Polarized emission spectra

Developments in instrumentation have allowed measurements of the single-crystal CD spectra of [(+ )D-Cr(en)3]3+ doped in [Ir(en)3]Cl3 between 7 and 293 K. Transitions to the excited states 4T2, 47i, ZE, 27i and 2T2 were observed.418 The discussion refers to much earlier work. The polarized electronic spectra of single crystals of several trans-Cr [XY(en)2] complexes have been assigned and ligand field parameters evaluated.419 CD, absorption and circularly polarized emission spectra have been reported for [Cr(en)3]3+ in the region of the A2g 2EgTig transitions.420... 

Fig. IS. Polarized spectra of single-crystal Ba[Pt(CN)4] 4 H20 (295 K, 1 bar)30,85, ll3). The intensities of the differently polarized emission spectra can be compared. The spectra are not corrected, neither due to the response of the apparatus nor due to reabsorption effects. Excitation X, = 364 mm, E c spectral resolution 10 cm-1 crystal thickness for the absorption measurements 10 pm...

The pressure dependence of the unpolarized emission is reported in Refs. 164, 166. Polarized emission spectra of BaCP133) at several pressures are shown in Fig. 33. A series of MCP compounds with other cations and thus with other R-values has also been investigated (see Table 9). 

Fig. 33. Polarized emission spectra of single-crystal Ba[Pt(CN)4] 4 H20 at different hydrostatic pressures (T = 295 K)133). The emission intensities at different pressures cannot be compared. The excitation wavelength was varied with pressure to fit approximately the maximum of the E c polarized reflectance. For the high pressure investigations a modified sapphire cell of Bridgman s opposed anvil type was used. The pressure was determined by the amount of red-shift of the Rt- and R2-lines167) of ruby crystals placed around the sample...

Fig. 42. Polarized emission spectra of Eu2[Pt(CN)4]3 18 H20 single crystals at different pressures at T = (100 10) K169>. (The E c polarized emission intensities of the spectra have to be multiplied by the given factors.)...

The polarized emission spectra and the apparatus response (recorded at emission wavelength) were sampled with a 0.12 ns channel width by the single-photon counting technique. Thanks to the stability of the pulses, the short-time limit of the experimental window is about 0.1 ns. The upper limit, imposed by the repetition rate of the pulses and the lifetime of the dye, is 2 ns. 

Stock and Yersin [221] reported polarized emission spectra up to 23 kbar for single crystals of Ba[Pt(CN)4] 4H2O. At ambient pressure, the intrachain Pt-Pt separation is 3.32 A and the peaks of the free exciton and self-trapped emission... 

Numerous experimental IR spectra of ultrathin films confirm the above theoretical interpretation (see Ref. [26] and literature therein). The jo-polarized emission spectra of a ZnSe film on the Si-Al BML substrate (Fig. 3.9) are given as an example [26]. As the Si underlayer thickness increases, the intensity of the ZnSe band near vtq (206 cm ) increases. In fact, according to the quasi-static interpretation, the dipole moment of mode 3 (mainly responsible for the band near vto) is directed along the x-axis, parallel to the interface. At small thicknesses, mode 3 will be quenched by the image in the metal (Section 1.8.2), but as the film thickness increases, this effect diminishes. Mode 2, whose band is near vlo (252 cm ) with polarization parallel to the surface, is unaffected by the presence of the underlayer. It is noteworthy that the experimental spectra agree so well with the spectral simulations from the Fresnel formulas. Another result, which agrees with the theoretical treatment, is the frequency dependence of the band near vto in spectra of the ZnSe layers on metals with different permittivities Sm... 

