Excitation anisotropy

Another important linear parameter is the excitation anisotropy function, which is used to determine the spectral positions of the optical transitions and the relative orientation of the transition dipole moments. These measurements can be provided in most commercially available spectrofluorometers and require the use of viscous solvents and low concentrations (cM 1 pM) to avoid depolarization of the fluorescence due to molecular reorientations and reabsorption. The anisotropy value for a given excitation wavelength 1 can be calculated as... 

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. 

As seen from Fig. 19, fluorescence excitation anisotropy is a very useful tool to predict the positions of 2PA bands as was concluded in Sect. 2.1. 

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. 

Grecco HE, Lidke KA, Heintzmann R, Lidke DS, Spagnuolo C, Martinez OE, Jares-Erijman EA, Jovin TM (2004) Ensemble and single particle photophysical properties (two-photon excitation, anisotropy, FRET, lifetime, spectral conversion) of commercial quantum dots in solution and in five cells. Microsc Res Tech 65 169-179... 

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. 7 Dependence of the limiting excitation anisotropy riPA on solvent polarity Af for compounds 6 (a) and 9 (b) for excitation into the first Sj (1), and second S2 (2) electronic excited states ( 5%) accuracy for each point...

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...

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 4a shows an example of an excitation anisotropy trace recorded for a single PFO chain in the (3 phase by collecting the fluorescence and rotating the plane of the exciting laser using a k/2 plate. From this curve... 

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.

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

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... 

AnisDifO iy (ccnr.) definition. 291-293 enTTi"t wavdengih and. 312 eaagy transfer and, 373-374 excitation anisotropy spectia. 295-298, A47-440... 

The excitation anisotropy scan the absorption band and detect separately at wavelengths corresponding to the two emission... 

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. 

Figure 2 The electric field induced excitation anisotropy of the fluorescence yield...

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