Excited State Fluorescence Research Articles

Electro optical emission measurements on a non-conjugated bichromophoric donor-acceptor molecule

Electro optical emission measurements have often been used for the determination of dipole moments of fluorescent excited states. In this publication results from such measurements on a non-conjugated bichromophoric donor-acceptor molecule in cyclohexane, benzene and dioxane solutions are reported and compared to literature values determined by quite different time resolved microwave conductivity (TRMC) measurements. The dipole moment of the first excited state is found to be around 80 × 10-30 cm, in benzene and in dioxane which is in very good agreement with the results from TRMC measurements, if a comparable description of the in solution effective electric field is used with both methods. The results in cyclohexane do not agree, most probably because of the short excited state life time in this solvent. In addition, the direction of the transition moment was determined from the electro optical measurements to be parallel to the excited state dipole moment. This information cannot be derived from TRMC m...

Human serum albumin-benzo[a]pyrene anti-diol epoxide adduct structure elucidation by fluorescence line narrowing spectroscopy.

Cryogenic (4-10 K) laser-induced vibrationless ground state and vibronic excited state fluorescence emission spectra of the adducts resulting from reaction in vitro of human serum albumin and the carcinogen (+-)-r-7,t-8-dihydroxy-c-9,c-10-epoxy-7,8,9,10- tetrahydrobenzo[a]-pyrene were recorded in order to determine the structures formed. Comparison of these fluorescence line-narrowed (FLN) spectra to those obtained from BaP-7,8,9,10- tetrahydrotetrols, synthetic N-t-BOC-alaninate ester, and N tau- and N pi-histidine amine anti-BaPDE adducts revealed that a mixture of adduct types are formed with the protein. Extensive dialysis of the adducted protein simplified the FLN spectrum, causing it to become nearly identical to the FLN spectrum obtained from the stable peptide adduct. Comparison of the FLN spectra of the synthetic histidine adducts to those obtained from peptide adducts isolated from enzymic digestion of the adducted protein indicated that only one of the imidazole nitrogens is the nucleophile which forms a stable adduct with anti-BaPDE. The FLN studies confirm that N tau-histidine adducts are formed between human serum albumin and the C-10 position of anti-BaPDE.

Luminescent Properties of Rare Earth Doped AlF3-Based Glasses

ABSTRACTRare earth ions can easily be incorporated into fluoride glasses in moderate to large concentrations and, due to their low phonon energy, these glasses appear to have many advantages over oxide glasses as hosts for rare earth ions used in optical amplifiers and lasers. We have therefore investigated the optical properties of Pr3+, Pr3+/Yb3+ and Pr3+/Yb3+/Lu3+ doped bulk AIF3-based glass samples as a function of rare earth ion concentration. We find that the addition of 2 wt% of Yb increases the fluorescence of Pr3+ at 1.32 μm by a factor of 35 when excited with 488 nm radiation. The fluorescence intensity and excited state lifetimes are found to be comparable to those measured for Pr in a ZBLAN host. Since it has also been demonstrated that optical fibers drawn from AIF3-based glasses exhibit relatively low loss (< 0.05 dB/m) and posses superior chemical durability compared to other fluotide glasses, it is possible that AIF3 glasses may become the fluoride glass of choice for practical fiber laser and amplifier applications.

Charge transfer emissive singlet excited states and photoinduced electron transfer properties in the diarylideneacetone compounds (RCHCH)2CO; R = phenyl, 1- and 2-naphthyl, 3-(N-ethylcarbazoyl), and 4-(C5H5)Fe(C5H4C6H4CHCH(CO)CHCHC6H5)

