Mechanism of quenching by oxygen of the excited states of ruthenium(II) complexes in aqueous media. Solvent isotope effect and photosensitized generation of singlet oxygen, O2(1Deltag), by [Ru(diimine)(CN)4]2- complex ions

In this study we report on the photophysical properties of some [RuL(CN)4](2-) complex ions where L = 2,2'-bipyridine (bpy), 5,5'-dimethyl-2,2'-bipyridine (dmb), 1,10-phenanthroline (phen), 1-ethyl-2-(2-pyridyl)benzimidazole (pbe), 2,2':6',2'-terpyridine (tpy) and [RuL3](2+) where L = bpy or phen. Measurements were carried out in H2O and D2O. The effect of the deuterium isotope effect on the lifetime of these complexes is discussed. It has also been found that the presence of cyano groups has a pronounced effect on the lifetime of the excited metal-to-ligand charge transfer ((3)MLCT) of these complexes. Quenching of the (3)MLCT states by oxygen is reported in H2O and D2O. The rate constants, k(q), for quenching of the (3)MLCT states of these ruthenium complex ions by molecular oxygen are in the range (2.55 to 7.01) x 10(9) M(-1) s(-1) in H2O and (3.38 to 5.69) x 10(9) M(-1) s(-1) in D2O. The efficiency of singlet oxygen, O2((1)Delta(g)), production as a result of the (3)MLCT quenching by oxygen, f(Delta)(T), is reported in D2O and found to be in the range 0.29-0.52. The rate constants, k(q)(Delta), for quenching of singlet oxygen by ground state sensitizers in D2O is also reported and found to be in the range (0.15 to 3.46) x 10(7) M(-1) s(-1). The rate constants and the efficiency of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition of a non-charge transfer (nCT) and a CT deactivation channel. nCT deactivation occurs from a fully established spin-statistical equilibrium of (1)(T1(3)Sigma) and (3)(T1(3)Sigma) encounter complexes by internal conversion (IC) to lower excited complexes that dissociate to yield O2((1)Delta(g)), and O2((3)Sigmag-). The balance between CT and nCT deactivation channels which is described by the relative contribution p(CT) of CT induced deactivation is discussed. The kinetic model proposed for the quenching of pi-pi* triplet states by oxygen can also be applied to the quenching of (3)MLCT states by oxygen.     

The effect of solvent polarity on the balance between charge transfer and non-charge transfer pathways in the sensitization of singlet oxygen by pipi triplet states

A large set of literature kinetic data on triplet (T(1)) sensitization of singlet oxygen by two series of biphenyl and naphthalene sensitizers in solvents of strongly different polarity has been analyzed. The rate constants and the efficiencies of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition of a non-charge transfer (nCT) and a CT deactivation channel. nCT deactivation occurs from a fully established spin-statistical equilibrium of (1)(T(1)(3)Sigma) and (3)(T(1)(3)Sigma) encounter complexes by internal conversion (IC) to lower excited complexes that dissociate to yield O(2)((1)Sigma(g)(+)), O(2)((1)Delta(g)), and O(2)((3)Sigma(g)(-)). IC of (1,3)(T(1)(3)Sigma) encounter complexes is controlled by an energy gap law that is generally valid for the transfer of electronic energy to and from O(2). (1,3)(T(1)(3)Sigma) nCT complexes form in competition to IC (1)(T(1)(3)Sigma) and (3)(T(1)(3)Sigma) exciplexes if CT interactions between T(1) and O(2) are important. The rate constants of exciplex formation depend via a Marcus type parabolic model on the corresponding free energy change DeltaG(CT), which varies with sensitizer triplet energy, oxidation potential, and solvent polarity. O(2)((1)Sigma(g)(+)), O(2)((1)Delta(g)), and O(2)((3)Sigma(g)(-)) are formed in the product ratio (1/6):(1/12):(3/4) in the CT deactivation channel. The balance between nCT and CT deactivation is described by the relative contribution p(CT) of CT induced deactivation calculated for a sensitizer of known triplet energy from its quenching rate constant. It is shown how the change of p(CT) influences the quenching rate constant and the efficiency of singlet oxygen formation in both series of sensitizers. p(CT) is sensitive to differences of solvent polarity and varies for the biphenyls and the naphthalenes as sigmoidal with DeltaG(CT). This quantitative model represents a realistic and general mechanism for the quenching of pipi triplet states by O(2), surpassing previous advanced models.     

