Proton and electron transfer in the excited state quenching of phenosafranine by aliphatic amines

The quenching of the excited singlet and triplet states of phenosafranine by aliphatic amines was investigated in acetonitrile and methanol. The rate constants for the quenching of the excited singlet state depend on the one-electron redox potential of the amine suggesting a charge transfer process. However, for the triplet state, quenching dependence on the redox potential either is opposite to the expectation or there is not dependence at all. Moreover, in MeOH the first-order rate constant for the decay of the triplet state, k(obs) presents a downward curvature as a function of the amine concentration. This behavior was interpreted in terms of the reversible formation of an intermediate excited complex, and from a kinetic analysis the equilibrium constant K(exc) could be extracted. The log K(exc) shows a linear relationship with the pKb of the amine. On the other hand, for the triplet state quenching in acetonitrile k(obs) varies linearly with the amine concentration. Nevertheless, the quenching rate constants correlate satisfactorily with pKb and not with the redox potential. The results were interpreted in terms of a proton transfer quenching, reversible in the case of MeOH and irreversible in MeCN. 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.     

QUENCHING OF OXONINE EXCITED STATES BY EDTA VIA ELECTRON TRANSFER AND DEACTIVATION PROCESSES. A COMPARATIVE STUDY OF EXCITED SINGLET AND TRIPLET STATE REACTIVITY

The quenching of excited singlet oxonine by EDTA in aqueous solution leads mainly to deactivation of the dye to the ground state and, to a lesser extent, to electron abstraction. The rate constants for these processes have been measured and compared to those for the same reactions involving the oxonine triplet state. The rate constant of electron abstraction is about ten times greater via the singlet state than via the triplet state. However, the rate constant of deactivation to the ground state is 10³-10⁴ times greater for the excited singlet state than for the triplet state, so that the efficiency of electron transfer is much smaller for the singlet state.     

DETERMINATION OF THE ACRYLAMIDE QUENCHING CONSTANT FOR PROTEIN AND MODEL INDOLE TRIPLETS

Abstract— The decay of the indole triplet of single tryptophan-containing proteins and model compounds can be readily measured at room temperature in aqueous solution by monitoring the triplet-triplet absorption or phosphorescence emission following a 265 nm exciting laser pulse. The quenching action of acrylamide on the triplet excited state of indole side chains was studied in an analogous fashion to that previously done at the singlet level (Eftink and Ghiron, 1977). The acrylamide triplet quenching constant (^tk_q) ranged from a high of 7.8 times 10⁸M^-1 s^-1 for the exterior indole of corticotropin (ACTH) to a low of 2 times 10⁵ Af^-1 s^-1 for the interior indole of ribonuclease T, (RNase T,). The ratio (7) of these values with their respective acrylamide singlet quenching constants (^tk_q),(γ=^tk_q⁸K_q) ranged from a high of 0.22 for ACTH to a low of 0.001 for RNase T₁,. Acrylamide is also an inefficient quencher of model indoles in various solvents (i.e. it has a γ less than 1). The magnitude of γ varied from a high of 0.3 in H₂0 to a low of 0.02 in acetonitrile, but did not correlate with viscosity, dielectric constant or polarity. The lower efficiency observed for internal indole groups can not be explained by that class of models which predict the presence of static quenching at the triplet level, since none was observed. The present results confirm the observation of Calhoun et al. of a large discrepancy between acrylamide's singlet and triplet quenching constants for buried indole side chains, but suggest that it may be largely explained by the fact that acrylamide is an inefficient quencher of the indole triplet state (1983). The magnitude of this inefficiency is probably determined by specific microenvironmental factors. Thus, unlike ⁸K_q, the environmentally sensitive ^lk_H cannot be easily used to characterize the dynamics of proteins.     

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.     

