ÉTUDE EN SPECTROSCOPIE PAR ÉCLAIR DES COLORANTS THIAZINIQUES EN SOLUTION AQUEUSE

Abstract— p K values of ionisation equilibrium of thiazines dyes in their triplet state have been measured in aqueous solutions by spectroscopy. The triplet-triplet absorption bands, in the red part of the spectrum, are given for thionine, azurs and methylene blue. It is shown that, in the pH range 4–9, the equilibria investigated correspond to a second protonation of thiazines dyes that occurs in the triplet state:
³TH⁺+ H⁺→³TH₂²⁺
designating by ³TH⁺ the thiazine cation similar to the species stable in neutral aqueous solutions.     

INCREASED QUANTUM YIELD OF PHOTOREDUCTION OF DYES (A CATALYTIC EFFECT)

Abstract— While studying the photoreduction of some dyes (D) by reducing agents (R), it was observed that the quantum yield of the photoreduction increases considerably upon addition of a third substance (C), whereas it is very small when the dye is photoreduced by C alone (catalytic effect), (see Table 1).
The system thionine (D), allylthiourea (R), and azulene (C) was investigated in detail using both flash photolysis and continuous illumination. On photolysis, thionine is converted into its photo-reduced form, leucothionine. Azulene reacts with the basic form of the thionine triplet ³ TH ⁺ to produce the semithionine radical. In the system thionine and azulene, most of these radicals revert back to thionine. When ATU (˜ 10^2- M ) is added to thionine and azulene (3 × 10^-4 M ), the semithionine radicals are reduced to leucothionine; the quantum yield of this reduction is considerably higher than in the system thionine and allylthiourea. Flash experiments demonstrate that allylthiourea does not react with the semithionine radicals.
At very high ATU concentrations (≥ 10^-1 M ), however, the primary reaction is between thionine triplet and allylthiourea; under these conditions the quantum yield is not influenced by azulene (3 × 10^-4 M ).     

TRANSIENT PHOTOCHEMISTRY OF NEUTRAL RED

Abstract— Flash photolysis of neutral red between pH 1.3 and pH 11 yields the triplet species ³DH₂⁺²³DH⁺ and ³D. Both ³DH₂⁺² and ³D exhibit first order decay with rate constants of 1.6 ± 0.3 × 10⁴ s^-1 but ³DH⁺ decays within the lifetime of the flash. Over the entire pH range, ascorbic acid quenches the triplet, forming the semireduced radicals DH₃⁺² DH₂⁺ and DH, all of which exhibit second order decay with k = 1.8 ± 0.4 ± 10⁸ M ^-1s^-1 most probably by recombination with semioxidized ascorbic acid. The dependence of the rate of decay of radical neutral red on the identity of reversible reductants supports the back-electron transfer mechanism, as does digital simulation of complex radical disproportionation schemes. In contrast to the efficient reduction of triplet neutral red by ascorbic acid, its reduction by EDTA is quite inefficient.     

RELATIVE REACTIVITIES OF THE EXCITED STATES OF FUROCOUMARINS FOR [2+2] PHOTOCYCLOADDITION REACTION WITH TETRAMETHYLETHYLENE

Abstract— The Stern-Volmer constants for fluorescence quenching by tetramethylethylene decrease in the order DMC ≫ DHP > F-2 > 8-MOP. The same order was observed for the quantum yields of [2+2] cycloaddition reaction with tetramethylethylene on direct irradiation. In [2+2] photocycloaddition of F-2 with tetramethylethylene in ethanol, the ratio of quantum yields deduced from singlet and triplet states of F-2; φ₃⁰/φ₁⁰, is about 5. The excited triplet state is the reactive state for the [2+2] photocycloaddition of F-2 with tetramethylethylene in solution but the excited singlet state of F-2 becomes very important in biological conditions.     

PHOTOPHYSICAL PROPERTIES OF NUCLEIC ACID COMPONENTS—II. EFFECT OF N, N'DIMETHYL SUBSTITUTION ON INTERSYSTEM CROSSING IN PYRIMIDINE BASES

Abstract— The intersystem crossing quantum yields to the lowest triplet states of N, N'dimethyluracil and N, N'dimethylthymine are 0.02 and ≥0.004, respectively. The very low yields are interpreted in terms of a ¹( n , π*) to ¹(π.π*) state order change compared with the unsubstituted parents.     