Photosensitized generation of singlet oxygen from ruthenium(II) and osmium(II) bipyridyl complexes

Photophysical properties for a number ruthenium(II) and osmium(II) bipyridyl complexes are reported in dilute acetonitrile solution. The lifetimes of the excited metal to ligand charge transfer states (MLCT) of the osmium complexes are shorter than for the ruthenium complexes. Rate constants, kq, for quenching of the lowest excited metal to ligand charge transfer states by molecular oxygen are found to be in the range (1.1-7.7) x 10(9) dm3 mol(-1) s(-1). Efficiencies of singlet oxygen production, fDeltaT, following oxygen quenching of the lowest excited states of these ruthenium and osmium complexes are in the range of 0.10-0.72, lower values being associated with those compounds having lower oxidation potentials. The rate constants for quenching of the excited MLCT states, kq, are found to be generally higher for osmium complexes than for ruthenium complexes. Overall quenching rate constants, kq were found to give an inverse correlation with the energy of the excited state being quenched, and also to correlate with the oxidation potentials of the complexes. However, when the contribution of quenching due exclusively to energy transfer to produce singlet oxygen, kq1, is considered, its dependence on the energy of the excited states is more complex. Rate constants for quenching due to energy dissipation of the excited MLCT states without energy transfer, kq3, were found to show a clear correlation with the oxidation potential of the complexes. Factors affecting both the mechanism of oxygen quenching of the excited states and the efficiency of singlet oxygen generation following this quenching are discussed. These factors include the oxidation potential, the energy of the lowest excited state of the complexes and spin-orbit coupling constant of the central metal.     

Quantitative determination of 1sigma(g)+ and 1delta(g) singlet oxygen in solvents of very different polarity. General energy gap law for rate constants of electronic energy transfer to and from O2 in the absence of charge transfer interactions

The quenching of excited triplet states of sufficient energy by O2 leads to O2(1sigma(g)+) and O2(1delta(g)) singlet oxygen and O2(3sigma(g)-) ground-state oxygen as well. The present work investigates the question whether in the absence of charge transfer (CT) interactions between triplet sensitizer and O2 the rate constants of formation of the three different O2 product states follow a generally valid energy gap law. For that purpose, lifetimes of the upper excited O2(1sigma(g)+) have been determined in a mixture of 7 vol % benzene in carbon tetrachloride, in chloroform, and in perdeuterated acetonitrile. They amount to 1.86, 1.40, and 0.58 ns, respectively. Furthermore, rate constants of O2(1sigma(g)+), O2(1delta(g)), and O2(3sigma(g)-) formation have been measured in these three solvents for five pi pi* triplet sensitizers with negligible CT interactions. The rate constants are independent of solvent polarity. After normalization for the multiplicity of the respective O2 product state, the rate constants follow a common dependence on the excess energies of the respective product channels. This empirical energy gap relation describes also quantitatively the rate constants of quenching of O2(1delta(g)) by 28 carotenoids. Therefore, it represents in the absence of CT interactions a generally valid energy gap law for the rate constants of electronic energy transfer to and from O2.     

