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.     

A LASER FLASH PHOTOLYSIS STUDY OF PYRENE-1-ALDEHYDE. INTERSYSTEM CROSSING EFFICIENCY, PHOTOREACTIVITY AND TRIPLET STATE PROPERTIES IN VARIOUS SOLVENTS

Abstract— Triplet-and singlet-related photoprocesses of pyrene-1-aldehyde (PA) in various solvents have been investigated in detail using 337.1 and 355 nm laser flash photolysis in conjunction with time-correlated determination of fluorescence lifetimes (τ_F) and steady-state photochemical and absorption-emission spectral measurements. In benzene, the lowest triplet of PA (43 < E_T < 46 kcal/mol) has a lifetime of about 50 µs (τ_T) and displays the absorption maximum at 443 nm with a maximum extinction coefficient (ε_max) of 21000 M ^-1cm^-1; the corresponding ketyl radical has a sharp absorption maximum at 428 nm (ε_max≥ 25000 M ^-1cm^-1). The quantum yields (φ_T) of lowest triplet occupation are high in nonprotic solvents (0.6–0.8), decrease in protic solvents (alcohols) as the polarity of the latter is increased, and maintain a complementary relationship with the quantum yields (φ_F) of fluorescence. Quantum yields (φ_PC) of loss of PA due to photoreactions in some solvents have also been determined under conditions of steady irradiation at 366 nm; φ_PC is in the range 0.1–0.2 in electron-rich olefinic solvents such as cyclohexene and tetramethylethylene. These results concerning τ_F, τ_T, φ_F. φ_T and φ_PC as well as the effects of 1,2,4-trimethoxybenzene and 2,5-dimethyl-2,4-hexadiene as quenchers for fluorescence, triplet yield, and photochemistry are discussed in the light of possible state orders for PA in polar and nonpolar environments.     

AN EPR STUDY ON THE TRIPLET STATE OF PURINE FREE BASE IN AQUEOUS GLASSES AT 77 K

Abstract— Since purine free base in aqueous solution exists in different ionic forms at different pHs and most of its photoreactions are likely to involve the triplet excited states of these different ionic forms, electron paramagnetic resonance studies (EPR) have been performed in order to determine intersystem crossing quantum yields and other characteristic parameters of the excited triplet state of these forms. Intersystem crossing yields decrease with a decrease in pH, being 0.62, 0.37 and 0.10 in 8 M NaOH, 8 M NaCIO₄ and 6M H₃P0₄ glasses, respectively. Differences in triplet decay lifetimes 3.4, 2.5 and 3.1 s, as well as in root mean-square zero-field splitting (ZFS) parameter, D_*, (0,1304, 0.1512 and 0.1353 cm_-1) are also observed for the anionic, neutral and cationic species of purine free base. The EPR signals of the triplet state of the neutral and anionic forms have been observed simultaneously in the pH range of7–10.     

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.     

QUANTUM YIELDS OF TRIPLET FORMATION OF SOME DERIVATIVES OF ANTHRAQUINONE

Abstract— Quantum yields of triplet formation for five amino substituted anthraquinones have been determined by the comparative actinometer method using laser flash photolysis. Substitution reduces the yields to values of ˜10^--². and the requirement of low laser intensities required high sensitivity of detection of triplet absorption. The ø_T values are compared with the quantum efficiencies of fluorescence and decomposition for the compounds, and the criteria for light stability discussed.     

THE PHOTOCHEMISTRY OF RIBOFLAVIN—VI. THE PHOTOPHYSICAL PROPERTIES OF ISOALLOXAZINES

Abstract— Many of the photophysical properties of riboflavin and several other N-10 substituted isoal-loxazines have been measured and these include: quantum yields of fluorescence at 77 K and 298 K, φ, quantum yields of phosphorescence at 77 K, φ_P, lifetimes of the triplet state by electron spin resonance and phosphorescence at 77 K, φp,. and the quantum yield of intersystem crossing. φ_isc. For riboflavin in an alcoholic matrix at 77 K the limiting values were: φ _J = 0.32, φ_P= 0.007, φp = 0.20s and φ_isc= 0.7. At 298 K, φ _f for riboflavin in water and alcohol were 0.25 and 0.32, respectively. The results for the photophysical processes are compared with several photochemical processes known to involve the triplet state of riboflavin in aqueous solution. It is concluded that the φ _isc decreases from 0.7 in alcohol to 0.6 in water at 298 K.     

