Emission Transition Moments Research Articles

Fluorescence spectroscopy of ancient sedimentary organic matter via confocal laser scanning microscopy (CLSM)

Fluorescence spectroscopy via confocal laser scanning microscopy (CLSM) was used to analyze ancient sedimentary organic matter, including Tasmanites microfossils in Devonian shale and Gloecapsomorpha prisca (G. prisca) in Ordovician kukersite from North American basins. We examined fluorescence emission as a function of excitation laser wavelength, sample orientation, and with respect to location within individual organic entities and in transects across bedded organic matter. Results from spectral scans of the same field of view in Tasmanites with different laser lines showed progressive red-shift in emission maxima with longer excitation wavelengths. This result indicates steady-state Tasmanites fluorescence emission is an overlapping combination of emission from multiple fluorophore functions. Stokes shift decreased with increasing excitation wavelength, further suggesting the presence of multiple fluorophore functions with different S1 → S0 transition energies. This observation also indicates that at longer excitation wavelengths, less absorbed light energy is dissipated via collisional transfer than at shorter excitation wavelengths and may suggest fewer polar functions are preferentially absorbing. Confirming earlier results, emission spectra observed from high fluorescence intensity regions (fold apices) in individual Tasmanites are blue-shifted relative to emission from other locations in the same microfossil. We suggest high intensity emission is from photoselective alignment of polarized excitation with the fluorophore absorption and emission transition moment. The blue shift observed in regions of high intensity emission may be due to relative absence or realignment of polar species, e.g., bridging ether or ester functions, although variations in O abundance could not be confirmed with preliminary time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis. Tasmanites occurring in consolidated sediments are flattened from original spherical morphology and, in optical microscopy, this burial deformation results in generally parallel extinction (strain-influenced) and positive elongation. The deformation also induces fluorescence anisotropy observed as variations in emission wavelength when individual Tasmanites are measured from their long axis parallel to bedding, whereas this effect is absent in bedding-normal view. Transects from G. prisca-rich source layers into adjacent reservoir layers show decrease in fluorescence intensity and spectral red-shift (increase in full-width half-maximum with increasing red portion of the half-width). These results may suggest an increase in fluorescence quenching across the source-to-reservoir transition zone, consistent with an increase in aromaticity following petroleum expulsion and migration. These observations are supported by increasing reflectance values measured across similar micro-scale transects. Our results highlight the applicability of CLSM as a broad and under-utilized approach for the characterization of sedimentary organic matter and are discussed with perspective toward petroleum processes and thermal indices research.

The orientation of the transition dipole moments of a polyene antibiotic Amphotericin B under UV–VIS studies

Amphotericin B (AmB) belongs to naturally occurring fluorescent antibiotics, commonly used in the treatment of life-threatening fungal infections. Open question regarding mechanism of action of this molecule calls for its orientation and organization studies in biomembranes. Here, we present studies on linear dichroism and fluorescence polarization of AmB embedded in isotropic and oriented poly(vinyl) alcohol films to characterize their transition dipole moments to low energy excited electronic transitions S1 (2Ag−) and S2 (1Bu+). The dichroic ratio and fluorescence anisotropy data were analyzed for stretched PVA films doped AmB. The results show that the transition moment for absorption makes an angle φ=27°±2° with the molecular axis of AmB defined by the film stretching direction. The angles between the absorption and emission transition moments have been found for both the low excited electronic states, S2 (β=4°±5°) and S1 (β=6°±5°). The fluorescence anisotropy analysis from the S2 state reveals additional component assigned to antiparallel AmB dimeric structure.

Characterization of trans-2-[4-[(Dimethylamino)styryl]benzothiazole as an Ultrafast Isomerization Probe and a Modifed Kramers Theory Analysis

The photophysics of trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT) was investigated by electronic structure calculations and steady-state and time-resolved emission spectroscopy in a wide range of solvents, including temperature and pressure dependence. DMASBT undergoes a facile photoinduced trans-cis isomerization similar to the reaction of trans-stilbene. The cis isomer has a lifetime of ∼1 ps and does not contribute appreciably to the steady-state emission spectrum. The absorption spectrum of the cis form overlaps that of the trans form such that considerable care is needed in determining correct emission quantum yields. The approximate equality of absorption and emission transition moments of DMASBT in all solvents indicates that absorption and emission involve a single excited state with high radiative rate. The low quantum yields of DMASBT in low-viscosity solvents reflect emission lifetimes in the 20-50 ps range. The nonradiative rates of DMASBT, which are assumed to measure the rate of isomerization in S1, depend upon both solvent viscosity and polarity. A modified Kramers analysis, which allows for a polarity-dependent barrier height, provides a satisfactory description of these rates but only if it is assumed that friction in alcohol solvents is more weakly dependent upon viscosity than in other types of solvents.