Cyclometalated iridium and platinum complexes as singlet oxygen photosensitizers: quantum yields, quenching rates and correlation with electronic structures

Photophysical properties are reported for a series of cyclometalated platinum and iridium complexes that can serve as photosensitizers for singlet oxygen. The complexes have the formula (C;N)(2)Ir(O;O) or (C;N)Pt(O;O) where C;N is a monoanionic cyclometalating ligand such as 2-(phenyl)pyridyl and 2-(phenyl)quinolyl, and O;O is the ancillary ligand acetylacetonate (acac) or dipivaloylmethane (dpm). Also examined were a series of (N;N)PtMe(2) complexes where N;N is a diimine such as 2,2'-bipyridyl. In general, the cyclometalated complexes are excellent photosensitizers for the production of singlet oxygen, while the (N;N)PtMe(2) complexes were ineffective at this reaction. Quantum yields of singlet oxygen production range from 0.9-1.0 for the cyclometalated Pt complexes and 0.5-0.9 for Ir complexes. Luminescence quenching and singlet oxygen formation of the Ir complexes occurs from a combination of electron and energy transfer processes, whereas the Pt complexes only react by energy transfer. For Ir complexes with low emission energy, physical deactivation of the triplet excited state becomes competitive with energy transfer to ground state dioxygen. The rates of singlet oxygen quenching for the complexes presented here are in the range 6 x 10(6)-2 x 10(7) M(-1) s(-1) for Pt complexes and 2 x 10(5)-2 x 10(7) M(-1) s(-1) for Ir complexes, respectively. Differences in the efficiency of both forming and quenching singlet oxygen between the Ir and Pt cyclometalates are believed to come about from the more exposed coordination geometry in the latter species.     

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.     

Heteroleptic copper(I) complexes coupled with methano[60]fullerene: synthesis, electrochemistry, and photophysics

Heteroleptic copper(I) complexes CuPOP-F and CuFc-F have been prepared from a fullerene-substituted phenanthroline ligand and bis[2-(diphenylphosphino)phenyl] ether (POP) and 1,1'-bis(diphenylphosphino)ferrocene (dppFc), respectively. Electrochemical studies indicate that some ground-state electronic interaction between the fullerene subunit and the metal-complexed moiety are present in both CuPOP-F and CuFc-F. Their photophysical properties have been investigated by steady state and time-resolved UV-vis-NIR luminescence spectroscopy and nanosecond laser flash photolysis in a CH2Cl2 solution and compared to those of the corresponding model copper(I) complexes CuPOP and CuFc and of the fullerene model compound F. Selective excitation of the methanofullerene moiety in CuPOP-F results in regular deactivation of the lowest singlet and triplet states, indicating no intercomponent interactions. Conversely, excitation of the copper(I)-complexed unit (405 nm, 40% selectivity) shows that the strongly luminescent triplet metal-to-ligand charge-transfer ((3)MLCT) excited state located at 2.40 eV is quenched by the carbon sphere with a rate constant of 1.6 x 10(8) s(-1). Details on the mechanism of photodynamic processes in CuPOP-F via transient absorption are hampered by the rather unfavorable partition of light excitation between the two chromophores. By determination of the yield of formation of the lowest fullerene triplet level through sensitized singlet oxygen luminescence in the NIR region, it is shown that the final sink of photoinduced processes is always the fullerene triplet. This can be populated via a two-step charge-separation charge-recombination process and a less favored (3)MLCT --> (3)C60 triplet-triplet energy-transfer pathway. In CuFc-F, both of the photoexcited copper(I)-complexed and fullerene moieties are quenched by the presence of the ferrocene unit, most likely via ultrafast energy transfer.     

Mechanisms of photoinitiated cleavage of DNA by 1,8-naphthalimide derivatives.

