Excitation Anisotropy Spectra

The changes in the fundamental anisotropy with excitation wavelength can be understood in terms of a rotation of the absorption transition moment. However, a more predse explanation is the changing contributions of two or more electronic transitions, each with a diffoent value of 

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Second, we analyze the nature of the next, strong 2PA bands. The positions of their final states correspond to one-photon symmetry forbidden bands and can be found from excitation anisotropy measurements, as illustrated in Figs. 6,19, and 23. Excitation anisotropy spectra for all cyanine-like molecules typically reveal a large alternation of maximum and minimum features suggesting the positions of the 1PA and 2PA transitions. Two-photon excitation into final states involves two dipole moments, fi0i and /i (i. 

Additional information about the nature of the absorption bands of flu-orenes can be revealed from their excitation anisotropy spectra [17,21-23]. The values of fluorescence anisotropy are commonly determined as [13] ... 

Fig. 18 Two-photon excitation anisotropy spectra (1, 1, top scales) for compound 6 (a, 1), 11 (a, 1 ), 9 (b, 1) in polyTHF and 17 (b, 1 ) in silicon oil. Solid lines (2, 2 ) are the corresponding linear absorption spectra...

Figure 7.11. Fluorescence excitation anisotropy spectra of apacid glycoprotein (a) and ai-acid glycoprotein s (b) at 20°C. Xem = 330 nm. Source Albani, J. R. 1998, Spectrochimica Acta Part A. 54, 175-183.

It is instructive to see how the anisotropy depends on the mode of excitation. Figure 10.34 diows die excitation anisotropy spectra of DPH. For one-photon exdtation... 

The excitation anisotropy spectra of chlorophyll a recorded at two different emission wavelengths are distinct and exhibit several crossover points, see figure 4. Similar results were found for chlorophyll b (data not shown). This indicates that at the corresponding wavelengths more than one transition moment is excited. 

As seen from (12) and Fig. 6, the peaks in the excitation anisotropy spectrum indicate a small angle between the absorption and emission transition dipoles suggesting allowed 1PA transitions while valleys indicate large angles between these two dipoles, suggesting a forbidden 1PA transition. Due to selection rules for symmetrical cyanine-like dyes, the valleys in the anisotropy spectrum could indicate an allowed 2PA transition as demonstrated in Fig. 6. Thus, an excitation anisotropy spectrum can serve as a useful guide to suggest the positions of the final states in the 2PA spectra. 

Fig. 4 Excitation and emission spectra of complex 16 in CHCI3, CH3CN, CH3OH and buffer at room temperature (A,ex = 400 nm). The solid line shows the excitation anisotropy spectrum in 100% glycerol at - 60 °C with the emission wavelength tuned to 550 nm [51]...

Figure 8 25 shows the excitation anisotropy spectrum of ai-acid glycoprotein at -45 C. The spectrum is typical of Tip residues. 

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). 

The complex anisotropy spectrum of indole was used to determine the absorption spectra corresponding to the 5 — L, and So—i La transitions. We present this example because of its didactic value and its importance for a detailed understanding of the fluorescence from tryptophan residues in proteins (Chapter 16). At any excitation wavelength X, the observed anisotropy is... 

Dependencies of luminescence bands (both fluorescence and phosphorescence), anisotropy of emission, and its lifetime on a frequency of excitation, when fluorescence is excited at the red edge of absorption spectrum. Panel a of Fig. 5 shows the fluorescence spectra at different excitations for the solutes with the 0-0 transitions close to vI vn, and vra frequencies. Spectral location of all shown fluorescence bands is different and stable in time of experiment and during lifetime of fluorescence (panel b)... 

Figure B9.3.1 shows the parallelism between the increase in emission spectrum displacement and fluorescence anisotropy observed for the red-edge of most vibronic bands and especially for the 0-0 one. It can be interpreted in terms of inhomogeous spectral broadening due to solvation heterogeneity. The decrease in energy transfer that is observed upon red-edge excitation is evidence that energy hopping is not chaotic but directed toward lower energy chromophores, as in photosynthetic antennae.

A Cu(II) complex containing two spin-labelled ligands was prepared with a rigid linkage between the terpyridine chelator and a nitroxyl ring.26 Due to the large anisotropy of the Cu(II) spectrum only a small fraction of the Cu(II) spins are excited by the pulses of a DEER experiment. Analysis of the echo modulation... 

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