Generation of 1O2 from O2 quenching of the lowest excited singlet and triplet states of 9,10-diphenylanthracene

Measurements of the quantum yield for photoperoxidation of 9,10-diphenylanthracene as a function of dissolved oxygen concentration in toluene at 20°C. are consistent with generation of ¹O₂ from both the lowest singlet and triplet states of the hydrocarbon.     

EXCITED SINGLET AND TRIPLET PRODUCTS GENERATED FROM OXIDATION OF THE -CO-CH- CONTAINING MOLECULES

Abstract— In a previous study. we found indirect chemiluminescence from air oxidation of various simple ketones in alkaline aprotic solvents containing 9,10-diphenylanthracene and 9,10-dibromoanthracene. By summarizing the experimental results. it was concluded that excited products. mainly in the triplet state. were generated from air oxidation of those simple compounds having a -CO-CH-group via dioxetane intermediates.
Recently. we have succeeded in observing an intense direct chemiluminescence due to the generation of the excited singlet product by the air oxidation of such anthracene derivatives having a -CO-CH-group as 9,10-diisobutyrylanthracene and related compounds. The finding lends strong support to the hypothesis that dioxetane or dioxetanone may be the critical intermediate to give fluorescent singlet products.     

Photostabilization of ethylene-propylene-ethylidenenorbornene terpolymer by hindered amine compounds

The effect of bis(2,2,6,6-tetramethylpiperidinyl)sebacate (NH) and the stable bisnitroxyl radical (NO^·) arising from it, on the 9,10-diphenylanthracene (DPA) sensitized photo-oxidation of ethylene-propylene-ethylidenenorbornene terpolymer (EPNB) has been examined. It was found that the formation of hydroperoxide, which is a result of the photo-oxidation of EPNB, is inhibited by both compounds. The stabilizing action of the bisnitroxyl radical is considerably greater, however. The radical NO^· was found to be able to quench the singlet excited state of DPA. Both the compounds quench singlet oxygen, but the effectiveness of NO^· is greater. The radical NO^·, in its electronically excited state, induces hydroperoxide decomposition. The hydroxylamine formed in this process can participate in the inhibition of polymer photo-oxidation.     

Nanosecond pulse radiolysis investigation of solute excited states formation in 3-methylpentane at low temperatures

The influence of temperature down to 90 K, on the formation of excited states in the nanosecond pulse radiolysis of aromatic solutes (9,10-diphenylanthracene, naphthalene) in 3-methylpentane was investigated. Lowering the temperature decreases the yield of solute singlets and triplets generated during the pulse and extends the time range of excited state formation up to milliseconds. The mechanism of solute excited state formation in irridiated saturated hydrocarbons is discussed.     

Oxidation of dienes by triphenylphosphine peroxopalladium

Thermal reaction of 1,3-diphenylisobenzofuran and tetramethylcyclopentadienone with PdLO₂ complex (L = PPh₃) gives compounds identical to those produced by singlet molecular oxygen. Photochemical reaction of 1,9-diphenylanthracene with PdLO₂ or PdL₃ in the presence of oxygen gives the 9,10-endoperoxide adduct.     

The Photolysis Of The Endoperoxide Of 9,10-Diphenylanthracene

During the photolysis of the endoperoxide of 9,10-diphenylanthracene, two different reactions are observed, depending on the irradiation wavelength: (i) Excitation of the S₁ band causes a homolytic cleavage of the peroxide bridge with a quantum yield Q₂ = 0.08. (ii) Irradiation of the S₂ band leads to an adiabatic photocleavage of the endoperoxide into 9,10-diphenylanthracene and singlet molecular oxygen with a quantum yield Q₁ = 0.28. Both reaction pathways confirm the theory of Kearns and Khan concerning the photolysis of endoperoxides     

Photocatalysts based on aluminum and zinc phthalocyanines supported on modified montmorillonite

The efficiency of the sensitizing action of photocatalysts prepared from montmorillonite modified by a cationic surfactant and treated with aluminum and zinc phthalocyanines was studied. The rates and quantum yields of acceptor (9,10-diphenylanthracene) consumption upon irradiation with the visible light of poly(methyl methacrylate) films and dispersions in toluene containing the photocatalysts were determined. The quantum yields of singlet oxygen generation were estimated on the basis of these data. The influence of the medium and concentration of phthalocyanines on the spectral characteristics of the photocatalysts was considered.     

