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We have investigated the response of a number of small organic molecules to intense infrared femtosecond laser pulses to explore the potential for strong-field post-ionisation in bio-imaging applications. The experimental system has been optimised and characterised using the well-documented ionisation behaviour of xenon. Preliminary work has involved a systematic study of the ionisation?dissociation characteristic of gas phase molecules, to optimise the ionisation step itself. The efficiency of molecular ion and fragment ion production as a function of laser wavelength and intensity has been studied. Here, we report on the behaviour of the toluene molecule, which is typical of several other small organics we have studied. With 800-nm irradiation, a clear transition in behaviour with increasing laser power is observed, favouring molecular ion production. At longer laser wavelengths, the ratio of parent molecular ion to fragment ions is increased by more than one order of magnitude. The molecular ion signal for toluene is also significantly increased on switching from 800-nm to 1300-nm irradiation. We discuss these observations in the context of the underlying ionisation mechanisms.

Photoluminescence (PL) of Gd-doped silica in the ultraviolet (UV) is investigated. The efficient emission detected at 314 nm is due to the 6P 7/2 to 8S 7/2 transition of a 4f electron of the Gd3+ ion.

Two carotenoid pigments have been linked as axial ligands to the central silicon atom of a phthalocyanine derivative, forming molecular triad 1. Laser flash studies on the femtosecond and picosecond time scales show that both the carotenoid S1 and S2 excited states act as donor states in 1, resulting in highly efficient singlet energy transfer from the carotenoids to the phthalocyanine. Triplet energy transfer in the opposite direction was also observed. In polar solvents efficient electron transfer from a carotenoid to the phthalocyanine excited singlet state yields a charge-separated state that recombines to the ground state of 1.

Artificial photosynthetic antenna systems have been synthesized based on carotenoid polyenes and polymer-polyenes covalently attached to tetrapyrroles. Absorption of light in the blue/green region of the spectra excites the polyenes to their S2 state, and ultrafast singlet energy transfer to the tetrapyrroles occurs when the chromophores are in partial conjugation. The additional participation of other excited states of the polyene in the energy-transfer process is a requirement for perfect antenna function. Analogs of photosynthetic reaction centers consisting of tetrapyrrole chromophores covalently linked to electron acceptors and donors have been prepared. Excitation of these constructs results in a cascade of energy transfer/electron transfer which, in selected cases, forms a final charge-separated state characterized by a giant dipole moment (>150 D), a quantum yield approaching unity, a significant fraction of the photon energy stored as chemical potential, and a lifetime sufficient for reaction with secondary electron donors and acceptors. A new antenna-reaction center complex is described in which a carotenoid moiety is located in partial conjugation with the tetrapyrrole π-system allowing fast energy transfer (

A model photosynthetic antenna system consisting of a carotenoid moiety covalently linked to a purpurin has been prepared to study singlet-singlet energy transfer from a carotenoid to a cyclic tetrapyrrole. Ultrafast fluorescence upconversion measurements of the carotenopurpurin dyad and an unlinked reference carotenoid demonstrate that the fluorescent S 2 excited state of the carotenoid model has a lifetime of 150 ′ 3 fs, whereas the corresponding excited state of the carotenoid in the carotenopurpurin dyad is quenched to 40 ′ 3 fs. This quenching is assigned to energy transfer from the S 2 state to the purpurin with a 73 ′ 6% efficiency, which is in accord with the 67 ′ 4% quantum yield obtained by steady-state fluorescence excitation measurements. Concomitant with the decay of the carotenoid S 2 excited state, a single-exponential rise of the excited S 1 state of the purpurin moiety was observed at 699 nm with a time constant of 64 fs. However, the decay of the fluorescence anisotropy was faster at this wavelength (40 fs) and isotropic rise times as short as 44 fs were determined at other emission wavelengths. The lifetime of the Si state of the carotenoid (7.8 ps) was the same in both the carotenoid model and the dyad. Taken together, these results unequivocally demonstrate that the S 2 state of the carotenoid moiety is the sole donor state in this efficient singlet-singlet energy transfer process. The simple dyad described in this work mimics the ultrafast energy transfer kinetics found in certain naturally occurring pigment protein complexes and is thus able to reproduce the high electronic coupling needed for efficient energy transfer from an extremely short-lived energy donor state.