THE PHOTOPHYSICS OF MEROCYANINE 540. A COMPARATIVE STUDY IN ETHANOL AND IN LIPOSOMES

Abstract— Absorption and fluorescence emission spectra, fluorescence lifetimes, fluorescence quantum yields, photoisomerization quantum yields and triplet quantum yields were measured for Merocyanine 540 (MC540) in ethanol and in large unilamellar dimyristoyl phosphatidylcholine vesicles. The major differences in the photophysics between the two media are the increase of the fluorescence quantum yield from 0.15 in ethanol to 0.6 in vesicles at 25° C, and the appearance of a second fluorescence decay with a lifetime of 1.87 ns in the latter medium. Upper and lower limits for the photoisomerization quantum yields were determined by combining the data from laser flash photolysis and optoacoustic spectroscopy. The decrease in photoisomerization quantum yield upon incorporation of the dye into the lipid bilayer by a factor 2 suggests that this process competes directly with fluorescence. The temperature dependence of the fluorescence and photoisomerization quantum yields in solution supports this model. In both media MC540 has a very low triplet quantum yield with values 0.002 > (> ø_T > 0.02 in ethanol and 0.01 > ø_T- > 0.09 in liposomes Our data are consistent with the model whereby the dye is incorporated into the lipid bilayer as a monomer with two different orientations and this model is adopted on the basis of the biexponential behaviour of the fluorescence and photoisomer decay.     

LASER FLASH PHOTOLYSIS OF INDOLE IN THE PRESENCE OF AMINO ACIDS

The laser flash photolysis of indole at 265 nm in the presence of glycine, proline and hydroxy proline was studied. The relative yields of c _aq, triplet state, and indole cation radical were determined in the absence and in the presence of the amino acids. The yields were determined as a function of laser intensity and the values at very low intensity were compared with the fluorescence quenching results. It was concluded that in these conditions the photoionization of indole occurs via the fluorescent state. From the curves of triplet yield vs laser intensity, the triplet quantum yield extrapolated at low laser intensity was obtained, φr = 0.55 φ 0.05, relative to the literature value of 0.15 for φ_eag. This gives φ_F+φ_eaq= 1.0 ± 0.1 at room temperature. When proline and hydroxy proline were used as singlet quenchers, the yield of In was greater than the yield of c_aq. This was considered as evidence that a fraction of the quenching processes leads to complete electron transfer from indole to the amino acids.     

ULTRAVIOLET IRRADIATION OF NUCLEIC ACIDS COMPLEXED WITH HEAVY ATOMS-II. PHOSPHORESCENCE AND PHOTODIMERIZATION OF DNA COMPLEXED WITH Ag*

Abstract— We have studied the luminescence (at 77°K) and photochemistry (at 298°K) of a variety of polynucleotides complexed with Ag⁺. In all cases we observed that Ag⁺ induces a 'heavy atom effect' as indicated by quenching of fluorescence, enhancement of phosphorescence, and reduction of triplet lifetimes. The enhancement by Ag⁺ of the phosphorescence intensity of poly (dT) and DNA paralleled a 20-fold enhancement of the rate of thymine dimerization. These results suggest a possible triplet precursor for thymine dimerization in Ag⁺ complexes. Dimers were also made with a variety of triplet sensitizers, and Ag⁺ increased the dimer yield two- to four-fold, depending on the sensitizer used.     

MEROCYANINE DYES: EFFECT OF STRUCTURAL MODIFICATIONS ON PHOTOPHYSICAL PROPERTIES AND BIOLOGICAL ACTIVITY

Abstract Merocyanine derivatives were prepared by structural alterations at the barbituric acid or chalcogenazole moieties. The photophysical properties of the dyes were markedly influenced by the presence of selenium rather than sulfur as a substituent at position 2 of the barbiturate. In methanol, quantum yields of both triplet state (φ_τ) and singlet oxygen sensitization (φ_Δ) were increased by over an order of magnitude, with a concomitant decrease in fluorescence, when selenium was present in the molecule. Photoisomerization, one of the dominant deactivation pathways in the sulfur- or oxygen-containing analogues, was completely absent in the selenium-containing derivatives. Efficient triplet state formation was observed for selenium-containing derivatives incorporated into L1210 cells by diffuse reflectance laser flash photolysis. Cytotoxicity studies, camed out using clonogenic assays on L1210 leukemia cells, showed a good correlation with φ_τ and φ_Δ, measured in solution. Experimental evidence provided by this paper supports a triplet state-, and probably singlet oxygen-, mediated phototoxic mechanism. Photoisomerization or singlet state mechanisms can be discounted.     

