Guilford Jones

Medium effects on fluorescence quantum yields and lifetimes for coumarin laser dyes

Abstract Fluorescence quantum yields and lifetimes of coumarin dyes are sharply reduced in polar solvents if amine substituent groups are free to rotate. The polar solvent effect is interpreted in terms of relaxation of excited dye from an initial planar conformation to a twisted zwitterionic state.

Solvent effects on photophysical parameters for coumarin laser dyes

Abstract The red shifts of absorption bands of coumarin dyes in polar solvents correlate with the solvent polarity-polarizability parameter π ∗ . Emission frequencies correlate well with the hydrogenbonding parameter, α and show a regular dependence on π ∗ if a solvent polarizability correction term is incorporated for chlorinated hydrocarbon and aromatic solvents. Quantum yield and lifetime measurements reveal that the reduced emission observed for dyes with non-rigid structures is associated with enhanced nonradiative decay in polar solvents. The solvent effect on fluorescence yield is interpreted in terms of the intervention of planar and non-planar excited species which appears to be general for polar dyes.

Modulation of transduction gain in light adaptation of retinal rods.

The effect of light adaptation on the period of photocurrent saturation induced by a bright stimulating flash was examined in rod photoreceptors of the larval-stage tiger salamander (Ambystoma tigrinum). Using suction electrodes, photocurrent responses to brief flashes were recorded from single, isolated rods in the presence and absence of steady background illumination. Background light decreased the saturation period (T) measured at fixed flash intensity (fixed If) and in this respect light-adapted the saturating response. Effects of the background on responses to weak (i.e. subsaturating) and bright flashes were compared with changes in a parameter, phi = e-delta T/TR*, where delta T is the decrease in saturation period, and where TR* is the slope of the line that relates T and ln If in a given state of adaptation. Dark- and light-adapted responses to flash intensities IDf and ILf, respectively, exhibited similar absolute peak photocurrent and falling-phase kinetics when IDf and ILf satisfied the relation, IDf = phi (ILf + IbTR*), where Ib is the background intensity. It is argued that phi approximates the relative PDE*/R* gain of transduction, i.e. the relative peak level of activated cGMP phosphodiesterase (PDE*) produced by a given, small amount of photoactivated visual pigment (R*). Interpreted on this view, the results imply that light adaptation derives largely from a decrease in PDE*/R gain, rather than from the stimulation of guanylate cyclase activity. The data are consistent with the possibility that modulation of the lifetime of PDE* underlies the background dependence of phi.

A time-resolved fluorescence study of dissolved organic matter in a riverine to marine transition zone

Abstract Time-resolved and steady-state fluorescence measurements were used to characterize dissolved organic matter (DOM) in bulk water samples from a fresh to marine transition zone. The region studied was the Shark River and Florida Bay in Everglades National Park in Southwestern Florida. Samples were taken from the fresh waters at the head of the Shark River, the mouth of the river, and in the fresh-water river plume as it mixed into the saline waters of Florida Bay. The salinity varied from 0.47 at the head of the river to 36.0 for this series of 11 samples. Steady-state fluorescence intensity decreased with increasing salinity from the head of the Shark River into Florida Bay. This is due to dilution by mixing, as changes in the fluorescence intensity also correlated with changes in the levels of total organic carbon (TOC). Fluorescence lifetime measurements were performed on the five most concentrated samples (salinity from 0.47 to 30.5). The decay of the time-resolved fluorescence could be reasonably fit to a multi-exponential function, with three lifetime components on the order of 0.5–0.8, 2–3, and 6–9 ns. These ranges are in agreement with previous literature results for humic acids, fulvic acids, and marine CDOM concentrated by ultrafiltration. This is the first study of CDOM fluorescence lifetimes in coastal waters in a fresh to marine transition zone. Although photochemical and biological transformations of CDOM occur in these zones, no statistically significant difference in lifetimes was observed with increasing salinity. However, irradiating fresh water samples from the head of the Shark River at 280 and 334 nm resulted in a significant decrease in the two shorter lifetime components (by a factor of ∼4), but only a slight decrease in the longest lifetime component.

