PHOTOSENSITIZATION VIA CHARGE TRANSFER OR REVERSIBLE ELECTRON TRANSFER. OXIRANE ISOMERIZATION AND SULFUR DIOXIDE EXTRUSION

Abstract— N, N, N^' N^'-Tetramethylbenzidine (NTMB) photosensitizes the cis-trans isomerization of stilbene oxiranes (SO) and the extrusion of SO₂ from dibenzyl sulfone (DBS). In acetonitrile solution it is found that in the absence of SO or DBS, singlet NTMB undergoes three processes: intersystem crossing to triplet NTMB (φ_ISC= 0.63, k _ISC= 6.3 × 10⁷s^-1), fluorescence (φ_f= 0.30, k _f= 3 × 10⁷s^-1), and formation of a cation by electron ejection (φ_ion= 0.09). Both singlet and triplet sensitization are observed. A charge transfer or reversible electron transfer mechanism is proposed to explain the results.     

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.     

TRIPLET STATES AND cis-trans PHOTOISOMERIZATION PROCESSES IN THE SCHIFF BASES OF RETINAL ISOMERS

Abstract —The triplet states of the n -butyl-amine Schiff bases of 11- cis , 9- cis , 13- cis and all- trans retinal are produced via triplet-triplet energy transfer. Their absorption spectra, peaking around 435 nm, and their decay kinetics are recorded using pulsed-laser photolysis. Direct-excitation (φ^D_ISO) and triplet-sensitized (φ^T_ISO) photoisomerization yields, determined using steady irradiation methods, are found to be: φ^T_ISO (9- cis ) = 0.06, φ^T_ISO (11- cis ) = 045, φ^T_ISO (13- cis ) = 008, φ^T_ISO (all- trans ) = 0.02-0.05, φ^D_ISO (11- cis , = (4 ± 1) × 10^-3, φ^D_ISO (all- trans ) = (2 ± 1) × 10^-3. The possible role of the triplet state in the isomerization of rhodospin is discussed.     

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.     

ACTINOMETRY IN MONOCHROMATIC FLASH PHOTOLYSIS: THE EXTINCTION COEFFICIENT OF TRIPLET BENZOPHENONE AND QUANTUM YIELD OF TRIPLET ZINC TETRAPHENYL PORPHYRIN

Abstract— The extinction coefficient ε_T, of triplet benzophenone in benzene has been directly determined by absolute measurements of absorbed energy and triplet absorbance, Δ D ⁰_T, under demonstrably linear conditions where incident excitation energy, E ₀, and ground state absorbance, A ₀, are both extrapolated to zero. The result, 7220 ± 320 M ^-1 cm^-1 at 530 nm, validates and slightly corrects many measurements relative to benzophenone of triplet extinction coefficients made by the energy transfer technique, and of triplet yields obtained by the comparative method.
As E ₀ and A ₀ both decrease, Δ D ⁰_T becomes proportional to their product. In this situation, the ratio R = (1/ A ₀)(dΔ D ⁰_T/d E ₀) = (ε_T - ε_G)φ_T. Measurements of R , referred to benzophenone, give (ε_T - ε_G)φ_T for any substance, without necessity for absolute energy calibration.
Both absolute and relative laser flash measurements on zinc tetraphenyl porphyrin (ε_T - ε_G at 470 nm = 7.3 × 10⁴ M ^-1 cm^-1) give φ_T= 0.83 ± 0.04.     

THE TRIPLET AND RADICAL SPECIES OF HAEMATOPORPHYRIN AND SOME OF ITS DERIVATIVES

Abstract— Nanosecond laser flash photolysis and pulse radiolysis have been used to generate and characterise the triplet state, and semioxidised and semireduced radicals of haematoporphyrin, and three 0 -acyl compounds derived from it (the monoacetate, the diacetate and the disuccinate).
After 347 nm irradiation in water containing 2% Triton X-100, haematoporphyrin forms the triplet state (φ_T= 0.92) and photoionises monophotonically (φ_I= 0.03). For the O -acyl derivatives, φ_T approaches unity and photoionisation is reduced. In acetone the triplet yield of all four compounds are close to unity. The difference and corrected spectra for the triplet species are presented and decay rates ( k ₁˜10⁴s^-1) and oxygen quenching constants ( k _Q˜1.5times10⁹ M ^-1s^-1) for the triplet state have been measured. The difference and corrected spectra for the semi-reduced species in methanol and semi-oxidised species in aqueous Triton X-100 are presented.
The photophysical characteristics in fluid solution of haematoporphyrin and its 0 -acyl derivatives are rather similar to those previously recorded for other photosensitising porphyrins.     

