1,3-diazole: a mechanistic study

Fluorescence quenching of CdS quantum dots (QDs) by 4‐azetidinyl‐7‐nitrobenz‐2‐oxa‐1,3‐diazole (NBD), where the two quenching partners satisfy the spectral overlap criterion necessary for Förster resonance energy transfer (FRET), is studied by steady‐state and time‐resolved fluorescence techniques. The fluorescence quenching of the QDs is accompanied by an enhancement of the acceptor fluorescence and a reduction of the average fluorescence lifetime of the donor. Even though these observations are suggestive of a dynamic energy transfer process, it is shown that the quenching actually proceeds through a static interaction between the quenching partners and is probably mediated by charge‐transfer interactions. The bimolecular quenching rate constant estimated from the Stern–Volmer plot of the fluorescence intensities, is found to be exceptionally high and unrealistic for the dynamic quenching process. Hence, a kinetic model is employed for the estimation of actual quencher/QD ratio dependent exciton quenching rate constants of the fluorescence quenching of CdS by NBD. The present results point to the need for a deeper analysis of the experimental quenching data to avoid erroneous conclusions.     

The excited states interaction of safranine-O with PAMAM and DAB dendrimers in methanol

The quenching of the excited singlet and triplet states of the synthetic dye safranine-O by low generation PAMAM and DAB dendrimers was investigated in methanol. The rate constants for the quenching of the excited singlet state depend on the number of primary amino groups in the dendrimer. The first-order rate constant for the decay of the triplet state presents a downward curvature as a function of the quencher concentration. This behavior was interpreted in terms of the reversible formation of an intermediate complex in the excited state. From a kinetic analysis of the quenching mechanism the equilibrium constant K_exc could be extracted. The values of K_exc may be related to the proton affinity of the quencher. The results were interpreted in terms of a reversible proton transfer quenching. This was further confirmed by the transient absorption spectra obtained by laser flash photolysis. The transient absorption immediately after the triplet state quenching could be assigned to the unprotonated form of the dye. At later times the spectrum matches the semireduced form of the dye. The overall process corresponds to a one-electron reduction of the dye mediated by the deprotonated triplet state.     

Dynamics and prototropic reactivity of electronically excited states in simple surfactant aggregates

Excitation of a molecule from the ground state to an electronically excited state can cause changes in its geometry, dipole moment, acidity or basicity, redox potentials and solvation. Bimolecular quenching of the excited state of the probe by other molecules present in the medium can be used to determine the mobilities of molecules and estimate microviscosities and encounter probabilities in the medium. Differences in excited state acidity or basicity relative to the ground state can be employed to investigate the dynamics of ultrafast proton transfer reactions. Three areas of current interest where fluorescent probes have served to elucidate important dynamic processes of molecules in simple self-aggregating surfactant systems such as aqueous micelles and reverse micelles are considered: (a) bimolecular quenching of excited states; (b) the dynamics of solvation of excited states and (c) ultrafast intermolecular excited state proton transfer (ESPT) reactions.     

Kinetic Analysis of the Light-induced Fluorescence Quenching in Light-harvesting Chlorophyll a/b Pigment-Protein Complex of Photosystem II

We carried out a kinetic analysis of the light-induced fluorescence quenching (AF) of the light-harvesting chlorophyll a/b pigment-protein complex of photosystem II (LHCII) that was first observed by Jennings et at (Pho-tosynth. Res. 27, 57–64, 1991). We show that during a 2 min light, 2 min dark cycle, both the light and dark phases exhibit biexponential kinetics; this is tentatively explained by the presence of two types of light-induced quenchers in different domains of aggregated LHCII. Quantitative analysis could be carried out on the faster kinetic component; the slower component that was not completed during the measurement was not amenable for quantitative analysis. Our analysis revealed that the rate of the light-induced decrease of the fluorescence yield depended linearly on the light intensity, which shows that the generation of the quencher originates from a reaction that is first order with respect to the concentration of the excited domains. As shown by the estimated rate constant, pho-togeneration of the quencher is a fast reaction that can compete with other excitation-relaxation pathways. Both the decay and the recovery time constants of AF depended strongly on the temperature. Thermodynamic analysis showed that the fast light-induced decline in the fluorescence was determined by a low fraction of the excited states. Recovery was associated with large decrease in the entropy of activation that indicated the involvement of large structural rearrangements. Macroaggregated LHCII exhibited larger ΔF than small aggregates, which is consistent with the proposed role of aggregated LHCII in thy-lakoid membranes in nonphotochemical quenching.     

