Abstract: We have employed diheteroarylethenes as acceptors for photochromic FRET (pcFRET), a technique introduced for the quantitative determination of fluorescence resonance energy transfer (FRET). In pcFRET, the fluorescent emission of the donor is modulated by cyclical transformations of a photochromic acceptor. Light induces a reversible change in the structure and, concomitantly, in the absorption properties of the acceptor. Only the closed forms of the selected diheteroarylethenes 2a and 2b have an absorption band overlapping the emission band of the donor, 1. The corresponding variation in the overlap integral (and thus critical transfer distance Ro) between the two states provides the means for reversibly switching the process of FRET on and off, allowing direct and repeated evaluation of the relative changes in the donor fluorescence quantum yield. The diheteroarylethenes demonstrate excellent stability in aqueous media, an absence of thermal back reactions, and negligible fatigue. The equilibration of th...

Abstract: We demonstrate very high-efficiency green phosphorescence from a single-layer dendrimer organic light-emitting diode formed by spin-coating. A first generation fac-tris(2-phenylpyridine) iridium cored dendrimer doped into a wide-gap 4,4′-bis(N-carbazole) biphenyl host displays a peak external quantum efficiency of 8.1% (28 Cd/A) at a brightness of 3450 Cd/m2 and a current density of 13.1 mA/cm2. A peak power efficiency of 6.9 lm/W was measured at 1475 Cd/m2 and 5 mA/cm2. We attribute this exceptionally high quantum efficiency for a single-layer device to the excellent film forming properties and high photoluminescence quantum yield of the dendrimer blend and efficient injection of charge into the emissive layer. These results suggest that dendrimers are an effective method for producing efficient phosphorescent devices by spin-coating.

Abstract: A series of derivatives of 5,10,15,20-tetrakis-(4-N-methylpyridyl)-porphine, where one N-methyl group was replaced by a hydrocarbon chain ranging from C6 to C22, were characterized for their photophysical and photosensitizing properties. The absorption and fluorescence features of the various compounds in neutral aqueous solutions were typical of largely monomeric porphyrins, with the exception of the C22 derivative, which appeared to be extensively aggregated. This was confirmed by the very low triplet quantum yield and lifetime of the C22 derivative as compared with 0.2–0.7 quantum yields and 88–167 μs lifetimes for the other porphyrins. The photophysical properties and photosensitizing activity toward N-acetyl-l-tryptophanamide of the C22 porphyrin became comparable to those typical of the other derivatives in 2% aqueous sodium dodecyl sulfate, where the C22 compound is fully monomerized. All the porphyrin derivatives exhibited at micromolar concentrations photoinactivation activity against bo...

Abstract: A series of spiro-linked oligofluorenes were synthesized that exhibit blue emission with photoluminescence quantum yields in some cases exceeding 0.50 in neat spin-coated films. Differential scanning calorimetry revealed highly variable glass transition temperatures. A longer conjugation length is desired for better stability against thermally activated crystallization, and a shorter pendant is desired for a higher glass transition temperature at a given conjugation length. In contrast to poly(2,7-(9,9-di-n-octylfluorene)), prolonged heating of spiro-oligo(fluorene) films resulted in no changes in morphology, emissive color, and photoluminescence quantum yield. The propensity of spiro-oligo(fluorene)s to thermally activated crystallization was rationalized with molecular shapes predicted by molecular mechanics simulations.

Abstract: Photodegradation of 12 polycyclic aromatic hydrocarbons was studied in aerated pure water, solutions of Suwannee River fulvic acid, and natural waters using polychromatic light (>290 nm). Quantum yields in pure water varied from 3.2 × 10-5 to 9.2 × 10-3. No obvious relationships were evident among the quantum yields and molecular properties. Photodegradation rate constants in solutions of Suwannee River fulvic acid or natural waters were largely unchanged compared to rate constants in pure water. Estimates of PAH photodegradation rates in natural waters can thus be obtained employing the quantum yields in pure water, PAH absorption, and solar irradiance. Calculated rate constants for photodegradation in surface waters during the summertime at mid-latitude varied from 3.2 × 10-3 to 7.6 h-1.