Kinetic Evidence for Methanol Trimer Assisted Proton Transfer: Transient Absorption Study on Excited Triplet State of 4-Hydroxy-4′-nitrobiphenyl

Hydroxyaromatic compounds have a wide range of applications in catalytic synthesis and biological processes due to their enhanced acidity upon photo-excitation. Most hydroxyaromatic compounds with a medium excited state acidity are unable to deprotonate in non-aqueous solvents such as alcohol due to their short-lived excited singlet states. The nitro group in 4-hydroxy-4'-nitrobiphenyl (NO₂-Bp-OH) increases the spin-orbit coupling between excited singlet states and the triplet state, resulting in ultrafast intersystem crossing and the formation of the long-lived lowest excited triplet state (T₁) with a high yield. Using transient absorption spectroscopy and kinetic analysis, we discover that, despite its moderate acidity, the T₁ state of NO₂-Bp-OH (³NO₂-Bp-OH) is able to transfer proton to methanol. Following the formation of the hydrogen-boned complex between ³NO₂-Bp-OH and three methanol molecules in a consecutive process, proton transfer occurs very fast. This finding suggests that the long lifetime of the photoacid excited state allows for the formation of alcohol oligomer with sufficient basicity to induce photoacid deprotonation.     

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.     

Photoinduced Electron Transfer in Dry Gelatin Containing Bound Methylene Blue

Excited states of methylene blue bound to dry gelatin undergo electron-transfer quenching reactions with neighboring amino acid residues to give reduced, methylene blue-free radicals. At the low loadings of methylene blue bound to gelatin used in this work, the absorption spectra indicated that the methylene blue existed principally in its monomeric form. The rates of methylene blue fluorescence quenching depended on the temperature at which the gelatin was dried. There was no detectable fluorescence characteristic of the localized excited singlet state of methylene blue when the dye is bound to gelatin dried at 15°C, where a significant proportion of the protein exists in its a -helical form. Instead, weak emission with a maximum at 645 nm is observed. It is inferred from this that the rate of quenching of any localized excited singlet states of the dye bound to a -helical gelatin is very fast thereby competing efficiently with radiative relaxation. The weak emission at 645 nm is ascribed to a fluorescent exciplex formed between bound methylene blue and a neighboring electron/proton donor residue of the gelatin. While fluorescence characteristic of the localized methylene blue singlet state was observed from dyed gelatin dried at 50°C, i.e. in its random coil form, the triplet state is subject to rapid quenching by electron/proton transfer with protein moieties.     

Intramolecular photoexcitation dynamics and magnetic field effects in an intermediate-case molecule

Molecular vibration and rotation play a significant role in the intramolecular photoexcitation dynamics of the so-called intermediate-case molecule, and the fluorescence intensity, decay and polarization of s-triazine vapor are shown to depend on the excited rovibronic level of the S₁ state. Fluorescence characteristics are interpreted by assuming three zero-order states: (1) a zero-order singlet state that carries the absorption intensity and emits fluorescence with sharp structure; (2) zero-order singlet states that do not carry the absorption intensity but emit broad fluorescence; and (3) zero-order triplet states. The interaction among these states depends not only on the vibrational level but also on the rotational level excited. It is suggested that the number of triplet states coupled to the singlet state increases with increasing excess vibrational energy. It is also suggested that K-scrambling occurs both in the triplet manifold following intersystem crossing (ISC) and in the singlet manifold following intramolecular vibrational energy redistribution (IVR). The fluorescence intensity and decay of s-triazine vapor are significantly influenced by a magnetic field, and the field effects are interpreted in terms of the spin decoupling in the triplet manifold following ISC; the role of external magnetic fields is to mix the spin sublevels of different rovibronic levels coupled to the excited singlet state. Magnetic depolarization of fluorescence also occurs because of the efficient interaction between the excited singlet state and the triplet state.     

INDICATION OF FAST TRIPLET FORMATION IN BIMOLECULAR QUENCHING OF EXCITED SINGLET STATES STUDIED BY CONVENTIONAL FLASH PHOTOLYSIS

The bimolecular quenching of the first excited singlet state of oxonine by allylthiourea leads to the formation of the triplet state of the dye. This has been proved by comparison with the triplet-triplet absorption spectrum of oxonine obtained by triplet-triplet energy transfer. The conventional flash experiments suggest that the dye triplet state is produced directly rather than by radical recombination.     