STUDIES ON THE TRIPLET EXCITED STATE OF 4-THIOURIDINE

Abstract— The absorption spectra, lifetimes, extinction coefficients and intersystem crossing quantum yields of the lowest triplet T ₁ of 4-thiouridine have been determined both in acetonitrile and in water. An ordering of ^1,3(n,π)* and ^1,3(π,π)* states is suggested. Triplet quenching rate constants with various pyrimidine bases or amino acids are reported.     

DETERMINATION OF THE pk VALUES OF THE LUMIFLAVIN TRIPLET STATE BY FLASH PHOTOLYSIS

Abstract— The triplet-triplet absorption spectra in aqueous solution of the acid (³LfH₂⁺), the neutral (³LfH) and the basic (³Lf^-) forms of lumifiavin (6,7,9-trimethylisoalloxazine) were measured by flash photolysis. The p K _a values of the corresponding protolytic equilibria of the lumifiavin triplet were found to be 4.4₅±0.1 and 9.8±0.15.     

PHOTOCHEMICAL HYDROXYLATION OF 7,8-DIMETHYLALLOXAZINE (LUMICHROME)-A FLASH PHOTOLYSIS STUDY

Abstract— Flash photolysis experiments on the hydroxylation of lumichrome (L) in aqueous 0.5 M H₂SO₄ solution in the presence of O₂ or Ni²⁺ as triplet quenchers and quantum yield measurements confirm the assignment of the photoreactive species to the protonated form of the excited singlet state. A mechamism concerning the photochemical step is proposed, accounting for the formation of protonated 9-hydroxy-5,10-dihydrolumichrome (LOH₃⁺). This primary stable photoproduct was characterized by spectral and kinetic data. The dark reactions originating from LOH₃⁺ were investigated, and data regarding the successive steps are presented. The reaction LOH₃⁺ L→ LO + LH₃⁺ is demonstrated to be a two-electron reduction. The rate constant for the reaction of LH₂⁺ with O₂ is much larger than that for the oxidation of LH₃⁺ by oxygen.     

TRIPLET EXCITED STATES OF PYRIMIDINE NUCLEOSIDES AND NUCLEOTIDES

Abstract— The triplet absorption spectra, lifetimes, extinction coefficients, e_TT, and intersystem crossing quantum yields to the lowest triplet T₁, ø_T1, of thymidine, thymidine monophosphate, uridine and uridine monophosphate, have been determined in various solvents at 300 K.
The effect of H-bonding on ø_T1, of these nucleosides and nucleotides and also of uracil has been determined and discussed. This effect allows, an ordering of ^l,3 n, π* and ^1,3 π, π* states in protic and aprotic solvents.     

CALCULATED RATES AND EQUILIBRIA OF THERMAL INTERCONVERSION OF TRIPLET (3g-) AND SINGLET (1Δg and g+) MOLECULAR OXYGEN

Abstract— From spectroscopic data and rate constants in the literature, equilibrium constants and rates of thermal formation of singlet oxygen (¹Δ_g and ¹Σ_g⁺) were calculated for a number of conditions. For the gas phase we estimate K _eq(¹Δ_g³Σ_g^-) = 1.67 exp(-94.31 KJ/RT) and K _eq(¹Σ_g⁺/³Σ_g^-) = 0.33 exp(-157.0 KJ/RT). The calculated rate constants for the ³Σ_g⁺→¹Δ_g transition of O₂ at 25°C varied from 2.5 × 10^-11 s^-1 in water to 4.8 × 10^-16 s^-1 in air, assuming equal solvent interactions with the ground and excited states. Physical quenchers for singlet oxygen are expected to be catalysts for its thermal formation. Equations are presented which allow one to estimate whether such catalysis by quenchers will result in a pro-oxidant effect.     

SOLVENT EFFECTS ON THE PHOTOPHYSICAL PROPERTIES OF DIBUCAINE: A QUINOLINE ANALOGUE

Abstract— Emission and absorption spectra of neutral, monocation and dication dibucaine were recorded in hydrocarbon and hydroxylic solvents. The spectral analysis indicates that the lowest electronic states of dibucaine originate from the quinoline analogue in the molecule. The fluorescence and phosphorescence emission of neutral dibucaine in all solvents at 77 K are best assigned as resulting from ¹ n ,π* and ³π,π*, respectively, whereas those of both monocation and dication dibucaines are found to have ¹π,π* and ³π,π*. The monocation and neutral dibucaines in ethanol solutions were shown to give identical emission spectral properties, indicating that dibucaine-HCl in ethanol is H⁺ dissociative; this was confirmed by FT-IR studies. The possible explanations for the deprotonation of dibucaine-HCl in ethanol environments are discussed.     

