THE LOWEST EXCITED TRIPLET STATES OF CODEINE AND MORPHINE AND OF THEIR MOLECULAR SUBUNITS VERATROLE AND GUAIACOL

Abstract— The lowest excited triplet T₁ states of the two alkaloids, morphine and codeine, have been investigated by ODMR and emission spectroscopy. The electronic character of their T₁ states could be determined by comparing the T₁ properties with those of the constituent parts guaiacol and veratrole. The T₁ properties, e.g. phosphorescence spectra, zero-field splitting parameters and triplet sublevel selective decay rates are very similar for the constituent parts and the alkaloids. Thus, in analogy to veratrole and guaiacol, the T₁ state of codeine and morphine is assigned to a ππ* state mainly localized on the benzene ring and the attached alkoxy and hydroxy groups. There is no evidence for a contribution from the other subunits, trialkylamine and unsaturated alcohol, to the T₁ state of codeine and morphine.     

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.     

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.     

LUMINESCENCE STUDIES OF QUINIZARIN AND DAUNORUBICIN

Abstract— The absorption and emission spectra of quinizarin (1,4-dihydroxy-anthraquinone) have been investigated in hydrocarbon and alcoholic solvents. Fluorescence spectra in 3 different Shpolskii matrices were recorded at 14 K. Vibrational analyses of these spectra revealed the presence of 3, 8, and 9 sites in octane, heptane, and hexane matrices, respectively. The fluorescence lifetime was found to be 6.5 ns in hexane and EPA. Fluorescence photoselection measurements in EPA (77 K) showed that the first 4 electronic transitions of quinizarin are polarized parallel, parallel, perpendicular, and parallel to the long molecular axis and can be assigned, in order of increasing energy, to ¹B₂, ¹B₂, ¹A₁ and ¹B₂ (ππ*)→¹A₁(C_2v,) transitions, respectively. The fluorescent transition is assigned as ¹B₁ (ππ*)→¹A¹. The absence of phosphorescence is attributed to the intramolecular hydrogen bonding present which displaces the parent anthraquinone n →* states above the ππ* states, thereby rendering the intersystem crossing (S₁-T₁) radiationless pathway inefficient.
Photoselection measurements on daunorubicin, a substituted quinizarin and known anticancer drug, revealed an absorption band polarization pattern identical to that of quinizarin. These results are in part at variance with assumptions used in previous work on the intercalation specificity of daunorubicin with DNA.     

FLUORESCENCE OF THYMINE IN AQUEOUS SOLUTION AT 300° K

Abstract— –Fluorescence of thymine in neutral aqueous solution at room temperature has been detected using the multiscaling operation of a multichannel analyzer. The emission maximum (2.96 μm^-1) and 0-0 transition energy (3.37-3.45 μm^-1) are close to those determined at liquid nitrogen temperature in mixed solvents. The quantum efficiency of fluorescence excited at 3.77 μm^-1 is calculated to be 1.04 × 10^-4.
The corrected relative excitation spectrum shows significant differences from the absorption spectrum when both are determined under identical conditions of concentration and spectral bandwidth on the same instrument. The quantum yield of fluorescence decreases about 2-fold as the energy of excitation is increased beyond the 0-0' transition and follows the relation 1/φ°α E _excit..
This behavior is discussed in terms of (a) n π* and ππ* states, (b) emission from a minor tautomer and (c) kinetics of competing deactivation processes.     

LASER FLASH PHOTOKINETIC STUDIES OF ROSE BENGAL SENSITIZED PHOTODYNAMIC INTERACTIONS OF NUCLEOTIDES AND DNA

Abstract— Laser flash photolysis studies of the production of the triplet state of the xanthene dye, rose bengal (RB), have been carried out. The reactions of this state with oxygen to form singlet oxygen and the superoxide anion radical have been observed and yields measured. Quenching of RB(T₁) by oxygen leads to approximately 75% singlet oxygen and 20% superoxide. The reactivity of these species-RB(T₁), O₂(¹Δ_g) and O₂^-—with four nucleotides and DNA have been determined. Only guanine residues showed any noticeable reaction at neutral pH. At higher pH guanine rate constants increased. The consequences to biological photodynamic processes are discussed.     