Excited state reactions of amphiphilic Zn(II) porphyrin incorporated into liposomes as models for biological electron transfer

The amphiphilic porphinato zinc(II) complex (ZnP) containing four substituted amphiphilic alkyl chains was embedded in the liposomes of l,2-dipalmitoyl-sn-glycero-3phosphocholine. The distance from the embedded ZnP to the outer phase was changed from 9 to 27 ? by changing the substituted alkyl chain length. The electron transfer from the excited Zn complex to methylviologen (MV²⁺) or benzoquinone (BQ) added in the outer aqueous phase was studied. At first, quenching reactions were analyzed based on dynamic and static reaction models of the excited state. For the MV²⁺ quencher, only the triplet excited state of the embedded ZnP reacted, and electron transfer occurred at a distance less than 12 ?. In BQ both the singlet and the triplet excited states reacted, and the reaction of the singlet state was a static one indicating that BQ is incorporated into the liposomes. The distribution of the BQ molecule in the quenching sphere of ZnP was presented based on calculations assuming a stepwise incorporation into the quenching sphere.     

Electron Transfer Fluorescence Quenching of Blepharisma japonicum Photoreceptor Pigments

Abstract— The hypericin analogs blepharismin (BP), oxyblepharismin (OxyBP) and stentorin (ST), the photosensing chromophores responsible for photomotile reactions in the ciliates Blepharisma japonicum (red and blue cells) and Stentor coeruleus, represent a new class of photoreceptor pigments whose chemical structures have recently been determined. In the case of ST it has been shown that the first excited singlet state can be deactivated by donation of an electron to an appropriate acceptor molecule (e.g. a quinone molecule). This charge transfer can be considered a possible mechanism for the primary photoprocess for the photomotile responses in S. coeruleus. To determine whether an electron transfer process also occurs in the deactivation of excited blepharismin, we studied the fluorescence quenching of OxyBP in dimethyl-sulfoxide (DMSO) and in ethanol using electron acceptors with different reduction potentials. Under our experimental conditions ground state and excited state complexes (like fluorescent exciplexes) are not formed between the fluorophore and the quenchers. In DMSO the bimolecular quenching constant values (k_q) calculated on the basis of the best fitting procedures clearly show that the quenching efficiency decreases with the quencher negative reduction potential, E⁰. The k_q (M^-1 s^-1) and E⁰ (V) values are, respectively, 7.8 times 10⁹ and -0.134 for 1,4-benzoquinone, 8.9 times 10⁹ and -0.309 for 1,4-naphthoquinone, 2.4 times 10⁹ and -0.8 for nitrobenzene, 0.009 times 10⁹ and -1.022 for azobenzene and 0 and -1.448 for benzophenone. These findings point to the conclusion that upon formation of the encounter complex between OxyBP and the quencher, an electron is released from excited OxyBP to the quencher, similar to what happens in ST. It is suggested that in the pigment granules such a light-induced charge transfer from excited blepharismin to a suitable electron acceptor triggers sensory transduction processes in B. japonicum.     

Effect of temperature and quencher on the fluorescence of 4-(5-methyl-3-furan-2-yl-benzofuran-2-yl)-7-methyl-chromen-2-one in different solvents

The effect of temperature on the fluorescence intensity of 4-(5-methyl-3-furan-2-yl-benzofuran-2-yl)-7-methyl-chromen-2-one (MFBMC) in different solvents, has been studied in the temperature range 293–333 K. A mechanism of fluorescence quenching with increase in temperature is discussed in terms of the relative location of lowest ¹(ππ*) and ³(nπ*) states, and the energy difference between them. The non-radiative deactivation of excited state in the absence of quencher is temperature-dependent; its activation energy has been found to be 9.453–27.893 kJ mole^?1. Further, the fluorescence quenching by aniline was investigated by both steady-state and time-resolved measurement (at 296 K). The quenching is found to be appreciable and shows positive deviation in the Stern–Volmer plots. This could be explained by static–dynamic quenching models. Various rate constants of the bimolecular quenching reaction have been determined by using ground-state complex formation and sphere of action static quenching model. The magnitude of these constants suggests that sphere of action static quenching model agrees very well with experimental results. Further, with the use of finite sink approximation model, it is concluded that the quenching mechanism is diffusion-limited.     