Four diarylideneacetone compounds ((RCHCH)2CO, where the aryl groups are phenyl (dba), 1-naphthyl (1-dNapha), 2-naphthyl (2-dNapha), and 3-(N-ethylcarbazoyl) (dNECa)), and 4-(C5H5)Fe(C5H4C6H4CHCH(CO)CHCH(C6H5) (dba-Fc) have been prepared and characterized. The compounds are found to be fluorescent and photochemically and reversibly electrochemically active. The lowest-energy absorption bands for the diarylideneacetones are assigned to a charge transfer (CT) electronic transition, except for dba-Fc, in which a ferrocenyl ligand field transition assignment is made. The 77 K CT absorption and fluorescence bands are vibrationally structured (vibrational spacings = 1260–1360 cm−1). While the fluorescence decay at 293 K is monoexponential, the excited state fluorescence lifetimes (τF) for the 77 K samples exhibit double exponential decays, the short component being 0.38–0.64 ns and the long one 3.5–10.9 ns. The photophysical results are interpreted in terms of excited state deactivation processes dominated by radiationless pathways that are associated with the presence of fluorescent species with different geometries. Only the dNECa compound is found to be fluorescent in solution at 298 K [Formula: see text]. Cyclic voltammetry and coulometry measurements suggest that a reversible one-electron reduction process and an irreversible higher potential one-electron reduction process take place in the −1 to −2 V vs. SSCE range. In addition, dba-Fc also exhibits an electrochemically reversible one-electron oxidation wave at 0.52 V vs. SSCE centered at the ferrocenyl group. These results together with the spectroscopic electronic data have permitted evaluation of the reduction potentials of the lowest singlet (CT excited states (E1−/*);they range from 1.4 to 2.2 V vs. SSCE, with dba being the strongest photooxidizing agent and dNECa the weakest. Photoinduced intermolecular electron transfer reactions have been investigated by steady state fluorescence techniques and picosecond flash photolysis spectroscopy for dNECa and dba, respectively. The bimolecular deactivation rate constants, kq, for the reductive photoinduced electron transfer reactions of dNECa with diphenylamine (DPA) (kq = (2.65 ± 0.25) × 107 M−1 s−1) and N, N, N′, N′-tetramethylphenylenediamine (TMPM) (kq = (1.38 ± 0.03) × 108 M−1 s−1) have been obtained in THF solutions at 293 K. No fluorescence quenching is observed when oxidative and energy transfer quenchers are used with dNECa. For the non-emissive dba compound at room temperature, picosecond flash photolysis experiments show that quenching of the broad dba transient band (~500 nm) does indeed occur between 5 and 10 ns. Keywords: dibenzylideneacetone, charge transfer, photoelectron transfer.

Chlorophyll-porphyrin heterodimers with orthogonal .pi. systems: solvent polarity dependent photophysics

The synthesis and properties of a series of fixed-distance chlorophyll-porphyrin molecules are described. These molecules consist of a methyl pyrochlorophyllide a moiety which is directly bonded at its 2-position to the 5-position of a 2,8,12,18-tetraethyl-3,7,13,17-tetramethyl-15-(p-tolyl)porphyrin. Steric hindrance between adjacent substituents rigidly positions the {pi} systems of both macrocycles perpendicular to one another. The macrocycles were selectively metalated with zinc to give the four possible derivatives, HCHP, ZCHP, HCZP, and ZCZP, where H, Z, C, and P denote free base, Zn derivative, chlorin, and porphyrin, respectively. The lowest excited singlet states of HCHP and ZCHP, which are localized on HC and ZC, respectively, exhibit lifetimes and fluorescence quantum yields that are solvent polarity independent and do not differ significantly from those of chlorophyll itself. ZCZP, however, displays a lowest excited singlet state and lifetime and fluorescence quantum yield that are strongly solvent polarity dependent.

Excited States Dipole Moments and Fluorescence Behaviour of Some Derivatives of Dimethylaminobenzonitrile

Abstract Two sterically hindered derivatives of dimethylaminobenzonitrile (DMCA) have been synthesized and the dipole moment of their fluorescent excited state has been determined in differently polar solvents. Its value increases with solvent polarity up to a limiting value of (50 + 2.5) • 10-30 Cm. The results are discussed with reference to the twisted intramolecular charge transfer (TICT) state formation in the class of molecules related to DMCA.