PHOTOPHYSICS AND PHOTOCHEMISTRY OF 4-DIHYDROPYRIDINONES

Dihydropyridinones (DHP) react photochemically with olefins to form cycloaddition products. The reactions proceed through the lowest excited state T ₁ of the DHPs, with rate constants which depend on the olefin and can approach the diffusional limit in the most favourable cases. Intra-and inter-molecular sensitization and quenching have been investigated, as well as spectroscopic properties such as absorption and luminescence spectra, and in particular electron energy loss spectra which have established the energy of the reactive T₁, state as 295 kj mol^-1 (3.07 eV).     

THE HABER-WEISS CYCLE

Abstract— The Haber-Weiss cycle:
was investigated at low pH by radiolysis of oxygen or nitrogen saturated solutions of hydrogen peroxide. It was found that reaction 2 has a low rate constant: k ₂= 3.0 ± 0.6 M ^-1 s^-1 (pH 2.3, 22°C). The rate determining step of reaction 2 is most probably the transfer of an electron from a π₈* orbital of HO₂ to the empty _u* orbital of H₂O₂. Overlap between these two orbitals is hindered by the filled π₈* orbitals of H₂O₂. Fe(HI)EDTA catalyses reaction 2.     

ABSORPTION AND EMISSION STUDIES OF ELECTRONIC STATES OF N-ARYLBENZAMIDES

Abstract— Absorption and emission spectra of several N-arylbenzamides have been measured. The quantum yields for their fluorescence were found to be dependent on matrix viscosity and temperature. Singlet-triplet splittings for these compounds were determined from their emission spectra and found to be abnormally small for π. π* states (˜ 1500 cm^-1). Indeed, the phosphorescence maxima of N-arylbenzamides occur slightly to the blue relative to their fluorescence maxima. Intersystem crossing efficiencies were determined for several of these compounds and are consistent with S₁→ S ₀ radiationless decay.     

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.     

THERMAL DENATURATION OF MONOMERIC AND TRIMERIC PHYCOCYANINS STUDIED BY STATIC AND POLARIZED TIME-RESOLVED FLUORESCENCE SPECTROSCOPY

C. Phycocyanin (PC) and allophycocyanin (APC). as well as the α-subunit of PC. have been isolated from the blue-green alga (cyanobacterium). Spirulina platensis . The effects of partial thermal denaturation of PC and of its state of aggregation have been studied by ps time-resolved, polarized fluorescence spectroscopy. All measurements have been performed under low photon fluxes(10¹³ photons/pulse cm²) to minimize singlet-singlet annihilation processes. A complex decay is obtained under most conditions, which can be fitted satisfactorily with a bi-exponential (T₁=70–400 ps, T₂ -1000–3000 ps) for both the isotropic and the polarized part, but with different intensities and time constants for the two decay curves. The data are interpreted in the framework of the model first developed by Teale and Dale [ Biochem. J. 116, 161 (1970)], which divides the spectroscopically different chromophores in (predominantly) sensitizing ( s ) and fluorescing ( f ), ones. If one assumes temperature dependent losses in the energy transfer from the s to the f and between f chromophores. both the biexponential nature of the isotropic fluorescence decay and the polarization data can be rationalized. In the isotropic emission (corresponding to the population of excited states) the short lifetime is related to the sf transfer. the longer one to the "free" decay of the final acceptor( s ) (= f ). The polarized part is dominated by an extremely short decay time. which is related to sf transfer, as well as to resonance transfer between the f -chromophores.     

