Photophysical properties of diphenyl-2,3-dihydroxychlorin and diphenylchlorin

The photophysical properties of 5,15-diphenyl-2,3-dihydroxychlorin (DPCOH) and 5,15-diphenyl-chlorin (DPC) in organic solution were studied. Absorption, fluorescence, triplet state and photobleaching experiments are reported. The ground states of both compounds show strong absorbance in red region (lambda = 638 nm, epsilon = 35,000 M(-1) cm(-1) and lambda = 645 nm, epsilon = 42,000 M(-1) cm(-1), respectively) and the singlet excited states show low fluorescence quantum yields of 0.0802 and 0.150 in benzene and the lifetimes are 7.38 and 10.18 ns, respectively. Absorption spectra of the triplet states were also measured and they have nearly the same triplet state lifetimes of 53 micros (DPCOH) and 50 micros (DPC). The triplet quantum yields are 0.82 and 0.75, respectively. The data of photobleaching quantum yields show that the presence of oxygen does not significantly affect the photobleaching. All the results demonstrate that both diphenylchlorines are good candidates for second-generation photosensitizer in photodynamic therapy.     

THE PHOTOPHYSICAL PROPERTIES OF PORPHYCENES: POTENTIAL PHOTODYNAMIC THERAPY AGENTS*

Porphycene and a tetra-n-propyl derivative remained unaltered on irradiation in toluene at room temperature. Quantum yields of fluorescence, S T intersystem crossing, and singlet molecular oxygen sensitization, as well as lifetimes of the singlet and triplet excited states were measured. In view of their structural relationship to porphyrin, their high absorption above 620 nm, their stability towards photooxidation, and their high quantum yields of fluorescence and singlet oxygen sensitization, these compounds qualify as potential agents for tumor marking and photodynamic therapy.     

Synthesis,photophysical and photochemical properties of tetra- and octa-substituted gallium and indium phthalocyanines

The synthesis, photophysical and photochemical properties of the tetra- and octa-[4-(benzyloxyphenoxy)] substituted gallium(III) and indium(III) phthalocyanines are reported for the first time. The new compounds have been characterized by elemental analysis, IR, ¹H NMR spectroscopy and electronic spectroscopy. General trends are described for quantum yields of photodegredation, fluorescence quantum yields and lifetimes, triplet lifetimes and triplet quantum yields as well as singlet oxygen quantum yields of these compounds in dimethylsulfoxide (DMSO). Substituted indium phthalocyanine complexes (7b–9b) showed much higher quantum yields of triplet state and shorter triplet lifetimes, compared to the substituted GaPc derivatives due to enhanced intersystem crossing (ISC) in the former. The gallium and indium phthalocyanine complexes showed phototransformation during laser irradiation due to ring reduction. The singlet oxygen quantum yields (Φ_Δ), which give an indication of the potential of the complexes as photosensitizers in applications where singlet oxygen is required (Type II mechanism) ranged from 0.51 to 0.94. Thus, these complexes show potential as photodynamic therapy of cancer.     

Photophysical Properties of Tin Ethyl Etiopurpurin I (SnET2) and Tin Octaethylbenzochlorin (SnOEBC) in Solution and Bound to Albumin

Abstract— The photophysical characteristics of two second-generation PDT photosensitizers, tin ethyl etiopurpurin I (SnET₂) and tin octaethylbenzochlorin (SnOEBC), have been measured in homogeneous solution and when bound to bovine serum albumin (BSA). The ground state and triplet state absorption spectra have been characterized, as have triplet lifetimes and quantum yields for intersys-tem crossing, singlet oxygen formation and photobleaching. In total, these parameters provide a complete set of data that can be used to quantitatively compare the photosensitizing efficiencies of these molecules. The photo-bleaching quantum yield of SnET₂ is increased dramatically when it is bound to BSA, thus limiting the production of singlet oxygen at incident fluences above 1 J/cm². In contrast, the quantum yield of photobleaching of SnOEBC is at least an order of magnitude lower than that of SnET₂ under these conditions and does not significantly limit the photosensitization process for typical in vivo or in vitro fluences. This difference is expected to play a significant role in determining the relative photosensitizing ability of these compounds in vivo.     