EFFECT OF TEMPERATURE AND pH ON THE QUENCHING OF TRIPLET LUMIFLAVIN IN THE PRESENCE AND ABSENCE OF FERRI- AND FERROCYANIDE

Abstract— –Flash photolysis at 450 nm over the temperature range 0.8–60°C was used to determine Arrhenius parameters for the first and second order disappearance of triplet lumiflavin (1.66 µ .M ) at a flash energy of 2 kj in deaerated phosphate buffer at varying pH:
³Lf → Lf₀
³Lf +³Lf → Lf₀+ Lf₀
Arrhenius parameters were also determined for the pseudo first-order quenching of triplet lumiflavin by 10 µ M ferri- and ferrocyanide ions,
³Lf + Fe³⁺→ Fe³⁺→ Lf₀+ Fe³⁺ (energy transfer)
³Lf + Fe²⁺→ Lf^-+ Fe³⁺ (electron transfer)
and for disappearance of the semireduced lumiflavin in the presence of ferrocyanide at pH 6.8, by the second-order reaction
Lf^-+Lf ^-→ Lf₀+ Lf⁼.     

TRANSIENT PHENOMENA IN THE PULSE RADIOLYSIS OF RETINYL POLYENES—7. RADICAL ANIONS OF VITAMIN A AND ITS DERIVATIVES

Abstract— Upon e^--pulse irradiation in nonprotic solvents, all- trans retinol (ROH) and retinylmethyl ether (ROMe) form transient species (τ= 0.5–7μs, λ_max=575–590 nm) identifiable as radical anions. Similar species are also formed upon laser pulse photoexcitation of these retinyl derivatives in the presence of N,N-dimethylaniline in acetonitrile. In contrast, electron transfer or attachment to all- trans retinyl acetate (ROAc) and palmitate (ROPa) results in 'instantaneous' loss of carboxylate anions from electron adducts giving the retinylmethyl radical (R-, λ_max= 395 nm, τ_k > 100 μ,s); the radical anions in these cases are too short-lived to be detected by nanosecond pulse radiolysis. The lifetimes of radical anions of ROH and ROMe are very sensitive to water and alcohols (e.g. k_q = 10⁷ M ^-1 s^-1 with methanol as quencher for ROH^- in tetrahydrofuran). Based on these findings, the spectral dissimilarity of the one-electron reduction products from ROH and ROAc in alcohols and aqueous micelles becomes explainable in terms of fast formation of protonated radical anions (RH(OH), τ_1/2, > 100 μs, λ_max=370–375 nm) in the case of ROH and of retinylmethyl radical via loss of AcO^- from radical anion in the case of ROAc. In tetrahydrofuran, the complexation of ROH^- with cations such as Na⁺ and Bu₄N⁺ affects the relative importance of its major decay modes, namely, protonation and dehydroxylation, the latter process being significantly enhanced by the presence of Na⁺.     

A FLASH PHOTOLYSIS STUDY OF 1-METHYLINDOLE

Abstract— The conventional flash photolysis of 1-methylindole in aqueous media was studied at Λ_excitation≥290 nm. The transients observed 20 μs after excitation consisted mainly of the radical cation (R⁺). the hydrated electron (e^-_aq) and the triplet state (T). Electron counting experiments indicate that photoionization is the only source of R⁺ with e^-_aq/R⁺= 1.07±0.09 in neutral media. Quenching of the R⁺ yield with H⁺ indicates that the fluorescent state is the precursor to 80% of the photoionization events with the remainder probably arising from a prefluorescent state. The triplet decays with a lifetime of 29 μs in deaerated neutral media. This decay is unchanged by N₂O saturation, but T reacts with acrylamide with k ≥2.8 × 10⁹ M ^-1. In 2 M Br^-, R⁺ and T yields are increased by factors of 2–3. Consideration of fluorescence quenching and T enhancement by Br-permits an estimate of φ_Isc between 0.33 and 0.49. The increased R⁺yield at high Br-concentrations cannot be accounted for by induced photoionization or triplet state reactions.     

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.     

PHOTOCHEMICAL cis/trans ISOMERIZATION OF AN AMPHIPHILIC MEROCYANINE DYE IN HOMOGENEOUS SOLUTION AND REVERSED MICELLES

Abstract— In the present study, the photochemical cis⇌trans isomerization of the amphiphilic merocy-anine dye (4-hydroxy-l-cetyl stilbazolium betaine C₁₆M) is reported. The structure of each isomer is established from high resolution FT ¹H-NMR analysis. The photochemical quantum yields φ_t→c, and φ_c→t are determined in methanol and in reversed micelles of the system (CDBA, benzene, water). The quantum yield φ_c→t increases in reversed micelles compared to that in methanol. The reverse results are obtained for φ_t→c. The micellar effect on the photochemical trans→cis isomerization will be 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.     