PICOSECOND LASER SPECTROSCOPY AND OPTICALLY DETECTED MAGNETIC RESONANCE ON A MODEL PHOTOSYNTHETIC SYSTEM

Fluorescence-detected magnetic resonance of triplets in zero magnetic field (FDMR), fluorescence fading (FF) due to triplet-formation, both at 4.2 K, and prompt fluorescence decay kinetics (FDK) at room temperature have been measured for free pheophorbide- a (f-Pheo) and bound (b-Pheo) to a synthetic polypeptide (L-L ys -L-A la -L-A la )_n, dissolved in dimethylformamide (DMF). Fluorescence decay kinetics measurements of f-Pheo in DMF yielded 1-5 ns lifetimes, for b-Pheo in DMF a ~ 50 ps decay-component was found emitting at 730–750 nm. Zero-field splitting parameters |D| and |E| of the lowest triplet state T₁ were determined from FDMR spectra as (337 and 24) 10^-4 cm^-1 for f-Pheo and (359 and 25) 10^-4 cm^-1 for b-Pheo, both in DMF. Decay rate constants of the three spin levels of T¹ of b-Pheo ( K _x= 1200 50 s^-1, k _y= 440 25 s^-1, k _z= 80 5 s^-1) and relative steady-state populations (N_x= 28 2%, N_y= 47 2%, N_z= 26 2%) determined from FF curves predict a fluorescence decrease at the D–E and D + E FDMR transitions, whereas experimentally a fluorescence increase is observed. The FDMR sign-inversion results from singlet-singlet energy transfer from b-Pheo monomers to their aggregates, followed by fast intersystem crossing to T₁. These results indicate that aggregates are formed by two or more b-Pheo molecules at different positions on the folded polypeptide chain. This situation resembles that in chlorophyll-proteins, containing low-lying traps, resulting from interaction of chromophores with other chromophores and with the protein environment.     

Energetics of Photoinduced Electron Transfer in the Indole-O2 Cluster in Gas Phase: Possible Consequences for Photoexcited Tryptophan in Solution

Abstract— A direct process for an activationless electron transfer from photoexcited tryptophan to molecular oxygen is proposed. By photodetachment of mass-selected indole-O_2- clusters in gas phase a neutral indole₊ O_2- charge-separated exciplex state is found at 2.25 0.2 eV above the neutral ground state. By theory also, the existence of an excited charge separated state at 3.05 0.2 eV is postulated. In gas phase both charge-separated cluster states are energetically below the first singlet states ₁L_b and ₁L_a and the lower even below the first triplet state T₁ of indole. In gas-phase clusters these energetics imply a very efficient quenching of photoexcited indole by fast electron transfer to oxygen. We discuss a similar mechanism for tryptophan-O₂ in aqueous environment and find it without activation barrier and presumably extremely fast. In the collisional tryptophan_*-O₂ complex the efficiency and the time scale of the charge transfer process should be mostly solvent independent. In polar solvent a complete charge separation and free superoxide formation are expected. We correlate this model with previous fluorescence and phosphorescence quenching data of excited tryptophan by O₂ and propose electron transfer to be the relevant process.     

ACTIVATED OXYGEN: SINGLET MOLECULAR OXYGEN AND SUPEROXIDE ANION

Abstract— Elusive processes associated with molecular oxygen in chemical and biological systems are interpreted in terms of two activated oxygen species, singlet molecular oxygen (¹Σ⁺_g/¹Δ_g) and superoxide anion (X²π_g). The generation and deactivation of singlet oxygen by interaction with organic triplet states are discussed within a comprehensive theoretical framework. Experimental results indicate the anomalous molecular oxygen enhanced luminescence from organic chromophores in polymer matrices results from the deactivation of singlet (¹Δ_g) oxygen by energy transfer to electronically excited states of the chromophore, and three types of oxygen enhanced luminescence have been identified in these systems. Properties of the superoxide anion relevant to its solution chemistry are briefly discussed. Electron transfer theory is used to theoretically examine the generation of singlet oxygen in disproportionation reactions of the superoxide anion, predicting that, depending on the number of water molecules present, the disproportionation reaction is a proficient source of singlet oxygen. A competing quenching process imposes a limit to the steady state concentration of singlet oxygen in most chemical systems. Available experimental results on the quenching of singlet oxygen by superoxide anion are in good agreement with theoretical results obtained via application of electron transfer theory.     