Sensitive optical detection of nitroaromatic vapours with diketo-pyrrolopyrrole thin films is reported for the first time and the impact of thin film crystal structure and morphology on fluorescence quenching behaviour demonstrated.

The solvent dependence of the steady-state and time-resolved fluorescence emission from the S2 (11Bu) state of all-trans-β-carotene was investigated in a range of polar and nonpolar solvents and in hexane/carbon disulfide mixtures. The steady-state absorption and fluorescence emission maxima in both polar and nonpolar solvents showed parallel shifts with increasing solvent polarizability, indicating that the emitting state is the S2 (11Bu) state. The lifetime of the S2 state was determined using the fluorescence upconversion technique, and lifetimes varying from 120 fs in quinoline to 177 fs in hexane were found. An intramolecular relaxation process occurring on a similar time scale was observed as a broadening in the reconstructed time-dependent emission spectra in hexane and as emission wavelength-dependent lifetimes in all the solvents studied. Correlations were observed between the S2 lifetimes, the π* solvatochromic parameter, and the ratio of the absorption fine structure. A correlation between the ...

Two closely related, rigidly linked porphyrin−naphthoquinone dyads have been prepared and studied using time-resolved fluorescence and absorption methods. Dyad 1, whose quinone carbonyl groups are relatively close to the porphyrin macrocycle, exhibits photoinduced electron-transfer rate constants that are virtually independent of solvent dielectric constant and temperature within the range 77−295 K. Dyad 2, which has a similar donor−acceptor linkage but whose quinone carbonyl groups are ∼2 A farther from the porphyrin, features photoinduced electron-transfer rate constants that decrease with decreasing solvent dielectric constant. Electron transfer in this molecule ceases at low temperatures. Photoinduced electron transfer in dyad 2 exhibits the usual dependence on free energy change and solvent reorganization observed in many similar porphyrin−quinone systems. The behavior of 1 may be attributed at least in part to the smaller separation of the porphyrin radical cation and the quinone radical anion, whic...

Two approaches to eliciting photoelectrochemical reactions from carotenoids are being pursued. One uses LB films of amphipathic carotenoids deposited on semiconducting electrodes which are immersed in electrolytes containing electron donors or acceptors. Photocurrents are observed and their action spectra implicate the excited carotenoid pigment, possibly in an aggregated form, as the photoactive species. An example of the second approach is a molecular dyad consisting of a carotenoid covalently linked to Gjo. Excitation of the carotenoid moiety generates a charge-separated species in high yield. More complex systems include a molecular mad (C-P-Q) containing a porphyrin moiety (P) linked to a quinone (Q) and to a carotenoid (C). Triads of this type have been incorporated unidirectionally into lipid bilayer membranes. Time-resolved fluorescence experiments reveal that excitation of C-P-Q in the membrane leads to electron transfer from 1P to give C-P'+-Q". A subsequent electron transfer from C to P" yields C'+-P-Q'-. In this species the carotenoid pigment acts as a molecular wire to conduct charge across the membrane. In the presence of appropriate cofactors, this system translocates protons across the bilayer lipid membrane and generates proton motive force.

Three dyad molecules, each consisting of a porphyrin (P) linked to a quinone (Q) through a rigid bicyclic bridge, have been prepared, and their photochemistry has been investigated using time-resolved fluorescence and absorption techniques. In all three molecules, photoinduced electron transfer from the porphyrin first excited singlet state to the quinone occurs with rate constants of approximately 10{sup 12} s{sup -1} in solvents ranging in dielectric constant from approximately 2.0 to 25.6 and at temperatures from 77 to 295 K. The transfer rate is also relatively insensitive to thermodynamic driving force changes up to 0.4 eV. This behavior is phenomenologically similar to photosynthetic electron transfer. The rapid rate of photoinduced electron transfer and its lack of dependence on environmental factors suggests that transfer is governed by intramolecular vibrations. Charge recombination of P{sup {center_dot}+} {sup -}Q{center_dot}, on the other hand, is substantially slower than charge separation and sensitive to both driving force and environmental conditions. Thus, by changing conditions, charge recombination rates can be varied over a wide range while photoinduced electron transfer rates are relatively unaffected. This suggests that rigid dyads of this general type may be useful building blocks for more complex molecular devices. 76 refs., 9 figs., 1more » tab.« less

