Eosin y triplet state as a probe of spatial heterogeneity in microcrystalline cellulose

The photophysical behavior of eosin Y adsorbed onto microcrystalline cellulose was evaluated by reflectance spectroscopy, steady-state fluorescence spectroscopy and laser induced time-resolved luminescence. On increasing the concentration of the dye, small changes in absorption spectra, fluorescence redshifts and fluorescence quenching are observed. Changes in absorption spectra point to the occurrence of weak exciton interactions among close-lying dye molecules, whereas fluorescence is affected by reabsorption and excitation energy trapping. Phosphorescence decays are concentration independent as a result of the negligible exciton interaction of dye pairs in the triplet state. Lifetime distribution and bilinear regression analyses of time-resolved phosphorescence and delayed fluorescence spectra reveal the existence of two different environments: long-lived, more energetic triplet states arise from dyes tightly entrapped within the cellulose chains, while short-lived, less-energetic states result from dyes in more flexible environments. Stronger hydrogen bond interactions between the dye and cellulose hydroxyl groups lead in the latter case to a lower triplet energy and faster radiationless decay. These effects, observed also at low temperatures, are similar to those encountered in several amorphous systems, but rather than being originated in changes in the environment during the triplet lifetime, they are ascribed in this case to spatial heterogeneity.     

Photophysics of Xanthene Dyes at High Concentrations in Solid Environments: Charge Transfer Assisted Triplet Formation

The photophysical behavior of two xanthene dyes, Eosin Y and Phloxine B, included in microcrystalline cellulose particles is studied in a wide concentration range, with emphasis on the effect of dye concentration on fluorescence and triplet quantum yields. Absolute fluorescence quantum yields in the solid‐state were determined by means of diffuse reflectance and steady‐state fluorescence measurements, whereas absolute triplet quantum yields were obtained by laser‐induced optoacoustic spectroscopy and their dependence on dye concentration was confirmed by diffuse reflectance laser flash photolysis and time‐resolved phosphorescence measurements. When both quantum yields are corrected for reabsorption and reemission of radiation, Φ _F values decrease strongly on increasing dye concentration, while a less pronounced decay is observed for Φ _T. Fluorescence concentration quenching is attributed to the formation of dye aggregates or virtual traps resulting from molecular crowding. Dimeric traps are however able to generate triplet states. A mechanism based on the intermediacy of charge‐transfer states is proposed and discussed. Calculation of parameters for photoinduced electron transfer between dye molecules within the traps evidences the feasibility of the proposed mechanism. Results demonstrate that photoactive energy traps, capable of yielding dye triplet states, can be formed even in highly‐concentrated systems with random dye distributions.     

Phosphorescence characteristics of acetophenone, benzophenone, p-aminobenzophenone and Michler's ketone in various environments

The phosphorescence characteristics (excitation and emission spectra and lifetimes) of acetophenone (AP), benzophenone (BP), p-aminobenzophenone (PABP) and Michler's ketone (MK) adsorbed on Whatman No. 1 filter paper were measured at various temperatures, and compared with the phosphorescence characteristics in different solvent glasses at 77 K. Both AP and BP phosphoresce on filter paper only at low temperature (208 K). The phosphorescence lifetimes of AP and BP are < 1 msec, indicating a (3)(n,pi(*)) lower triplet level for paper substrates. With PABP, the low lying triplet state in polar solvents is (3)(CT) and in non-polar solvents is (3)(n, pi(*)); PABP on filter paper results in spectral characteristics similar to those of PABP in polar solvents at 77 K. The lifetime of PABP is longer than that of BP, indicating a (3)(CT) low-lying triplet state. MK, like PABP, has strongly environment-dependent photophysical properties. MK, when adsorbed on filter paper, has an intense long-lived luminescence at room temperature, resulting in a limit of detection of 3 ng ml or 3 pg, and a linear dynamic range of over 3 orders of magnitude. MK appears to be strongly hydrogen-bonded to the filter paper. In studies in ethanol and other solvents, MK adsorbed on filter paper shows a dramatic change in its phosphorescence spectrum when the temperature is lowered from 298 K to 208 K; the phosphorescence peak moves to longer wavelengths and the intensity decreases. The temperature effect could arise from the presence of several conformers of MK or be due to different environmental sites or E-type delayed fluorescence. The low-lying triplet state of MK on filter paper is most likely a (3)(CT) state. Lowering the temperature appears to increase the phosphorescence intensity for ketones which phosphoresce in the (3)(n,pi(*)) triplet state, but affects it only slightly for analytes which phosphoresce in the (3)(pi,pi(*)) triplet state. Room-temperature phosphorescence seems to arise for aromatic ketones and aldehydes with low-lying (3)(pi, pi(*)) or (3)(CT) triplet states.     