Abstract: The photophysics and photochemical behavior of the phenoxazin-3-one dyes, resazurin and resorufin, have been studied in aqueous solutions. The irradiation of resazurin in the presence of amines leads to deoxygenation of the N-oxide group, giving resorufin. This photoreaction is highly dependent on the amine structure and is efficient only in the presence of tertiary aliphatic amines. The absorption and fluorescence properties of these dyes are dependent on pH. At pH above 7.5 both dyes are in their anionic form. For resorufin this form is highly fluorescent (ΦF = 0.75). At lower pH the fluorescence is strongly reduced. The N-oxide dye presents a very weak fluorescence quantum yield (0.11), which also is reduced at low pH. Flash photolysis experiments allowed characterization of the triplet state and the transients formed after irradiation of these dyes in the absence and presence of amines. The triplet quantum yields are 0.08 for resazurin and 0.04 for resorufin. The photodeoxygenation of N-oxide...

Abstract: The quantitation of fluorescence radiance may at first suggest the need to obtain the number of fluorophore that are responsible for the measured fluorescence radiance. This goal is beset by many difficulties since the fluorescence radiance depends on three parameters 1) the probability of absorbing a photon (molar extinction), 2) the number of fluorophores, and 3) the probability of radiative decay of the excited state (quantum yield). If we use the same fluorophore in the reference solution and the analyte then, to a good approximation, the molar extinction drops out from the comparison of fluorescence radiance and we are left with the comparison of fluorescence yield which is defined as the product of fluorophore concentration and the molecular quantum yield. The equality of fluorescence yields from two solutions leads to the notion of equivalent number of fluorophores in the two solutions that is the basis for assignment of MESF (Molecules of Equivalent Soluble Fluorophore) values. We discuss how MESF values are assigned to labeled microbeads and by extension to labeled antibodies, and how these assignments can lead to the estimate of the number of bound antibodies in flow cytometer measurements.

Abstract: On-off kinetics observed in fluorescence intensity traces of single organic dye molecules have been assigned to molecules being either continuously emitting photons (on state) or occupying the triplet state (off state). To date this was mainly substantiated by the oxygen dependence of this behavior. By means of various experimental approaches, we show that indeed the occupation of the triplet state principally causes the off periods in some cyanine dyes structurally related to frequently used Dil or Cy3. Comparing recorded traces for various cyanine dyes bearing different atoms at the 1- and 1'-positions, we demonstrate for the first time the consequences of the internal heavy atom effect on the on-off dynamics of single organic dye molecules. An additional pathway to populate the triplet state is proposed in order to explain the enhanced intersystem crossing quantum yield for the transition S 1 to T 1 for these molecules, which demand less free volume for rotation.

Abstract: Self-assembly of rhenium(I)-based molecular rectangles containing long alkyl chains has been achieved in one-pot synthesis by solvothermal methods. An enormous enhancement in the emission intensity, quantum yield, and lifetime of the rectangles has been observed when the solvent medium is changed from organic to aqueous. Addition of water favors the aggregation of Re(I) molecular rectangle resulting in the luminescence enhancement, and this phenomenon has been traced out using light scattering techniques.

Abstract: The absorption and emission behavior of flavin mononucleotide (FMN) in the light-, oxygen- and voltage-sensitive (LOV) domain LOV1 of the photoreceptor Phot1 from the green alga Chlamydomonas reinhardtii was studied. The results from the wild-type (LOV1-WT) were compared with those from a mutant in which cysteine 57 was replaced by serine (LOV1-C57S), and with free FMN in aqueous solution. A fluorescence quantum yield of phi(F) = 0.30 and a fluorescence lifetime of tau(F) = 4.6 ns were determined for FMN in the mutant LOV1-C57S, whereas these quantities are reduced to about phi(F) = 0.17 and tau(F) = 2.9 ns for LOV1-WT, indicating an enhanced intersystem crossing in LOV1-WT because of the adjacent sulfur of C57. A single-exponential fluorescence decay was observed in picosecond laser time-resolved fluorescence measurements for both LOV1-WT and LOV1-C57S as expected for excited singlet state relaxation by intersystem crossing and internal conversion. An excitation intensity dependent fluorescence signal saturation was observed in steady-state fluorescence measurements for LOV1-WT, which is thought to be because of the formation of a long-lived intermediate flavin-C(4a)-cysteinyl adduct in the triplet state (few microseconds triplet lifetime, adduct lifetime around 150 s). No photobleaching was observed for LOV1-C57S, because no thiol group is present in the vicinity of FMN for an adduct formation.