Porphyrin-C60 dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels-Alder reaction. The porphyin singlet lifetimes of both the zinc (Pzn-C60) and free base (P-C60) dyads, determined by time-resolved fluorescence measurements, are ≦17 ps in toluene. This substantial quenching is due to singlet-singlet energy transfer to C60 The lifetime of Pzn-1C60 is -5 ps in toluene, whereas the singlet lifetime of an appropriate C60 model compound is 1.2 ns. This quenching is attributed to electron transfer to yield Pznbull;+-C60bull;-. In toluene, P-1C60 is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 ran and a lifetime of-10 μs was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C60 triplet. In the more polar benzonitrile, P-1C60 underoes photoinduced electron transfer to give P•+-C60bull;-. The electron transfer rate constant is −2 × 1011 s−1.

The first representative of a new class of porphyrins featuring a tetramethine bridge joining the 2 and 20 positions of the macrocycle has been prepared and found to undergo the Diels-Alder reaction, yielding structures in which functional groups are linked to the porphyrin macrocycle through a rigid, bicyclic bridge.

Time-resolved fluorescence and subpicosecond transient absorption experiments have been carried out on a triad molecular device (CPQ) comprising a porphyrin (P) covalently linked to an electron-donating carotenoid pigment (C) and a quinone (Q), which acts as an electron acceptor. Rate constants governing the intramolecular electron transfer processes originating from the excited singlet state of the porphyrin have been determined. Excitation of the porphyrin moiety of the triad in benzonitrile solution results in photoinduced electron transfer to give CP .+ Q .− ( k 1 =2.5 × 10 9 s −1 ) followed by rapid non-photochemical electron transfer to yield C .+  PQ .− ( k 3 ≈ 1 × 10 11 s −1 ). The intermediate CP .+ Q .− , also decays by charge recombination to the ground state with a rate constant k 2 of about 6 × 10 11 s −1 . Nanosecond laser flash experiments were used to measure a quantum yield of 0.13 for the long-lived (370 ns) charge-separated species C .+ PQ .− . Excitation of the carotenoid moiety of the triad results in singlet energy transfer to the attached porphyrin with a quantum yield of about 0.07.

A series of molecular pentads, each consisting of a porphyrin dyad (P-P) covalently linked to a carotenoid polyene (C) and a diquinone moiety (Q[sub A]-Q[sub B]), have been prepared, and the photochemical properties of these molecules have been studied using steady-state and transient absorption and emission spectroscopies. Each of the pentads undergoes photoinduced electron transfer from the C-P-[sup 1]P-Q[sub A]-Q[sub B] singlet state to yield the charge-separated state C-P-P[sup [sm bullet]+]-Q[sub A][sup [sm bullet][minus]]-Q[sub B]. Competing with charge recombination of this species are additional electron-transfer reactions operating in series and in parallel which converge on a final C[sup [sm bullet]+]-P-P-Q[sub A]-Q[sub B][sup [sm bullet][minus]] state. The electron-transfer rate constants and the quantum yields of the various charge-separated species are sensitive functions of the state energies and the electronic coupling between the porphyrin and diquinone moieties. One of the pentads undergoes photoinduced electron transfer to produce the final C[sup [sm bullet]+]-P-P-Q[sub A]-Q[sub B][sup [sm bullet][minus]] state with a quantum yield of 0.83 and a lifetime of 55 [mu]s. This example of an artificial photosynthetic reaction center preserves about half of the initial excited singlet state energy as chemical potential. Other pentads have charge-separation lifetimes of several hundred microseconds. 28 refs., 10more » figs., 1 tab.« less