Fluorescence quenching of naphthols by Cu2+ in micelles

Effect of the micelles of anionic, cationic and non-ionic surfactants on the fluorescence quenching of 1- and 2-naphthols has been studied in the presence of copper ion. The excited state lifetime, dynamic and static quenching constants for these systems have been determined. Fluorescence quenching in water and SDS micelle is due to the collision of the fluorophore with the quencher with a small static component. The negatively charged naphtholate ions in the excited state are quenched with significantly higher rates than the neutral naphthol molecules, which are located further inside the mesophase. CTAB micelle is less effective than the SDS micelle for fluorescence quenching. The effect of CTAB on water-assisted excited-state deprotonation has been investigated in the presence of ZnSO₄. For TX-100 micelle there is negligible quenching even at higher concentration of the quencher.     

Effects of Charge and Structure of Hyaluronic Acid on the Luminescence Quenching in Aqueous Solution

The quenching of electronically excited Ru(bpy)₃²⁺ (bpy = Tris-2,2′-bipyridine) by methylviologen (MV) and ferricyanide (FC) in aqueous solutions of hyaluronic acid (HA) was studied. The structural and viscosity changes occuring with increasing HA concentration were found to influence the photophysical and photochemical properties of the sensitizer. Different kinetic models had to be used for the quenchers studied. The kinetics of the quenching of *Ru(bpy)₃²⁺ by MV can be described by the pseudophase model, which indicates that the rate for the exchange of the quencher between the microdroplets is higher than that for the excited state decay of the Ruthenium complex. In contrast, the quenching by the negatively charged quencher, FC, can be described by the Infelta-Tachiya equation, which indicates that the distribution of this quencher on the aqueous microdroplets is of the Poisson type and there is no exchange of quencher molecules during the lifetime of the sensitizer. The lifetimes of the excited Ruthenium complex, the unimolecular constants for its quenching by FC and the average concentration of the aqueous microdroplets increase with increasing HA concentration, reflecting the change in the solution structure during the transition from semidilute to concentrated regions. For MV no significant dependence of the quenching constant on the HA content of the solution was found. The reaction behavior of charged reactants in HA solution depends strongly on the sign of the charge.     

Photoinduced Energy Transfer from Poly(N‐vinylcarbazole) to Tricarbonylchloro‐(2,2′‐bipyridyl)rhenium(I)

This work investigates the photoinduced energy transfer from poly(N‐vinylcarbazole) (PVK), as a donor material, to fac‐(2,2′‐bipyridyl)Re(CO)₃Cl, as a catalyst acceptor, for its potential application towards CO₂ reduction. Photoluminescence quenching experiments reveal dynamic quenching through resonance energy transfer in solid donor/acceptor mixtures and in solid/liquid systems. The bimolecular reaction rate constant at solution–film interfaces for the elementary reaction of the excited state with the quencher material could be determined as 8.8(±1.4)×10¹¹ L mol^?1 s^?1 by using Stern–Volmer analysis. This work shows that PVK is an effective and cheap absorber material that can act efficiently as a redox photosensitizer in combination with fac‐(2,2′‐bipyridyl)Re(CO)₃Cl as a catalyst acceptor, which might lead to possible applications in photocatalytic CO₂ reduction.     

CHLOROPHYLL PHOTOSENSITIZED ELECTRON TRANSFER IN PHOSPHOLIPID BILAYER VESICLE SYSTEMS: EFFECTS OF CHOLESTEROL ON RADICAL YIELDS AND KINETIC PARAMETERS*

Abstract The quenching of the triplet state of chlorophyll a (Chl) by asymmetrically located electron acceptors was examined in vesicle systems containing egg yolk phosphatidylcholine and 0–50 mole % cholesterol. The incorporation of cholesterol had two main effects: (1) the distribution of Chl within the vesicle wall shifted from one favoring the inner monolayer to one favoring the outer monolayer, and (2) the Chi molecules (both ground and excited states) became more accessible to water and to the quencher molecules. This latter property was probably due to the creation of space between the phospholipid head groups by insertion of cholesterol. These phenomena required cholesterol concentrations in excess of 15 mol %. In general, the addition of cholesterol caused increases in the apparent bimolecular rate constant for triplet quenching, in the probability that quenching produced radicals, and in the rate of radical recombination. Some of the specific effects of cholesterol depended upon whether or not the quencher molecules were amphiphilic.     