1H-INDENYLFURAN AND THIOPHENE DERIVATIVES– A NEW CLASS OF SINGLET OXYGEN SENSITIZERS

Abstract— Photophysical and photochemical properties of 1H-indenylfuran and thiophene derivatives were studied. UV spectra showed that all the compounds tested absorbed UV light in the region 350–380 nm. The fluorescence spectra of 4a-e showed bands in the region 410–470 nm and quantum yields (φ_F) in the range 0.25–0.88. Attempts to calculate the triplet energy of 4a-e failed except for 4c and 4d , which showed E_T= 43–44 kcal mol^-1. These compounds are a new class of singlet oxygen sensitizers. The sensitized reaction of 2,5-dimethylfuran with singlet oxygen was followed showing that 4a and 4e are more reactive than α-terthiophene (4.0 and 2.5 times respectively), while the other compounds show the same reactivity as 1 . This behavior can be explained considering the different intersystem crossing quantum yields of compounds 4a-e . Diazabicyclo[2. 2. 2]octane is a quencher of singlet oxygen in this reaction, while we can exclude superoxide ion formation using the photooxidation of α, α-dimethylstilbene.     

PHOTOPHYSICAL PROPERTIES OF NUCLEIC ACID COMPONENTS—1. THE PYRIMIDINES: THYMINE, URACIL, N, N-DIMETHYL DERIVATIVES AND THYMIDINE

Abstract— Low-temperature (and some room temperature) absorption and emission, fluorescence and phosphorescence, data including quantum yields and lifetimes have been obtained from the title pyrimidine bases as a function of the nature of the solvent environment. Modest vibrational resolution has been observed for the first time in the absorption spectra, particularly for thymine and uracil. The excitation spectra also show structure. The quantum yields of fluorescence (φ_F) and phosphorescence are independent of the excitation wavelength. Thymine, thymidine and uracil have profoundly different photophysical properties in polar-aprotic vs polar-protic solvents. The N, N-dimethyl substitution of thymine and uracil produces photophysical changes comparable to the solvent change for the unsubsti-tuted bases. The species involved in the emission processes is the keto (lactam) form. It is probable that ^1,3(n,π*) state(s) has(have) changed order relative to a lowest ¹(π,π*) state as a consequence of both the solvent change and N, N-dimethyl substitution. The lowest triplet state is assigned as ³(n π*). We propose that an important factor contributing to the previously reported excitation wavelength dependence of φ_F and φ_T1 (φ_isc) for nucleic-acid components is the equilibrium coexistence of H-bonded and non-H-bonded forms each having different photophysical properties. Consideration is given of the impact of the significantly different photophysical properties of nucleic-acid bases as a function of the nature of the solvent upon the photochemical properties.     

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.     

THE IN VITRO PHOTOCHEMISTRY OF BIOLOGICAL MOLECULES—II. THE TRIPLET STATES OF β-CAROTENE AND LYCOPENE EXCITED BY PULSE RADIOLYSIS

Abstract— –Pulse radiolysis has been used to excite the triplet states of β-carotene (τ# 9μ sec) and lycopene (τ= 8μsec) in hexane solution, both in the presence and absence of naphthalene as a triplet sensitiser. The absorption spectra of both triplets have been measured in the range 430–550 nm and have thus been extended into the region of the corresponding singlet absorptions. The overlap of the triplet and singlet spectra is discussed in relation to in vivo studies. Extinction coefficients of 1.3±0.1 × 10⁵ l/mole cm for β-carotene triplet 515 nm and 3.9±0.2 × 10⁵ l/mole cm for lycopene triplet at 525 nm were obtained. Isomerisation of the all- trans polyenes used was detected and preliminary measurements indicate that the yield of isomerisation was greater than the triplet yield. The rate of triplet energy transfer from naphthalene to β-carotene was estimated to be 1.5 × 10¹⁰ l/mole sec. The corresponding value for lycopene was 1.4× 10¹⁰ l/mole sec. The measured efficient quenching of triplet β-carotene by oxygen may occur by an energy transfer mechanism, leading to the formation of singlet oxygen (¹Δ_g. This would suggest that the triplet energy level of β-carotene lies between 121 and 94 kJ mole^-1.     

EXCITED TRIPLET-SINGLET INTERSYSTEM CROSSING IN ISOALLOXAZINES

Abstract— Higher excited triplet states originating from the lowest triplet state of isoalloxazines by absorption of light with Λ - 600 nm undergo "inverse" intersystem crossing to the singlet manifold [φ( T_n-S_m ) = 8 × 10^-3] followed by rapid internal conversion and "normal" fluorescence S₁-S₀ with Λ_m= 540 nm.     

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.     

THE CHARACTERISATION OF THE TRIPLET STATE OF CROCETIN, A WATER SOLUBLE CAROTENOID, BY NANOSECOND LASER FLASH PHOTOLYSES

Abstract— The triplet state of crocetin, which is a water soluble carotenoid, has been sensitized by psoralen. The triplet extinction coefficient, ε_T (73000 dm³ mol^-1 cm^-1 at 470 nm), the triplet-triplet spectrum and the quantum yield of triplet formation, φ_T (less than 1%) are reported in aqueous solution.
In order to calculate the extinction coefficient of crocetin it was necessary to obtain ε_T for psoralen in water (10000dm³ mol^-1 cm^-1 at 450 nm). This latter value was obtained using the complete conversion technique and is reported with the triplet-triplet spectrum.     

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.