The Influence of Temperature on Coumarin 153 Fluorescence Kinetics

The influence of temperature varied in the range 183 K–323 K on the fluorescence quantum yield, fluorescence lifetime, absorption and emission transition moments and non-radiative deactivation rate was determined for the well known and largely used dye Coumarin 153, dissolved in 1-chloropropane. The Kennard-Stepanov relation connecting the absorption and emission spectra was used to check for the presence of more than one absorbing/emitting species and to investigate whether intramolecular vibrational redistribution completes in the C153 excited S 1 state before the emission takes place. The emission spectrum corresponding to S 1→S 0 transition, was fitted at each temperature to the model function including the information on the dye vibrational modes coupling. In this way the displacement in equilibrium distance for the most active vibrational mode was determined for C153 in S 1 and in S 0. Using the temperature dependence of the fluorescence decay time and quantum yield, the non-radiative deactivation rate was determined. Its temperature dependence was compared to that calculated using the theoretical model with the most active vibrational mode displacement values taken from steady-state spectra analysis. The somewhat surprising dependence of the fluorescence decay time and quantum yield on temperature was related to non-trivial coupling between low-frequency vibrational modes of C153 in the excited and ground states.

Rapid internal conversion in a symmetric molecule LD 700 studied by means of femtosecond fluorescence depletion

The rapid internal conversion dynamics at room temperature is determined by using the femtosecond time-resolved fluorescence depletion measurements of a complex solvated molecule of LD 700 (rhodamine 700) combined with steady-state absorption and fluorescence spectroscopy, as well as quantum chemical calculation. The molecule is excited by a 50 fs laser pulse at 400 nm which directly populated the highly excited singlet state, the rapid internal conversions (ICs) are observed, which leads to the directional changes of the emission transition moment following photoexcitation to the highly excited singlet state S5 of LD 700.

Electrolyte-Concentration and Ion-Size Dependence of Excited-State Intramolecular Charge-Transfer Reaction in (Alkylamino)benzonitriles: Steady-State Spectroscopic Studies

Steady-state spectroscopic studies have been performed with three intramolecular charge-transfer molecules, 4-(1-azetidinyl)benzonitrile (P4C), 4-(1-pyrrolidinyl)benzonitrile (P5C), and 4-(1-piperidinyl)benzonitrile (P6C), in ethyl acetate and acetonitrile in presence of lithium perchlorate (LiClO(4)) at room temperature to investigate the effects of electrolytes on excited-state intramolecular charge-transfer reaction. Electrolyte-concentration and ion-size dependences of several spectroscopic properties such as quantum yield, absorption and emission transition moments, radiative and nonradiative rates, and changes in reaction free energies associated with LE --> CT conversion have been determined for these molecules and reported. For P4C, quantum yield decreases by a factor of approximately 7 at the highest electrolyte concentration relative to that in pure ethyl acetate whereas it is a factor of approximately 4 for both P5C and P6C. However, in acetonitrile with 1.0 M LiClO(4), quantum yield reduces to almost half of that in the pure solvent. Formation of a charge-transfer (CT) state is found to be strongly favored over the locally excited (LE) state as the electrolyte (LiClO(4)) concentration is increased, electrolyte effects being more pronounced in ethyl acetate than in acetonitrile. Relative to pure ethyl acetate, reaction free energy change (-DeltaG(r)) increases by a factor of approximately 5, approximately 4, and approximately 2 for P4C, P5C, and P6C, respectively, at 2.5 M LiClO(4) in this solvent. -DeltaG(r) for P4C exhibits a change in sign (from negative to positive) upon addition of electrolyte in ethyl acetate. In acetonitrile, however, these changes are within a few percent, except for P4C where it is about 4 times greater at 1.0 M LiClO(4) than that in pure acetonitrile. The electrolyte-induced total red shift of the CT band of these TICT molecules is 3 times higher in ethyl acetate than in acetonitrile. Although both the quantum yield and CT emission peak frequency decrease linearly with the increase in ion size, -DeltaG(r) remains largely insensitive. Further studies using a nonreactive probe (coumarin 153) in concentrated electrolyte solutions also show qualitatively similar results.