Studies on the fluorescence quenching of polysilane copolymers by chlorohydrocarbons

The room-temperature solution fluorescence quenching of polysilane copolymers by chlorohydrocarbons such as CCl₄, CHCl₃, C₂Cl₆, and Cl₂CHCHCl₂ was studied. The existence of dynamic quenching was preliminarily demonstrated by the experiment of fluorescence lifetime quenching. The fluorescence quenching data were in conformity with the equation: F₀/F = (1+K_SV[Q])exp(NV[Q]), where F and F₀ are the fluorescence intensity with and without the addition of quencher, K_SV is the Stern-Volmer constant, [Q] is the quencher concentration, N is the Avogadro constant, and V is the volume of the active sphere. The fluorescence quenching by the first three chlorohydrocarbons was attributed to the contemporaneous effect of dynamic quenching and static quenching. There exists, at least mathematically, a critical quencher concentration [Q]_C. When the quencher concentration [Q] < [Q]_C, the fluorescence quenching is dominated by the dynamic quenching part; when [Q] > [Q]_C, it is dominated by the static quenching part. However, the fluorescence quenching by Cl₂CHCHCl₂ was attributed to only static quenching. Furthermore, it was proposed that the dynamic quenching may be related with the electrical positivity of the central carbon nucleus of the quenching molecules while the static quenching may be caused by the “outside heavy atom effect” of the Cl element. © 1996 John Wiley & Sons, Inc.     

Influence of quenchers on excited complexes upon triplet-triplet annihilation of tetrapyrrole molecules in liquid solutions

The influence of triplet quenchers on the kinetics of triplet-triplet annihilation (TTA) for chlorophylla and tetraphenylporphine was investigated. It was found that the rate constants for the quenching of triplet states by TTA increase with increasing the quencher concentration [Q]. The greatest values for the triplet deactivation parameter are proportional to [Q]. Experimental results are consistent with the rationalization of the triplet annihilation through the formation of complexes from the triplet molecules. The linear dependence of the degradation rate constant of the triplet-triplet complexes to the ground-state molecules on [Q]² suggests that two quencher molecules are necessary for the quenching of one complex molecule. This means that two locally excited triplet states exist in the complex. It is likely the spin correlation time of the triplet states is longer than the lifetime of the complexes.     

Amplified luminescence quenching of phosphorescent metal-organic frameworks

Amplified luminescence quenching has been demonstrated in metal-organic frameworks (MOFs) composed of Ru(II)-bpy building blocks with long-lived, largely triplet metal-to-ligand charge-transfer excited states. Strong non-covalent interactions between the MOF surface and cationic quencher molecules coupled with rapid energy transfer through the MOF microcrystal facilitates amplified quenching with a 7000-fold enhancement of the Stern-V?lmer quenching constant for methylene blue compared to a model complex.     

Quenching of the Singlet and Triplet Excited States of Pterin by Amino Acids

Unconjugated oxidized pterins accumulate in the skin of patients suffering from vitiligo and, under UVA irradiation, photosensitize the oxidation of amino acids. In this work, we study the interaction of the singlet and triplet excited states of pterin (Ptr), the parent compound of oxidized pterins, with four oxidizable amino acids: tryptophan (Trp), tyrosine (Tyr), histidine (His) and methionine (Met). Steady‐state and time‐resolved fluorescence measurements and laser flash photolysis experiments were performed to investigate the quenching of the Ptr excited states by the amino acids in aqueous solution. The singlet excited states of Ptr are quenched by Met mainly via a dynamic process and by Trp via a combination of dynamic and static processes. His does not quench singlet excited states of Ptr, and quenching by Tyr could not be investigated due to the low solubility of this amino acid. The triplet excited states of Ptr are quenched by the four studied amino acids, and the corresponding bimolecular quenching rate constants are in the range of diffusion controlled limit. The assessment of the results in the context of the Ptr‐photosensitization of amino acids suggests that triplet excited state of Ptr is the species that initiates the photochemical processes.     