A molecular tetrad consisting of a free-base porphyrin (P) linked to a carotenoid polyene (C) and a diquinone moiety (Q A –Q B ) has been synthesized, and its photochemistry has been investigated using time-resolved techniques. Excitation of the porphyrin moiety of the tetrad in dichloromethane solution is followed by photoinduced electron transfer to yield an initial C-P + -Q − A –Q B state, which is formed with arate constant of 2.3×10 9 s −1 and a quantum yield of 0.87. In chloroform, the rate is 4.1×10 9 s −1 and the quantum yield is 0.94. Transient absorption studies show that this state evolves by subsequent electron transfer pathways to a final C + -P-Q A –Q − B charge-separated state whose lifetime is 7.4 μs in dichloromethane and 740 ns in chloroform. The quantum yield of the final state is 0.49 in dichloromethane and 0.57 in chloroform. The yield of the final state is substantially higher than that in a related, previously-reported tetrad in spite of the fact that the quantum yield of the initial C-P + -Q − A –Q B species is lower. This fact is interpreted in terms of the rates of charge-separation reactions relative to those of charge recombination. It is shown that yields of charge separation in multicomponent molecules may be altered in a predictable fashion using the basic tenets of electron transfer theory.

Both steady-state and time-resolved luminescence measurements at 1270 nm indicate that, when illuminated, isolated reaction centres of Photosystem II can generate singlet oxygen, O 2 ( 1 Δ g ). The oxygen dependent component of the luminescence signal, which was measured in a D 2 O buffer, was quenched by either azide or H 2 O. Singlet oxygen was detected both when primary charge separation was functioning and after it had been inactivated, suggesting that 1 O 2 can be generated from triplet states formed by radical pair recombination (P680 chlorophyll triplet) and by intersystem crossing (accessory chlorophylls). Neither azide nor H 2 O was found to protect against light-induced oxygen dependent irreversible bleaching of chlorophyll and pheophytin of reaction centres. We suggest that the pool of singlet oxygen detected by steady-state luminescence at 1270 nm and quenched by azide and water is located in the bulk medium rather than in the protein matrix of the reaction centre, where the photodamage to pigments occurs, and the singlet oxygen is initially generated.

A series of molecular pentads, each consisting of a porphyrin dyad (P-P) covalently linked to a carotenoid polyene (C) and a diquinone moiety (Q[sub A]-Q[sub B]), have been prepared, and the photochemical properties of these molecules have been studied using steady-state and transient absorption and emission spectroscopies. Each of the pentads undergoes photoinduced electron transfer from the C-P-[sup 1]P-Q[sub A]-Q[sub B] singlet state to yield the charge-separated state C-P-P[sup [sm bullet]+]-Q[sub A][sup [sm bullet][minus]]-Q[sub B]. Competing with charge recombination of this species are additional electron-transfer reactions operating in series and in parallel which converge on a final C[sup [sm bullet]+]-P-P-Q[sub A]-Q[sub B][sup [sm bullet][minus]] state. The electron-transfer rate constants and the quantum yields of the various charge-separated species are sensitive functions of the state energies and the electronic coupling between the porphyrin and diquinone moieties. One of the pentads undergoes photoinduced electron transfer to produce the final C[sup [sm bullet]+]-P-P-Q[sub A]-Q[sub B][sup [sm bullet][minus]] state with a quantum yield of 0.83 and a lifetime of 55 [mu]s. This example of an artificial photosynthetic reaction center preserves about half of the initial excited singlet state energy as chemical potential. Other pentads have charge-separation lifetimes of several hundred microseconds. 28 refs., 10more » figs., 1 tab.« less

Our knowledge of the diverse peripheral light-harvesting complexes (LHCs) found in species of algae which contain chlorophyll (Chi) c as an accessory pigment is reviewed. Sequencing of genes encoding intrinsic LHCs with three putative transmembrane helices is proceeding rapidly in all groups, but basic biochemistry, particularly of the LHC components attached to PS I-enriched complexes, is currently neglected. All LHCs appear to have two different environments for Chi c, and the longer wavelength forms (> 640 nm) have their Qy transition in the plane of the membrane, or the long axis of the particle. A limited amount of time-resolved spectroscopic data has been interpreted as excluding Chi c as an intermediate in energy transfer between either phycobilins or carotenoids and Chi a, but this may be premature. All the light-harvesting proteins seem to be encoded by multigene families, and for diatoms and brown algae, many genes have been sequenced. This will allow the role of individual gene products in determining the adaptive responses to environmental variation to be appraised.

The first excited state of molecular oxygen, O2(1Δg), lying at 0.98 eV above the ground state, O2(3Σg-), is relatively long-lived as a consequence of the multiple forbiddenness of the O2(1Δg) → O2(3Σg-) transition. As a result, O2(1Δg-) shows high reactivity to organic substrates, and irreversible photo-oxidative damage may occur. A typical example is that singlet oxygen is believed to be the cytotoxic species generated in the photodynamic therapy (PDT) of cancer.