PHOTOPHYSICAL AND PHOTOSENSITIZING PROPERTIES OF BENZOPORPHYRIN DERIVATIVE MONOACID RING A (BPD-MA)*

The photophysical properties of benzoporphyrin derivative monoacid ring A (BPD-MA), a second-generation photosensitizer currently in phase II clinical trials, were investigated in homogeneous solution. Absorption, fluorescence, triplet-state, singlet oxygen (O₂(¹Δ_g)) sensitization studies and photobleaching experiments are reported. The ground state of this chlorin-type molecule shows a strong absorbance in the red (λ≈ 688 nm, ?≈ 33 000 M^?1 cm^?1 in organic solvents). For the singlet excited state the following data were determined in methanol: energy level, E_s= 42.1 kcal mol^?1, lifetime, Φ_f= 5.2 ns and fluorescence quantum yield, Φ_f= 0.05 in air-saturated solution. The triplet state of BPD-MA has a lifetime, τ_f >. 25 ns, an energy level, E_T= 26.9 kcal mol^?1 and the molar absorption coefficient is ?_T= 26 650 M^?1 cm^?1 at 720 nm. A dramatic effect of oxygen on the fluorescence (φ_f) and intersystem crossing (φ_T) quantum yields has been observed. The BPD-MA presents rather high triplet (φ_T= 0.68 under N₂-saturated conditions) and singlet oxygen (φ_Δ= 0.78) quantum yields. On the other hand, the presence of oxygen does not significantly modify the photobleaching of this photostable compound, the photodegradation quantum yield (φ_Pb) of which was found to be on the order of 5 × 10^?5 in organic solvents.     

Towards tuning PDT relevant photosensitizer properties: comparative study for the free and Zn2+ coordinated meso-tetrakis[2,6-difluoro-5-(N-methylsulfamylo)phenyl]porphyrin

The spectroscopic, photochemical, and biological studies of 5,10,15,20-tetrakis[2,6-difluoro-5(N-methylsulfamylo)phenyl]porphyrinate Zn(II) (ZnF₂PMet) were carried out including absorption and fluorescence spectra, fluorescence quantum yields, triplet absorption spectra, triplet lifetimes, singlet oxygen quantum yield, and reactive oxygen species (ROS) detection under biological conditions and compared with its free-base analog (F₂PMet). Zinc coordination into the porphyrin ring results in decrease of hydrophobicity and in higher cellular uptake. F₂PMet localized specifically in endoplasmic reticulum and mitochondria while the ZnF₂PMet is more diffused all over the cell, bonded to membrane proteins, as assessed by fluorescence microscopy. Zn-porphyrin exhibits greater singlet oxygen quantum yield than its free-base analog. Studies with fluorescent probes confirm that the ZnF₂PMet produces mostly singlet oxygen, whereas F₂PMet generates more hydroxyl radicals as the ROS. F₂PMet is a more effective photosensitizer in vitro than its zinc complex, thus, the final photodynamic effect depends more on the nature of ROS than on the higher cellular uptake.     

On the triplet state of poly(N-vinylcarbazole)

Triplet state properties including transient triplet absorption spectrum, intersystem crossing yields in solution at room temperature and phosphorescence spectra, quantum yields and lifetimes at low temperature as well as singlet oxygen yields were obtained for poly(N-vinylcarbazole) (PVK) in 2-methyl-tetrahydrofuran (2-MeTHF), cyclohexane or benzene. The results allow the determination of the energy value for the lowest lying triplet state and also show that triplet formation and deactivation is a minor route for relaxation of the lowest excited singlet state of PVK. In addition, they show the triplet state is at higher energy than reported heavy metal dopants used for electrophosphorescent devices, such that if this is used as a host it will not quench their luminescence.     

Absorption and emission spectroscopic characterization of Ir(ppy)3

The absorption and emission spectroscopic behaviour of cyclometalated fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy)₃] is studied at room temperature. Liquid solutions, doped films, and neat films are investigated. The absorption cross-section spectra including singlet–triplet absorption, the triplet–singlet stimulated emission cross-section spectra, the phosphorescence quantum distributions, the phosphorescence quantum yields and the phosphorescence signal decays are determined. In neat films fluorescence self-quenching occurs, in diluted solid solution (polystyrene and dicarbazole-biphenyl films) as well as deaerated liquid solution (toluene) high phosphorescence quantum yields are obtained, and in air-saturated liquid solutions (chloroform, toluene, tetrahydrofuran) the phosphorescence efficiency is reduced by triplet oxygen quenching. At intense short-pulse laser excitation the phosphorescence lifetime is shortened by triplet–triplet annihilation. No amplification of spontaneous emission in the phosphorescence spectral region was observed indicating higher excited-state absorption than stimulated emission.     

