Phloxine B as a probe for entrapment in microcrystalline cellulose

The photophysical behaviour of phloxine B adsorbed onto microcrystalline cellulose was evaluated by reflectance spectroscopy and laser induced time-resolved luminescence in the picosecond-nanosecond and microsecond-millisecond ranges. Analysis of the absorption spectral changes with concentration points to a small tendency of the dye to aggregate in the range of concentrations under study. Prompt fluorescence, phosphorescence and delayed fluorescence spectral decays were measured at room temperature and 77 K, without the need of sample degassing because cellulose protects triplet states from oxygen quenching. In all cases, spectral changes with time and lifetime distribution analysis were consistent with the dye coexisting in two different environments: dyes tightly entrapped between polymer chains in crystalline regions of cellulose showed longer fluorescence and phosphorescence lifetimes and more energetic triplet states, while dyes adsorbed in more amorphous regions of the support showed shorter lifetimes and less energetic triplet states. This behaviour is discussed in terms of the different dye-support interactions in both kinds of adsorption sites.     

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.     

Aggregation properties of heavy atom substituted squaraine dyes: evidence for the formation of J-type dimer aggregates in aprotic solvents

The squaraine dye bis(2,4,6-trihydroxyphenyl)squaraine (SqH) was earlier reported to form J-type dimer aggregates in acetonitrile solutions at higher concentrations. Subsequent studies have suggested that concentration-dependent changes in the absorption spectrum of SqH in acetonitrile could be attributed to shifts in the acid-base equilibrium due to the presence of water as an impurity. In this work, we describe our studies on the effect of varying acid and dye concentration on the absorption spectra of the bromo and iodo substituted dyes, bis(3,5-dibromo-2,4,6-trihydroxyphenyl)squaraine (SqBr) and bis(3,5-diiodo-2,4,6-trihydroxyphenyl)squaraine (SqI). Analysis of the changes in the absorption spectra as a function of dye concentration and the nature of the solvent composition confirmed the formation of J-type dimer aggregates in aprotic solvents in this class of dyes. Further confirmation for the formation of the J-type dimer aggregates could be obtained by comparing the differences in the triplet excited state properties of the neutral and aggregated forms of the dyes using time-resolved spectroscopy.     

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.     

Amplified quenching of a conjugated polyelectrolyte by cyanine dyes

The conjugated polyelectrolyte PPESO3 features a poly(phenylene ethynylene) backbone substituted with anionic 3-sulfonatopropyloxy groups. PPESO3 is quenched very efficiently (KSV > 10(6) M(-1)) by cationic energy transfer quenchers in an amplified quenching process. In the present investigation, steady-state and picosecond time-resolved fluorescence spectroscopy are used to examine amplified quenching of PPESO3 by a series of cyanine dyes via singlet-singlet energy transfer. The goal of this work is to understand the mechanism of amplified quenching and to characterize important parameters that govern the amplification process. Steady-state fluorescence quenching of PPESO3 by three cationic oxacarbocyanine dyes in methanol solution shows that the quenching efficiency does not correlate with the Forster radius computed from spectral overlap of the PPESO3 fluorescence with the cyanines' absorption. The quenching efficiency is controlled by the stability of the polymer-dye association complex. When quenching studies are carried out in water where PPESO3 is aggregated, changes observed in the absorption and fluorescence spectra of 1,1',3,3,3',3'-hexamethylindotricarbocyanine iodide (HMIDC) indicate that the polymer templates the formation of a J-aggregate of the dye. The fluorescence dynamics in the PPESO3/HMIDC system were probed by time-resolved upconversion and the results show that PPESO3 to HMIDC energy transfer occurs on two distinctive time scales. At low HMIDC concentration, the dynamics are dominated by an energy transfer pathway with a time scale faster than 4 ps. With increasing HMIDC concentration, an energy pathway with a time scale of 0.1-1 ns is active. The prompt pathway (tau < 4 ps) is attributed to quenching of delocalized PPESO3 excitons created near the HMIDC association site, whereas the slow phase is attributed to intra- and interchain exciton diffusion to the HMIDC.     

Effect of Concentration on the Photophysics of Dyes in Light‐Scattering Materials.

Photoactive materials based on dye molecules incorporated into thin films or bulk solids are useful for applications as photosensitization, photocatalysis, solar cell sensitization and fluorescent labeling, among others. In most cases, high concentrations of dyes are desirable to maximize light absorption. Under these circumstances, the proximity of dye molecules leads to the formation of aggregates and statistical traps, which dissipate the excitation energy and lower the population of excited states. The search for enhancement of light collection, avoiding energy wasting requires accounting the photophysical parameters quantitatively, including the determination of quantum yields, complicated by the presence of light scattering when particulate materials are considered. In this work we summarize recent advances on the photophysics of dyes in light‐scattering materials, with particular focus on the effect of dye concentration. We show how experimental reflectance, fluorescence and laser‐induced optoacoustic spectroscopy data can be used together with theoretical models for the quantitative evaluation of inner filter effects, fluorescence and triplet formation quantum yields and energy transfer efficiencies.     