PHOTOCHEMISTRY ON SURFACES: TRIPLET-TRIPLET ENERGY TRANSFER ON MICROCRYSTALLINE CELLULOSE STUDIED BY DIFFUSE REFLECTANCE TRANSIENT ABSORPTION and EMISSION SPECTROSCOPY*

Triplet-triplet energy transfer has been studied between benzophenone and an oxazine dye (2,7-bis(diethyl-amino)-phenazoxonium chloride) co-adsorbed on the surface of microcrystalline cellulose. Ground state absorption and fluorescence measurements provide evidence for dimer formation of the oxazine dye when adsorbed on cellulose in contrast to the behaviour in ethanol solution where no dimerization is observed. The equilibrium constant for dimerization, which is found to be (1.0 × 0.1) × 10⁶ mol^?1 g (2560 × 250 dm³ mol^?1) for oxazine alone on cellulose decreases in the presence of co-adsorbed benzophenone. Fluorescence is detected from excited monomeric but not from excited dimeric oxazine. The absorption spectrum of the triplet state of oxazine adsorbed on cellulose was obtained and its extinction coefficient evaluated relative to that of triplet benzophenone which was used as a sensitizer. The lifetime of adsorbed triplet oxazine is 4.3 ms which is 300 times longer than that in acetonitrile solution. The efficiency of energy transfer from triplet benzophenone to oxazine on cellulose was studied using both time resolved sensitized absorption and phosphorescence intensity measurements as a function of oxazine concentration. Lifetime measurements show that the energy transfer process involves static quenching since the benzophenone lifetime is independent of oxazine loading at the surface. A mechanism is proposed to explain the results in which one oxazine molecule is suggested as being able to quench phosphorescence from a “pool” consisting of 2 to 3 benzophenone molecules.     

Luminescence properties of acridine dyes in micellar sodium dodecyl sulfate solutions containing thallium ions

The effect of external heavy Tl^I ion on the luminescence properties of Trypaflavine, Acridine Yellow, and Acridine Orange solubilized in sodium dodecyl sulfate micelles was studied. An increase in the concentration of thallium ions results in a decrease in the intensity of prompt fluorescence, an increase in the intensity of delayed fluorescence, the appearance of phosphorescence at 20 °C, and a shortening of the triplet state lifetime of the dyes. The effective and micellar Stern—Volmer constants of fluorescence quenching of the dyes by thallium ions were determined. The effective and micellar quenching rate constants of triplet states of the dyes by Tl(i) ions and lifetimes of the triplet states of the dyes in the absence of thallium ions were calculated on the basis of kinetic measurements.     

Luminescence quantum yield determination for molecules adsorbed onto solid powdered particles.

A new methodology for the determination of the fluorescence quantum yield of dyes adsorbed onto microcrystalline cellulose is presented and applied to rhodamine 101, cresyl violet and auramine O. It is based on a previously reported method by Ruetten and Thomas (J. Phys. Chem., 1998, 102, 598-606), which is not applicable to the dyes used in the present study. It uses ground-state diffuse reflectance spectra obtained with and without filters, which prevents the luminescence of the dye from reaching the integrating sphere and the photodetector. New equations are presented here, correcting for the fluorescence emission of the dye, which depends on the detector sensitivity. Cut-on filters, which have a transmittance close to unity in the absorption region, and close to zero in the emission region, of the dye are used to obtain corrected reflectance spectra. The influence of the substrate was also taken into account. This methodology may be applied to other probes and surfaces or emissions of a different nature (i.e., phosphorescence or delayed fluorescence), and constitutes a very simple and general procedure to solve the important problem of luminescence quantum yield determination of probes adsorbed onto solid powdered surfaces.     