Triplet formation of 4-thiothymidine and its photosensitization to oxygen studied by time-resolved thermal lensing technique

Excited-state dynamics of 4-thiothymidine (S4-TdR) and its photosensitization to molecular oxygen in solution with UVA irradiation were investigated. Absorption and emission spectra measurements revealed that UVA photolysis of S4-TdR gives rise to a population of T1(pipi*), following S2(pipi*) --> S1(npi*) internal conversion. In transient absorption measurement, the 355 nm laser photolysis gave broad absorption (380-600 nm) bands of triplet S4-TdR. The time-resolved thermal lensing (TRTL) signal of S4-TdR containing the thermal component due to decay of triplet S4-TdR was clearly observed by the 355 nm laser excitation. The quantum yield for S1 --> T1 intersystem crossing was estimated to be unity by a triplet quenching experiment with potassium iodide. In the presence of molecular oxygen, the photosensitization from triplet S4-TdR gave rise to singlet oxygen O2 (1Deltag) with a quantum yield of 0.50. Therapeutic implications of such singlet oxygen formation are discussed.     

QUANTUM YIELDS AND KINETICS OF THE PHOTOBLEACHING OF HEMATOPORPHYRIN, PHOTOFRIN II, TETRA(4-SULFONATOPHENYL)-PORPHINE AND UROPORPHYRIN

Porphyrins used as sensitizers for the photodynamic therapy (PDT) of tumors are progressively destroyed (photobleached) during illumination. If the porphyrin bleaches too rapidly, tumor destruction will not be complete. However, with appropriate sensitizer dosages and bleaching rates, irreversible photodynamic injury to the normal tissues surrounding the tumor, which retain less sensitizer, may be significantly decreased. This paper surveys the quantum yields and kinetics of the photobleaching of four porphyrins: hematoporphyrin (HP), Photofrin II (PF II), tetra(4-sulfonatophenyl)porphine (TSPP) and uroporphyrin I (URO). The initial quantum yields of photobleaching, as measured in pH 7.4 phosphate buffer in air, were: 4.7 x 10(-5), 5.4 x 10(-5), 9.8 x 10(-6), and 2.8 x 10(-5) for HP, PF II, TSPP and URO respectively; thus, the rates of photobleaching are rather slow. Low oxygen concentration (2 microM) significantly reduced the photobleaching yields. However, D2O increased the yields only slightly, and the singlet oxygen quencher, azide, had no effect, even at 0.1 M. Photosensitizing porphyrins in body fluids, cells and tissues may be closely associated with various photooxidizable molecules and electron acceptors and donors. Therefore, selected model compounds in these categories were examined for their effects on porphyrin photobleaching. A number inhibited and/or accelerated photobleaching, depending on the compound, the porphyrin and the reaction conditions. For example, 1.0 mM furfuryl alcohol increased the photobleaching yields of HP and URO more than 5-fold, with little effect on PF II or TSPP. In contrast, the electron acceptor, methyl viologen, increased the photobleaching yield of TSPP more than 10-fold, with little accelerating effect on the other porphyrins. These results suggest that the mechanism(s) of the photobleaching of porphyrin photosensitizers in cells and tissues during PDT may be complex.     

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.     

QUANTUM YIELDS AND KINETICS OF THE PHOTOBLEACHING OF HEMATOPORPHYRIN, PHOTOFRIN II, TETRA(4-SULFONATOPHENYL)-PORPHINE AND UROPORPHYRIN