Absorption,fluorescence and stimulated fluorescence by polaritons in thin anthracene crystals. A re-interpretation of spectroscopic properties of anthracene

The transmission, absorption (excitation spectra) and fluorescence spectra of thin (? 47 nm) free mounted anthracene flakes have been measured. True absorption in b polarization in the region of the lowest exciton state occurs as a result of scattering by phonons. It has a minimum near the transverse exciton frequency and a maximum near the longitudinal exciton frequency, in agreement with expected polariton behaviour. Thickness dependent polariton states have finite absorption and fluorescence transition probabilities (due to crystal inhomogeneities) and are observed below the transverse exciton frequency. These polariton states represent the energy reservoir for excitation energy in the bulk of the crystal. A surface-induced exciton state is the origin of the sharp line fluorescence from pure crystals and accounts for the high efficiency of stimulated fluorescence at low temperatures. Stimulated fluorescence can also be observed from the polariton modes when excitation occurs in these modes. Resonance interactions between polariton modes and impurity levels represent an important pathway for fluorescence quenching in crystalline anthracene.     

Excitons in semiconducting carbon nanotubes: diameter-dependent photoluminescence spectra

Semiconducting single-walled carbon nanotubes are one-dimensional (1D) quantum nanostructures and their unique optical responses arise from stable 1D excitons with huge binding energies. Here we review recent studies on optical properties of semiconducting carbon nanotubes. The diameter dependence of luminescence spectra and dynamics are revealed by single-nanotube spectroscopy and time-resolved optical spectroscopy. Short-range Coulomb interactions play a crucial role in energy structures of dark, triplet, and charged excitons. Enhanced exciton-exciton interactions in 1D semiconductor nanostructures determine nonlinear optical responses. We present generic configurations of neutral and charged excitons and discuss exciton optics of single-walled carbon nanotubes.     

Non-radiative deactivation pathways of subphthalocyanine and subnaphthalocyanine dyes and of their mixture

Subphthalocyanine and subnaphthalocyanine dyes and their mixture were investigated by means of the spectroscopic and photoelectric methods. Absorption, fluorescence, steady-state and time-resolved photothermal measurements for the dyes and their mixture were done in order to get information about the radiative and non-radiative deactivation processes as competetive processes to charge separation. It was shown that energy transfer between the dyes improved the photocurrent generation in photoelectrochemical cells (PEC) based on In(2)O(3) and SnO(2) as an electrode. The possible participation of the dye triplet states in non-radiative processes was discussed.     

Temperature-dependent relaxation of excitons in tubular molecular aggregates: Fluorescence decay and stokes shift

We report temperature-dependent steady-state and time-resolved fluorescence studies to probe the exciton dynamics in double-wall tubular J-aggregates formed by self-assembly of the dye 3,3'-bis(3-sulfopropyl)-5,5',6,6'-tetrachloro-1,1'-dioctylbenzimidacarbocyanine. We focus on the lowest energy fluorescence band, originating from the inner cylindrical wall. At low temperatures, the experiments reveal a nonexponential decay of the fluorescence, with a typical time scale that depends on the emission wavelength. At these temperatures we also find a dynamic Stokes shift of the fluorescence spectrum and its nonmonotonic dependence on temperature under steady-state conditions. All these data indicate that below about 20 K the excitons in the lowest fluorescence band do not reach thermal equilibrium before emission occurs, while above about 60 K thermalization on this time scale is complete. By comparing the two lowest fluorescence bands, we also find indications for fast energy transfer from the outer to the inner wall. We show that the Frenkel exciton model with diagonal disorder, which previously has been proposed to explain the absorption and linear dichroism spectra of these aggregates, yields a quantitative explanation to the observed dynamics. To this end, we extend the model to account for weak phonon-induced scattering of the localized exciton states; the spectral dynamics are then described by solving a Pauli master equation for the exciton populations.     

Ultrafast Exciton Self-Trapping and Delocalization in Cycloparaphenylenes: The Role of Excited-State Symmetry in Electron-Vibrational Coupling

Upon photon absorption, π-conjugated organics are apt to undergo ultrafast structural reorganization via electron-vibrational coupling during non-adiabatic transitions. Ultrafast nuclear motions modulate local planarity and quinoid/benzenoid characters within conjugated backbones, which control primary events in the excited states, such as localization, energy transfer, and so on. Femtosecond broadband fluorescence upconversion measurements were conducted to investigate exciton self-trapping and delocalization in cycloparaphenylenes as ultrafast structural reorganizations are achieved via excited-state symmetry-dependent electron-vibrational coupling. By accessing two high-lying excited states, one-photon and two-photon allowed states, a clear discrepancy in the initial time-resolved fluorescence spectra and the temporal dynamics/spectral evolution of fluorescence spectra were monitored. Combined with quantum chemical calculations, a novel insight into the effect of the excited-state symmetry on ultrafast structural reorganization and exciton self-trapping in the emerging class of π-conjugated materials is provided.     