Features of fluorescence spectra of polyethylene 2,6‐naphthalate films

AbstractThe characteristics of the fluorescence spectra of polyethylene 2,6‐naphthalate (PEN) films were compared with those of poly(ethylene terephthalate) (PET). The fluorescence spectra of PEN films were found, by photoselection, to consist of two components, one peaking at 425 nm and the other at 445 nm, with their emission transition moments perpendicular to each other. The integrated excitation spectra of the PEN films were much more intense than those of PET. After the resolution of the integrated excitation spectra of PEN, most, but not all, of their component peaks corresponded to those of the absorption spectra. On the basis of this fact together with previous data on photodegradation, the spectral component peaking at 445 nm originated, at least partly, from traces of photochemical products generated by exposure during the measurements. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 105: 114–121, 2007

Compartmental Modeling of the Fluorescence Anisotropy Decay of a Cylindrically Symmetric Brownian Rotor: Identifiability Analysis

We present the results of the deterministic identifiability analysis based on similarity transformation for models of one-state excited-state events of cylindrically symmetric rotors in isotropic environments undergoing rotational diffusion described by Brownian reorientation. Such an analysis on error-free time-resolved fluorescence (anisotropy) data can reveal whether the parameters of the considered model can be determined. The fluorescence delta-response functions I(parallel)(t) and I(perpendicular)(t), for fluorescence polarized respectively parallel and perpendicular to the electric vector of linearly polarized excitation, are used to construct, in convenient matrix form, expressions of the sum S(t) = I(parallel)(t) + 2I(perpendicular)(t), the difference D(t) = I(parallel)(t) - I(perpendicular)(t), and the time-resolved fluorescence anisotropy r(t) = D(t)/S(t). The identifiability analysis of r(t) demonstrates that the rotational diffusion coefficients D(parallel) and D(perpendicular) for rotation respectively about and perpendicular to the symmetry axis can be uniquely resolved. However, the polar and azimuthal angles defining the absorption and emission transition moments in the molecular reference frame are not individually identifiable. Nevertheless, the difference between the polar angles of these transition moments is uniquely determined.

Photophysics of trans-4-(Dimethylamino)-4‘-cyanostilbene and Its Use as a Solvation Probe

Electronic structure calculations, steady-state electronic spectroscopy, and femtosecond time-resolved emission spectroscopy are used to examine the photophysics of trans-4-(dimethylamino)-4'-cyanostilbene (DCS) and its solvent dependence. Semiempirical AM1/CI calculations suggest that an anilino TICT state is a potential candidate for the emissive state of DCS in polar solvents. But observation of large and solvent-independent absorption and emission transition moments in a number of solvents (M(abs) = 6.7 +/- 0.4 D and M(em) = 7.6 +/- 0.8 D) rule out the involvement of any such state, which would have a vanishingly small transition moment. The absorption and steady-state emission spectra of DCS evolve in a systematic manner with solvent polarity, approximately as would be expected for a single, highly polar excited state. Attempts to fit the solvatochromism of DCS using standard dielectric continuum models are only partially successful when values of the solute dipole moments suggested by independent measurements are assumed. The shapes of the absorption and emission spectra of DCS change systematically with solvent polarity in a manner that is semiquantitatively reproduced using a coupled-state model of the spectroscopy. Kerr-gate emission measurements show that the emission dynamics of DCS down to subpicosecond times reflect only solvent relaxation, rather than any more complicated electronic state kinetics. The spectral response functions measured with DCS are well correlated to those previously reported for the solvation probe coumarin 153, indicating DCS to be a useful alternative probe of solvation dynamics.

The Tetrafluoro Analogue of DMABN: Anomalous Fluorescence and Mechanistic Considerations

Absorption and emission properties of DMABN-F4, the tetrafluoro analogue of DMABN, have been investigated and compared with the parent compound. Unlike in DMABN, this new compound exhibits only a highly solvatochromic and strongly red-shifted charge transfer (CT) fluorescence and is characterized by the absence of an LE band even in nonpolar solvents. This evidences the faster formation of CT in the excited state as compared to DMABN. The low quantum yield values of DMABN-F4 suggest that the high rate of nonradiative decay takes place via internal conversion (IC) rather than intersystem crossing (ISC) as no phosphorescence is observed in rigid glass solvents at 77 K in contrast to DMABN. The emission transition moment and radiative rate constant values of DMABN-F4 in medium and highly polar solvents point to a forbidden emission in the excited state similar to that of DMABN. Electronic structure and twist potentials were also studied by quantum chemical calculations using ab initio and semiempirical methods. In contrast to DMABN, the dimethylamino group in DMABN-F4 is found to be twisted by around 30-50 degrees, but the photophysics are concluded to be analogous to DMABN with the addition of a very fast IC channel.

