SINGLET OXYGEN GENERATION BY FUROCOUMARIN TRIPLET STATES—I. LINEAR FUROCOUMARINS (PSORALENS)

Abstract— Triplet extinction coefficients and hence singlet → triplet intersystem crossing quantum yields have been measured in benzene for a number of linear furocoumarins including pseudopsoralen, 5, 8-dimethoxypsoralen, 4, 5', 8-trimethylpsoralen and 3-carbethoxypseudopsoralen. These triplet yields were then used in conjunction with the corresponding quantum yields of singlet oxygen formation, measured in oxygenated solution, to estimate the fractions of furocoumarin triplets which when quenched by ground state oxygen produce singlet excited oxygen, similar data being obtained for psoralen, 5-methoxypsoralen, 8-methoxypsoralen and 3-carbethoxypsoralen. The superoxide anion radical was not detected from these oxygen quenching reactions, nor was a contribution to the singlet oxygen yield found from furocoumarin excited singlet state quenching by oxygen. The fraction of furocoumarin-oxygen quenching interactions leading to singlet oxygen varied between 0.13 (for 5, 8-dimethoxypsoralen) and unity (for 3-carbethoxypsoralen), and thus needs to be taken into account, as well as the triplet quantum yields, in assessing photobiological processes involving singlet oxygen.     

Fluorescence of the antharacenes following Tn → T1 excitation studied by a double excitation method

The fluorescence from the lowest excited singlet state following excitation of the lowest triplet state was observed for anthracene, 9-methylanthracene, and 9-phenylanthracene in ethanol by a newly devised double excitation method which is essentially the combination of flash and laser photolysis. The quantum yield of intersystem crossing from the excited triplet state, T_n(n ? 2), to the lowest excited singlet state was markedly increased by methyl- and phenyl-substitution at the meso-position.     

THE PHOTOSENSITIZERS BENZOPHENOXAZINE and THIAZINES: COMPREHENSIVE INVESTIGATION OF PHOTOPHYSICAL and PHOTOCHEMICAL PROPERTIES

Eight differently substituted title dye compounds have been investigated regarding intersystem crossing, triplet state, fluorescence and singlet excited state pKa properties. In general, non-halogenated oxazines and thiazines as well as a mono bromooxazine show very low triplet quantum yields, phi tau (less than 0.03) and relatively long triplet lifetimes (approximately 40 microseconds) in acidic methanol. The phi tau data correlate well with known singlet oxygen yields. In basic methanol no triplet transient is observed but a significant yield of a ground state transient protonated (base dye) form is produced with a short lifetime, approximately 400 ns. Fluorescence can be seen simultaneously from both the excited base and the protonated base dye forms in basic methanol. For iodinated oxazine or thiazines, the triplet yield increases and can be as high as 0.5 (diiodo case) in acidic methanol. The triplet lifetimes are further shortened to approximately 10 microseconds compared to the non-iodinated derivatives above. The triplet yields of the iodo compounds are higher or equal to known singlet oxygen yields. In basic methanol triplet yields up to 0.2 can be seen, the triplet lifetime are shortened still further to 1 microsecond but no observable protonated form is produced (in distinction to the non-iodinated cases). Consideration is given to the correlation of triplet and singlet oxygen yields, ground and excited pKa properties, spin-orbit coupling and internal conversion properties, solvent effects, and phototherapeutic activity of these dyes.     

Electronic spectra and luminescence properties of 1,2-benzotetraphene in crystalline matrices at 77 K

Quasilinear absorption and luminescence spectra of 1,2-benzotetraphene were obtained in polycrystalline matrices at 77 K. Tne energies of successive excited singlet states as well as the energy of the lowest excited triplet state were found experimentally and compared with those calculated by the PPP CI method. The fluorescence lifetime and quantum yield were determined experimentally. Moreover, the radiationless transition probabilities, lifetime of triplet state and phosphorescence quantum yield were estimated employing the Siebrand-Williams model. The results obtained suggest that radiationless ISC processes are the main deactivation channel of the S₁ and T₁ states. The vibrational analysis of quasilinear absorption and luminescence spectra was performed and fundamental frequencies of ground and first excited singlet states were determined.     