Fluorescence of acridinic dyes in anionic surfactant solution

The interaction of the cationic dyes acridine, 9-aminoacridine (9AA), and proflavine, with sodium dodecyl sulfate (SDS) was studied by electronic absorption, steady-state and time-resolved fluorescence spectroscopies. The dyes interact with SDS in the pre-micellar region leading in two cases to dimerization in dye-surfactant aggregates, but with distinct molecular arrangements. For proflavine, the observed red shift of the electronic absorption band indicates the presence of J-aggregate, which are nonfluorescent. In the case of 9AA, the aggregates were characterized as nonspecific (neither J- nor H-type is spectroscopically observed). The time-resolved emission spectra gives evidences of the presence of weakly bound dimers by the recovery of three defined decay times by global analysis: dye monomer (tau1 = 16.4 ns), dimer (tau2 = 7.1 ns), and a faster component (tau3 = 2.1 ns) ascribed to intracluster energy migration between monomer and dimer. Acridine has a weak interaction with SDS forming only an ion pair without further self-aggregation of the dye.     

PHTHALOCYANINE FLUORESCENCE AT HIGH CONCENTRATION: DIMERS OR REABSORPTION EFFECT?

Abstract— For tetrasulfonated aluminum phthalocyanine (AlPcS₄), dimer formation is characterized in the absorption spectrum by a broadening of the Q-band and the appearance of a new band at the red edge of the spectrum. The high concentrations required to produce dimers, however, often leads to anomalous observations in fluorescence spectroscopy. In the present study, we have examined the photophysical characteristics of two dye systems; AlPcS₄ in a 66% ethanol/water mixture and disulfonated aluminum phthalocyanine in methanol. Using absorption spectroscopy, the formation of dimers is shown to be prevalent only in the case of AlPcS₄. The fluorescence emission spectra in both cases, however, exhibit similar spectral changes with increasing dye concentration. The measured fluorescence decay profiles for both dyes also show similar trends: They are monoexponential, invariant with emission wavelength and have decay times that increase with dye concentration. These distortions are sometimes incorrectly attributed to dimer fluorescence. We find no evidence for the existence of dimer fluorescence and demonstrate that these data can be readily explained, by taking into consideration the effects of reabsorption of fluorescence.     

Energy transfer in unsymmetrical phenylene ethynylene dendrimers

We have applied the fluorescence upconversion technique to explore the electronic excitation energy transfer in unsymmetrical phenylene ethynylene dendrimers. Steady-state emission spectra show that the energy transfer from the dendrons to the core is highly efficient. Ultrafast time-resolved fluorescence measurements are performed at various excitation wavelengths to explore the possibility of assigning absorption band structures to exciton localizations. We propose a kinetic model to describe the time-resolved data. Independent of the excitation wavelength, a typical rise-time value of 500 fs is measured for the fluorescence in the dendrimer without an energy trap, indicating initial delocalized excitation. While absorption is into delocalized exciton states, emission occurs from localized states. When an energy trap such as perylene is introduced on the dendrimer, varying the excitation wavelength yields different energy-transfer rates, and the excitation energy migrates to the trap through two channels. The interaction energy between the dendrimer backbone and the trap is estimated to be 75 cm(-1). This value is small compared to the vibronic bandwidth of the dendrimer, indicating that the monodendrons and the energy trap are weakly coupled.     

Spectroscopic studies of a near-infrared absorbing pH sensitive aminodienone-carbocyanine dye system

Synthetic red and near-infrared absorbing dyes may be used as probe molecules in a large number of applications. Dyes exhibiting spectral changes with hydrogen ion concentration are useful as pH probes. Those dyes which have their absorption and fluorescence maxima in the long wavelength region of the visible spectral region are specially valuable because of decreased interference and semiconductor laser applications. In this paper we have evaluated an aminodienone dyes 1 which demostrates pH dependent absorption and fluorescence spectra as well as solvent polarity dependence. In organic solvents the long wavelength absorption band of the dye is in the reduced interference region. The absorption maximum is at 535 nm in neutral or alkaline solutions in methanol. The absorption spectra undergo a strong bathochromic shift in the presence of acids (lambda(max) = 709 nm) with a concomitant change in the fluorescence spectra. This pH sensitive dye was found to be specially especially useful for organic solvents. The analytical utility of this and similar near-infrared absorbing dyes is discussed.     