Abstract: A series of hemicyanine dimers, (B3, B5, and B12) whose two chromophores are linked by different numbers of methylene groups, are synthesized. Negative slovatochromic behaviors are observed, and the fluorescence quantum yield is sensitive to the polarity of solvent. In chloroform, the fluorescence intensity is much more intensive than in other polar solvents, and the decay lifetime is much longer. This fluorescence sensitivity to solvent is due to the twisted intramolecular charge transfer (TICT) state formation, which can be blocked in weak polar solvent. The two chromophores in one dimer molecule are correlated and the TICT formation of dimer is relatively difficult compared with monomer. When the linkage is long enough, this correlation can be reduced. The dynamic fluorescence spectra of the chloroform solutions of these dyes support our assumption. The time-resolved fluorescence studies of their LB films provide a new perspective on the excited-state deactive process. The two-dimensional Forster energ...

Abstract: The production of singlet oxygen (1O2) by bacteriochlorin a (BCA) was studied in phosphate buffer and in dimyristoyl-L-α-phosphatidylcholine (DMPC) unilamellar liposomes. The comparative method used to measure 1O2 production was a quantitative analysis of photooxidation reactions leading to the loss of absorbance of the water-soluble specific probe: anthracene-9,10-dipropionic acid. Rose Bengal, whose 1O2 quantum yield (ΦRB) is well known in alcohols and phosphate buffer, was used as the standard for the quantification of the BCA singlet oxygen production. Our results confirm quantitatively that solubilization of BCA in liposomes leads to an increase in 1O2 production. Indeed, the quantum yield of 1O2 production by BCA (ΦBCA) is 0.05 in phosphate buffer and 0.33 in DMPC liposomes. Furthermore, the diffusion characteristics of 1O2 produced by BCA bound to liposome were also examined using the isotopic lifetime enhancement effect of D2O. It was shown that 1O2 spent at least 70% of its lifetime in the vesicular environment.

Abstract: The yield of hydroxyl radicals has been determined by illumination of TiO 2 layers immersed in air saturated aqueous methanol solutions. This yield is equal to half the measured formaldehyde yield in the pH range 7-13. A detailed mechanism is proposed, accounting for the lack of accumulation of hydrogen peroxide. The effect of changing methanol concentration, pH and light intensity (the latter by three orders of magnitude) is in agreement with a very simple mechanism. In contrast to hydroxyl radicals, which react via hydrogen abstraction, leading to formation of HCHO, there is no sign for reaction of methanol with mobile holes. Thus, the limiting quantum yield observed at high methanol concentration is related to the maximum yield of ○OH ads under air saturated conditions at the given pH and light intensity. The effect of light intensity shows the expected inverted square root dependency. The yield of ○OH ads is nearly constant in the range 7 < pH < 12. This system may be useful for comparative tests of different TiO 2 preparations.

Abstract: Four novel poly(aryl ether)s consisting of alternate isolated hole-transporting carbazole and electron-transporting 1,3,4-oxadiazole segments were synthesized from the nucleophilic displacement reaction of bis(fluoride) monomers with bis(phenol) monomers. These poly(aryl ether)s are soluble in common organic solvents and exhibit good thermal stability with 5% weight loss temperature above 400 °C under a nitrogen atmosphere. The photoluminescent (PL) spectra and quantum yields of these polymers are dependent on the composition of the two isolated fluorophores. The formation of exciplex in P3 was observed in the film and solution state and resulted in the lower quantum yield. The quantum yields of P4 in solutions can increase from 0.04 of P3 to 0.36, due to the dilute effect, by introducing the inert bisphenol A segments. However, the PL spectra of P4 only showed a little blue shift in the film state. This means the interchain exciplex still dominated the emission of polymeric films. The HOMO and LUMO energ...