Photochemistry and photophysics of thienocarbazoles

Two methylated thienocarbazoles and two of their synthetic nitro-precursors have been examined by absorption, luminescence, laser flash photolysis and photoacoustic techniques. Their spectroscopic and photophysical characterization involves fluorescence spectra, fluorescence quantum yields and lifetimes, and phosphorescence spectra and phosphorescence lifetimes for all the compounds. Triplet-singlet difference absorption spectra, triplet molar absorption coefficients, triplet lifetimes, intersystem crossing S1 --> T1 and singlet molecular oxygen yields were obtained for the thienocarbazoles. In the case of the thienocarbazoles it was found that the lowest-lying singlet and triplet excited states, S1 and T1, are of pi,pi* origin, whereas for their precursors S1 is n,pi*, and T1 is pi,pi*. In both thienocarbazoles it appears that the thianaphthene ring dictates the S1 --> T1 yield, albeit there is less predominance of that ring in the triplet state of the linear thienocarbazole, which leads to a decrease in the observed phiT value.     

PROMPT AND DELAYED FLUORESCENCE OF SOME DNA ADSORBATES

Abstract— The absorption and prompt fluorescence spectra of twelve cationic compounds in the free and bound states were surveyed, and four compounds selected for detailed study, viz: proflavine, benzoflavine, acridine orange and thioflavine T. The absorption spectra, prompt fluorescence spectra and delayed fluorescence and phosphorescence spectra of these four compounds in fluid solutions in the free and bound states were measured. The shifts in the promp fluorescence bands of the diaminoacridines on adsorption were found to differ significantly from those of the other compounds, and the behaviour of acridine orange was found to be different from that of the other two diaminoacridines. All three diaminoacridines gave E -type delayed fluorescence in the free and bound states. The ordered structure of the strongly binding adsorption sites in native DNA is associated with a higher intensity of delayed fluorescence than is observed from the diaminoacridines in the free state or adsorbed on denatured DNA. Quenching experiments with proflavine indicated that, in the weakly binding sites, its triplet is easily quenched by unadsorbed quenchers, but in the strongly binding sites its triplet is quenched only with great difficulty.     

Laser-Induced Luminescence of Flavinium Salts

Flavinium salts dissolved in an ethanolic glass exhibit blue fluorescence and orange-red phosphorescence upon excitation with a UV. line from an argon ion laser equipped with UV. optics. This arrangement enables the wavelength distribution and the time-dependence of the phosphorescence to be measured in a relatively short time. Four cationic flavins were investigated. In spite of the small difference in the chemical structure of the compounds studied, large differences in the spectral shapes and in the ratio of quantum efficiencies of phosphorescence and fluorescence became apparent. The phosphorescence lifetimes were of the same order of magnitude, which indicated a similar rate of depletion of the lowest excited triplet state for all four cations. However, the efficiency of triplet formation (intersystem crossing) is affected by slight structural modifications in the pyrimidine subnucleus of the flavinium salt. The results point to a possible role of vibronic spin-orbit coupling in the intersystem crossing.     

Excited state properties of oligophenyl and oligothienyl swivel cruciforms

A comprehensive study has been undertaken of the electronic spectral and photophysical properties of two oligophenyl (BPH and BPHF) and one oligothienyl (BTF) swivel cruciforms involving measurements of absorption, fluorescence, and phosphorescence spectra, quantum yields of fluorescence (phiF), phosphorescence (phiPh) and triplet formation (phiT), lifetimes of fluorescence (tauF) and of the triplet state (tauT), and quantum yields of singlet oxygen production (phiDelta). From these, all radiative kF and radiationless rate constants, kIC and kISC, have been obtained in solution. The energies of the lowest lying singlet and triplet excited states were also determined at 293 K. Several of the above properties have also been obtained at low temperature and in the solid state (thin films). In general, for the phenyl oligophenyl (BPH) and for the oligothienyl (BTF) compounds, the radiationless decay channels (phiIC+phiISC) are the dominant pathway for the excited-state deactivation, whereas with the fluorene based oligophenyl BPHF the radiative route prevails. In contrast to the general rule found for related oligomers (and polymers) where radiative emission from T1 is absent, with the compounds studied, phosphorescence has been observed for all of the compounds, indicating that this type of functionalization can lead to emissive triplets. Time-resolved fluorescence decays with picosecond resolution revealed multiexponential (bi- and triexponential) decay laws compatible with the existence of more than one species or conformation in the excited state. These results are discussed on the basis of conformational flexibility in the excited state.     