Fluorescent solvent excited states from hole-electron recombination in cis- and trans-decalin irradiated with high energy electrons

The formation of the fluorescent solvent excited states in cis- and trans-decalin in the presence of various electron and hole scavengers has been studied by measuring the fluorescence resulting from pulsed irradiation. In order to be able to determine the effect of the charge scavengers on the yield of formation of the fluorescent excited state the quenching of the excited state by the scavengers has been studied in detail. Diffusion coefficients of the solutes have been determined and reaction radii for the quenching reactions are given. The ratio of the depression of the yield of formation of fluorescent excited states and the yield of charge scavenging has been found to be approx. 0.9 for both electron and hole scavengers and shows a tendency to decrease at higher concentrations. The results are consistent with earlier independent measurements of the mobilities of electron holes and excess electrons. The ratio of approx. 0.9 represents the average probability of singlet excited state formation from ion recombination for the relatively longer lived ion pairs, which indicates a relatively large contribution of single ion pairs. The average probability of singlet formation from the shorter lived ion pairs is expected to be lower.

Photophysical Properties of The DCM and DFSBO Styryl DyesConsequence for Their Laser Properties

The two styryl dyes, 4‐dicyanomethylene‐2‐methyl‐6‐P‐dimethylaminostyryl‐4H‐pyran (DCM) and 7‐ dimethylamino‐3‐(p‐formylstyryl)‐l, 4‐benzoxazin‐2‐one (DFSBO) exhibit similar solvent‐induced shifts of their absorption and emission spectra related to a large intramolecular charge transfer (ICT) in the first singlet excited state. From the Stokes shift values (vA − vF) and a vectorial analysis of their ground state dipole moment (μg= 6.1 D for DCM and 5.8 D for DFSBO), and using the Lippert‐Mataga theory, we have estimated the dipole moments of their fluorescent excited states S1 (μe = 26.3 D for DCM and 27.6 D for DFSBO). Intersystem crossing to the triplet state is totally inefficient in DCM but significant in DFSBO. Moreover the absorption of the DFSBO triplet is quite large in the emission band (600–650 nm), which makes of DFSBO a poor laser dye. Although DCM trans‐cis photoisomerization can be quite efficient in non polar solvents (chloroform, tetrahydrofuran), DFSBO does not photoisomerize probably due to steric hindrance and to the S1 character which should be more "benzoxazinone" than ethylenic. DFSBO is also shown to exhibit rotamerism.

Characterization of ground-state neutral and ion transport during laser ablation of Y1Ba2Cu3O7−x using transient optical absorption spectroscopy

Transient optical absorption spectroscopy has been utilized for the first time to study the transport of ground-state Y, Ba, Cu, and Ba+ following excimer laser ablation of Y1Ba2Cu3O7−x pellets. Spectral broadening of the atomic lines monitored in both absorption and emission is reported, indicating the existence of gas phase collisions in the plume of ejected material. Time-of-flight velocity distributions of the nonemitting neutrals and ions determined by the absorption technique are broadened and shifted to lower velocities than the velocity distributions inferred from excited-state fluorescence in the plume. Absorption by ground-state Y+, YO, BaO, and CuO also has been observed with this technique. The absorption technique, and its application as an in situ monitor of neutral and ion transport during deposition of superconducting thin films, is described.

ChemInform Abstract: Hydrogen‐Abstraction Reactivity of Excited‐State Para‐Substituted Benzyl Radicals in Solution at Room Temperature.

AbstractIn contrast to almost no H‐abstraction reactivtiy of the ground‐state p‐X‐benzyl radicals (X: ‐CN, ‐Cl, ‐F, ‐O‐Me) toward 1,4‐cyclohexadiene (CHD), a significant fluorescence quenching by CHD in hexane at room temp. is confirmed for their fluorescent excited states, which may be assigned to 2B2 for p‐cyanobenzyl and 1A2 for the other three p‐X‐benzyls.