FORMATION AND CHEMILUMINESCENT DECOMPOSITION OF DIOXETANES IN THE GAS PHASE

Abstract— High resolution chemiluminescence spectra have been obtained of the singlet electronically excited products of O₂(¹Δ) plus alkene, dioxetane forming, reactions. The experiments were conducted in a flow apparatus at pressures of 1–5 torr. The spectra are a measure of the unrelaxed initial distribution of energy in the excited product. Results are reported for ethylene, 1, 1-difluoroethylene. methyl vinyl ether, ethyl vinyl ether, n -butyl vinyl ether, ketene, ketene-d₂, allene, unsymdimethyl allene, dimethyl ketene, 2-methoxy propene, 1-ethoxy propene, 2-bromo propene, and N, N- dimethyl isobutenyl amine. Chemiluminescence activation energies, representing the cycloaddition process, and absolute quantum yields for singlet excited product, ranging from 10^--⁴ to 2.5 × 10^--². are reported for 10 alkenes. Several of the reactions, 1,1-difluoroethylene, ketene, ethylene and allene give formaldehyde ¹ nπ* product with excess vibrational-rotational energy and a higher quantum yield than reactions not displaying this phenomenon. This is an indication of at least partially statistical partitioning of the energy in excess of that needed to electronically excite the formaldehyde. The experiments with ketene and ketene-d₂ provide the first evidence for the existence of unsubstituted 1,2-dioxetanone. The results from several of the experiments, particularly those with 2-methoxy propene and I-ethoxy propene are consistent with the mechanism of Goddard, which predicts regioselective and stereoselective attack of O₂(¹Δ) upon alkoxy substituted alkenes having allylic hydrogen.     

POTENTIAL PHOTOSENSITIZERS FOR PHOTODYNAMIC THERAPY—II. PHOTOPHYSICAL PROPERTIES OF [26] PORPHYRIN

Abstract— –Photophysical properties of [26] porphyrin (26 P) were investigated in chloroform. The quantum yields of fluorescence, of S₁→ T₁ intersystem crossing and singlet oxygen formation were measured. The purity, stability, the strong absorption in the red (δ_max= 783 nm; ε_max= 28 000 M ¹ cm^-1) and the ability of singlet oxygen formation recommend 26 P as potential photosensitizer for tumor therapy.     

THE PHOTOPHYSICS OF p-AMINOBENZOIC ACID

Abstract— The lowest-lying allowed UV transition in p -aminobenzoic acid (PABA) is assigned Γ→¹L_a based on quantitative absorption and fluorescence studies, as well as semiempirical PM3 multielec-tron configuration interaction calculations. The oscillator strengths, fluorescence quantum efficiencies and lifetimes are reported for PABA in several polar, nonpolar, protic and aprotic solvents (aerated) at 296 K. Reasonable agreement is found between the observed radiative rate constant and that calculated from the absorption and fluorescence spectra. Shifts in the absorption and fluorescence spectra in aprotic solvents are analyzed in terms of the Onsager reaction field model; results are consistent with an increase in dipole moment of ca 4 D between the relaxed S₀ and S₁, states. No evidence is found for the emission from the amino-twisted form of PABA in all solvents studied although calculations show that the amino-twisted S, state is highly polar, but higher in energy by ca 35 kJ/mol ( in vacuo ). The fluorescence efficiency is excitation wavelength independent in both methylcyclohexane and water. The temperature dependence of the nonradiative rate constant (from S₁) was studied in several solvents. Nonradiative decay may be due to intersystem crossing, which would be fast enough to compete with thermally activated intramolecular NH₂ twisting. The phosphorescence spectrum and lifetime obtained in an EPA glass at 77 K are reported, and the triplet energy of PABA is estimated.     