Abstract: The electrostatic binding of a series of anionic disulfopropyl-thiacyanine (THIA) derivatives to poly(allylamine hydrochloride) (PAH) in aqueous solution is investigated by optical spectroscopy. From strong variations of the absorption, fluorescence, and excitation spectra of the dyes upon binding to PAH, conclusions are drawn about the binding modes. The unsubstituted THIA forms a ground state complex with PAH which exhibits a blue shift in absorption, like H-aggregates, but also a strongly enhanced fluorescence with respect to the free dye. The methyl derivative behaves similarly. The absorption of the methoxy derivative also exhibits a hypsochromic shift upon addition of PAH but its fluorescence is quenched. In the case of the dichloro-derivative, addition of PAH induces the formation of J-aggregates. The fluorescence quantum yield is strongly enhanced from 0.008 for the monomer to 0.48 for the PAH-J-aggregate. The monochloro-THIA shows a competition between the J-aggregate formation and the dye–PAH complex formation. Similarly to the methoxy-THIA, the fluorescence is quenched upon addition of PAH. The stability of the PAH-J-adduct to replacement by polyanions is larger than for the H-adduct, although the aggregation tendency of the dyes in pure water displays an opposite behaviour. Results show that the optical properties and the aggregate structures of sensitizing cyanine molecules can be tuned widely by interaction with polyelectrolytes.

Abstract: A series of novel CVD films of titanium(IV) oxide of different thicknesses, spanning the range 10-91 nm, are prepared on quartz, via the reaction of titanium(IV) chloride and ethyl acetate, using a CVD technique The films are clear, mechanically robust and comprise thin layer of nanocrystalline anatase titania of different thicknesses that absorb UV light The UV-Visible spectral profiles of all the CVD TiO2 films of different thickness are the same and obey Lambert's law (absorbance is porportional to film thickness) A plot of the reciprocal length for the TiO2 coating versus wavelength is reported The photocatalytic activity of each film to mediate the destruction of a thin layer of stearic acid is investigated The rate depends directly upon the fraction of light absorbed and the apparent quantum yield for the overall process is 000035, which appears low compared with that for sol-gel TiO2 films

Abstract: Halogenated phenylbacteriochlorins are synthesized with high yields in a two-step procedure. They have strong absorbances in the red and are very stable to air and light at room temperature. Flash photolysis measurements show that the triplet states of these bacteriochlorins have 30 μs lifetimes in deaerated toluene, that are quenched with diffusion-controlled rate constants by molecular oxygen. Time-resolved photoacoustic measurements, with nanosecond and nanocalorie resolution, show that these bacteriochlorins sensitize the formation of singlet oxygen with nearly unity quantum yield. However, singlet-oxygen phosphorescence measurements indicate that physical quenching occurs before the singlet-oxygen molecules diffuse into solution, and nearly half of the sensitized singlet states are lost.

Abstract: We report the photophysical characterization of the two tautomers (1e and 1i) of 5,10,15,20-tetraphenyl N-confused free-base porphyrin, potential building blocks in assemblies designed for artificial photosynthesis, using a combination of steady state and time-resolved techniques. Tautomer 1e was found to have a significantly higher quantum yield of fluorescence than tautomer 1i (ΦFl = 0.036 vs ΦFl = 0.0016, respectively), despite the fact the fluorescence lifetimes were quite similar (i.e., 1.98 ns vs 1.60 ns, respectively). These differences were attributed to a more rapid rate of internal conversion or intersystem crossing in 1i due to steric conditions in the interior of the macrocycle. The absorption spectra of these porphyrins were also examined using the results from B3LYP/6-31G(d)//B3LYP/3-21G(d) calculations and interpreted using the four electron four orbital model.

Abstract: The dependence of the quantum yield of photogeneration of singlet oxygen with aluminum sulfophthalocyanines on the degree of their sulfonation was studied. Experiments showed that Fotosens preparation containing, on the average, about three sulfo groups in the macroring in aqueous buffer solution (pH 7.4) exists in the monomeric form and sensitizes singlet oxygen with a quantum yield ϕΔ 0.42±0.06. Aluminum tetra-4-sulfophthalocyanine forms dimers, and correspondingly ϕΔ is lower: 0.22±0.03. Sulfonated aluminum phthalocyanine containing, on the average, about two sulfo groups in the macroring is aggregated to a significant extent. Addition of Triton X-100 detergent results in partial deaggregation of the dye, and ϕΔ increases from ∼0.01 to 0.15±0.02. In the series including previously unknown aluminum phthalocyanines containing residues of phosphonic acids or their esters, and also aluminum and zinc octacarboxyphthalocyanines, the influence of the dye structure on the quantum yield of generation of singlet oxygen suggests participation of axial hydroxy groups in associate formation. These associates are not manifested spectroscopically and generate singlet oxygen inefficiently. The quantum yields ϕΔ for the monomers and associates are 0.3 and 0.1, respectively.