Intramolecular excited state charge transfer and fluorescence decay of p-Cyano-N,N-dimethylaniline in mixed hydrocarbon/halocarbon solvents

Fluorescence spectra, quantum yields and lifetimes of p-cyano-N,N-dimethylaniline (CDMA) were studied in various binary mixtures of n-hexane with chlorinated hydrocarbons. Solvent effects on the equilibrium between normal emission and fluorescence originating from the highly dipolar twisted internal charge transfer (TICT) state arise predominantly from non-specific solute-solvent interactions. The non-radiative deactivation rate of the TICT state is affected primarily by the solvent composition and slows down with the stabilization of the TICT state in environments of high polarity.

Torsional dynamics of molecules on barrierless potentials in liquids. III. Pressure dependent picosecond studies of triphenyl-methane dye solutions in a diamond anvil cell

Pressure dependent measurements of the nonradiative torsional dynamics of triphenyl-methane dyes are reported. The viscosity dependence of the excited state absorption and fluorescence decays of crystal violet, dissolved in methanol, ethanol, and n-propanol, are studied over a viscosity range of between 0.5 and 500 cP (pressures up to 1.5 GPa ∼15 000 atm). The overall viscosity dependence of the excited state decay time is linear. The nonlinear viscosity dependencies observed in earlier studies may have resulted from the complex nature of the solvents used and, in the case of fluorescence quantum yield measurements, from a pressure dependence of the radiative rate. Differences between the viscosity dependence observed when varying pressure, temperature, or solvent type at low viscosities are tentatively ascribed to pressure and temperature induced conformational changes.

APUD cell development in grafts from chick or quail presumptive enteric endoderm combined with a fragment of notochord : E.L.M.J. Wiertz-Hoessels and J. Drukker. Department of Anatomy/Embryology, Medical Faculty, University of Limburg, Maastricht, the Netherlands

Characterization of the internal environment of a sol–gel matrix is an important area of investigation in optical biosensors. In the present study, different sol–gel compositions were prepared by varying the water (H2O) to tetraethyl-orthosilicate (TEOS) ratio (R) from 1 to 16 and the changes in the internal environment of the sol–gel both in bulk and thin films as a function of aging (storage) were investigated using fluorescence spectroscopy. We focussed on the fluorescence characteristics , viz. emission and excited state lifetime of Hoechst 33258 (H258), a bisbenzimidazole derivative, which was used as fluorescence probe entrapped in the TEOS derived sol–gel bulk and thin films. These sols were prepared at a low pH (∼2.0) and the thin films were coated by dip coating technique at withdrawal speeds of 1 cm/min and 0.1 cm/min. Usually, uniform thin films were obtained at a high speed (1 cm/min) and partially cracked film at a low speed (0.1 cm/min) as observed by fluorescence microscope. These observations did not change during aging. On the contrary, three months long observations on steady-state fluorescence emission measurements on H258 depicted a blue shift from 535 nm to 508 nm at R = 1 in the sol–gel bulk, whereas at higher ratios this was not prominent. At all ratios, dual emission bands were observed in thin films. This may be due to faster sol–gel to xerogel transition during aging depending on the ratio (R). Analysis of the excited state decay profiles of H258 revealed a double exponential fitting having a short (τ1) and a long (τ2) component in both fresh and during aging, in the sol–gel bulk and thin films, indicating heterogeneity in the internal environment. The value of τ1 increased from 0.4 ns to 1.2 ns whereas τ2 attained a value from 3.0 ns to 3.6 ns at R = 1 upon aging in the sol–gel bulk. The corresponding values of τ1 and τ2 in thin films were 0.3 ns and 3.5 ns, respectively. The values of these decay components in thin films did not alter much due to storage, but their relative contributions showed more systematic changes in the thin films. The observed changes could be correlated to rigidification in the bulk depending on the ratio (R). This process was very slow at R ≥ 4. The heterogeneity in the internal environment of bulk and thin films upon aging appeared to be different as revealed from analysis of excited-state lifetime. Thus, the bisbenzimidazole derivative H258 appears to be very useful probe for characterizing the internal environment of both the sol–gel bulk and thin films.