The fluorescence quantum yield of zinc porphyrin (ZnP) covalently linked to 9,10-bis(phenylethynyl)anthracene (AB) is strongly dependent upon the solvent properties. The bichromophoric system ZnP-AB exhibits 'normal' zinc porphyrin fluorescence in solvents that cannot coordinate to the central zinc atom. In contrast, if a Lewis base, such as pyridine, is added to a sufficiently polar solvent, the fluorescence is significantly quenched. Picosecond transient absorption measurements, in conjunction with fluorescence quenching and cyclic voltammetric measurements, suggest that the quenching mechanism is intramolecular electron transfer from ZnP to AB. The charge separated state, ZnP.+-AB(.-), has a lifetime of not more than 220 ps before recombining. If a secondary electron acceptor, iron(III) porphyrin (FeP), is covalently connected to the AB unit, a second electron transfer from AB(.-) to FeP occurs and the charge separated state, ZnP.+-AB-FeP.-, has a lifetime of at least 5 ns. This demonstrates that electron transfer might be sensitively tuned (switched on) by specific solvent effects. (C) 2001 Elsevier Science BN. All rights reserved.

Photoinduced electron transfer in donor-bridge-acceptor systems with zinc porphyrin (or its pyridine complex) as the donor and gold(III) porphyrin as the acceptor has been studied. The porphyrin moieties were covalently linked with geometrically similar bridging chromophores which vary only in electronic structure. Three of the bridges are fully conjugated pi -systems and in a fourth, the conjugation is broken. For systems with this bridge, the quenching rate of the singlet excited state of the donor was independent of solvent and corresponded to the rate of singlet energy transfer expected for a Forster mechanism. In contrast, systems with a rr-conjugated bridging chromophore show a solvent-dependent quenching rate that suggests electron transfer in the Marcus normal region. This is supported by picosecond transient absorption measurements, which showed formation of the zinc porphyrin radical cation only in systems with pi -conjugated bridging chromophores. On the basis of the Marcus and Rehm-Weller equations, an electronic coupling of 5-20 cm(-1) between the donor and acceptor is estimated for these systems. The largest coupling is found for the systems with the smallest energy gap between the donor and bridge singlet excited states. This is in good agreement with the coupling calculated with quantum mechanical methods, as is the prediction of an almost zero coupling in the systems with a nonconjugated bridging chromophore.

The deactivation pathways of the singlet excited state of a series of zinc or free-base donor porphyrins covalently linked by a bridge to a paramagnetic iron(III) chloride porphyrin acceptor have been studied. These donor-bridge-acceptor systems all share a similar geometry (25 Angstrom donor-acceptor center-to-center distance), but the bridges vary in electronic structure. In previously reported investigations of zinc/iron porphyrin systems, the fluorescence quenching of the donor has predominantly been assigned to electron transfer. However, for the porphyrin systems studied in this paper, we show that the dominant deactivation channels are enhanced intersystem crossing and singlet energy transfer. In both series, the intersystem crossing rate (S-1 --> T-1) of the donor moiety is almost doubled in the presence of a paramagnetic high-spin metal-porphyrin acceptor. The significant spectral overlap of the donor fluorescence and acceptor absorption in both series allows for efficient singlet energy transfer (Forster mechanism). Furthermore, the bridging chromophores mediate energy transfer and the enhancement is inversely dependent upon the energy gap between the donor and bridge excited states. Although Marcus theory predicts thermodynamically favorable electron transfer to occur in the systems investigated, the quenching rate constants were found to be independent of solvent polarity, and no charge-separated state could be detected, indicating very small electronic coupling for election transfer.