Abstract Porphyrins used as sensitizers for the photodynamic therapy (PDT) of tumors are progressively destroyed (photobleached) during illumination. If the porphyrin bleaches too rapidly, tumor destruction will not be complete. However, with appropriate sensitizer dosages and bleaching rates, irreversible photodynamic injury to the normal tissues surrounding the tumor, which retain less sensitizer, may be significantly decreased. This paper surveys the quantum yields and kinetics of the photobleaching of four porphyrins: hematoporphyrin (HP), Photofrin II (PF II), tetra(4-sulfonatophenyOporphine (TSPP) and uroporphyrin I (URO). The initial quantum yields of photobleaching, as measured in pH 7.4 phosphate buffer in air, were: 4.7 × 10^-5, 5.4 × 10^-5, 9.8 × 10^-5, and 2.8 × 10^-5 for HP, PF II, TSPP and URO respectively; thus, the rates of photobleaching are rather slow. Low oxygen concentration (2 μM) significantly reduced the photobleaching yields. However, D₂O increased the yields only slightly, and the singlet oxygen quencher, azide, had no effect, even at 0.1 M. Photosensitizing porphyrins in body fluids, cells and tissues may be closely associated with various photooxidizable molecules and electron acceptors and donors. Therefore, selected model compounds in these categories were examined for their effects on porphyrin photobleaching. A number inhibited and/or accelerated photobleaching, depending on the compound, the porphyrin and the reaction conditions. For example, 1.0 mM furfuryl alcohol increased the photobleaching yields of HP and URO more than 5-fold, with little effect on PF II or TSPP. In contrast, the electron acceptor, methyl viologen, increased the photobleaching yield of TSPP more than 10-fold, with little accelerating effect on the other porphyrins. These results suggest that the mechanism(s) of the photobleaching of porphyrin photosensitizers in cells and tissues during PDT may be complex.     

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.     

Photophysics and photochemistry of new verdin compounds

Five different verdins, including one zinc metal chelate, were examined by laser flash techniques. Triplet molar absorption coefficients, triplet and singlet oxygen quantum yields and triplet lifetimes were determined. Zinc methyl pyroverdin (ZNMPV), copro II verdin trimethyl ester (CVTME) and deuteroverdin methyl ester (DVME) have the highest triplet and singlet oxygen quantum yields. ZNMPV and CVTME have the longest triplet lifetimes. Our data are consistent with singlet oxygen as the primary modality for phototherapy and it is suggested that DVME and CVTME may be useful agents.     

Aza‐BODIPY Derivatives: Enhanced Quantum Yields of Triplet Excited States and the Generation of Singlet Oxygen and their Role as Facile Sustainable Photooxygenation Catalysts

A new series of aza‐BODIPY derivatives ( 4 a – 4 c , 5 a , c , and 6 b , c ) were synthesized and their excited‐state properties, such as their triplet excited state and the yield of singlet‐oxygen generation, were tuned by substituting with heavy atoms, such as bromine and iodine. The effect of substitution has been studied in detail by varying the position of halogenation. The core‐substituted dyes showed high yields of the triplet excited state and high efficiencies of singlet‐oxygen generation when compared to the peripheral‐substituted systems. The dye 6 c , which was substituted with six iodine atoms on the core and peripheral phenyl ring, showed the highest quantum yields of the triplet excited state (Φ_T=0.86) and of the efficiency of singlet‐oxygen generation (Φ_Δ=0.80). Interestingly, these dyes were highly efficient as photooxygenation catalysts under artificial light, as well as under normal sunlight conditions. The uniqueness of these aza‐BODIPY systems is that they are stable under irradiation conditions, possess strong red‐light absorption (620–680 nm), exhibit high yields of singlet‐oxygen generation, and act as efficient and sustainable catalysts for photooxygenation reactions.     

Halogenated Squaraine Dyes as Potential Photochemotherapeutic Agents. Synthesis and Study of Photophysical Properties and Quantum Efficiencies of Singlet Oxygen Generation*

Abstract— We describe the synthesis and photophysical studies, including measurements of quantum yields of triplet excited states and singlet oxygen generation of bis(3,5-dibromo-2,4,6-trihydroxyphenyl)squaraine (2) and bis(3,5-diiodo-2,4,6-trihydroxyphenyl)squaraine (3). These dyes exist in solution in the protonated, neutral, single and double depro-tonated forms, depending on pH. The pK_a values of these dyes were found to be relatively lower than those of the parent bis(2,4,6-trihydroxyphenyl)squaraine (1). Only the single deprotonated forms (Sq) of 2 and 3 showed measurable fluorescence. In microheterogeneous media such as in the presence of β-cyclodextrin, cetyltrunethylammonium bromide and polyvinylpyrrolidone), bathochromic shifts in the absorption and emission spectra of Sq were observed with a substantial enhancement in their fluorescence yields. Triplet excited states are the main transient intermediates obtained upon 532 nm laser excitation of the various forms of 2 and 3 in methanol. These triplets have lifetimes in the range from 0.061 to 132 μs. The triplet quantum yields of double deprotonated forms are low (φT = <0.01), whereas the neutral and Sq^?forms of 2 (φr = 0.12 and 0.22) and 3 (φ_T= 0.24 and 0.5), respectively, exhibited significant triplet yields. Quantum yields of singlet oxygen generation by Sq^?forms of 2 and 3 were determined in methanol and were found to be 0.13 and 0.47, respectively, which are in good agreement with the triplet yields obtained in these systems.     