Photosensitized generation of singlet oxygen

This work gives an overview of what is currently known about the mechanisms of the photosensitized production of singlet oxygen. Quenching of pi pi* excited triplet states by O2 proceeds via internal conversion of excited encounter complexes and exciplexes of sensitizer and O2. Both deactivation channels lead with different efficiencies to singlet oxygen generation. The balance between the deactivation channels depends on the triplet-state energy and oxidation potential of the sensitizer, and on the solvent polarity. A model has been developed that reproduces rate constants and efficiencies of the competing processes quantitatively. Sensitization by excited singlet states is much more complex and hence only qualitative rules could be elaborated, despite serious efforts of many groups. However, the most important deactivation paths of fluorescence quenching by O2 are again directed by excess energies and charge-transfer interactions similar to triplet-state quenching by O2. Finally, two recent developments in photosensitization of singlet oxygen are reviewed: Two-photon sensitizers with particular application potential for photodynamic therapy and fluorescence imaging of biological samples and singlet oxygen sensitization by nanocrystalline porous silicon, a material with very different photophysics compared to molecular sensitizers.     

Delayed fluorescence from the lowest 1B+3u state of anthracene,due to hetero-triplet—triplet annihilation of 3anthracene* and 3xanthone*

The P-type delayed fluorescence (DF) S_i→S_o of aromatic compounds results from the population of excited singlet states S_i by triplet—triplet annihillation (TTA) of molecules in their lowest and metastable triplet state T₁ : T₁ + T₁

S_i + S_o; S_i may be any excited singlet state whose excitation energy E(S_i ? 2 E(T₁). TTA of unlike molecules A and B (hetero-TTA) may lead to excited singlet states either of A or of B. In particular, if E(T_A¹) < E(T₁^B), hetero-TTA may lead to excited singlet states S_k^A which are not accessible by TTA of 2 T₁^A. In the present paper we report the first example of the detection of the DF from a very short-lived upper excited singlet state S_k^A which has been populated by hetero-TTA. The systems investigated are liquid solutions of A = anthracene-h₁₀ or anthracene-d₁₀ or 9,10-dimethylanthracene and B = xanthone in 1,1,2-trichlorotrifluoroethane at 243 K. S_k^A is the lowest ¹B_3U⁺ state (B_b state) of anthracene.     

Low power upconversion using MLCT sensitizers

Selective low energy excitation of the metal-to-ligand charge transfer (MLCT) transition in [Ru(dmb)(3)](2+)(dmb = 4,4'-dimethyl-2,2'-bipyridine) in the presence of anthracene or 9,10-diphenylanthracene yields easily visualized upconverted singlet fluorescence resulting from triplet-triplet annihilation at low excitation power.     

Theoretical investigation of anthracene-9,10-endoperoxide vertical singlet and triplet excitation spectra

The electronic absorption spectrum of anthracene-9,10-endoperoxide (APO) has been investigated by means of multiconfigurational multi-state second order perturbation theory on complete active space self-consistent field wavefunctions (MS-CASPT2/CASSCF) and two single reference methods: time-dependent density functional theory (TD-DFT) and coupled cluster of second order (CC2). After testing several active spaces and basis sets, a CAS (14,12) active space together with an ANO-S basis set was found an appropriate choice to describe the vertical singlet and triplet electronic states of APO. Unfortunately, TD-DFT and CC2 methods cannot reproduce the MS-CASPT2 and experimental spectrum. Our MS-CASPT2//CASSCF(14,12)/ANO-S calculations predict a predominant pi*(OO)sigma*(OO) character for the lowest singlet excited state S(1) at 3.85 eV. Accordingly, the lowest singlet state of APO should be responsible for homolysis of the endoperoxide group. The next two absorbing excited states, experimentally proposed to be responsible for singlet oxygen production and therefore connected to the biological interest of APO, have been computed vertically at 4.34 and 4.59 eV and assigned to pi(CC)pi*(CC) and pi*(OO)pi*(CC) transitions, respectively. The vertical triplet electronic spectrum follows the singlet vertical spectrum ordering. The high density of triplet and singlet excited states of different nature within few eV points to the possibility of intersystem crossings between potential energy surfaces of different multiplicity.     