LASER PHOTOLYSIS STUDY OF TRIPLET EOSIN AND THIONINE REACTIONS IN PHOTOSENSITIZED OXIDATIONS

Abstract —Nanosecond laser photolysis was used to determine the rate constants of quenching (including chemical reactions) of the triplet states of eosin and thionine, at pH 7, by imidazole, l-histidine, l-methionine, l-phenylalanine, l-tryptophan, 3-indole-acetic acid, cytidine, guanosine, α-chymotrypsin, lysozyme, allylthiourea, thiourea, p -nitrosodimethylaniline, I^- N₃^- and Fe(CN)₆^4-. The quenching rate constants for thionine were found to be up to three orders of magnitude higher than those for eosin. The mechanism of photosensitized oxygenations (free radical vs singlet oxygen) is discussed on the basis of the present results.     

MECHANISM OF PHOTOREDUCTION OF THIAZINE DYES BY EDTA STUDIED BY FLASH PHOTOLYSIS—III CONSEQUENCES OF A NEWLY FOUND pKT OF THIONINE ON THE MECHANISM IN BASIC SOLUTIONS

Abstract— Evidence for an acid-base equilibrium between the neutral and monocationic forms of the triplet state of thionine, ³ T and ³ TH ⁺ (p K _T# 8–9) has been found. This equilibrium, which has no counterpart in the methylene blue system can explain the different behaviours of these two similar dyes concerning their photoreduction by ethylene diamine tetracetic acid (EDTA) in basic solutions. With reasonable assumptions about the reactivity of the various species present in solution, it is now possible to calculate over the whole pH range the quantum yield of semireduced dye formation φs, which, according to the previously proposed mechanism, must be proportional to the overall photoreduction quantum yield φR. The very good fit between the calculated curve φs = f (pH) and the experimental one φR = f (pH) for both dyes strongly supports the proposed mechanism.     

THE INTERACTION OF THE EXCITED STATES OF SAFRANINE T WITH ALIPHATIC AMINES IN ORGANIC SOLVENTS

Abstract—
The reactions of the excited states of safranine T with aliphatic amines have been studied in methanol and acetonitrile. Quenching of the singlet and triplet states occurs by different mechanisms. Whereas the former excited state is quenched by a charge-transfer mechanism, the triplet state is quenched through proton transfer from the excited dye to the amine. This process leads to the unprotonated form of the dye in the triplet state, which is later quenched by amines to form the corresponding semireduced species. The monoprotonated triplet also undergoes self-quenching in both solvents (k = 1.2 × 10⁸ M ^-1 s^-1).     

Photosensitizing properties of triplet beta-lapachones in acetonitrile solution

The photochemical reactivity of β-lapachone ( 1 ), nor -β-lapachone ( 2 ) and 1,2-naphthoquinone ( 3 ) towards amino acids and nucleobases or nucleosides has been examined employing the nanosecond laser flash photolysis technique. Excitation (λ = 355 nm) of degassed solutions of 1 – 3 , in acetonitrile, resulted in the formation of their corresponding triplet excited states. These transients were efficiently quenched by l -tryptophan, l -tryptophan methyl ester, l -tyrosine, l -tyrosine methyl ester and l -cysteine ( k _q  ≈ 10⁹ L mol⁻¹ s⁻¹). For l -tryptophan, l -tyrosine and their methyl esters new transients were formed in the quenching process, which were assigned to the corresponding radical pair resulting from an initial electron transfer from the amino acids or their esters to the excited quinone, followed by a fast proton transfer. No measurable quenching rate constants could be observed in the presence of thymine and thymidine. On the other hand, efficient rate constants were obtained when 1 – 3 were quenched by 2'-deoxyguanosine ( k _q  ≈ 10⁹ L mol⁻¹ s⁻¹). The quantum efficiency of singlet oxygen (¹O₂) formation from 1 to 3 was determined employing time-resolved near-IR emission studies upon laser excitation and showed considerably high values in all cases (Φ_Δ = 0.6), which are fully in accord with the ππ* character of these triplets in acetonitrile.     