Dynamics of geminate ions in the pulse radiolysis of cyclohexane and n-hexane containing aromatic solutes studied by streak camera measurements of fluorescence and absorption

The nature of geminate ion reactions in cyclohexane or n-hexane solutions irradiated with 30 ps electron pulses has been investigated by observing solute excited state fluorescence and absorption of solute ions. For the fluorescence measurements, 4-Br,4″hexa(5-decyl)- p-terphenyl(BHTP) was used as the solute. The short fluorescence lifetime (0.3 ns) of the excited singlet state of this compound (BHTP 1) facilitates the interpretation of the results because BHTP 1 formed in an excitation process occuring during the pulse decays rapidly enough that the slower production of BHTP 1 by reaction of BHTP ⨪ with the radical cation of the solvent is more easily discernible than in the case of scintillators with longer lifetimes. Comparison of the results with a model indicates a qualitative correspondence with the known properties of ions and electrons in the two solvents. In particular, the results support a higher mobility cation derived from the solvent in cyclohexane, compared with n-hexane. Quantitatively, however, the experimental data show a greater amount of excitation occuring within the first 20 ps (during the pulse) than is expected from the model. Results on the formation of the pyrene radical cation in cyclohexane and n-hexane solutions of pyrene also show significant differences between the two solvents which indicate a slower formation of the pyrene cation in n-hexane.

Steric Effects in the Electronic Spectra of the 3,5-Diarylaminobenzene Derivatives

Absorption and fluorescence properties, excited state lifetimes and fluorescence quantum yields were determined for a series of 3,5-diarylaminobenzene derivatives in solvents of different polarities. The role of the nitrile, methyl, phenyl and naphthyl substituents is discussed. Especially the steric effects on the spectroscopic behaviour of the investigated molecules are studied.

Lateral distribution of a pyrene-labeled phosphatidylcholine in phosphatidylcholine bilayers: fluorescence phase and modulation study.

The lateral distribution of 1-palmitoyl-2-[10-(1-pyrenyl)decanoyl]phosphatidylcholine (PyrPC) was studied in small unilamellar vesicles of 1,2-dipalmitoyl-, 1,2-dimyristoyl-, and 1-palmitoyl-2-oleoyl-phosphatidylcholine (DPPC, DMPC, and POPC, respectively) under anaerobic conditions. The DPPC and DMPC experiments were carried out over temperature ranges above and below the matrix phospholipid phase transition temperature (Tm). The excimer to monomer fluorescence intensity ratio (E/M) was determined as a function of temperature for the three PyrPC/lipid mixtures. Phase and modulation data were used to determine the temperature dependence of pyrene fluorescence rate parameters in gel and in liquid-crystalline bilayers. These parameters were then used to provide information about excited-state fluorescence in phospholipid bilayers, calculate the concentration of the probe within liquid-crystalline and gel domains in the phase transition region of PyrPC in DPPC, and simulate E/M vs. temperature curves for three systems whose phase diagrams are different. From the simulated curves we could determine the relationship between the shape of the three simulated E/M vs. temperature curves and the lateral distribution of the probe. This information was then used to interpret the three experimentally derived E/M vs. temperature curves. Our results indicate that PyrPC is randomly distributed in pure gel and fluid phosphatidylcholine bilayers.(ABSTRACT TRUNCATED AT 250 WORDS)

Picosecond laser study of the collisionless photodissociation of dimethylnitramine at 266 nm

Abstract We report a picosecond pump-probe study of the UV photolysis of gaseous dimethylnitramine. After photolysis by picosecond laser pulses at 266 nm, efficient monophotonic collision-free photodissociation occurs within 6 ps. NO 2 fragments are formed in the ground electronic state and in a fluorescent excited state; the quantum yields for both of these channels are estimated.

