A. L. Moore

Making electrical contacts to molecular monolayers

Electrical contacts between a metal probe and molecular monolayers have been characterized using conducting atomic force microscopy in an inert environment and in a voltage range that yields reversible current-voltage data. The current through alkanethiol monolayers depends on the contact force in a way that is accounted for by the change of chain-to-chain tunnelling with film thickness. The electronic decay constant, βN, was obtained from measurements as a function of chain length at constant force and bias, yielding βN = 0.8±0.2 per methylene over a ±3 V range. Current-voltage curves are difficult to reconcile with this almost constant value. Very different results are obtained when a gold tip contacts a 1,8-octanedithiol film. Notably, the current-voltage curves are often independent of contact force. Thus the contact may play a critical role both in the nature of charge transport and the shape of the current-voltage curve.

Carotenoporphyrins as selective photodiagnostic agents for tumours

The covalent binding of a carotene moiety to one phenyl ring and meso-tetraphenyl-substituted porphyrins (see Figure 1) efficiently quenches the photosensitising activity of the porphyrin while a relatively large yield of fluorescence emission around 650 nm is retained. Pharmacokinetic studies performed with two carotenoporphyrins (CPs) and the corresponding porphyrins (Ps) in Balb/c mice bearing an MS-2 fibrosarcoma show that the two Ps give a high selectivity of tumour localisation (tumour/peritumoral tissue ratios of dye concentration ranging between c. 30 and 90 at 24 h after injection of 4.2-8.4 mumol kg-1 in a Cremophor emulsion) and photosensitive tumour necrosis upon red light irradiation. For the same injected doses, the two CPs show no tumour-photosensitising activity even though they localise in the tumour in concentrations of the order of 10-40 micrograms g-1 at 24 h with tumour/peritumoral ratios larger than 10. Thus, the fluorescence emitted by these CPs in the tumour can be used for photodiagnostic purposes with no risk of skin photosensitisation. However, this approach is presently limited by the large accumulation and prolonged retention of the CPs in the liver and spleen.

Laser-induced fluorescence in malignant and normal tissue in mice injected with two different carotenoporphyrins.

Laser-induced fluorescence (LIF) was used to characterise the localisation of an intravenously administered trimethylated carotenoporphyrin [CP(Me)3] and a trimethoxylated carotenoporphyrin [CP(OMe)3] in an intramuscularly transplanted malignant tumour (MS-2 fibrosarcoma) and healthy muscle in female Balb/c mice, 3, 24, 48 and 96 h post injection. The fluorescence was induced with a dye laser pumped by a nitrogen laser, emitting light at 425 nm. The fluorescence spectra were recorded in the region 455-760 nm using a polychromator equipped with an image-intensified CCD camera. The tumour/peritumoral muscle ratio was about 5:1 for CP(Me)3 and about 6:1 for CP(OMe)3 in terms of the background-free fluorescence intensity, which peaked at about 655 nm. By including the endogenous tissue fluorescence, the contrast was further enhanced by a factor of approximately 2.

Energy transfer and spin polarization of the carotenoid triplet state in synthetic carotenoporphyrin dyads and in natural antenna complexes

A series of carotenoporphyrin dyads, in which the carotenoid is covalently linked to a tetraarylporphyrin at the ortho, meta or para position of one of the meso aromatic rings, has been studied using Time-Resolved Electron Paramagnetic Resonance (TREPR) spectroscopy. In parallel, an investigation has been carried, on two different photosynthetic antenna systems, the B800–B850 complex ofR. acidophila and the LHCII complex of higher organisms. The initial spin polarization of the carotenoid triplet-state, populated indirectly by laser excitation, has been detected. It has been demonstrated that the initial polarization is not a characteristic property of the carotenoid triplet-state, as previously stated, but depends on the donor-acceptor mutual orientation. The triplet energy transfer to the carotenoid from a chlorophyll or porphyrin triplet state is discussed on the basis of the observed spin polarization.

Laser-induced fluorescence studies of the biodistribution of carotenoporphyrins in mice.