ELECTRON PARAMAGNETIC RESONANCE OF SOME TRIPHENYLMETHANE DYES IN THE TRIPLET STATE

Abstract— Electron paramagnetic resonance spectra of the triplet states of several triphenylmethane dyes in glassy solutions at 90 K have been measured and their zero-field splitting parameters estimated. Crystal violet and para rosaniline do not possess trigonal symmetry in their triplet states, and the unusually broad absorptions in the Δ M ₈= 1 region of the spectra have been attributed to the presence of different rotational isomers of the dye cations. A number of malachite green derivatives were investigated, but absorption of the triplet states of these dyes was only observed in the low field Δ M ₈= 2 region of the spectrum.     

Two-step resonance energy transfer between dyes in layered silicate films

Single- and two-step fluorescence resonance energy transfer (FRET) was investigated between laser dyes rhodamine 123 (R123), rhodamine 610 (R610), and oxazine 4 (Ox4). The dye molecules played the role of molecular antennas and energy donors (ED, R123), energy acceptors (EA, Ox4), or both (R610). The dye cations were embedded in the films based on layered silicate laponite (Lap) with the thickness of several μm. Optically homogeneous films were prepared directly from dye/Lap colloids. Dye concentration in the films was high enough for FRET to occur but sufficiently low to prevent the formation of large amounts of molecular aggregates. The films were characterized by absorption and fluorescence spectroscopies, and their optical properties were compared with colloid precursors and dye aqueous solutions. The phenomenon of FRET was confirmed by means of steady-state and time-resolved fluorescence spectroscopies. Significant quenching of ED emission in favor of the luminescence from EA molecules was observed. FRET led to the decrease in the lifetimes of excited states of ED molecules. Molecular orientation of dye molecules was determined by polarized absorption and fluorescence spectroscopies. Almost parallel orientation with respect to silicate surface (～30°) was determined for all fluorescent species of the dyes. Theoretical model on relationship between anisotropy and molecular orientation of the fluorophores fits well with measured data. The analysis of anisotropy measurements confirmed the significant role of FRET in the phenomenon of light depolarization.     

Photoprocesses of Xanthene Dyes Bound to Lysozyme or Serum Albumin

The ground and excited state processes of eosin, erythrosin and rose bengal in aqueous solution were studied in the presence of lysozyme or bovine serum albumin (BSA). Noncovalent protein-dye binding was analyzed by circular dichroism (CD), fluorescence and UV–Vis absorption spectroscopy. The effects of protein concentrations and pH were studied. Fluorescence quenching of the dye takes place due to binding to lysozyme and fluorescence enhancement due to low loading to BSA. The effects of proteins on the xanthene triplet state and its precursor were observed by time-resolved 530 nm photolysis. The triplet lifetime is quenched by lysozyme and prolonged by loading to BSA. Light-induced damages on both the dyes and proteins were observed under exclusion of oxygen. Photo-oxidation is efficient for lysozyme and lower for BSA. The CD signal of the eosin/BSA system is maximum at pH 4, where the photo-oxidation is minor.     

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.     

New Ru(II) chromophores with extended excited-state lifetimes

We describe the synthesis, electrochemical, and photophysical properties of two new luminescent Ru(II) diimine complexes covalently attached to one and three 4-piperidinyl-1,8-naphthalimide (PNI) chromophores, [Ru(bpy)(2)(PNI-phen)](PF(6))(2) and [Ru(PNI-phen)(3)](PF(6))(2), respectively. These compounds represent a new class of visible light-harvesting Ru(II) chromophores that exhibit greatly enhanced room-temperature metal-to-ligand charge transfer (MLCT) emission lifetimes as a result of intervening intraligand triplet states ((3)IL) present on the pendant naphthalimide chromophore(s). In both Ru(II) complexes, the intense singlet fluorescence of the pendant PNI chromophore(s) is nearly quantitatively quenched and was found to sensitize the MLCT-based photoluminescence. Excitation into either the (1)IL or (1)MLCT absorption bands results in the formation of both (3)MLCT and (3)IL excited states, conveniently monitored by transient absorption and fluorescence spectroscopy. The relative energy ordering of these triplet states was determined using time-resolved emission spectra at 77 K in an EtOH/MeOH glass where dual emission from both Ru(II) complexes was observed. Here, the shorter-lived higher energy emission has a spectral profile consistent with that typically observed from (3)MLCT excited states, whereas the millisecond lifetime lower energy band was attributed to (3)IL phosphorescence of the PNI chromophore. At room temperature the data are consistent with an excited-state equilibrium between the higher energy (3)MLCT states and the lower energy (3)PNI states. Both complexes display MLCT-based emission with room-temperature lifetimes that range from 16 to 115 micros depending upon solvent and the number of PNI chromophores present. At 77 K it is apparent that the two triplet states are no longer in thermal equilibrium and independently decay to the ground state.