FEMTOSECOND STUDIES OF HEMATOPORPHYRIN DERIVATIVE IN SOLUTION: EFFECT OF pH AND SOLVENT ON THE EXCITED STATE DYNAMICS

Abstract We report direct femtosecond measurements of the excited state dynamics of hematoporphyrin derivative (HpD) in solution. The dynamics are found to be very sensitive to the solvent and pH of aqueous solutions. The decay of the excited singlet states is much faster in acidic and pH 7 buffer aqueous solutions (<230 ps) than in basic aqueous solutions or organic solvents (> 10 ns). The dynamical results show strong correlation with static fluorescence measurements: weaker fluorescence in acidic and pH 7 buffer solutions corresponding to shorter-lived excited states. A new fast decay component with a time constant around 5 ps is identified both in acidic aqueous solutions and in organic solvents such as acetone and attributed to internal conversion from the second to the first excited singlet state of aggregates or certain oligomers in HpD, in accord with the observation that the fast decay component is larger at a higher concentration. Oxygen is found to have no effect on the dynamics on the time scale investigated, 1 ns, indicating that oxygen quenching of the singlet excited states is insignificant on this time scale. The sensitive solvent and pH dependence of the excited state dynamics has important clinical implications in the use of HpD as a photosensitizing agent.     

Time-Resolved Studies of the Excited-State Dynamics of meso-Tetra(hydroxylphenyl)chlorin in Solution

Meso-tetra(hydroxyphenyl)chlorin (m-THPC) is a new photosensitizer developed for potential use in photodynamic therapy (PDT) for cancer treatment. In PDT, the accepted mechanism of tumor destruction involves the formation of excited singlet oxygen via intermolecular energy transfer from the excited triplet-state dye to the ground triplet-state oxygen. Femtosecond transient absorption measurements are reported here for the excited singlet state dynamics of m-THPC in solution. The observed early time kinetics were best fit using a triple exponential function with time constants of 350 fs, 80 ps and > or = 3.3 ns. The fastest decay (350 fs) was attributed to either internal conversion from S2 to S1 or vibrational relaxation in S2. Multichannel time-resolved absorption and emission spectroscopies were also used to characterize the excited singlet and triplet states of the dye on nanosecond to microsecond time scales at varying concentrations of oxygen. The nanosecond time-resolved absorption data were fit with a double exponential with time constants of 14 ns and 250 ns in ambient air, corresponding to lifetimes of the S1 and T1 states, respectively. The decay of the T1 state varied linearly with oxygen concentration, from which the intrinsic decay rate constant, ki, of 1.5 x 10(6) s-1 and the biomolecular collisional quenching constant, kc, of 1.7 x 10(9) M-1 s-1 were determined. The lifetime of the S1 state of 10 ns was confirmed by fluorescence measurements. It was found to be independent of oxygen concentration and longer than lifetimes of other photosensitizers.     

Interplay of computational chemistry and transient absorption spectroscopy in the ultrafast studies

The primary physical and chemical processes in the photochemistry of 1-(trideuteromethyl)-2,3,4-trideutero (1) and 1-acetoxy-2-methoxy-(2) 9,10-anthraquinones were studied using femtosecond transient absorption spectroscopy and computational chemistry. Excitation of 1 and 2 at 270 nm results in the population of a set of highly excited singlet states which decay within the laser pulse by internal conversion and vibrational energy redistribution. The transient absorption spectra of the lowest singlet and triplet excited states of substituted anthraquinones 1 and 2 as well as the triplet excited and ground states of the products were detected. The assignments of the transient absorption spectra were performed on the basis of quantum chemical calculations of the electronic absorption spectra of the intermediates. Time-dependent density functional theory or CASSCF/CASPT2 procedure were used to calculate the spectroscopic properties of the intermediates.     

Photoactivation mechanism of orthovanadate-like (V=O)O3 species

The photoluminescence spectrum and action spectrum for the photooxidation of orthovanadate-like (V=O)O₃ species exhibiting photoluminescence at 520 nm indicate that the triplet excited state T₁ of the orthovanadate-like species, which is formed from the singlet excited states S₁ and S₂ by intersystem crossing, is directly involved in the photooxidation of cyclohexane into cyclohexanone in the presence of molecular oxygen.     

ENERGY TRANSFER-ENHANCED CHEMILUMINESCENCE OF ADAMANTANONE (n,π*) AND ESTER (π,π*) SINGLET AND TRIPLET EXCITED STATES IN THE THERMOLYSIS OF SILYLOXYARYL-SUBSTITUTED SPIROADAMANTYL DIOXETANES†