Metal phthalocyanine-sensitized photoreduction of nitro-compound

Metal phthalocyanine-sensitized photoreduction of dimethyl 4-nitrophthalate with ascorbic acid has been investigated. The primary photoreaction products are the corresponding amino-and hydroxylamino-compounds. The azoxy-compound is formed by coupling of the nitrosocompound with hydroxylamino-compound in the presence of air through secondary dark reaction. The redox potential and fluorescence quantum yield are also determined. The variation of the quantum yield of the sensitized photoreduction, the relative fluorescence quantum yield and their product with the concentration of nitro-compound has been examined. The efficiency of photoreduction sensitized by the excited singlet and triplet state of metal phthalocyanine has been also calculated. It is believed that electron transfer from the excited metal phthalocyanine to the nitro-compound is the initial process in the sensitized photoreduction. Quenching by electron transfer involves creation of an ion pair. Charge separation and back electron transfer is then a competitive process. Due to the spin selection rules, the efficiency of photoreduction sensitized by excited triplet state of metal phthalocyanine is higher than excited singlet state. Thus, a necessary requirement for a good sensitizer is that the triplet state is populated in high yield. An alternative way and also the aim of our work is to design a suitable phthalocyanine skeleton to overcome geminate recombination of the ion pair, in order to increase the efficiency of photoreduction sensitized by sir glet excited state of the sensitizer, so as to increase the quantum yield of the total sensitized photoreduction.     

Absolute fluorescence quantum yields for vapour-phase benzene and naphthalene,and comments on the non-radiative processes

Optic—acoustic measurements have been employed in the determination of absolute quantum yields for benzene and naphthalene. Heat yields are measured by a method using oxygen quenching of both triplet and singlet states. For vibrationally relaxed excited singlet states the fluorescence quantum yields, φ^B_f, are 0.16 ± 0.02 and 0.79 ± 0.02 for benzene and naphthalene respectively. For 0.07 torr naphthalene at room temperature with 248 nm excitation, φ_f = 0.35 ± 0.03 and the quantum yield of internal conversion is less than 0.05. The decay of the highly vibrationally excited triplet state is dominated by vibrational relaxation for 0.07 torr naphthalene, but for benzene, even at high pressures, strong competition comes from an indirect coupling process to the ground state.     

取代锌酞菁的合成及光物理性质

取代锌酞菁的合成及光物理性质张先付,许慧君（中国科学院感光化学研究所,北京,１００１０１）关键词取代酞菁,合成,光物理性质,电荷转移癌症的光动力疗法及其机制是目前光医学、光生物学及光化学的前沿课题［１］。临床应用的光疗药物──血卟啉有一些难以克服的致...     

Photophysical properties of a 1,2,3,4,5,6-hexasubstituted fullerene derivative

The photophysical properties of a novel 1,2,3,4,5,6-hexasubstituted fullerene derivative (1) are examined in this study. In addition to the ground state absorption spectrum of 1, we report its triplet-triplet absorption spectrum and molar extinction coefficient (Deltae(T-T)), as well as the triplet quantum yield (PhiT), lifetime (tauT), and energy (ET). The saturation of a single six-member ring on the fullerene cage results in significant changes in the triplet state properties as compared to that of pristine C60. The triplet-triplet absorption spectrum shows a hypsochromic shift in long wavelength absorption, and both the triplet state lifetime and the triplet quantum yield are decreased. The triplet energy was found to be similar to that of C60. In addition, the quantum yield (PHI(delta)) of singlet oxygen generated by 1 was calculated and is found to be significantly less than in the case of C60.     

Femtosecond ground state recovery: measuring the intersystem crossing yield of polyspirobifluorene

Measurement of the quantum yield of triplet formation has been made for the prototypical conjugated polymer polyspirobifluorene in solution and solid state. An updated method has been described based on femtosecond time resolved ground state recovery following photoexcitation of the polymer. The two components to the recovery of the ground state due to the decay of the singlet and triplet excited states are clearly visible and from these it is possible to calculate Phi(T)=0.05+/-0.01 in solution, this gives k(isc)=5.4 x 10(7) s(-1) which compares favorably with other conjugated polymers. In polymer films an increased triplet yield of Phi(T)=0.12+/-0.02 is found to be independent of temperature, the increased yield is attributed to triplet recombination from charged states.     