Polarized emission spectra, obtained under isotropic excitation at 365 nm and polarized detection in either p- or j-mode are shown in Figures 2b and 7. In binary UHMW-PE / DMC films, the emission from DMC, centred around 400 nm, exhibits only minor polarization, expressed by an emission dichroic ratio, of 2.3, consistent with the low degree of orientation of die sensitizer. In the ternary blend, importantly, the DMC emission is almost fully suppressed, while the emission from EHO-OPPE is highly polarized (DRe= 16). The fact that DR is somewhat lower in the ternary than in a comparable binary UHMW-PE / EHO-OPPE blend (DRe= 27) is explained with a plastisizing effect of DMC on EHO-OPPE that reduces the efficiency of the orientation process. 

Figure 14. Polarized emission spectra with polarized excitation.

Polarized emission spectra were measured on a single KAu(CN)2 crystal oriented with the incident laser beam polarized parallel to the crystallographic c axis. The c axis in this case is the axis perpendicular to the in-plane Au-Au layers. Figure 10 shows a plot of the polarization P of the high-energy emission band as a function of temperature. P is defined by Eq. 16,... 

The behavior of the low temperature Elc polarized emission spectrum of a typical MCP salt, BaCP, for different magnetic field strengths is presented in Fig. 4394). The vector orientation is H c, HIO and Ole, where O denotes the direction of observation. A relatively large blue shift of the peak maximum of the Elc emission by —270 cm-1 (Fig. 44) and a drastic increase of the intensity (Fig. 45) are observed as the magnetic field is raised from 0 to 6 T. Above = 3 T the spectral position of the Elc emission remains nearly constant, whereas no saturation effect of the intensity growth is found up to 6 T. If the excitation wavelength is changed from 364 to 458 nm the increase in intensity is lowered by a factor of 3. 

Fig. 43. Eic polarized emission spectrum of single-crystal Ba(Pt(CN)4]i 4 H20 at T = 1.9 K for different magnetic field strengths. Vector orientations HIE, HIO, die. O direction of observation. (X = 364 mn)M)...

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... 

Additional evidence for conformational changes in the transporter has come from measurement of the intrinsic fluorescence of the protein tryptophan residues, of which there are six, in the presence of substrates and inhibitors of transport. The fluorescence emission spectrum of the transporter has a maximum at about 336 nm, indicating the presence of tryptophan residues in both non-polar environments (which would emit maximally at about 330 nm) and in polar environments (which would emit at 340-350 nm) [154], The extent of quenching by the hydrophilic quencher KI indicates that more than 75% of the fluorescence is not available for quenching, and so probably stems from tryptophan residues buried within the hydrophobic interior of the protein or lipid bilayer [155]. Fluorescence is quenched... 

Fluorescent probes are divided in two categories, i.e., intrinsic and extrinsic probes. Tryptophan is the most widely used intrinsic probe. The absorption spectrum, centered at 280 nm, displays two overlapping absorbance transitions. In contrast, the fluorescence emission spectrum is broad and is characterized by a large Stokes shift, which varies with the polarity of the environment. The fluorescence emission peak is at about 350 nm in water but the peak shifts to about 315 nm in nonpolar media, such as within the hydrophobic core of folded proteins. Vitamin A, located in milk fat globules, may be used as an intrinsic probe to follow, for example, the changes of triglyceride physical state as a function of temperature [20]. Extrinsic probes are used to characterize molecular events when intrinsic fluorophores are absent or are so numerous that the interpretation of the data becomes ambiguous. Extrinsic probes may also be used to obtain additional or complementary information from a specific macromolecular domain or from an oil water interface. 

Figure 6.1. Jablonski-type diagram for pyrazine. The zero-field splittings (between tx, tV) t2) are not drawn to scale. Spin polarization ( x x x) resulting from the most probable intersystem crossing routes and part of the emission spectrum where different vibronic bands (v = /,/, k) have different zf origins are schematically indicated. (After El-Sayed.(17))...

Dichroic ratios are very useful to describe the PL emission from a physiological point of view (e.g., brightness of a device as perceived by the human eye), and we have attempted to consistently employ this representation throughout this text. Most authors express DRE as a ratio of intensities at a given wavelength, usually at a maximum of the emission spectrum, and, unless otherwise stated, we here follow this convention. However, quite frequently the emission spectra of p- and s-polarized are not identical, so that it would be more appropriate to compare the integrated emission spectra in order to allow for an apple-to-apple comparison. 

