Two-photon fluorescence anisotropy

Anisotropy properties of the molecular fluorescence under two-photon excitation reflect the nature of 2PA processes and may provide additional information on the electronic structure of the molecules, including the peculiarities of the 2PA mechanism. In general, the measurements of two-photon fluorescence anisotropy are more sensitive than at one-photon excitation due to a broader range of anisotropy values [13] that in some cases provide extra advantages for practical applications of2PA [65]. 

This relationship is modified by two constants the molecular shape factor/ (a function of the molecular dimensions) and the boundary coefficient C, which takes into account the interaction between the solvent and the solute. In principle, two-photon fluorescence anisotropy decays in isotropic media should yield the same diffusion times as for single photon excitation, but with significantly increased initial fluorescence anisotropy this can be seen in Figure 11.17, which compares single- and two-photon anisotropy decays for the fluorescent probe rhodamine 6G in ethylene glycol. Rotational drflusion times for small molecular probes vary from nanoseconds to hundreds of picoseconds for isotropic rotational drflusion in low viscosity solvents. 

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