PHOTOPHYSICAL PROPERTIES OF A CHLORIN, A POTENT SENSITIZER FOR PHOTOCHEMOTHERAPY

Abstract— Photophysical and photodynamic properties of a ehlorin type molecule derived from hydroxyethylvinyldeuteroporphyrin are presented. It photosensitizes singlet oxygen production as efficiently as mesotetraphenylporphin. The high absorptions of both its ground and triplet states in the red (660 nm) make it a potent photosensitizer which might act not only by photo-oxidation via singlet oxygen but also by radicals produced via sequential biphotonic absorption.     

Investigation of photobleaching of hypocrellin B in non-polar organic solvent and in liposome suspension

Hypocrellin B (HB) is a natural pigment with a promising application in the photodynamic therapy (PDT) for anticancer treatment. The photobleaching of HB in non-polar organic solvents and in liposomes in aqueous solution were investigated by the measurements of absorption spectra, quenching experiments and determination of photoproducts. Control experiments indicated that the sensitizer, oxygen and light were all essential for the photobleaching of HB, which suggested that it was mainly self-sensitized photooxidation. The illumination of HB with visible light in aerobic non-polar solvent generated singlet oxygen efficiently [Phi(1O(2))=0.76] which then attacked the sensitizer HB with formation of an endoperoxide product. The endoperoxide of HB was unstable at room temperature and underwent predominantly loss of singlet oxygen with regeneration of parent HB. The singlet oxygen released from the endoperoxide of HB was detected with chemical trapping experiments. When HB was embedded in EPC liposomes, no endoperoxide product and no singlet oxygen release from the photobleaching process of HB were detected. The quenching experiments indicated that the singlet oxygen mechanism (type II) played an important role in the non-polar solvent and the free radical mechanism (type I) was predominant in liposomal aqueous solution for the photobleaching of HB.     

Synthesis and Characterization of Near‐Infrared Absorbing Water Soluble Squaraines and Study of their Photodynamic Effects in DLA Live Cells

Here, we report the synthesis, photophysical properties and photodynamic effects in DLA live cells of three water soluble squaraine dyes, viz. bisbenzothiazolium squaraine dyes SQMI and SQDI with iodine in one and both benzothiazolium units, respectively, and an unsymmetrical squaraine dye ASQI containing iodinated benzothiazolium and aniline substituents. The diiodinated SQDI showed an anomalous trend in both fluorescence and triplet quantum yields over the monoiodinated SQMI, with SQDI showing higher fluorescence and lower triplet quantum yields compared to SQMI. Nanosecond laser flash photolysis of SQDI and SQMI indicated the formation of triplet excited states with quantum yield of 0.19 and 0.26, respectively. On photoirradiation, both the SQDI and SQMI generate singlet oxygen and it was observed that both dyes undergoing oxidation reactions with the singlet oxygen generated. ASQI which exhibited a lower triplet quantum yield of 0.06 was, however, stable and did not react with the singlet oxygen generated. In vitro cytotoxicity studies of these dyes in DLA live cells were performed using Trypan blue dye exclusion method and it reflect an order of cytotoxicity of SQDI>SQMI>ASQI. Intracellular generation of the ROS was confirmed by dichlorofluorescein assay after the in vitro PDT.     

Photochemistry of kynurenine, a tryptophan metabolite: properties of the triplet state

Photolysis of aqueous kynurenine (KN) solutions results in the formation of triplet kynurenine TKN. In low pH solutions, triplet formation occurs with almost 100% efficiency, while in neutral solutions the triplet quantum yield is PhiT = 0.018 +/- 0.004. The dissociation constant of TKN, which is attributed to deprotonation of the anilino group, has a pKa value of 4.7. Similar triplet absorption spectra were obtained under direct and acetone-sensitized photolysis. The large difference in quantum yields as a function of pH is attributed to excited-state properties of the first excited singlet state of KN. The rate constant quenching for TKN by oxygen is kq = 2 x 10(9) M(-1) s(-1).