The biodistribution of two recently developed tumour markers, trimethylated (CP(Me)3) and trimethoxylated (CP(OMe)3) carotenoporphyrin, was investigated by means of laser-induced fluorescence (LIF) after i.v. injection into 38 tumour-bearing (MS-2 fibrosarcoma) female Balb/c mice. At 3, 24, 48 or 96 h after administration, the carotenoporphyrin fluorescence was measured in tumoral and peritumoral tissue, as well as in the abdominal, thoracic and cranial cavities. The fluorescence was induced by a nitrogen laser-pumped dye laser, emitting light at 425 nm, and analysed by a polychromator equipped with an image-intensified CCD camera. The fluorescence was evaluated at 490, 655 and 720 nm: the second and third wavelengths represent the carotenoporphyrin (CP)-related peaks, whereas the first one is close to the peak of the tissue autofluorescence. The tumour and the liver were the two tissue types showing the strongest carotenoporphyrin-related fluorescence, whereas the cerebral cortex and muscle consistently exhibited weak substance-related fluorescence. In most tissue types, the fluorescence intensities decreased over time. A few exceptions were observed, notably the liver, in which the intensity remained remarkably constant over the time period investigated.

Photosynthesis mimics as molecular electronic devices

As illustrated in this brief survey, the field of artificial photosynthesis has developed to the point where molecular scale photovoltaics giving high quantum yields of long-lived, energetic charge separated states can be designed using the principles governing natural photosynthesis, and then synthesized and studied. In contrast to the large progress in this area, the study of how to incorporate such devices into electrical circuits is still in its infancy. Finally, these same principles may be employed in the design of relatively sophisticated optoelectronic switches and logic elements. Although only a few prototypes have as yet been reported. The field is ripe for further development.<<ETX>>

Influence of Tumour Depth, Blood Absorption and Autofluorescence on Measurements of Exogenous Fluorophores in Tissue

We investigated the influence of tumour depth and differences in blood concentration and autofluorescence between tumour and normal tissue on the fluorescence of a tumour-localising agent. Carotenoporphyrin, CP(Me)3, was injected into rats and nude mice with intradermal tumours. On the tumours an incision was made, uncovering 2 mm2 of the tumour, and fluorescence measurements, including excitation-emission maps and fluorescence ratios, were made on skin, covered and uncovered tumour. The measured fluorescence ratio in the uncovered tumour showed a three- to tenfold increase compared to the covered tumour. We used a one-dimensional layered tissue model to analyse the data. In conclusion, even with a high tumour-selectivity deeper lying tumours cannot always be detected, particularly if the tumour has a high blood concentration or low autofluorescence intensity.

Model systems for photosynthesis acting as photoinduced molecular wires in bilayers

Abstract Sequential electron transfer are the key steps which determine the efficiency of the energy conversion processes in photosynthesis. An attempt at modelling the active natural reaction centers in the membrane environment is presented. Various triads and one tetrad, made up of electron donor and electron acceptor molecules, have been synthesized and incorporated into lipid bilayers. Photoelectrochemical measurements evidence a vectorial intramolecular and transmembrane electron transfer. The influence of the structure of the compounds on the photocurrents is discussed.

Porphyrin and pyropheophorbide phosphorescence in synthetic molecules that mimic photosynthetic triplet energy transfer

Abstract Phosphorescence emission and excitation spectra and triplet lifetimes at 77 K have been obtained for a carotenoid-porphyrin-pyropheophorbide triad model for the photosynthetic triplet energy transfer relay and seven porphyrin and pyropheophorbide model compounds including methyl pyropheophorbide- a . The results confirm that triplet energy from the pyropheophorbide moiety of the triad is transferred to the carotenoid by a triplet energy relay involving endergonic transfer from the pyropheophorbide to the porphyrin, whose triplet is rapidly quenched by the carotenoid polyene.

Nitroxyl free radical enhancement of the forbidden O2(3Σ−g) ← O2(1Δg) radiative transition in chlorinated hydrocarbon solvents

Abstract Utilizing transient near-IR luminescence spectroscopy, we have shown that free radicals like TEMPO (2,2,6,6- tetramethylpiperidine-N-oxyl) increase the rate of the forbidden radiative transition of singlet oxygen in chlorinated hydrocarbon solvents. The quantum yield for emission has been shown to depend on the nature of the substituent group in the 4′ position. Concomitant with this effect is a slight high energy shift in the steady state singlet oxygen phosphorescence spectrum. Additional experiments with TEMP (2,2,6,6-tetramethylpiperidine) have shown that the nitroxyl group is necessary for this effect. Using the kinetic equations associated with a collision complex model, we have investigated a curve crossing (isophotic) point observed in the 1270 nm luminescence decays at various TEMPO concentrations. This information indicates that we are observing a contact complex characterized by a weak interaction energy and a short lifetime. We have also determined the conditions under which the isophotic point can be observed.