Laser Flash Photolysis Studies of the UVA Sunscreen Mexoryl SX

Abstract The results of a nanosecond laser flash photolysis investigation of the UVA sunscreen Mexoryl* SX in various solvent environments and within a commercial sunscreen formulation are reported. To the best of our knowledge this is the first laser flash photolysis study of a commercial suncare formulation. In each of these environments kinetic UV-visible absorption measurements following nanosecond 355 nm laser excitation reveals a short-lived species with a solvent-dependent absorption maximum around 470–500 nm and a solvent-dependent lifetime of 50–120 ns. This transient absorption is attributed to the triplet state of Mexoryl* SX on the basis that it is quenched by molecular oxygen leading to the formation of singlet oxygen in acetonitrile. The singlet oxygen quantum yield (φ_Δ), determined by comparative time-resolved near-infrared luminescence measurements and extrapolated to the limit of complete triplet state quenching, is estimated as 0.09 ± 0.03 in acetonitrile. In aqueous solution the shorter triplet state lifetime combined with lower ambient oxygen concentrations precludes significant triplet state quenching. For the commercial sunscreen formulation there was no observable difference in the measured triplet lifetime between samples exposed to oxygen or argon, suggesting that the singlet oxygen quantum yield in such environments is likely to be orders of magnitude lower than that measured in acetonitrile.     

Photoinduced Electron Transfer‐based Halogen‐free Photosensitizers: Covalent meso‐Aryl (Phenyl,Naphthyl, Anthryl,and Pyrenyl) as Electron Donors to Effectively Induce the Formation of the Excited Triplet State and Singlet Oxygen for BODIPY Compounds

Pristine BODIPY compounds have negligible efficiency to generate the excited triplet state and singlet oxygen. In this report, we show that attaching a good electron donor to the BODIPY core can lead to singlet oxygen formation with up to 58 % quantum efficiency. For this purpose, BODIPYs with meso ‐aryl groups (phenyl, naphthyl, anthryl, and pyrenyl) were synthesized and characterized. The fluorescence, excited triplet state, and singlet oxygen formation properties for these compounds were measured in various solvents by UV/Vis absorption, steady‐state and time‐resolved fluorescence methods, as well as laser flash photolysis technique. In particular, the presence of anthryl and pyrenyl showed substantial enhancement on the singlet oxygen formation ability of BODIPY with up to 58 % and 34 % quantum efficiency, respectively, owing to their stronger electron‐donating ability. Upon the increase in singlet oxygen formation, the fluorescence quantum yield and lifetime values of the aryl‐BODIPY showed a concomitant decrease. The increase in solvent polarity enhances the singlet oxygen generation but decreases the fluorescence quantum yield. The results are explained by the presence of intramolecular photoinduced electron transfer from the aryl moiety to BODIPY core. This method of promoting T₁ formation is very different from the traditional heavy atom effect by I, Br, or transition metal atoms. This type of novel photosensitizers may find important applications in organic oxygenation reactions and photodynamic therapy of tumors.     

Triplet state spectroscopic studies on some 5,10,15,20-tetrakis(methoxyphenyl)porphyrins

Extensive triplet state spectroscopic investigations were carried out with a series of 5,10,15,20-tetrakis(methoxyphenyl)porphyrins. Triplet absorption spectra, triplet lifetime, triplet quantum yield and quantum yield for singlet oxygen production were determined with different absorption and emission techniques, using the frequency-doubled beam of a Nd:YAG laser. It has been found that these synthetic porphyrins are effective photosensitizers which can be used as model compounds to investigate the theoretical and instrumental aspects of PDT.     

PHOTOPHYSICAL PROPERTIES OF 3,3'-DIALKYLTHIACARBOCYANINE DYES IN HOMOGENEOUS SOLUTION

The photophysical properties of 3,3′-dialkylthiacarbocyanine iodides and chlorides were measured in various solvents. It was found that photoisomerization and fluorescence are the major contributors to the deactivation of the excited singlet state; intersystem crossing occurs with only a very low efficiency. In ethanol, a triplet yield of 0.004 and a singlet oxygen quantum yield of 0.002 were determined. The photophysical parameters of these dyes are not substantially influenced by the length of the alkyl chain or the size of the halide counterion. The substitution of an ethyl with an octadecyl-chain only slightly hinders photoisomerization, and the replacement of the chloride with an iodide reduces only marginally the fluorescence lifetimes and fluorescence quantum yields in chloroform. A significant external heavy-atom effect is observed using dibromoethane as a solvent: triplet and singlet oxygen yields increase7–10-fold, and the triplet lifetime decreases from 55 μs to 15 mUs.     

Ultrafast Decay of the Excited Singlet States of Thioxanthone by Internal Conversion and Intersystem Crossing

The experimental ultrafast photophysics of thioxanthone in several aprotic organic solvents at room temperature is presented, measured using femtosecond transient absorption together with high‐level ab initio CASPT2 calculations of the singlet‐ and triplet‐state manifolds in the gas phase, including computed state minima and conical intersections, transition energies, oscillator strengths, and spin–orbit coupling terms. The initially populated singlet ππ* state is shown to decay through internal conversion and intersystem crossing processes via intermediate nπ* singlet and triplet states, respectively. Two easily accessible conical intersections explain the favorable internal conversion rates and low fluorescence quantum yields in nonpolar media. The presence of a singlet–triplet crossing near the singlet ππ* minimum and the large spin–orbit coupling terms also rationalize the high intersystem crossing rates. A phenomenological kinetic scheme is proposed that accounts for the decrease in internal conversion and intersystem crossing (i.e. the very large experimental crescendo of the fluorescence quantum yield) with the increase of solvent polarity.     