Abstract— The thermal generation of singlet and triplet excited states from silyloxyaryl-substituted spiroadamantyl dioxetanes lab and the adamantylidineadamantane dioxetane (1c) was investigated by direct and enhanced chemiluminescence (CL). 9,10-Diphenylanthracene (DPA) and 9-fluorenone were used as energy acceptors in the singlet-singlet (S-S), naphthalene and europium chelate Eu(TTA)₃Phen (TTA = thenoyltrifluoroacetone, Phen = 1,10-phenanthroline) in the triplet-triplet (T-T) and 9,10-di-bromoanthracene (DBA) in triplet-singlet (T-S) energy transfer experiments. The direct chemiluminescence observed in the thermolysis of dioxetanes lab consisted of fluorescence derived from the singlet-excited adamantanones 2a,b. In the presence of naphthalene, selective T-S energy transfer with DBA (napthalene as quencher) displayed the adamantanone triplets 2a,b and with Eu(TTA)₃Phen (naphthalene as mediator) also the silyloxyaryl ester 3 triplets. From the Stern-Volmer constants (k_TNT_T⁰) the triplet lifetimes t⁰_t of these triplet state products were assessed. By using the Hastings-Weber standard, the total triplet excitation yield (φ^t) was estimated to be ca 20%. The energies of the first excited singlet and triplet states of the adamantanones 2a,b and the silyloxyaryl ester 3, the products of the thermally induced decomposition of dioxetanes la-c , were determined by semiempirical calculations (AMI-based configuration interaction), which included explicitly solvent effects on the excitation energies in terms of a self-consistent reaction field approach. The calculations revealed that the first excited singlet and triplet states of the adamantanones 2a,b are expectedly n,π*-type excitations while the silyloxyaryl ester 3 possesses π,π* character. The semiempirical computations suggest that excitation of the adamantanones 2a,b as well as the silyloxyaryl ester 3 is feasible in the thermolysis of the spiroadamantyl dioxetanes lab , which has been confirmed by the experimental energy transfer studies.     

Spectral and photochemical properties of tetraphenylporphyrin tetrasulfonate supported on modified montmorillonite nanoparticles

A photocatalyst that effectively sensitizes the oxidation of 9,10-diphenylanthracene in toluene under irradiation with visible light has been prepared by the treatment of cationic surfactant-modified montmorillonite nanoparticles with a solution of meso-tetrakis(4-sulfonatophenyl)porphyrin. Reaction quantum yields and singlet oxygen generation probability have been determined. The shifts and intensity changes observed for the absorption bands of meso-tetrakis(4-sulfonatophenyl)porphyrin in the catalyst as compared with aqueous solutions are associated with solvation effects. The meso-tetrakis(4-sulfonatophenyl)porphyrin triplet state has been detected using the laser photolysis technique, and the rate constant of its quenching by oxygen on the surface of nanoparticles has been measured.     

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.     

Photophysical Properties of Some Methyl-Substituted Angelicins: Fluorometric and Flash Photolytic Studies

This paper describes the results of a study of the photophysical properties of various methyl-angelicins (MA) in solvents of different polarity and proticity. The behavior of their excited singlet and triplet states was investigated by fluorometry and nanosecond laser flash photolysis. On the basis of semiempirical (ZINDO/S-CI) calculations and the solvent effect on the absorption and fluorescence properties, the lowest excited singlet state (S₁) is assigned to a partially allowed π, π* state. The close lying S₂ state is n,π* in nature. The efficiency of the decay pathways of S₁ (fluorescence, intersystem crossing and internal conversion) strongly depends on the energy gap between the S₁ and S₂ states consistent with the manifestation of “proximity effect.” Thus, MA in cyclohexane decay only through S₁→ S₀ internal conversion, while in acetonitrile and ethanol, where the n, π* state is located at higher energy, their fluorescence and intersystem crossing increase significantly. The lowest excited triplet states (T₁) were characterized in terms of their absorption spectra, decay kinetics, molar absorption coefficients and formation quantum yields. The interaction of T₁ MA with molecular oxygen leads to an efficient formation of singlet oxygen, as evidenced by the appearance of characteristic IR phosphorescence centered at 1269 nm.     

Effect of aza substitution on the photophysical and electrochemical properties of porphycenes: Characterization of the near-IR-absorbing photosensitizers 2,7,12,17-tetrakis(p-substituted phenyl)-3,6,13,16-tetraazaporphycenes

The need for new photodynamic-therapy photosensitizers has stimulated the search of new families of compounds absorbing strongly in the 700-900 nm range, the region where tissue is most transparent to radiation capable to induce the photodynamic effect. Using computational chemistry techniques, 3,6,13,16-tetraazaporphycenes were previously identified as interesting target candidates. This work reports on the photophysical and electrochemical properties of selected members of this new family of macrocycles. Compared to porphycenes, the tetra-aza counterparts show stronger absorption in the near-infrared, lower-lying singlet and triplet excited states, and substantially larger internal conversion quantum yield (Phi(IC) = 0.93). Energy transfer to oxygen is observed, which results in the formation of the cytotoxic species singlet oxygen. The process is found to be reversible, consistent with a triplet-energy value close to that of singlet oxygen.