Graphene as a quencher of electronic excited states of photochemical probes

Graphene sheets quench the singlet and triplet excited states of a series of six photochemical probes including pyrene, acridine orange, tris(2,2?-bipyridyl)ruthenium(II) dichloride, methylene blue, meso-tetrakis(phenylsulphonate)porphyrin, and 5,10,15,20-tetraphenyl-21H,28H-porphine zinc. It was found that Stern-Volmer fluorescence quenching can fit to one or two different quenching regimes depending on the probe. In addition, the quenching can be either static or dynamic depending on the fluorophore. The occurrence of several quenching regimes has been interpreted considering that quenching arises from the crowding of the fluorophore on both graphene faces, or site isolation on the graphene sheets. Laser flash photolysis has shown that the triplet lifetime of the probes generally decreases due to graphene quenching and that no new transients appear except in the case of methylene blue, where a new absorption spectrum characterized by a continuous absorption band is observed and attributed to graphene radical ion. This spectroscopic evidence suggests that the most general quenching mechanism is energy transfer from the singlet or triplet excited state of the dye to graphene. This raises the issue of determining the energy of the electronic excited states of graphene.     

Study of singlet—triplet coupling in glyoxal by level anticrossing spectroscopy. V. Nature of singlet—triplet interaction

We observe in glyoxal cooled in a supersonic free jet the fluorescence of individual rotational levels of the S₁ state excited by a cw laser. We use the technique of singlet—triplet magnetic resonance near an anticrossing to measure matrix elements V₃₁ as a function of rotational quantum numbers N_s, N_t, K_s, K_t. The experimental results are compared with theoretical models of singlet—triplet couplings and we show that the spin-vibronic interaction is the dominant singlet—triplet interaction in glyoxal.     

Photosensitizing properties of octacarboxy metallophthalocyanines in aqueous medium and their interaction with bovine serum albumin

Photosensitizing properties of aluminium, silicon, zinc and germanium octacarboxy phthalocyanines ((OH)AlOCPc, (OH)₂SiOCPc, ZnOCPc and (OH)₂GeOCPc) were studied in aqueous medium and in the presence of bovine serum albumin (BSA). Triplet quantum yields increased with increasing atomic number of the central metals of the metallophthalocyanine. The efficiency of singlet oxygen generation via energy transfer from the excited triplet state of the octacarboxy metallophthalocyanines (MOCPcs) to ground state oxygen increased markedly in the presence of BSA. The triplet state lifetimes of the MOCPc complexes in the presence of BSA were found to be longer than in the absence of BSA, ranging from 110 to 580 μs. These complexes bind readily to BSA. Stern–Volmer quenching constant K_SV as well as the binding constant k_b values were calculated. The probable mechanism of quenching of BSA fluorescence by the MOCPc complexes is by static 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.     