Photochemistry of naphthalimide photoacid generators

The photophysical properties of a series of 1,8-naphthalimide photoacid generators were studied by steady state fluorescence and phosphorescence spectroscopy. Emission and excitation anisotropies, triplet quantum yields in polar and nonpolar solvent and photoacid generation were evaluated. The singlet excited state exhibits a low polarity and is strongly deactivated by an efficient intersystem crossing process. In protic solvent, a homolytic singlet cleavage of the N-O bond occurs and leads to the acid production. The existence of a triplet state close to the singlet state was clearly evidenced. The presence of close singlet excited states is supported by fluorescence anisotropy and picosecond laser spectroscopy experiments. Results of DFT calculations well confirm the experimental contentions and yield important information about the cleavage process involved in such compounds.     

Effect of sensitizer protonation on singlet oxygen production in aqueous and nonaqueous media

The yield of singlet molecular oxygen, O2(a(1)Delta(g)), produced in a photosensitized process can be very susceptible to environmental perturbations. In the present study, protonation of photosensitizers whose chromophores contain amine functional groups is shown to adversely affect the singlet oxygen yield. Specifically, for bis(amino) phenylene vinylenes dissolved both in water and in toluene, addition of a protic acid to the solution alters properties of the system that, in turn, result in a decrease in the efficiency of singlet oxygen production. In light of previous studies on other molecules where protonation-dependent changes in the yield of photosensitized singlet oxygen production have been ascribed to changes in the quantum yield of the sensitizer triplet state, Phi(T), and to possible changes in the triplet state energy, E(T), our results demonstrate that this photosystem can respond to protonation in other ways. Although protonation-dependent changes in the amount of charge-transfer character in the sensitizer-oxygen complex may influence the singlet oxygen yield, it is likely that other processes also play a role. These include (a) protonation-dependent changes in sensitizer aggregation and (b) nonradiative channels for sensitizer deactivation that are enhanced as a consequence of the reversible protonation/deprotonation of the chromophore. The data obtained, although complicated, are relevant for understanding and ultimately controlling the behavior of photosensitizers in systems with microheterogeneous domains that have appreciable pH gradients. These data are particularly important given the use of such bi-basic chromophores as two-photon singlet oxygen sensitizers, with applications in spatially resolved singlet oxygen experiments (e.g., imaging experiments).     

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.     

Absorption and emission spectra of isomeric tolunitriles

Absorption and emission characteristics ofo-, m- andp-tolunitriles in polar and non-polar solvents under different conditions have been investigated in detail. Solvatochromic shifts of band origin of these molecules in non-polar solvents show that their dipolemoments in the first excited singlet state are almost the same while its value in the second excited singlet is larger in the metathan in the para-isomer. Vibronic analyses of the low temperatures absorption, fluorescence and phosphorescence spectra of all the three molecules have provided evidence that these molecules are slightly distorted in the first excited singlet state while such distortion in the phosphorescence emitting triplet state is larger. The data on fluorescence and phosphorescence quantum yield and phosphorescence lifetime of the tolunitriles are reasonably interpreted as showing that in these molecules, particularly m-andp- tolunitriles, the internal conversion rate from the first excited singlet to the ground state is probably small and that the charge transfer character of the triplet state in thep-isomer is larger than that in the meta.     

Effect of pH on methylene blue transient states and kinetics and bacteria photoinactivation

We have measured directly by time-resolved spectroscopy the transient spectra and kinetics of the methylene blue (MB) excited singlet and triplet state as a function of pH from a few picoseconds to several microseconds. The data show that the acidic triplet state (3)MBH(2+) is the protonated analogue of the basic (3)MB(+). It is also shown that the singlet oxygen formation quantum yield is much higher in basic than in acidic media. The transient spectra and their kinetics suggest that because pH exerts a large influence in singlet oxygen and radical formation, it may also be important in bacteria inactivation. Therefore, we performed experiments, which showed that the rate of gram-positive and gram-negative bacteria inactivation at pH 9 is 3-25 times higher than the rate at pH 5.