Fluorescence excitation spectra, polarization

Figure 9.29 Two-photon fluorescence excitation spectrum of 1,4-difluorobenzene. The upper and lower traces are obtained with plane and circularly polarized radiation, respectively, but the differences are not considered here. (Reproduced, with permission, Ifom Robey, M. J. and Schlag, E. W., Chem. Phys., 30, 9, 1978)...

It is possible, however, that the electrochromic response of some styrylpyridi-nium probes, for example, RH421 (see Fig. 2), is enhanced by a reorientation of the dye molecule as a whole within the membrane. There is a steep gradient in polarity on going from the aqueous environment across the lipid headgroup region and into the hydrocarbon interior of a lipid membrane. Therefore, any small reorientation of a probe within the membrane is likely to lead to a change in its local polarity and hence a solvatochromic shift of its fluorescence excitation spectrum. Such a... 

Since ANS dissolved in a polar medium does not fluoresce, one cannot record its fluorescence excitation spectrum. For tryptophan, ethidium bromide, and riboflavin, one can see that for each molecule, the absorption spectrum looks like the fluorescence... 

Fig. 26.8. Fluorescence-excitation spectrum from an individual RC-LHl complex from Rps. palustris as a function of the polarization of the excitation light, (a) Top Stack of 312 individual spectra recorded consecutively. Between two successive spectra the polarization of the incident radiation has been rotated by 6.4°. The horizontal axis corresponds to the photon energy, the vertical axis to the scan number or equivalently to the polarization angle and the intensity is given by the gray scale. The excitation intensity was 10 W/cm. Bottom Spectrum that corresponds to the average of the 312 consecutively recorded spectra, (b) Top Fluorescence intensity of the three bands marked by the arrows in the lower part as a function of the polarization of the incident radiation (dots) together with cos -type functions fitted to the data black). Bottom Two fluorescence-excitation spectra from the stack that correspond to mutually orthogonal polarization of the excitation light. The spectra where chosen such that the horizontal polarization yielded maximnm intensity for the narrow feature at the low-energy side. Adapted from [62]...

Fluorescence and Chemiluminescence Spectroscopy. - The fluorescence excitation spectrum of PF3 at 9-13 eV, using monochromatised synchrotron radiation, has been examined to resolve the pyramidal geometry of the X Ai ground state of the PFs" cation, which was also confirmed by ab initio calculations. Dimethylamino-substituted triphenylphosphines exhibit dual fluorescence in polar solvents, and fluorescence-decay measurements have shown that the photo-induced intramolecular charge-transfer process occurs in a few picoseconds, even in weakly-polar solvents. 

Fig. 14. Low-temperature (at 1.5 K) (r-polarized fluorescence excitation spectrum of pentacene in p-terphenyl. Note that the absorption of the O3, O4 sites in this picture is artificially reduced by the detection setup. In a straight absorption experiment all four sites absorb equally.

Fig. 15. Low-temperature (1.5 K) fr-polarized fluorescence excitation spectrum of pentacene in naphthalene. 

Figure 2 Two-photon polarized fluorescence excitation spectrum of NO (Reproduced by permission from Chem. Phys. Letters, 1975, 33, 1)...

Further information can be obtained from measurements of the polarization of the photofragment fluorescence excitation spectrum. The directions and magnitudes of the fluorescence polarization, which can be measured following photodissociation at any wavelength in the excitation spectrum, reflect the orientation of the electric dipole transition moment in the parent molecule and its lifetime with respect to pre-dissociation into the observed products . ... 

Figure 12. Fluorescence-excitation spectrum of pentacene-d 4 in / terphenyl-d 4. The strong lines, labelled Oi and O2, coimected by the brackets, correspond to pentacene-du molecules in the 0 and O2 spectra sites of p-terphenyl. The O2 line appears weaker in the spectrum because the polarization of the incident laser was adjusted to give a maximum signal for the Oi line. The two lines connected by the dashed bracket correspond to the Oi and O2 ensemble lines of pentacene molecules which contain a single proton bound to the carbon in the 7 position. Similarly the two lines connected by the dashed-dotted bracket result from pentacene molecules containing two protons each of them bound to a carbon in the 7 position of pentacene.

