Polarization excitation spectra

Spectroscopy Excitation polarization spectra distinction between excited states... 

Fig. B9.3.1. A absorption spectrum of the B excitation polarization spectra of the model multi-chromophoric cyclodextrin CD7(6) and compound NAEt and CD7(6). Solvent mixture variations in the emission maximum as a func- (9 1 v/v) of propylene glycol and 1,4-dioxane at tion of the excitation wavelength (broken line). 200 K (adapted from Berberan-Santos et al.a)).

Absorption spectra, emission spectra and excitation polarization spectra were recorded in a propylene glycol-dioxane glass at 200 K. Comparison was made with the reference chromophore 2-ethylnaphthoate (NAEt). 

Figure 5.19. Absorption (A) and emission spectra (F and P) of phenanthrene in ethanol at 93 K. FP and PP denote the polarization spectra of fluorescence and phosphorescence, AP(F) and AP(P) the excitation polarization spectra of fluorescence and phosphorescence, respectively (by permission from D6rr, 1966).

Fig. S.8. Excitation polarization spectrum of perylene in propane-1,2-diol at — 60 °C.

The long wavelength absorption band of indole consists of two electronic transitions 1La and b, whose transition moments are almost perpendicular (more precisely, they are oriented at —38° and 56° to the long molecular axis, respectively). Figure B5.2.1 shows the excitation spectrum and the excitation polarization spectrum in propylene glycol at —58 °C. 

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 ).

Since electronic transitions differ from one excitation wavelength to another, the value of P would change with excitation wavelength. Emission generally occurs from the lowest excited state Si Vo, and so one can measure anisotropy or polarization along the absorption spectrum at a fixed emission wavelength. We obtain a spectrum called the excitation polarization spectrum or simply the polarization spectrum (Figure 11.2). 

Figure 11.2 shows the excitation polarization spectrum of protoporphyrin IX in propylene glycol at —55°C (full line) and bound to the heme pocket of apohemoglobin recorded at 20°C (dotted line). One can see that polarization at a low temperature is higher than that observed when porphyrin is embedded in the heme pocket of apohemoglobin. This is the result of fluorophore local motions within the pocket, independently of the global rotation of the protein. 

Figure 11.2 Excitation polarization spectrum of protoporphyrin IX in propylene glycol at —55°C (full line)...

Figure 5.4. Excitation polarization spectrum of protoporphyrin (X in propylene-glycol at -55 C (full ine) and bound to the heme pocket of apohemoglobin recorded at 20°C (dotted line).

The excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

The case of indole and tryptophan is peculiar because the low-lying absorption bands overlap. Box 5.2 shows how the indole absorption spectrum can be resolved into two bands from the combined measurement of the excitation spectrum and the exdtation polarization spectrum. 

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. 

The reasons for the less satisfactory agreement between the theoretical and experimental polarized Raman spectra may be both on the theoretical and on the experimental sides. The experimental polarized spectrum is obtained as the difference of two nearly equal signals excited with different beam polarizations 409 410, and the accuracy of the polarized intensities deduced from the experiment is rather poor. On the other hand, the theoretical values of the interaction-induced trace may suffer from the size-inconsistency of the cisd method or from the basis-set superposition error. 

Here and Ij are the intensities of the components of the emitted light parallel and perpendicular to the electric vector of the exciting light, respectively. The curves P(A) or P(v) are called the polarization spectrum. Depending on whether the measurement is carried out with constant excitation... 

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