Abstract: The rhenium(I) and ruthenium(II) complexes of a fullerene-substituted bipyridine ligand have been prepared. Electrochemical studies indicate that some ground state electronic interaction between the fullerene subunit and the metal-complexed moiety are present in the ReI but not the RuII complex. The photophysical properties have been investigated by steady-state and time-resolved UV/Vis-NIR luminescence spectroscopy and nanosecond laser flash photolysis in CH2Cl2 solution, and compared to those of the corresponding model compounds. Excitation of the methanofullerene moiety in the dyads does not lead to excited state intercomponent interactions. Instead, excitation of the metal-complexed unit shows that the lowest triplet metal-to-ligand-charge-transfer excited state (3MLCT) centered on the ReI- or RuII-type unit is quenched with a rate constant of about 2.5×108 s−1. The quenching is attributed to an electron-transfer (ElT) process leading to the reduction of the carbon sphere, as determined by luminescence spectroscopy for the RuII dyad. Experimental detection of electron transfer in the ReI dyad is prevented due to the unfavorable absorption of the metal-complexed moiety relative to the fullerene unit. However, it can be postulated on the basis of energetic/kinetic arguments and by comparison with the RuII-type array. The primary ElT process is followed by charge-recombination to give the lowest-lying fullerene triplet excited state (3C60) with quantitative yield, as determined by sensitized singlet oxygen luminescence experiments. Direct 3MLCT→3C60 triplet–triplet energy-transfer (EnT) does not successfully compete with ElT since it is highly exoergonic and located in the Marcus inverted region. The quantum yield of singlet oxygen sensitization (ΦΔ) of the ReI-based dyad is found to be lower (0.80) than for the corresponding RuII derivative (1.0). This is likely to be the consequence of different conformational structures for the two dyads, rather than a different yield of 3C60 formation.

Abstract: The treatment of aqueous phenol (100 mg l −1 ) and nitrobenzene (50 mg l −1 ) solutions using iron(III) and UV radiation has been carried out. The quantum yield of photodegradation of both compounds has been determined ( φ phenol =11.2±2.0 mmol einstein −1 , φ NB =6.0±0.5 mmol einstein −1 ). The effect of the amount of ferric ion in the degradation rate has been studied in the range of molar iron(III) to organic compound ratios from 0 to 6. The reaction rate showed to follow a first-order kinetic with respect to the substrate and the kinetic constant depends on the Fe(III)/organic ratio. Optimal values of these kinetic constants have been found.

Abstract: The spectroscopic properties of 2-(2‘-tosylaminophenyl)benzimidazole (TPBI) have been studied in a series of different solvents. As revealed by absorbance, steady-state, and time-resolved emission spectroscopy, the molecule undergoes fast excited-state intramolecular proton transfer (ESIPT) to yield emission of the corresponding tautomeric species with a large quantum yield (0.5). The fluorescence emission shows monoexponential decay kinetics (τ = 4.5 ns) regardless of the nature of the solvent. The ground-state equilibrium is dominated by a single rotamer and small amounts of the deprotonated anion. The X-ray structure of TPBI shows a substantial out-of-plane twist along the aryl−benzimidazole bond axis, which is presumably due to π-stacking interactions between the tosylamide and benzimidazole rings. Ab initio calculations suggest a different structure in the gas phase without π-stacking interactions and a substantially reduced twist angle. A large energy barrier for interconversion of the cis- and tran...

Abstract: Excited-state lifetime and quantum yield of a fluorophore are determined by the radiative and nonradiative decay rates. There is an extensive literature about factors affecting the nonradiative dec...

Abstract: The title complexes displayed a marked dependence of their luminescence properties upon pyridine substitution. The ligand with the 4-carbamoyl-substituted pyridine showed a long-lived luminescence of near-unity quantum yield for Tb3+ and quite reasonable values for Eu3+, Sm3+ and Dy3+, thus making it an excellent candidate for a luminescent probe to be attached to biomolecules by a peptide-like linkage.