PHOTOCHEMICAL STABILITY OF BIOLOGICALLY ACTIVE COMPOUNDS. IX. CHARACTERIZATION OF THE SPECTROSCOPIC PROPERTIES OF THE 4-AMINOQUINOLINES, CHLOROQUINE AND HYDROXYCHLOROQUINE, AND OF SELECTED METABOLITES BY ABSORPTION, FLUORESCENCE AND PHOSPHORESCENCE MEASUREMENTS

Abstract The spectroscopic properties of the antimalarials chloroquine, hydroxychloroquineand selected metabolites were investigated by absorption, fluorescence and phosphorescence measurements. The antimalarials exhibit both fluorescence and phosphorescence. Fluorescence dominates for the deprotonated forms of the 4-aminoquinolines, while phosphorescence dominates when the compounds are protonated. The phosphorescence lifetimes of the 4-aminoquinolines were measured. Both the protonated and the deprotonated forms have long phosphorescence lifetimes indicating a long-lived triplet state.     

Phosphorimetric Determination of Polycyclic Aromatic Hydrocarbons in Clinoptilolite-Filled Fibers at Room Temperature

The phosphorescence of polycyclic aromatic hydrocarbons (PAHs) adsorbed on natural zeolite (clinoptilolite) and clinoptilolite-filled acrylic fibers was studied at room temperature. The optimal conditions were selected for the phosphorescence of PAHs extracted from water and water–dioxane solutions at room temperature using clinoptilolite-filled fibers. The phosphorescence of PAHs in the clinoptilolite-filled fibers at room temperature is characterized by a high quantum yield and long lifetimes of PAH triplet states, as was the case at low temperatures (77 K). The results obtained indicate that the adsorbents used offer promise for the analytical chemistry of PAHs.     

p-Hertz excitation spectroscopy of an optical antenna, optical transmitter-receiver mechanism

o-Xylene sensitized biacetyl fluorescence and phosphorescence have been investigated and photosensitized fluorescence and phosphorescence lifetimes of biacetyl in the vapor phase have been determined. Attempts to detect the triplet of biacetyl by its absorption spectrum were unsuccessful, primarily due to, it is believed, the low extinction coefficients of the triplet, and the low triplet concentrations produced by the optical pumping device at room temperature.     

Temperature effects on the solid-matrix luminescence properties of 4-phenylphenol adsorbed on filter paper

Experimental values of fluorescence quantum yield, phosphorescence quantum yield, and phosphorescence lifetime were obtained at temperatures from 23 degrees to -180 degrees for 4-phenylphenol adsorbed on filter paper. From the experimental values, rate constants for phosphorescence and radiationless transition from the triplet state were calculated along with the triplet formation efficiency. The data revealed several important aspects that are responsible for the room-temperature fluorescence and phosphorescence of 4-phenylphenol adsorbed on filter paper.     

Environmental effects on the absorption and luminescence properties of aromatic amines: Aniline

Emission and absorption spectra of aniline have been studied in different solvents at 300 K and 77 K. This molecule exhibits broad and structureless fluorescence and poorly resolved phosphorescence in glassy polar ethanol and non-polar methylcyclohexane and n-hexane matrices at 77 K. Unlike the behaviour in a polar medium, fluorescence is found to be more intense than phosphorescence in non-polar media. In contrast to the case of glassy matrices, aniline shows highly stuctured emission and excitation spectra in crystalline cyclohexane and benzene matrices at 77 K. Excited state dipole moment calculation shows that both the singlet states contain charge transfer character, which is responsible for the strong phosphorescence in polar glassy medium. Spectral analysis indicates that the aniline molecule is slightly distorted in both the singlet and triplet states. The marked reduction in phosphorescence intensity of this molecule observed in non-polar matrices, particularly in a benzene crystalline matrix, relative to that in an ethanol matrix, is attributable to the decreased intersystem crossing rate and increased non-radiative rate from the emitting triplet state.     

Spectroscopic studies of the intermolecular interactions of Congo red and tinopal CBS with modified cellulose fibers

The adsorption of Congo red and tinopal CBS dyes on cellulose fibers was investigated using electronic absorption and fluorescence spectroscopies. Hydrogen bonds appear to be relevant for the dye-fiber interactions as indicated by the solvatochromism of Congo red in water, methanol, and dimethyl sulfoxide solutions, and when adsorbed on cellulose fibers. We also demonstrate that electrostatic interactions play an important role in the dye-medium interaction, through the analysis of absorption spectra of Congo red and fluorescence spectra of tinopal in aqueous solutions containing salt and in layer-by-layer nanostructured films with poly(allylamine hydrochloride). For instance, dye adsorption was enhanced when salt was added to the dipping solution, which was explained by the synergistic effect between the conformational changes of the cellulose and changes in the solvation layer around the cellulose chains and around dye molecules. On the basis of the fluorescence results for tinopal CBS, we inferred that dye aggregation is not relevant for adsorption on the fibers. In addition, fluorescence spectroscopy is proven very sensitive for studying the organization of dye molecules in layer-by-layer films, particularly those undergoing irreversible structural changes.     