Using water-soluble 1,8-naphthalimide derivatives, the mechanisms of photosensitized DNA damage have been elucidated. Specifically, a comparison of rate constants for the photoinduced relaxation of supercoiled to circular DNA, as a function of dissolved halide, oxygen and naphthalimide concentration, has been carried out. The singlet excited states of the naphthalimide derivatives were quenched by chloride, bromide and iodide. In all cases the quenching products were naphthalimide triplet states, produced by induced intersystem crossing within the collision complex. Similarly, the halides were found to quench the triplet excited state of the 1,8-naphthalimide derivatives by an electron transfer mechanism. Bimolecular rate constants were < 10(5) M-1 s-1 for quenching by bromide and chloride. As expected from thermodynamic considerations quenching by iodide was 6.7 x 10(9) and 8.8 x 10(9) M-1 s-1 for the two 1,8-naphthalimide derivatives employed. At sufficiently high ground-state concentration self-quenching of the naphthalimide triplet excited state also occurs. The photosensitized conversion of supercoiled to circular DNA is fastest when self-quenching reactions are favored. The results suggest that, in the case of 1,8-naphthalimide derivatives, radicals derived from quenching of the triplet state by ground-state chromophores are more effective in cleaving DNA than reactive oxygen species or radicals derived from halogen atoms.     

The photophysics of monomeric bacteriochlorophylls c and d and their derivatives: properties of the triplet state and singlet oxygen photogeneration and quenching

Measurements of pigment triplet-triplet absorption, pigment phosphorescence and photosensitized singlet oxygen luminescence were carried out on solutions containing monomeric bacteriochlorophylls (Bchl) c and d, isolated from green photosynthetic bacteria, and their magnesium-free and farnesyl-free analogs. The energies of the pigment triplet states fell in the range 1.29-1.34 eV. The triplet lifetimes in aerobic solutions were 200-250 ns; they increased to 280 +/- 70 microseconds after nitrogen purging in liquid solutions and to 0.7-2.1 ms in a solid matrix at ambient or liquid nitrogen temperatures. Rate constants for quenching of the pigment triplet state by oxygen were (2.0-2.5) x 10(9) M-1 s-1, which is close to 1/9 of the rate constant for diffusion-controlled reactions. This quenching was accompanied by singlet oxygen formation. The quantum yields for the triplet state formation and singlet oxygen production were 55-75% in air-saturated solutions. Singlet oxygen quenching by ground-state pigment molecules was observed. Quenching was the most efficient for magnesium-containing pigments, kq = (0.31-1.2) x 10(9) M-1 s-1. It is caused mainly by a physical process of singlet oxygen (1O2) deactivation. Thus, Bchl c and d and their derivatives, as well as chlorophyll and Bchl a, combine a high efficiency of singlet oxygen production with the ability to protect photochemical and photobiological systems against damage by singlet oxygen.     

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.     

Involvement of type I and type II mechanisms in the linoleic acid peroxidation photosensitized by tiaprofenic acid

Analysis of the photomixtures resulting from irradiation of aqueous solutions of linoleic acid sensitized by tiaprofenic acid (TPA) or its major photoproduct (DTPA) by HPLC has shown the formation of all the four possible conjugated dienic hydroperoxides. According to laser flash photolysis experiments the rate constants for hydrogen abstraction from linoleic acid by the excited triplet states of TPA and DTPA are 2 x 10(5) and 3.2x 10(5) M(-1) s(-1), respectively. These data, together with the known rate constants for oxygen quenching of triplet (D)TPA and for the reaction of singlet oxygen with linoleic acid, show that the mechanism is mixed type I/type II. Finally, typical radical scavengers such as BHA and singlet oxygen quenchers such as DABCO and sodium azide are efficient quenchers of the triplet excited state of DTPA. This shows the risk of assigning mechanisms based on indirect 'evidences' using 'specific' additives.     

QUENCHING OF OXONINE EXCITED STATES BY EDTA VIA ELECTRON TRANSFER AND DEACTIVATION PROCESSES. A COMPARATIVE STUDY OF EXCITED SINGLET AND TRIPLET STATE REACTIVITY

The quenching of excited singlet oxonine by EDTA in aqueous solution leads mainly to deactivation of the dye to the ground state and, to a lesser extent, to electron abstraction. The rate constants for these processes have been measured and compared to those for the same reactions involving the oxonine triplet state. The rate constant of electron abstraction is about ten times greater via the singlet state than via the triplet state. However, the rate constant of deactivation to the ground state is 10³-10⁴ times greater for the excited singlet state than for the triplet state, so that the efficiency of electron transfer is much smaller for the singlet state.     