Squaraine Rotaxane as a Reversible Optical Chloride Sensor

A mechanically interlocked squaraine rotaxane is comprised of a deep‐red fluorescent squaraine dye inside a tetralactam macrocycle. NMR studies show that Cl^? binding to the rotaxane induces macrocycle translocation away from the central squaraine station, a process that is completely reversed when the Cl^? is removed from the solution. Steady‐state fluorescence and excited‐state lifetime measurements show that this reversible machine‐like motion modulates several technically useful optical properties, including a three‐fold increase in deep‐red fluorescence emission that is observable to the naked eye. The excited states were characterized quantitatively by time‐correlated single photon counting, femtosecond transient absorption spectroscopy, and nanosecond laser flash photolysis. Cl^? binding to the rotaxane increases the squaraine excited singlet state lifetime from 1.5 to 3.1 ns, and decreases the excited triplet state lifetime from >200 to 44 μs. Apparently, the surrounding macrocycle quenches the excited singlet state of the encapsulated squaraine dye and stabilizes the excited triplet state. Prototype dipsticks were prepared by adsorbing the lipophilic rotaxane onto the ends of narrow, C18‐coated, reverse‐phase silica gel plates. The fluorescence intensity of a dipstick increased eighteen‐fold upon dipping in an aqueous solution of tetrabutylammonium chloride (300 mM ) and was subsequently reversed by washing with pure water. It is possible to develop the dipsticks for colorimetric determination of Cl^? levels by the naked eye. After dipping into aqueous tetrabutylammonium chloride, a dipstick’s color slowly fades at a rate that depends on the amount of Cl^? in the aqueous solution. The fading process is due primarily to hydrolytic bleaching of the squaraine chromophore within the rotaxane. That is, association of Cl^? to immobilized rotaxane induces macrocycle translocation and exposure of the electrophilic C₄O₂ core of the squaraine station, which is in turn attacked by the ambient moisture to produce a bleached product.     

Etude theorique de la premiere protonation de la proflavine

A comparison between theory and experiment (the last performed by J. H. Lhoste) gives consitent results pertaining to the difference in acid-base behaviour of proflavin in the ground state and in the excited singlet and triplet states. In particular, several facts emerge. An extraordinary inequality between the different pK′s of acid-base equilibria are indicated: p_triplet^K < p_fondamental^K < p_singulet^K A change occured by protonation in the electronic configuration of the lowest triplet state of that molecule and the variation of solvation energy during the absorption ground state-first excited singlet state plays an important part.     

Time-Resolved Studies of the Excited-State Dynamics of meso-Tetra(hydroxylphenyl)chlorin in Solution

Meso-tetra(hydroxyphenyl)chlorin (m-THPC) is a new photosensitizer developed for potential use in photodynamic therapy (PDT) for cancer treatment. In PDT, the accepted mechanism of tumor destruction involves the formation of excited singlet oxygen via intermolecular energy transfer from the excited triplet-state dye to the ground triplet-state oxygen. Femtosecond transient absorption measurements are reported here for the excited singlet state dynamics of m-THPC in solution. The observed early time kinetics were best fit using a triple exponential function with time constants of 350 fs, 80 ps and > or = 3.3 ns. The fastest decay (350 fs) was attributed to either internal conversion from S2 to S1 or vibrational relaxation in S2. Multichannel time-resolved absorption and emission spectroscopies were also used to characterize the excited singlet and triplet states of the dye on nanosecond to microsecond time scales at varying concentrations of oxygen. The nanosecond time-resolved absorption data were fit with a double exponential with time constants of 14 ns and 250 ns in ambient air, corresponding to lifetimes of the S1 and T1 states, respectively. The decay of the T1 state varied linearly with oxygen concentration, from which the intrinsic decay rate constant, ki, of 1.5 x 10(6) s-1 and the biomolecular collisional quenching constant, kc, of 1.7 x 10(9) M-1 s-1 were determined. The lifetime of the S1 state of 10 ns was confirmed by fluorescence measurements. It was found to be independent of oxygen concentration and longer than lifetimes of other photosensitizers.     

The reactive state in the photo-rearrangement of o-nitrobenzaldehyde

The primayy step of the o-nitrobenzaldehyde-o-nitrosobenzoic acid photorearrangement in solution has been studied by flash absorption with 35 ps 355 nm light pulses. Flash photolysis of o-nitrobenzaldehyde in acetonitrile or THF solutions produces a transient absorption with a maximum at ca. 440 nm. Formation of the transient was < 35 ps, the laser pulse width, and within experimental error, no furthrr buildup was observed. The transient which decayed at nanosccond times is attributed to a remarkably reactive ketene intermediate formed by H abstraction of the aldehydic hydrogen by the excited state of the nitro group. Decay of the ketene was more rapid in water-acetonitrile, methanol-acetonitrile, tert-butyl alcohol and in THF than in acetonitrile solution. It is suggested that the intramolecular reaction of the ketene intermediate is enhanced in THF relative to acetonitrile because of the ability of THF to faciliaate proton transfer associated with the reaction. The addition of the triplet quencher cis-piperylene to a solution of o-nitrobenzaldehyde in THF did not accelerate decay of the transient nor reduce its yield. The n,π* triplet excited state band observed in the 625–650 nm region for a number of the nitroaromatic compounds was not observed in the case of o-nitrobenzaldehyde. The results provide evidence that in the direct irradiation on o-nitrobenzaldehyde in THF or acetonitrile solutions, the intramolecular reaction occurs from the singlet rather than the triplet excited state.     