To qualify the environment into which the colorant molecule is embedded, the actual fluorescence spectrum is compared with the one under standard conditions. If the fluorescence emission spectrum is shifted to longer wavelengths (bathochromic shift), it can be concluded that the molecular enviromnent is of a more polar nature or is polarized by the excited fluorophore. Conversely, a fluorescence shift to shorter wavelengths (hypsochromic shift) indicates a transfer of the fluorophore from a polar... 

Figure 1. Fluorescence emission-excitation spectrum (bottom) and the degree of polarization P (top) for 1 in 3-methylpentane at 25 C. (Reproduced from Ref. 19. Copyright 1986 American Chemical Society.)...

Fig. B5.2.1. Corrected excitation spectrum (broken line) and excitation polarization spectrum of indole in propylene glycol at -58 °C. The fluorescence is observed through a cut-off filter (Corning 7-39 filter) (reproduced with permission from Valeur and Weber3 ).

Figure 8.2 presents the fluorescence of pyrene on silica gel. The loading is low so that pyrene is predominantly adsorbed as nonaggregated monomers (Mi). The backward fluorescence spectrum Fb of this sample is very comparable to the spectrum in polar solvents and not distorted by reabsorption. However, the forward spectrum Ft is almost completely suppressed in the region of overlap with the o -transition and hot sidebands of the weak first absorption band Si. The absorption coefficients of the sample vary widely from k" = 0.1 cm 1 (Si-band, Aa = 350-370 nm) to k = 25 cm-1 (S2-band, 1 290-340 nm), and in a first approximation the excitation spectrum of Fh reflects this variation correctly (Figure 8.2, left). The Ff-excitation spectrum, however, has only little in common with the real absorption spectrum of the sample. 

Figure 4.17 A. Fluorescence polarization spectrum. A general case of a molecule with two absorption bands a and b with transition moment vectors at right angles to each other. (1) Absorption spectrum (2) Polarization of flourescence excitation spectrum. 

Increasing the solvent polarity results in a red shift in the -t -amine exciplex fluorescence and a decrease in its lifetime and intensity (113), no fluorescence being detected in solvents more polar than tetrahydrofuran (e = 7.6). The decrease in fluorescence intensity is accompanied by ionic dissociation to yield the t-17 and the R3N" free radical ions (116) and proton transfer leading to product formation (see Section IV-B). The formation and decay of t-17 have been investigated by means of time resolved resonance Raman (TR ) spectroscopy (116). Both the TR spectrum and its excitation spectrum are similar to those obtained under steady state conditions. The initial yield of t-1 is dependent upon the amine structure due to competition between ionic dissociation and other radical ion pair processes (proton transfer, intersystem crossing, and quenching by ground state amine), which are dependent upon amine structure. However, the second order decay of t-1" is independent of amine structure... 

The fluorescence polarization excitation spectrum has been measured for thymine in aqueous solution. " The depolarization at the red edge is attributed to the hidden n, ir transition. Ionization of the lowest excited singlet and triplet states have been determined by the effect of pH on the absorption, fluorescence, and phosphorescence spectra of purines and pyrimidines. " Spectral, polarization, and quantum yield studies of cytidylyl-(3, 5 )-adenosine have also been published. Intermediates in the room-temperature flash photolysis of adenine and some of its derivatives have been identified hydrated electron, radical cations and anions, and neutral radicals resulting from their reactions have been assigned. Photoionization occurs via the triplet state. FMN encapsulated in surfactant-entrapped water pools interacts with polar head groups, entrapped water molecules, and outer apolar solvent. ... 

In the bulk, the low concentration of ground-state pairs excludes their observation by absorption. The formation of the excited-state complex, termed exciplex, is a collisional process electronic excitation of either the acceptor or the donor leads to the formation of a locally excited state (for instance, in hydrocarbon molecules, it is a nn state). During the lifetime of this state, a collision with the other partner (which is in the ground state) leads to the formation of the exciplex. This mechanism is compatible with the fact that the absorption and fluorescence excitation spectra of the system are identical with those obtained by superimposing the spectra of the individual components. At the same time, the fluorescence emission spectrum changes drastically—a broad band, red shifted with respect to the bare molecule s emission spectrum, appears. It is usually devoid of vibrational structure, and is shifted to longer wavelengths as the solvent polarity increases [1],... 