Previously, the spatial arrangement of the carotenoid and bacteriochlorophyll molecules in the peripheral light-harvesting (LH2) complex from Rhodopseudomonas acidophila strain 10050 has been determined at high resolution. Here, we have time resolved the energy transfer steps that occur between the carotenoid's initial excited state and the lowest energy group of bacteriochlorophyll molecules in LH2. These kinetic data, together with the existing structural information, lay the foundation for understanding the detailed mechanisms of energy transfer involved in this fundamental, early reaction in photosynthesis. Remarkably, energy transfer from the rhodopin glucoside S2 state, which has an intrinsic lifetime of ~120 fs, is by far the dominant pathway, with only a minor contribution from the longer-lived S1 state., Swedish Natural Science Research Council. European Science Foundation. Gatsby Charitable Trust. Biochemical Society. Kempe Foundation. International Human Frontiers of Science Program. Biotechnology and Biological Sciences Research Council

The light-harvesting antenna of purple bacteria are ideal complexes in which to study the energy transfer function of carotenoids. Light-harvesting complexes which accumulate a single type of carotenoid can be readily isolated and the carotenoid can be selectively excited in the blue-green spectral region. Furthermore, nature assembles a variety of purple bacteria antenna complexes, with a range of energy transfer efficiencies, by packing bacteriochlorophylls into spectroscopically distinct ring-like structures in close proximity to a range of carotenoids with different conjugation lengths (1).

Ultrafast singlet-singlet energy transfer with a ~60% yield has been directly measured from the S2 state of a carotenoid to a covalently-linked pyropheophorbide by fluorescence upconversion. Transient absorption measurements indicate there is no energy transfer from the S1 state of the carotenoid to the pyropheophorbide.

We demonstrate that pulse shaping of a broadband Ti:sapphire laser can result in almost an order of magnitude increase in the sensitivity and signal to background ratio (SBR) of multiphoton total internal reflection fluorescence (TIRF) microscopy. We produced transform-limited pulses of 15 fs duration at the sample, and observed a 8-fold enhancement in the fluorescence of CdSe/ZnS quantum dots via two-photon objective-type TIRF excitation. There was a concomitant 6-fold increase of the SBR upon compression of the pulse duration. Enhancement of non-linear evanescent imaging has recently been demonstrated using surface-plasmons [Opt. Express 17, 5987 (2009)] and structured substrates [Opt. Express 18, 23218 (2010)]. Our approach of ultrafast pulse shaping could be used alone or combined with these new methods to offer significant gains in image quality.

The Z–E-isomerisation of bilirubin upon excitation with visible light is a fundamental step in phototherapy of newborns with neonatal jaundice. Here we report results of an ultrafast optical spectr ...

The emission peaks and solvent-induced spectral shifts of the singlet oxygen transitions to the ground and first vibrational level of the ground triplet state of oxygen have been determined in a large number of solvents. The spectra were measured with a substantially higher accuracy than previously reported, by use of an FT spectrometer and a filtered white-light source.The results were interpreted in terms of the formation of van der Waals (vdW) complexes of singlet oxygen with four or six solvent molecules and existing theory satisfactorily accounts for the observed shifts in a number of solvents. For perfluorocarbons, no vdW complex is formed and it is suggested that weak long-range repulsive forces dominate, leading to small blue shifts in the emission peaks. The theory as used takes no account of specific interactions such as charge transfer, which could explain why the shifts predicted for the dispersion forces are smaller than the observed shifts in solvents of low ionisation potential. In solvents such as benzene, pyridine, toluene, hexachlorobutadiene, tetrachloroethene and iodopentafluorobenzene, charge-transfer interactions between all the solvent molecules and singlet oxygen are apparently important and lead to a further stabilisation of the complex.The bandwidths of the transitions did not correlate particularly well with the radiative rates, but did show a good correlation with the solvent spectral shifts. A number of other subtle solvent perturbations were observed. The ratio of the intensities of the (0–0) to (0–1) transitions showed a solvent dependence, as did the vibrational spacing of the ground triplet state of oxygen. These results indicate the existence of weak ground-state complexes and a relaxation of the poential-energy surfaces describing the complex states.

Ground-state, fluorescence, triplet-state and singlet-oxygen luminescence studies are reported for two purpurins, a metallo (SnIV) purpurin and a porphyrin tautomer, NTW, containing an unsaturated meso-substituent. Fluorescence lifetimes of the purpurins are short (ca. 2 ns) and the fluorescence yields are low ( 0.9) for NTW is suggested to be free rotation/isomerisation of the unsaturated meso-substituent. These results are discussed with respect to the possible role of purpurins, the tautomeric porphyrin, NTW, and the photo-oxidation products as sensitisers for photodynamic therapy of cancer.