Polarization of the Emitted Light

A depolarization measurement consists of exciting a fluorescent sample with linearly polarized light and measuring the polarization of emitted light at right angles to the plane of excitation. The polarization of the emitted light is defined as... 

If a sample is irradiated with polarized light, only those molecules with absorption axes parallel to the plane of polarization will absorb appreciable energy. The emission from the molecule is also polarized, and its plane of polarization will be fixed in relation to its absorption axis. If the molecule has not moved between the absorption and emission processes, all the emitted radiation will be in one plane of polarization. The spread in the plane of polarization of the emitted light is a function of the lifetime of the excited state and the rate of molecular movement. Polarization data give information on molecular size and shape and may be obtained by a combination of spectrum scanning with modulation of the emission signal by rotation of a polarizing film interposed between the sample and detector (K7). Most manufacturers supply a simple, manually operated attachment for polarization studies. 

Another method for the study of the conformational mobility of polymers utilized the dependence of the polarization of the emitted light on the rotational diffusion of the fluorophore. This technique was pioneered by Anufrieva and her collaborators who used polymers with anthracene built into the chain backbone. They showed, for instance, that conformational mobility in polymethacrylates is much smaller than in polyacrylates. 

So, the behavior of the vector U allows one to define the polarization of the emitted light. 

Use of polarized light to excite fluorescence, and measurement of the state of polarization of the emitted light introduce another set of measurable parameters that can characterize structures and dynamics of molecules. The anisotropy of the polarization of fluoresence after excitation by linearly polarized light provides the rotational diffusion coefficient, or rotational correlation time, of the fluorophore. When there is fluorescence energy transfer, analysis of the anisotropy of both donor and acceptor can reveal the relative orientation, and the relative motion. Measurement of fluorescence after excitation by circularly polarized light provides the fluorescence-detected circular dichroism. This measurement characterizes the chiral environment of the ground state of the fluorophore. If the circular polarization of the fluorescence is measured, the circularly polarized luminescence is obtained. This measurement characterizes the chirality of the excited state. 

As a technique complementary to AFM, near-field scanning optical microscopy (NSOM) studies have reported the nanoscale topographic and fluorescence features of poly(fluorene)s [156-158]. From the NSOM experiments, it is possible to quantify the film optical anisotropy on the local scale by measuring the polarization of the emitted light. The intensity of fluorescence is found to be the most when collected perpendicular to the fibril axis. Since the fluorescence is polarized along the conjugated backbone, this indicates that the ribbons are indeed composed of poly(fluorene) chains stacked orthogonal to the ribbon axis. 

The peculiar properties of lanthanide luminescence also make these ions attractive in the generation of CPL. In this technique, a pulse of light is used for excitation and circular polarization of the emitted light is measured using a photomodulator, allowing one to select alternatively left- and right-handed circularly polarized light. 

The power of this method was greatly increased when instrumentation became available which made it possible to follow the polarization of the emitted light as a function of time after excitation by a light flash. In this case it is convenient to express data by the emission anisotropy, r = (I - I )/( + 2Ij ), where r should exhibit simple... 

The origin of FA is the polarization of electronic transition of molecules to each transition is associated a vector called transition moment (see Sect. 1.5) which as a given orientation with respect to the molecular structure, In Fig. 6.15, the absorption transitions Sq —> 5i and 5o —> S2 of perylene are depicted. Transition 5i —> Sq responsible for the fluorescence is almost parallel to 5q —> 5i. In general, when the deactivation of an excited state takes place radiatively, the emitted photon is polarized parallel to the transition moment. Hence, if a single molecule is observed, the polarization of the emitted light is parallel to the direction defined by the transition responsible for the fluorescence. 

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