CHEMILUMINESCENT REACTIONS OF SO

Abstract— The rapid bimolecular reaction SO + O₃= SO₂+O₂+ 106 kcal/mole
yields electronically excited SO₂ in the ³B ₁ and ¹B₂ states with some vibrational excitation, as well as SO₂ in its electronic ground state. It is shown that k₁ = 1.5 x 10¹² exp (-2100/ RT ) cm² mole^-1 s-¹ and that the formation of electronically excited SO₂ involves higher activation energies.     

REACTIVITY OF ACRIDINE DYE TRIPLET STATES IN ELECTRON TRANSFER REACTIONS

Abstract—Rate constants, k _q , for the reaction of cationic and neutral acridine orange and 10-methylacridine orange triplet states (³AOH ⁺, ³AO, ³MAO⁺) with a series of electron donors have been measured. Two different protolytic forms of the semireduced dye radical are produced by acridine orange triplet quenching at various pH^M values in methanolic solution.
It is found that k ₄ decreases with increasing oxidation potential of the reducing agent for all triplet states in a manner which is expected for electron transfer reactions. The different reactivities of the cationic and neutral triplet forms can, therefore, be attributed to the difference in reduction potentials of these species. The difference in reduction potentials is related to the p K ^M values of triplet state, p K _T^M , and semireduced dye radical, p K ^M_S , by thermodynamic consideration. p K _T^M (³AOH⁺/³AO) is determined to be 11.2. From thisp K _S^M (AOH./AO;) is estimated to be 17–18. This is in striking contrast to the protolytic equilibrium of the semireduced dye radicals found to be pK^F= 4.1. We conclude that the last value represents the second protonation equilibrium (AOH⁺₂./AOH). This conclusion is confirmed by spectroscopic data.     

FLASH PHOTOLYSIS STUDIES OF OROTIC ACID

Abstract— The triplet state of orotic acid has been studied by flash photolysis. The rate for dimerization has been observed to vary from 2 × 10⁹ M ^-1 sec^-1 at pH 1 where both the triplet and ground state molecules are neutral, to under 10⁸ M^-1 sec^-1 above pH 9 where both the triplet and ground state molecules are doubly ionized. The p K of the triplet state has been measured as 4.6. The rate of oxygen quenching for the triplet is 2–3 × 10⁹ M^-1 sec^-1 while the rate of radiationless decay in solution is 0.73 × 10⁴ sec^-1. The triplet absorption spectra have been measured for the two ionic forms of the triplet.     

Photophysical properties of the lowest excited singlet and triplet states of thio- and seleno-psoralens

The decay processes of the lowest excited singlet and triplet states of five heteropsoralens (HPS) were investigated by steady-state and shift-phase fluorometry and by laser-flash photolysis in different solvents. The emission spectra of HPS are detectable only in trifluoroethanol (TFE), where fluorescence lifetimes (τ_F) and quantum yields (φ_F) were measured. The triplet lifetimes (τ_T), triplet (φ_T) and singlet-oxygen production (φ_Δ) quantum yields were determined in benzene, ethanol and TFE by laser-flash photolysis. Semiempirical (INDO/1-CI) calculations allowed the nature of the lowest excited singlet and triplet states and transition probabilities to be obtained. Theoretical and experimental results indicate that the two lowest excited singlet states S₁ and S₂ of HPS are close-lying and different in nature (π,π* and n,π*). The "proximity effect" between these two states controls the photophysical properties of HPS as it does for the other furocoumarins. However, HPS have a peculiar behavior with respect to the related compounds because they are fluorescent and have, in three cases, detectable intersystem crossing only in TFE. This behavior can be tentatively explained by a different energy gap and/or order between the S₁ and S₂ states.     

THE ELECTRONIC SPECTROSCOPY OF PYRIMIDINES: THE EFFECT OF COVALENTLY BONDED SULFUR ON THE PHOSPHORESCENCE AND ABSORPTION SPECTRA

Abstract— The phosphorescence of uracil, 2-thiouracil, 2 ,4-dithiouracil and 2-mercaptopyrimidine was studied at 77 K. 2-Thiouracil and 2,4-dithiouracil showed phosphorescence quantum yields of 0.65 and 0. 1 , respectively. The emitting triplet states of these compounds have been assigned as the ³(π, π*) type. The enhancement of spin-orbit coupling through the involvement of the 3d orbitals on sulfur has been invoked in describing emission characteristics of thiouracils.