THE PHOTOPHYSICAL AND PHOTOCHEMICAL PROCESSES OF TRYPTOPHAN IN INTERACTION WITH POLYNUCLEOTIDES: LASER FLASH PHOTOLYSIS STUDY

Abstract— The influence of nucleotides or polynucleotides on the photophysics and the photochemistry of tryptophan (Trp) derivatives has been investigated in aqueous solutions using the 265 nm laser flash photolysis technique. In solutions containing mixtures of N -acetyltryptophanamide and uridine monophosphate (UMP) or mercurated dUMP, the Trp triplet and the hydrated electron (e_aq) are quenched at almost diffusion controlled rates by the nucleotides leading to uracil reduction. Lysyl-tryptophyl-α-lysine (Lys-Trp-Lys) forms stable complexes in solution with normal or mercurated poly(uridylic acid) [poly(U)]. In the Poly(rU)-Lys-Trp-Lys complex the Trp triplet state is completely quenched, whereas the Trp triplet formation quantum yield is enhanced in complexes with mercurated poly(U). In this last case, the 'heavy atom effect' is characterized by a shortening of the Trp triplet lifetime in agreement with low temperature experiments. Our results also show that photoionization of Trp does occur in the complexed state with both polymers. The e_aq lifetime is however longer with the complexed than with the free peptide.     

MECHANISM OF THE PHOTOCHEMICAL REACTION OF DIPHENYLAMINES WITH CARBON TETRACHLORIDE

Abstract. The quantum yields of HCI (φ_HC1) formation have been measured for the photolysis of N -methyldiphenylamine (MeDPA), triphenylamine (TPA) and diphenylamine (DPA) in the presence of CCl₄ in polar solvents. The quantum yields of N-methylcarbazole formation (φ_mφca) have also been determined for the system MeDPA-CCl₄. With increasing CCl₄ concentration, φ_HCl increases as φ_MeCA decreases, and φ_HCl reaches maximum values 2.7 at 1 M CCl₄. Using laser photolysis, transient spectra have been recorded for MeDPA in the absence and presence of CCl₄ in polar and non-polar solvents, and for TPA. Transient absorption due to the triplet states and photocyclization products (without CCU), exciplexes, the (C₆H₅)₂ NCHi radical, the MeDPA⁺ cation radical, the (TPA⁺., CCl₄) ion pair, and the TPA⁺ cation radical have been identified. The mechanistic implications of these results are discussed.     

THE FLAVIN-SENSITIZED PHOTOOXIDATION OF NUCLEOTIDES — III. THE PARTICIPATION OF OXYGEN

Abstract— Quantum yields for the lumiflavin-sensitized oxidation of guanosine monophosphate and adenosine monophosphate in solution have been measured as functions of oxygen and nucleotide concentration. The quantum yield increases with oxygen concentration at low oxygen concentrations, but quenching of the excited flavin molecule by oxygen results in a fall in quantum yield at higher concentrations. It has also been established that the reciprocal of the quantum yield is linearly related to the reciprocal of the nucleotide concentration. A mechanism in which molecular oxygen reacts with an excited complex formed between triplet lumiflavin and the nucleotide is consistent with these observations.
A value for the second-order rate constant for the quenching of triplet lumifiavin by oxygen of 2·65 × 10⁹ M ^-1 sec^-1 has been obtained.     

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.     

EXCITED STATES OF BILIRUBIN

Abstract— The excited states of bilirubin (BR) in a variety of environments have been studied by 347 nm laser flash photolysis. Quantum yields of formation of triplet BR have been shown to be less than 0.005 in solution in water ( p H 9–11), methanolic ammonia, 10% aqueous mulgofen and in cetyl trimethyl-ammonium bromide. In benzene the quantum yield was 0.01 although this diminished to less than 0.005 on addition of triethylamine. Permanent products are formed with benzene and with 1% methanolic ammonia. With BR in HSA a transient decaying with k = 3.5 × 10⁵ s^-1 is formed by a monophotonic process together with a permanent product. Neither species is affected by oxygen or by iodide ion. Both originate from BR molecules in the strongest binding site in the HSA. The yields of both species are unaffected by salt but are temperature dependent. The decay of the transient is strongly temperature dependent corresponding to an activation energy of about 50–60 kj mol^-1. If this transient is a triplet it is formed with a quantum yield of 0.13 ± 0.01. The relevance of these results to an understanding of the photo therapeutic process is discussed.     

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.     

FLUORESCENCE AND PHOTOOXIDATION OF 1—ANILINONAPHTHALENE—8—SULFONIC ACID IN REVERSED MICELLAR SOLUTIONS AND THE EFFECT OF CCl4

Abstract— The 1-anilinonaphthalene-8-sulfonic acid solubilized in dodecylammonium propionate reversed micellar cyclohexanic solutions, emitted a strong fluorescence, and was photooxidized under aerobic conditions. Carbon tetrachloride (CCl₄) highly quenched the fluorescence and remarkably enhanced the oxidation reaction. The fluorescence quenching obeyed the Stern-Volmer relation, and the photooxidation was caused by the singlet oxygen generated by the photosensitization of the dye. From the kinetic analysis, it was known that the intersystem crossing rate from the dye excited singlet to triplet was enhanced by CCl₄. Carbon tetrachloride did not quench the triplet state. The ratio of quantum yields for the oxidation in the presence and absence of CCl₄ was independent of the oxygen concentration in the reaction mixture. The fluorescence quenching constant and the intersystem crossing rate were obtained at various solubilized water contents.     