Anisotropy measurements of intrinsic fluorescence of prenyllipids reveal much higher mobility of plastoquinol than alpha-tocopherol in model membranes

As an alternative to a fluorescent probe approach, the intrinsic fluorescence of reduced forms of prenylquinones has been exploited, which offers a convenient means of determining directly motional properties of these molecules. The steady-state fluorescence anisotropy measurements of plastoquinols (PQH 2) and α-tocopherol (α-Toc) incorporated into phospholipid liposomes have been performed. The effect of prenyllipid concentration, PQH 2 side chain length and the composition of the membranes has been studied. For the data interpretation, the fundamental anisotropy of α-Toc, PQH 2, ubiquinol-10 and α-tocopherolquinol, as well as the angles between the absorption and emission transition moments have been also determined. It was concluded that α-Toc shows very low mobility in the lipid bilayer, whereas PQH 2-9 displays significant motional freedom in dipalmitoylphosphatidylcholine vesicles and even higher in egg yolk lecithin membranes.

Time-Resolved Emission Anisotropy of Anthracene Fluorophore in the Backbone of Stereoregular Poly(methyl methacrylate)

Syndiotactic fluorescent poly(methyl methacrylate) labeled with anthracene in the middle of the polymer chain was prepared by living anionic polymerization (s-PMMA-A). Absorption and emission transition moments of the label are oriented in the direction of the polymer backbone, therefore the fluorophore really reflects the mobility of polymer segments. Stereocomplex formation between isotactic poly(methyl methacrylate) (i-PMMA) and s-PMMA-A was studied by time-resolved fluorescence depolarization method in solvents with different complexing ability and at different i-PMMA/s-PMMA-A ratios. The stoichiometry of the stereocomplex i-PMMA/s-PMMA-A was 1/2 and 1/1.5 in N,N-dimehtylformamide and 1,4 dioxane, respectively. The stereocomplex was not formed in chloroform. Determination of the rotational correlation times at different temperatures enabled us to evaluate the height of the potential barrier to rotational motion of the anthracene fluorophore (ΔU r ) or activation enthalpy and entropy (ΔH ¬= , ΔS ¬= ) of this process in the presence and absence of the stereocomplex. Time-resolved emission anisotropy data, r(t), were analyzed by intermediate zone formula of the torsion dynamics theory for stiff macromolecules. The torsional rigidity of the stereocomplex i-PMMA/s-PMMA-A, a = 1.9 w 10 -18 N.m in N,N-dimethylformamide at 30°C, is larger than that obtained for DNA (a = 3.8 x 10 -19 N.m). Self-aggregation of s-PMMA takes place probably in N,N-dimethylformamide.

Photophysical Properties of Pseudo-Dihydro Derivatives of C70

The first photophysical study of addition derivatives of C 70 is reported. The spectroscopic and photophysical properties of the singlet and triplet states of these compounds have been determined using a combination of time-resolved and steady-state techniques, including picosecond single photon timing, laser flash photolysis, and low-temperature polarized fluorescence. Electronic absorption, fluorescence, and phosphorescence spectra are red-shifted with respect to those of C 70. As with the parent fullerene, the main deactivation channel of the excited singlet state is intersystem crossing, and the yield of singlet oxygen formation is close to unity. Most of the measured photophysical parameters such as the fluorescence lifetime, and fluorescence quantum yield, are similar, but not identical, to those of C 70. Fluorescence polarization shows that the emission transition moment is located in the xy plane, like in C70, and that the effective fluorophore symmetry remains high.

Photophysics of 4-dimethylamino cinnamic acid in different environments

4-dimethylamino cinnamic acid (DMACA) has been studied by steady state and time-resolved fluorescence spectroscopy in different environments in order to get the information about its photophysics and photochemistry. Dual fluorescence in polar medium has been assigned to be arising out of delocalized excited state (DE) and twisted intramolecular charge transfer (TICT) state from different experimental and theoretical considerations. The computed excited state dipole moments in different twisted geometries evince that a twist of 90° of N(CH 3) 2 produces minimum energy state and maximum dipole moment change. The experimentally measured value (9.23 D) for dipole moment change from ground to excited state tallies with the corresponding value computed theoretically (7.5 D) for twisted geometry. Polarization studies in polar, nonpolar solvents and in β-CD cavity suggest that the emission transition moment is perpendicular to the molecular axis confirming the presence of twisted conformer. Weak acid protonates the carbonyl group while strong acid favors protonation of amino nitrogen. The solvent dependent absorption and emission spectra were found to be due to intramolecular structure change rather than ‘solvent coordinate’ effect. TICT band intensity is enhanced in β-CD inclusion complex due to modulation of photophysics by it while the normal (DE) band failed to appear. The fluorescence dependence of DMACA inclusion complex on polarity and viscosity has been discussed in detail. Quantum chemical calculations with AM1 Hamiltonian have been performed to find the actual geometry producing twisted intramolecular charge transfer state by twisting N(CH 3) 2 group to different angles and to confirm experimental findings qualitatively.