FREE RADICALS INDUCED BY PHOTOLYSIS OF FROZEN SOLUTIONS OF THYMIDINE-5'-MONOPHOSPHATE: AN ESR STUDY

Abstract— The photolysis (Δ < 220 nm) of thymidine-5'-monophosphate was studied by electron-spin resonance (ESR) in acidic and alkaline phases. In both cases, the H–addition radical at the C₆ position is detected at 77°K. At 225°K, a triplet 1:2:1 is observed, which suggests a H abstraction radical from the CH₃ group. When oxygen is present during irradiation, a peroxide–type radical is observed, which results partly from a reaction like R + O₂→ ROO and partly from an energy transfer from thymidine-5'-monophosphate to oxygen, probably in the ¹π₀ state.     

Chemically Initiated Electron Exchange Luminescence of Silyloxyaryl-Substituted Spiroadamantyl Dioxetanes: Kinetics and Excited State Yields

Abstract— The kinetics of the fluoride-induced decomposition of the thermally stable silyloxyaryl-substituted spiroadamantyl dioxetanes 1a,b and the excited state formation of this chemically initiated electron exchange luminescence (CIEEL) have been investigated. Two limiting kinetic regimes flash and glow have been identified, which depend on the fluoride concentration, the first at high, the second at low [F^-] triggering, whose detailed kinetic analysis affords the rate constants for the deprotected dioxetanes 2a,b cleavage in acetonitrile and dimethyl sulfoxide and chemiluminescence measurements the CIEEL and phen-olate 4 (CIEEL emitter) excitation yields. Chloro-substi-tution in the spiroadamantyl dioxetane does not affect the deprotection step k ₂ but leads to a ca five-fold faster cleavage of the deprotected dioxetane 2, while the chemiexcitation yield is the same for both dioxetanes. The energies of the first excited singlet and triplet states of the emitting phenolate 4 were estimated by AM1 configuration interaction calculations with explicit consideration of acetonitrile as solvent (self-consistent reaction field approach). The first excited singlet and triplet state of the CIEEL emitter phenolate 4 possess π,π* character, as suggested by the π-type molecular orbitals and the large singlet-triplet energy gap. The chemiexcitation of both singlet and triplet states of the excited phenolate 4 is feasible during the dioxetanes 1a,b cleavage, but the experimentally determined high singlet excitation yields suggest that preferentially the phenolate 4 singlet state is populated in the fluoride ion-triggered CIEEL process.     

HO2 ELIMINATION FROM α-HYDROXYALKYLPEROXYL RADICALS IN AQUEOUS SOLUTION

Abstract— In aqueous solutions α-hydroxyalkylperoxyl radicals undergo a spontaneous and a base catalysed HO₂ elimination. From kinetic deuterium isotope effects, temperature dependence, and the influence of solvent polarity it was concluded that the spontaneous reaction occurs via an HO₂ elimination followed by the dissociation of the latter into H⁺ and O₂^-. The rate constant of the spontaneous HO₂ elimination increases with increasing methyl substitution in α-position ( k (CH₂(OH)O₂) < 10s^-1 k (CH₃CH(OH)O₂) = 52s^-1 k ((CH₃)₂C(OH)O₂) = 665 s^-1). The OH^- catalysed reaction is somewhat below diffusion controlled. The mixture of peroxyl radicals derived from polyhydric alcohols eliminate HO₂ at two different rates. Possible reasons for this behaviour are discussed. The mixture of the six peroxyl radicals derived from d -glucose are observed to eliminate HO₂ with at least three different rates. The fastest rate is attributed to the HO₂ elimination from the peroxyl radical at C-l ( k > 7000s^-1). Because of the HO₂ eliminations the peroxyl radicals derived from d -glucose do not undergo a chain reaction in contrast to peroxyl radicals not containing an α-OH group. In competition with the first order elimination reactions the α-hydroxylalkylperoxyl radicals undergo a bimolecular decay. These reactions are briefly discussed.