Photoinduced intramolecular electron transfer and triplet energy transfer in a steroid-linked norbornadiene-carbazole dyad

Bichromophoric compound 3 beta-((2-(methoxycarbonyl)bicyclo[2.2.1]hepta-2,5-diene-3-yl)carboxy)androst-5-en-17 beta-yl-[2-(N-carbazolyl)acetate] (NBD-S-CZ) was synthesized and its photochemistry was examined by fluorescence quenching, flash photolysis, and chemically induced dynamic nuclear polarization (CIDNP) methods. Fluorescence quenching measurements show that intramolecular electron transfer from the singlet excited state of the carbazole to the norbornadiene group in NBD-S-CZ occurs with an efficiency (Phi SET) of about 14 % and rate constant (kSET) of about 1.6 x 10(7) s-1. Phosphorescence and flash photolysis studies reveal that intramolecular triplet energy transfer and electron transfer from the triplet carbazole to the norbornadiene group proceed with an efficiency (TET + TT) of about 52 % and rate constant (kTET + kTT) of about 3.3 x 10(5) s-1. Upon selective excitation of the carbazole chromophore, nuclear polarization is detected for protons of the norbornadiene group (emission) and its quadricyclane isomer (enhanced absorption); this suggests that the isomerization of the norbornadiene group to the quadricyclane proceeds by a radical-ion pair recombination mechanism in addition to intramolecular triplet sensitization. The long-distance intramolecular triplet energy transfer and electron transfers starting both from the singlet and triplet excited states are proposed to proceed by a through-bond mechanism.     

ON THE MECHANISM OF QUENCHING OF SINGLET OXYGEN IN SOLUTION

Abstract— Bimolecular rate constants for the quenching of singlet oxygen O*₂(¹Δ_g), have been obtained for several transition-metal complexes and for β-carotene. Laser photolysis experiments of aerated solutions, in which triplet anthracene is produced and quenched by oxygen, yielding singlet oxygen which then sensitizes absorption due to triplet carotene, firmly establishes diffusion-controlled energy transfer from singlet oxygen as the quenching mechanism in the case of β-carotene. The efficient quenching of singlet oxygen by two trans-planar Schiff-base Ni(II) complexes, which have low-lying triplet ligand-field states, most probably also occurs as a result of electronic energy transfer, since an analogous Pd(II) complex and ferrocene, which both have lowest-lying triplet states at higher energies than the O*₂(¹Δ_g), state, quench much less effectively.     

PHOTOPHYSICAL PROPERTIES OF meso-TETRAPHENYLPORPHYRIN and SOME meso-TETRA(HYDROXYPHENYL)PORPHYRINS

Abstract— The o-, m-, and p-isomers of 5, 10, 15, 20- tetra(hydroxyphenyl)-porphyrin have been of recent interest as potential second-generation sensitisers in tumour phototherapy. Fluorescence spectroscopy, nanosecond laser flash photolysis and pulse radiolysis have been used to characterise the singlet and triplet excited states of tetraphenylporphyrin and the o, m-, and p-isomers of tetra(hydroxyphenyl)porphyrin. This has included evaluation of fluorescence yields and lifetimes, triplet spectra, lifetimes, oxygen quenching rate constants, extinction coefficients, and yields and singlet oxygen yields. Whilst the fluorescence quantum yields were low, the triplet yields were all 0.7 ± 10% and the singlet oxygen yields 0.6 ± 10%: all these parameters are in the ranges shown by other efficient porphyrin photosensitisers. The similar photophysical properties found for these compounds suggest that their differing tumour sensitising efficiencies are likely to be due to other factors.     