The first observation of the effect of dendritic structure to produce the triplet excited state of the core stilbene by dendron excitation

We report that both singlet and triplet energy transfers in stilbene-cored benzophenone dendrimers (trans-BPST) took place quite efficiently. On excitation (290 nm) of stilbene group, the intramolecular singlet energy transfer from the excited core stilbene to the benzophenone part (99.7%) was confirmed by quenching of the fluorescence from the core stilbene. The benzophenone in the excited singlet state is known to undergo intersystem crossing to give its excited triplet state quantitatively. However, the very weak phosphorescence from benzophenone part in trans-BPST was observed even at 77 K. The phosphorescence intensity of trans-BPST is only 1% of that of model compound (4-methylbenzophenone) at 77 K. During the irradiation, the absorption spectra also changed due to the trans-cis isomerization. This is probably due to the ultrafast triplet energy transfer from the benzophenone to produce the triplet state stilbene.     

EFFECT OF THE BROMINE ATOM UPON THE DISSOCIATION CONSTANTS OF BROMOPYRIDINES,BROMOQUINOLINES AND 4-BROMOISOQUINOLINE IN THEIR LOWEST EXCITED SINGLET AND TRIPLET STATES*

Abstract— The lowest excited singlet-state dissociation constants (pK^S_a) of bromosubstituted pyridines, quinolines, and isoquinolines were determined from the pH-dependent shifts in their electronic absorption spectra. The lowest excited triplet-state dissociation constants (pK^T_a) of bromosubstituted quinolines and 4-bromoisoquinoline were obtained from the shifts of the 0–0 phosphorescence bands measured in rigid aqueous solution at 77 K. The pK^S_a values indicate that the basicity of these brominated nitrogen heterocycles is increased in the lowest excited singlet state by 2 to 10 orders of magnitude as compared with the ground state. The pK^T_a values are found to be significantly different from the corresponding ground-state pK_a values, indicating that the basicity of bromoquinolines is increased in the lowest excited triplet state by 1.7 to 3.0 pK units. The enhancement of the excited singlet-and triplet-state basicity of brominated nitrogen heterocycle derivatives as compared with the unsuhstituted parent compounds is attributed to the increased electron-donor conjugative interactions of the bromine atom pπ orbitals with π orbitals in the lowest excited singlet and triplet state.     

Proton transfer in 3-hydroxyflavone: The influence of physical excitation conditions and dynamic quenching

The influence of excitation conditions, temperature, and fluorescence quencher on the properties of 3-hydoxyflavone excited states is considered. Two-band fluorescence spectra of 3-hydroxyflavone formed in excitation by electromagnetic radiation in the region of the S ₁ and S ₂ absorption bands over the temperature range 20–80°C were recorded and analyzed. The TEMPO spin quencher was used as an excited state quencher. An analysis of the fluorescence parameters showed that solution heating from room temperature to 60°C increased the rate of proton transfer by 1.24 times at standard excitation into the main absorption band. The rate increased still more rapidly (by 6.9 times) in excitation into the second absorption band. The presence of the quencher caused a decrease in the yields of both fluorescence bands according to the diffusion mechanism and a noticeable growth in the rate of proton transfer. The latter increased by 1.16 times at room temperature and by 1.25 times at 80°C. The corresponding changes were more substantial, especially at elevated temperatures, if the second singlet band was excited. They then amounted to 1.24 and 3.5 times over the same temperature range.