Energy storage in organic photoisomers

Abstract Criteria for the successful photochemical storage of solar energy as latent heat in organic materials are outlined. Photoisomerization reactions which have some potential for storage of photon energy in kinetically stable products are surveyed. Emphasis is placed on well-known internal cyclo-additions which display thermal reversibility, large storage capacities and high chemical and quantum efficiencies. Chemicals available on an industrial scale, which are known to undergo valence isomerization, are identified. Attempts to drive these reactions via exciplexes (complexes involving strong electron donor or electron acceptor sensitizers) are described. The sensitized isomerization of dimethylnorbornadiene-2,3-dicarboxylate (7) and a model compound, hexamethyldewarbenzene, are identified as bona fide exciplex isomerizations. Triplet sensitizers have been employed in the sensitization of 7 to visible light (to 500 nm). The potential importance of endothermic energy transfer in triplet sensitization (the upconversion of very low energy triplets) is discussed. The review includes developments by several research groups in the spectral sensitization of isomerizable substrates, the use of heterogeneous photosensitizers, and photocalorimetric techniques.

Photoisomerization of bis(9-anthryl)methane and other linked anthracenes. The role of excimers and biradicals in photodimerization

A series of linked anthracenes capable of storing photon energy through endoergic valence photo-isomerization have been studied. Photophysical and photochemical characteristics of the systems have been completely characterized by measurement of fluorescence quantum yields and lifetimes, and efficiencies for forward and reverse isomerization. The release of energy stored in photoisomers has been measured using kinetic and calorimetric techniques. From emission and lifetime data the respective roles of excimers and biradicals in anthracene photodimerization have been defined. (Author)

Intramolecular photoinduced electron transfer for cations derived from azole-substituted coumarin dyes

Abstract The conjugate acids of 7-aminocoumarins that are substituted with benzimidazole or benzothiazole groups display long wavelength absorption and emission associated with low-lying charge transfer states; rapid radiationless deactivation is proposed to proceed via TICT-type intermediates.

Photon energy storage in organic materials— The case of linked anthracenes☆

Criteria for the photochemical storage of solar energy as latent heat are outlined. Energy-storing valence isomerizations which may be driven by irradiation and which may be reversed by heating with or without a catalyst are described. Data for photoisomerization which utilize 300-500 nm radiation with storage capacities of 50-250 cal/g and with storage efficiencies of 5-10% are summarized. New data concerning linked anthracenes which photoisomerize with phi = 0.2-0.4 are provided. A photocalorimeter for the measurement of storage enthalpies is described. New systems for the practical conversion of solar energy are suggested.

Products of Photodegradation for Coumarin Laser Dyes.

Abstract The products of conventional photolysis of the coumarin laser dyes, C1, C35, C153, and C152 have been investigated. The previously reported dealkylation of C1 is documented for the fluorinated dyes, C35, and C152 in deaerated solvents. In addition, a reduction product is identified for C1, consistent with a radical mechanism for decomposition. Evidence is provided that the concentration quenching (self quenching) of singlet dye is important to the degradation mechanism. For the rigid dye, C153, a photooxidation product involving the amine functionality results from decomposition in aerated media. For several dyes, very low triplet yields have been measured.

Azole-linked coumarin dyes as fluorescence probes of domain-forming polymers

The binding of 7-aminocoumarins, substituted in the 3-position with heterocyclic benzimidazole or benzothiazole groups by domain-forming polymers in water has been studied. The acrylic polyelectrolyte, poly(methacrylic) acid (PMAA) was used as a solubilizing agent for coumarin dyes 6, 7, and 30 in water. The acid-base properties of these bound coumarin dyes were monitored spectroscopically on titration of aqueous solutions. Alterations in the fluorescence wavelength and intensity, quantum yields, lifetimes, and polarization are consistent with the preferential binding of the dyes in compact hydrophobic domains that form at a pH regime in which the polymer is in its protonated (uncharged) state. In this pH range (<4.0), coumarins 7 and 30 are bound as monocations, whereas coumarin 6 remains in its neutral form. Reduced quantum yields and lifetimes of fluorescence for cationic coumarins can be understood in terms of the imposition of a low-lying electron transfer state, an example of a twisted intramolecular charge transfer (TICT) intermediate. Effects of polymer microenvironment on the rate of TICT state decay (a reverse electron transfer) are observed. Coumarins of the azole type may find use as fluoroprobes of the microenvironments of proteins and other biological macromolecules and as agents for pH sensing.