On the (uninteresting) dependence of the absorption and emission transition moments of coumarin 153 on solvent

The absorption and emission transition moments of coumarin 153 as well as fluorescence lifetimes and quantum yields were measured over a broad range of solvents in which the absorption and emission frequencies varied substantially. The same value was found for both the absorption and emission transition moments, and both also were found to be independent of solvent. These results suggest that a simple two electronic state model for the S 0↔S 1 transition in coumarin 153 is entirely appropriate.

Investigations of Chl a aggregates cross-linked by dioxane in 3-methylpentane

In this work, dioxane-bound aggregates of chlorophyll a are prepared in 3-methylpentane. The properties of the aggregates are studied by using steady-state and time-resolved spectroscopies. The Q y-region absorption spectrum of the chlorophyll a-dioxane aggregate shows four clearly resolvable narrow bands with comparable intensities. The band maxima are located at 683, 689, 698 and 702 nm. The emission spectrum consists of two emission bands centred at 699 and 702 nm suggesting the presence of two types of aggregates. High degree of fluorescence polarization is detected yielding the angles between the absorption transition moments with respect to the 702 nm emission transition moment. The circular dichroism signal is strong and the sign sequence follows that of the polarization spectrum. ψ-type effect is discovered due to long-range interactions within the sizable aggregate frame. The insensitivity of the time-resolved emission kinetics to the temperature indicate the excitation delocalization length to be short. The rapid loss of time-resolved anisotropy at certain wavelengths supports the energy transfer to take place. The independence of the excitation spectrum over a broad range of emission wavelengths suggests that the two diverse spectral types are found within the same macro-aggregate and that they are coupled together by the energy transfer.

Picosecond time-resolved and steady-state studies of the polarization of the fluorescence of C60 and C70

From a measurement of the time-resolved and steady-state fluorescence anisotropies of C60 and C70 in low-temperature organic glasses, it is concluded that the fluorescence of C60 is intrinsically unpolarized, and that the emission transition moment of C70 is located in the molecular xy plane. Also for C70, but at higher energies, some transitions occur mainly along the z molecular axis.

Time-resolved polarized fluorescence of the potential-sensitive dye RH421 in organic solvents and micelles

Fluorescence lifetimes and time-resolved fluorescence anisotropy decays of the voltage-sensitive styrylpyridinium dye RH421 have been measured in various organic solvents and when bound to both anionic and cationic detergent micelles. At least two and often three exponential functions were necessary to fit the decay data. In organic solvents the form of the lifetime and anisotropy decays were dependent on the emission wavelength. In the red part of the emission spectrum an initial rise in fluorescence after the excitation pulse could be observed. This indicates the presence of a precursor-successor mechanism, in which a second fluorescent species is formed in an excited-state reaction. The initial fluorescence anisotropy (at zero time) was also higher for blue-detected emission than at the red edge, indicating a change in the angle of the emission transition moment of about nine degrees during the excited state lifetime. The results are discussed in terms of the formation of a fluorescent twisted intramolecular charge transfer state, occurring simultaneously with the relaxation of the surrounding solvent cage. The results in micellar systems suggest a distribution of dye over different sites within the micelle.

Photoselection of Luminescent Molecules in Anisotropic Media in the Case of Two-Photon Excitation. Part I. Theoretical Considerations

Abstract Photoselection of elongated luminescent molecules oriented in a stretched polymer film is considered theoretically for the case of two- and three-photon excitation, having in mind the employment of this phenomenon for determining the angle between the absorption and emission transition moments. Two-photon photoselection provides information on electronic excited states which are not detectable by conventional one-photon spectroscopy due to selection principles.

Determination of the directions of the transition dipoles in tetrabutylperylene in stretched polymers

The directions of the transition dipole moments of 2,5,8,11,-tetra-butylperylene were determined from angle-resolved fluorescence depolarization experiments on molecules embedded in a stretched anhydrous nitrocellulose matrix. The absorption transition moments lies almost parallel to the elongated axis of the molecule, but the emission transition moment makes an angle of 20° with the axis. The orientational distribution of the molecules in the polymer indicates significant deviations from a circular form.