Fig. 26.3. Fluorescence-excitation spectra of LH2 complexes of Rp. acidophila. The top traces show the comparison between an ensemble spectrum (dotted line) and the sum of spectra recorded from nineteen individual complexes (full line). The lower trace displays the spectrum from a single LH2 complex. The spectra have been averaged over all polarizations of the incident radiation. All spectra were measured at 1.2 K at 20W/cm with LH2 dissolved in a PVA-buffer solution. Adapted from [39]...

Fig. 26.5. Fluorescence-excitation spectra from RC-LHl complexes from Rps. palustris. The top traces show the comparison between an ensemble spectrum black line) and the sum of about 41 spectra recorded from individual complexes gray line). The lower traces show spectra from single RC-LHl complexes. For each individual complex two spectra, recorded with mutually orthogonal polarization of the excitation light, are displayed. The vertical scale is valid for the two lowest traces, the other spectra have been scaled by a factor of two and are offset for clarity. Adapted from [55]...

In order to analyze the spectral bands from the individual core complexes in more detail, we recorded the fluorescence-excitation spectra as a function of the polarization of the incident radiation. The excitation spectra have been recorded in rapid succession and the polarization of the excitation light has been rotated by 6.4° between consecutive scans. An example of this protocol is shown in the top part of Fig. 26.8a in a two-dimensional representation where 312 individual scans are stacked on top of each other. The horizontal axis corresponds to photon energy, the vertical axis to the individual scans, or equivalently to the polarization of the excitation, and the detected hu-orescence intensity is coded by the gray scale. The sum spectrum of these scans is presented at the bottom of Fig. 26.8a and shows two broad bands at 11,253 and 11,398 cm with a linewidth of 250 and 153 cm (FWHM),... 

Fluorescence-detected circular dichroism (FDCD) is a chiroptical technique in which the spectrum is obtained by measuring the difference in total luminescence obtained after the sample is excited by left- and right-circularly polarized light. For the FDCD spectrum of a given molecular species to match its CD spectrum, the luminescence excitation spectrum must be identical to the absorption spectrum. 

Fluorescence studies of the commercially important l,3-diphenyl-2-pyrazoline derivatives have been extended to a wide range of phenyl- and methyl-substituted derivatives. The fluorescence yields in non-polar solvents are all close to unity, with the exception of l,3,5,5-tetraphenyl-2-pyrazoline. This is not the case in methanol, when a large variation in yields is observed.85 (See the section on oxidation for other photochemical processes in these compounds.) By an examination of the fluorescence spectra and fluorescence excitation spectra of 9-amino-acridine at 4.2 K, the fine-structure fluorescence spectrum of the neutral molecule has been identified and investigated using laser excitation.88... 

Figure 4 shows the polarized excitation spectra of the B873 form at low temperature. The value of 0.12 for the polarization is in agreement with values reported for LHl complexes [3]. The fluorescence spectra of the different subunits from Rb. sphaeroides all confirmed to the shown spectra of Rs, rubrum.. Only a more pronounced increase in the polarization was observed in the red edge of the excitation spectrum of Rb. sphaeroides (not shown). 

Figure 10.7 Normalized fluorescence (a), excitation (b) and optically pumped PL spectra (c, d) taken from platelet crystals of BP1T. The spontaneous fluorescence (a), AST (c) and SRRS (d) spectra were taken at Xex = 365, 355 and 460 nm, respectively. The excitation spectrum (b) was taken for the fluorescence band at 493 nm. Reproduced from H. Yanagi, I. Sakata, A. Yoshiki, 5. Hotta and 5. Kobayashi, Polarization dependence of Stimulated resonance Raman scattering from a single crystal of bi-phenyl-capped thiophene, Jpn. J. Appl. Phys., 45, 483-487 (2006) with permission from The Institute of Pure and Applied Physics...

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