Fast and slow processes of thermal deactivation of excited stilbazolium merocyanine dyes

The long living triplet states play important role in sensitizing action in all photochemical reactions. The yield of generation of triplet states of dyes can be evaluated on the basis of measurements of their slow (microsecond) thermal deactivation (TD). All experiments were carried out in the oxygen presence, it means under quenching dye triplets. The pulse dye laser generates in the investigated solution pressure signal. The high of the amplitudes of first maximum of this pressure wave-form signal in the solution of the investigated dye and in the reference sample were measured. Reference sample exhibits only fast processes of TD. The comparison of the first maximum of wave-form photothermal signal of sample and of reference enable to calculate part of energy exchanged into heat in time longer than time resolution of arrangement. The fluorescence yields of investigated dyes were also established. On the basis of such data, using procedure described in literature, the yield of singlet–triplet intersystem crossing (ISC) was evaluated. It was shown that this yield depends on the length of stilbazolium merocyanine chain. The product of triplet state yield and energy was lower for merocyanines with longer chains. At lower temperatures the yield of fluorescence increases and amount of excitation exchanged in short time into heat decreases. The slow TD process increases in low temperature because of the decrease in the quenching of the dyes triplet states by oxygen. The amount of energy exchanged into heat in a time longer than time resolution of apparatus is due predominantly through TD of the dye triplet states.     

Sensitized phosphorescence studies of p-xylene+biacetyl system, an optical antenna

p-Xylene sensitized biacetyl fluorescence and phosphorescence has been investigated and photosensitized phosphorescence lifetimes of biacetyl in the vapor phase has been determined. Attempts to detect the triplet of biacetyl by its absorption spectrum were unsuccessful, due to primarily, it is believed, to the low extinction coefficients of the triplet, and the low triplet concentrations produced by the optical pumping device at room temperature.     

ANION AND SOLVENT EFFECTS ON THE RATE OF REDUCTION OF TRIPLET EXCITED THIAZINE DYES BY FERROUS IONS

Abstract— The lifetimes of the triplet excited states of thionine and methylene blue were measured in aqueous and 50 v/v% aqueous acetonitrile solutions acidified with 0.01 N sulfuric or trifluoromethyl-sulfonic acid. The rate constants for reaction of the triplet excited dyes with ferrous ions were measured in the same solutions. The triplet lifetimes in the absence of added quenchers were insensitive to a change in acid from trifluoromethylsulfonic to sulfuric or to a change in solvent from water to 50v/v% aqueous acetonitrile (τ for triplet thionine ˜7.5 μs, τ for triplet methylene blue ˜4.5 μs). In contrast, the rate constant for reaction of the triplet dyes with ferrous ions increased by nearly a factor of 10 with a change in acid from trifluoromethylsulfonic to sulfuric. In solutions containing sulfate ions this reaction rate constant increased with increasing sulfate concentration and with a change in solvent from water to 50 v/v% aqueous acetonitrile. The results are discussed in terms of the possibility of association of the positively charged reactive ions with sulfate anions. Quenching of the triplet excited dyes by ferric ions or by ground state dye molecules was shown to be negligible at the concentration used for the ferrous ion quenching study.     

Correlation between phosphorescence lifetimes and ratios of steady state phosphorescence to fluorescence intensity for biacetyl in various solvents

The phosphorescence of biacetyl in fluid solutions is quenched not only by oxygen but also by impurities resulting from the distillation procedure and from the freeze-pump-thaw degassing technique. A linear correlation between the measured phosphorescence lifetimes and the ratios of the phosphorescence to the fluorescence intensities is established; the slope affords reliable calculations of the biacetyl triplet lifetimes from steady state measurements and additionally the determination of the rate constant k_p of radiative triplet deactivation. This constant is dependent on the solvent, whereas the fluorescence constant k_F is not.