螺吡喃激发态的反应活性

研究了6′-硝基-1,3,3-三甲基吲(口乃木)苯骈螺吡喃,Ⅰ,和6′-硝基-1-苯基-3,3-二甲基吲(口乃木)苯骈螺吡喃,Ⅱ,的光化学,Ⅰ的直接光解和敏化光解均得到份菁结构的光解产物Ⅲ,量子产率分别是0.42和0.32.Ⅱ直接光解时生成相应的有色光解产物Ⅳ,量子产率是0.40,氮杂蒽酮为敏化剂的敏化光解量子产率是0.58,无疑地,单线态和三线态均参予螺吡喃的发色反应,用这些结果和折算关系式得到,在Ⅰ的直接光解中,Ⅲ主要来自单线态,而在Ⅱ的直接光解中,Ⅳ主要来自三线态.从Ⅰ或Ⅱ的敏化联乙酰燐光实验中求得三线态参数.它们是:φ_ISC^Ⅰ＝0.11,φ_ISC^Ⅱ＝0.48;³K_dt^Ⅰ＝3×10⁴秒^-1,³K_dt^Ⅱ＝9×10⁴秒^-1,类似地,³K_r^Ⅰ＝0.12×10⁴秒^-1,³K_r^Ⅱ＝2.52×10⁴秒^-1,这样,³φ_r^Ⅰ值较小是由于³φ_ISC^Ⅰ值较小,而三线态寿命随结构变动不大.最后,取苯乙烯为模型,用它的电子能量与乙烯双键扭曲角度关系图推断出,来自单线态的双离子具有螺环构型,而来自三线态者具有平面状构型.     

QUANTIFICATION OF THE SOLVENT EFFECTS ON THE TRIPLET QUANTUM YIELD OF PSORALEN BY THE "LINEAR SOLVATION ENERGY RELATIONSHIP"

Triplet formation quantum yields (Φ_τ) of psoralen in a set of 17 pure solvents ranging from n -hexane to water and in dioxane: water mixtures were obtained by nanosecond laser flash photolysis. The triplet yield increases with solvent polarity. The extremum values are 0.009 and 0.545 in n -hexane and water, respectively. Good correlations of the experimental Φ_τ values with empirical "polarity" scales (Dimroth/Reichardt's E_T [30], Kamlet/Taft's solva-tochromic parameters β, and α, and Swains acity/basity A_S/B_S) were obtained: Ln(φ_T^-1 - 1) = 8.86 - 0.143E_T(30) Ln(φ_T^-1 - 1) = 4.40 - 2.34_τ - 1.70α Ln(φ_T^-1 - 1) = 4.65 - 3.72A_s - 1.12B_s The results are discussed in terms of the sensitivity of psoralen triplet quantum yield to solvent polarity and hydrogen-bonding abilities.     

PHOTOCHEMISTRY OF FLAVINS. II. PHOTOPHYSICAL PROPERTIES OF ALLOXAZINES AND ISOALLOXAZINES

Abstract. Pulsed laser photolysis at 347nm has been used to study the transient spectroscopy of alloxazine, lumichrome, lumiflavin, and riboflavin in acidic (pH 2.2) aqueous solution and in ethanol. Intersystem crossing quantum yields (φ_ISC) were determined by a modification of the comparative laser excitation method which utilizes the variation of the triplet yield with intensity in conjunction with a kinetic model for the various photophysical and photochemical processes occurring during the pulse. Fluorescence quantum yields and lifetimes are also reported. Correction for quenching of the excited singlet state by H⁺ ions shows that, in neutral aqueous solution, intersystem crossing for flavins is an efficient process (φ_ISC˜ 0.7) which, in conjunction with fluorescence, accounts for the fate of all absorbed photons. For alloxazine (φ_ISC˜ 0.45) and lumichrome (φ_ISC˜ 0.7) the results are more difficult to interpret owing to interconversion between alloxazine and isoalloxazine structures in the singlet excited state. For all four compounds, the quantum yield of products derived from the singlet excited state is estimated as ˜0.04. There is evidence of biphotonic product formation at high laser energies. In ethanol, where φ_ISC for lumichrome is about twice that of lumiflavin, internal conversion between the excited singlet and ground states appears to be a significant process. Complete triplet-triplet absorption spectra in the region 260–750nm are reported. For lumichrome at pH 2.2 there is spectral evidence for isomeric triplet states which appear to be in equilibrium.