Change in spin state and enhancement of redox reactivity of photoexcited states of aromatic carbonyl compounds by complexation with metal ion salts acting as Lewis acids. Lewis acid-catalyzed photoaddition of benzyltrimethylsilane and tetramethyltin via photoinduced electron transfer

The lowest excited state of aromatic carbonyl compounds (naphthaldehydes, acetonaphthones, and 10-methylacridone) is changed from the n,pi triplet to the pi,pi singlet which becomes lower in energy than the n,pi triplet by the complexation with metal ions such as Mg(ClO(4))(2) and Sc(OTf)(3) (OTf = triflate), which act as Lewis acids. Remarkable positive shifts of the one-electron reduction potentials of the singlet excited states of the Lewis acid-carbonyl complexes (e.g., 1.3 V for the 1-naphthaldehyde-Sc(OTf)(3) complex) as compared to those of the triplet excited states of uncomplexed carbonyl compounds result in a significant increase in the redox reactivity of the Lewis acid complexes vs uncomplexed carbonyl compounds in the photoinduced electron-transfer reactions. Such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C-C bond formation between benzyltrimethylsilane and aromatic carbonyl compounds via the Lewis-acid-promoted photoinduced electron transfer. The quantum yield determinations, the fluorescence quenching, and direct detection of the reaction intermediates by means of laser flash photolysis experiments indicate that the Lewis acid-catalyzed photoaddition reactions proceed via photoinduced electron transfer from benzyltrimethylsilane to the singlet excited states of Lewis acid-carbonyl complexes.     

Synthesis and photophysical properties of silicon phthalocyanines with axial siloxy ligands bearing alkylamine termini

Eleven silicon phthalocyanines which can be grouped into two homologous series [SiPc[OSi(CH3)2(CH2)(n)N(CH3)2]2, n = 1-6 (series 1), and SiPc[OSi(CH3)2(CH2)3N((CH2)(n)H)2]2, n = 1-6 (series 2)] as well as an analogous phthalocyanine, SiPc[OSi(CH3)2(CH2)3NH2]2, were synthesized. The ground state absorption spectra, the triplet state dynamics, and singlet oxygen quantum yields of 10 of these phthalocyanines were measured. All compounds displayed similar ground state absorption spectral properties in dimethylformamide solution with single Q band maxima at 668 +/- 2 nm and B band maxima at 352 +/- 1 nm. Photoexcitation of all compounds in the B bands generated the optical absorptions of the triplet states which decayed with lifetimes in the hundreds of microseconds region. Oxygen quenching bimolecular rate constants near 2 x 10(9) M(-1) s(-1) were measured, indicating that energy transfer to oxygen was exergonic. Singlet oxygen quantum yields, phi(delta), were measured, and those phthalocyanines in which the axial ligands are terminated by dimethylamine residues at the end of alkyl chains having four or more methylene links exhibited yields near > or = 0.35. Others gave singlet oxygen quantum yields near 0.2, and still others showed singlet oxygen yields of <0.1. The reduced singlet oxygen yields are probably caused by a charge transfer quenching of the 1pi,pi* state of the phthalocyanine by interaction with the lone pair electrons on the nitrogen atoms of the amine termini. In some cases, these can approach and interact with the electronically excited pi-framework, owing to diffusive motions of the flexible oligo-methylene tether.     

Theoretical analysis of singlet and triplet excited states of nickel porphyrins

Local density and generalized gradient approximation time-dependent density functional methods have been used for calculation of the singlet and triplet excited states of nickel-porphine, Ni-tetraphenyloporphine, and Ni-octaethyloporphyrine. Special attention is paid to metal-ligand transitions and d-d transitions. It is shown that the lowest exited singlet states of the three compounds can be described as a transfer of an electron from the porphine ring to the d(x2-y2) orbital of the nickel atom. On the other hand, the lowest excited triplet state arises from promotion of an electron between two nickel d orbitals, an occupied d(z2) and an empty d(x2-y2). It is proposed that a rapid quenching of the excited singlet states is due to an ultrafast intersystem crossing between 1Eg)and 3Eg or 3B1g states.     