Finally, there is a specific red-edge effect related to non-radiative energy transfer between a donor fluorophore whose emission spectrum overlaps the absorption spectrum of an acceptor fluorophore in rigid polar solutions, there is a lack of energy transfer upon excitation at the red-edge. This effect, called Weber s effect, will be described in Section 9.4.3. 

Plastocyanin from parsley, a copper protein of the chloroplast involved in electron transport during photosynthesis, has been reported to have a fluorescence emission maximum at 315 nm on excitation at 275 nm at pH 7 6 (2°8) gjncc the protein does not contain tryptophan, but does have three tyrosines, and since the maximum wavelength shifts back to 304 nm on lowering the pH to below 2, the fluorescence was attributed to the emission of the phenolate anion in a low-polarity environment. From this, one would have to assume that all three tyrosines are ionized. A closer examination of the reported emission spectrum, however, indicates that two emission bands seem to be present. If a difference emission spectrum is estimated (spectrum at neutral pH minus that at pH 2 in Figure 5 of Ref. 207), a tyrosinate-like emission should be obtained. 

Yttrium aluminum borate, YAlj (603)4 (abbreviated to YAB), is a nonlinear crystal that is very attractive for laser applications when doped with rare earth ions (Jaque et al, 2003). Figure 7.9 shows the low-temperature emission spectrum of Sm + ions in this crystal. The use of the Dieke diagram (see Figure 6.1) allows to assign this spectrum to the " Gs/2 Hg/2 transitions. The polarization character of these emission bands, which can be clearly appreciated in Figure 7.9, is related to the D3 local symmetry of the Y + lattice ions, in which the Sm + ions are incorporated. The purpose of this example is to use group theory in order to determine the Stark energy-level structure responsible for this spectrum. 

When tryptophan is dissolved in water, it shows the fluorescence characteristics illustrated in O Figure 5-2. However, one especially useful property of tryptophan fluorescence is that its emission spectrum is highly sensitive to the polarity of its environment. In less polar solvents (alcohols, alkanes, etc.), the emission... 

The charge transfer and the h,tt states are affected differently by a change in solvent. In polar solvents, the charge transfer state is attained in nonpolar solvents the n,n state results and 4-aminobenzophenone is reduced upon irradiation in cyclohexane.68 This change in chemical reactivity is reportedly paralleled by a change in the phosphorescence emission spectrum.68 This solvent-dependent reactivity, however, is not observed in the photocycloaddition reaction. Irradiation of 4-aminobenzophenone with isobutylene37 in cyclohexane solution failed to produce either the oxetane or the reduction product. The... 

Indole in cyclohexane and ethanol is excited at 270 nm populating a mixture of the La and Lb states. The La and Lb states differ in their spectral structure (Fig. la)) and Stokes shifts [3]. The unstructured spectrum of the La state shows a large Stokes shift due to the large change of the dipole moment upon electronic excitation. The dipole moment of the Lb state is similar to the ground state value. In nonpolar solvents like cyclohexane, the Lb state is energetically below the La state and its emission spectrum exhibits vibronic structure. In the polar solvent ethanol state reversal occurs after the electronic excitation and the La state becomes responsible for the more red shifted fluorescence [4],... 

The compound bis-(4,4 -dimethylaminophenyl)-sulfone (DMAPS) and related compounds show multiple fluorescences in polar solvents due to excited state charge transfer (Rettig and Chandross [144]). Su and Simon [84,85] have examined the intramolecular electron transfer reaction in DMAPS, in alcohol solution over the temperature range from — 50°C to + 30°C. They observe that the decay of the local excited state is nonexponential and significantly faster than the longitudinal relaxation time of the solvent. In addition, they observed that the emission spectrum of the TICT state... 

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