Fluorescence quenching of anthracene by indole derivatives in phospholipid bilayers

The quenching of anthracene fluorescence by indole, 1,2-dimethylindole (DMI), tryptophan (Trp) and indole 3-acetic acid (IAA) in palmitoyloleoylphosphatidylcholine (POPC) lipid bilayers was investigated. A very efficient quenching of the anthracene fluorescence in the lipid membrane is observed. Stern-Volmer plots are linear for DMI but present a downward curvature for the other quenchers. This was interpreted as an indication of the presence of an inaccessible fraction of anthracene molecules. By a modified Stern-Volmer analysis the fraction accessible to the quenchers and the quenching constant were determined. The changes in the fluorescence emission spectrum of indole and DMI have been used to calculate the partition constants of these probes into the membranes, and bimolecular quenching rate constants were determined in terms of the local concentration of quencher in the lipid bilayer. The rate constants are lower than those in homogeneous solvents, which may be ascribed to a higher viscosity of the bilayer. No changes in the emission spectra of Trp and IAA are observed in the presence of vesicles, indicating that these probes locate preferentially in the aqueous phase, or in close proximity to the vesicular external interface in a medium resembling pure water. In these cases quenching rate constants were determined in terms of the analytical concentration. In the quenching by DMI a new, red shifted, emission band appears; it is similar to that observed in non-polar solvents and it is ascribable to an exciplex emission. The exciplex band is absent in the quenching by IAA and Trp and only very weakly present when the quencher is indole. From the position of the maximum of the exciplex emission, a relatively high local polarity could be estimated for the region of the bilayer where the quenching reaction takes place.     

HETEROEXCIMER SYSTEMS IN AQUEOUS MICELLAR SOLUTIONS

Fluorescence quenching due to charge transfer interactions in the excited state has been studied in aqueous micellar solutions. Fluorescers used were pyrene, various pyrene derivatives and several other conjugated π-electronic systems, and quenchers were N, N-dimethylaniline, N. N-dimethylaniline sulfonate, dicyanobenzenes and cyanopyridine. Strong quenching of the aromatic hydrocarbon fluorescence with dimethylanilines as well as dicyanobenzenes was observed, while no heteroexcimer emission was detected. The dependences of relative fluorescence yield and fluorescence decay curve upon the quencher concentration have been explained with equations derived on the basis of a simple model. Based on the obtained results, some discussions were given on the nature of micelle 'interior' and micelle 'surface'.     

MECHANISMS OF QUENCHING OF THE FLUORESCENCE OF A BENZO[a]PYRENE TETRAOL METABOLITE MODEL COMPOUND BY 2'-DEOXYNUCLEOSIDES

Abstract— The hydrophobic interactions of bulky polycyclic aromatic hydrocarbons with nucleic acid bases and the formation of noncovalent complexes with DNA are important in the expressions of the mutagenic and carcinogenic potentials of this class of compounds. The fluorescence of the polycyclic aromatic residues can be employed as a probe of these interactions. In this work, the interactions of the (+)-trans stereoisomer of the tetraol 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene (BPT), a hydrolysis product of a highly mutagenic and carcinogenic diol epoxide derivative of benzo[a]pyrene, were studied with 2′-deoxynucleosides in aqueous solution by fluorescence and UV spectroscopic techniques. Ground-state complexes between BPT and the purine derivatives 2′-deoxyguanosine (dG), 2′-deoxyadenosine (dA), and 2′-deoxyinosine (dI) are formed with association constants in the range of ～40–130 M^?1 Complex formation with the pyrimidine derivatives 2′-deoxythymidine (dT), 2′-deoxycytidine (dC), and 2′-deoxyuridine (dU) is significantly weaker. Whereas dG is a strong quencher of the fluorescence of BPT by both static and dynamic mechanisms (dynamic quenching rate constant k_dyn= [2.5 ± 0.41 × 10⁹M¹ s ¹, which is close to the estimated diffusion-controlled value of ～ 5 × 10⁹M^{? 1} s^?1), both dA and dI are weak quenchers and form fluorescenceemitting complexes with BPT. The pyrimidine derivatives dC, dU, and dT are efficient dynamic fluorescence quenchers (K_dyn～ [1.5–3.0] × 10⁹M^?1 s^?1), with a small static quenching component due to complex formation evident only in the case of dT. None of the four nucleosidcs dG, dA, dC and dT are dynamic quenchers of BPT in the triplet excited state; the observed lower yields of triplets are attributed to the quenching of single excited states of BPT by 2′-deoxynucleosides without passing through the triplet manifold of BPT. Possible fluorescence quenching mechanisms involving photoinduced electron transfer are discussed. The strong quenching of the fluorescence of BPT by dG, dC and dT accounts for the low fluorescence yields of BPT-native DNA and of pyrene-DNA complexes.