Ligand-Localized Triplet-State Photophysics in a Platinum(II) Terpyridyl Perylenediimideacetylide

The synthesis, electrochemistry, and photophysical behavior of a Pt(II) terpyridyl perylenediimide (PDI) acetylide (1) charge-transfer complex is reported. The title compound exhibits strong (ε ≈ 5 × 10(4) M(-1)cm(-1)) low-energy PDI acetylide-based π-π* absorption bands in the visible range extending to 600 nm, producing highly quenched singlet fluorescence (Φ = 0.014 ± 0.001, τ = 109 ps) with respect to a nonmetalated PDI model chromophore. Nanosecond transient absorption spectroscopy revealed the presence of a long excited-state lifetime (372 ns in 2-methyltetrahydrofuran) with transient features consistent with the PDI-acetylide triplet state, ascertained by direct comparison to a model Pt(II) PDI-acetylide complex lacking low-energy charge-transfer transitions. For the first time, time-resolved step-scan FT-IR spectroscopy was used to characterize the triplet excited state of the PDI-acetylide sensitized in the title compound and its associated model complex. The observed red shifts (～30-50 cm(-1)) in the C═O and C≡C vibrations of the two Pt(II) complexes in the long-lived excited state are consistent with formation of the (3)PDI acetylide state and found to be in excellent agreement with the expected change in the relevant DFT-calculated IR frequencies in the nonmetalated PDI model chromophore (ground singlet state and lowest triplet excited state). Formation of the PDI triplet excited state in the title chromophore was also supported by sensitization of the singlet oxygen photoluminescence centered at ～1275 nm in air-saturated acetonitrile solution, Φ((1)O(2)) = 0.52. In terms of light emission, only residual PDI-based red fluorescence could be detected and no corresponding PDI-based phosphorescence was observed in the visible or NIR region at 298 or 77 K in the Pt(II) terpyridyl perylenediimideacetylide.     

Dark and photoinduced interactions between Trolox, a polar-solvent-soluble model for vitamin E, and riboflavin

The aerobic riboflavin (Rf)-sensitized visible-light irradiation of Trolox (TX), a polar-solvent-soluble model for vitamin E, has been studied employing stationary photolysis, polarographic detection of oxygen uptake, stationary and time-resolved fluorescence spectroscopy, and laser flash photolysis. Results indicate that in methanolic solution, no dark complexation exists between Rf and TX. The latter quenches singlet and triplet states of Rf, with rate constants of 6.2 x 10(9) M(-1) s(-1) and 4.7 x 10(9) M(-1) s(-1), respectively. The photodecomposition of Rf, a known process taking place from triplet Rf, has been found to depend on the concentration of dissolved TX: at >/=30 mM very slight Rf photodecomposition occurs due to the massive quenching of excited singlet Rf, while at TX concentrations < or =1 mM triplet Rf is photogenerated and subsequently quenched either by oxygen, giving rise to O(2)((1)Delta(g)), or by TX, yielding semireduced Rf through an electron transfer process. Complementary experiments performed in pure water employing superoxide dismutase and sodium azide inhibition of the oxygen uptake, in coincidence with flash photolysis data, indicate that superoxide anion and singlet molecular oxygen are generated, likely by the reaction of the anion radical from Rf with dissolved oxygen, also yielding neutral, ground state Rf or by energy transfer from triplet Rf to ground-state oxygen, respectively. The final result is that both TX and Rf are photodegraded, likely through oxidation with activated oxygen species. In the absence of oxygen no degradation of TX can be detected, but Rf photodegradation is favoured because Rf regeneration is avoided.