A study on the excited states of liquid p-dioxane by means of picosecond pulse radiolysis

The flourescence phenomena in liquid p-dioxane systems were investigated by pulse radiolysis technique, and at the same time, comparative 200 nm steady-state photoexcitation experiments were carried out. The spectra ontained by the two excitation methods are in good agreement with each other. This indicates that the same fluorescent excited states are formed. The pulse radiolysis of neat p-dioxane showed that the fluorescence has a clear growth in 1 ns after irradiation and its time profile is independent of wavelength. The formation of the state could be explained well by geminate ion recombination of holes and electrons. The lifetime and G-value of the state were determined to be 2.6 ± 0.2 ns and 1.0 ± 0.1 at 18°C, respectively. In the presence of a polar solute, such as water or alcohol, the emission spectrum is shifted to a longer wavelength region. It has been found that the growth of emission is dependent on wavelength, while the lifetime is independent. On the other hand, addition of a non-polar solute, such as isooctane or cyclohexane, makes a slight shift of the emission spectrum to a shorter wavelength region and the decay becomes faster. For all systems, the yields of emission and the lifetimes for the excited states were measured as a function of concentration of additives. Furthermore, measurements were made for ternary mixture systems ( p-dioxane, water and isooctane or cyclohexane). Based on the above experimental results, the excitation process in the liquid p-dioxane systems is discussed.

Structural features of the binding site of cholera toxin inferred from fluorescence measurements.

The dependence on pH of the fluorescence of cholera toxin and its A and B subunits has been studied at 25 degrees C. The fluorescence intensity of cholera toxin is highly pH-dependent. In the pH range 7-9.5 it reaches a maximum corresponding to a quantum yield of 0.076. In the pH range 4-7 a strong increase in fluorescence intensity is observed (delta Q/Qmax = 0.64). Evaluation of the pH sensitivity of the fluorescence intensity of the A and B subunits reveals that the B subunit is mainly responsible for the observed pH effect (delta Q/Qmax for B subunit = 0.64). The intensity changes are paralleled by similar although less pronounced changes in the average fluorescence excited state life-time tau (delta tau/tau max = 0.33 for cholera toxin). Fluorimetric titration of the B subunit, which is related to the indole fluorescence of the lone Trp-88, reveals that the fluorescence intensity changes in the pH range 4-7 are due to reaction of two types of ionizable quencher displaying apparent pKa values of 4.4 and 6.2, respectively. It is suggested that the increase in fluorescence intensity with a midpoint at pH 6.2 is the result of deionization of the imidazolium side-chain of one or two out of the four histidine residues present in each beta-polypeptide chain, whereas a deionized carboxyl group is responsible for the quenching with midpoint at pH 4.4. Complex formation of cholera toxin or B subunit with the monosialoganglioside GM1 or the oligosaccharide moiety of GM1 (oligo-GM1) completely prevents the quenching by both quenchers. Addition of 6 M urea also eliminates the pH effect. The quenching is not the result of the dissociation of the B subunit into its constituent monomers. Upon fluorimetric titration of cholera toxin or B subunit above pH 9, a progressive drop in both fluorescence intensity and tau occurs. This decrease could be due to energy transfer from the indole moiety of Trp-88 to ionized tyrosines or by quenching through an unprotonated epsilon-amino group of lysine. Fluorimetric titration of the A subunit indicates that the tryptophan fluorescence is only moderately altered by ionizable groups displaying a pKa in the range 4 to 9. Activation of A subunit does not affect this lack of pH sensitivity. Above pH 9, however, a much more significant drop in the fluorescence intensity of activated A subunit occurs. The structural implications of the results are discussed.

Upper excited state reactivity and fluorescence of fused 8-cyanoheptafulvenes

Upper excited state reactivity and fluorescence of fused 8-cyanoheptafulvenes