Absorption and emission spectroscopy with polarized light

Absorption and Emission Spectroscopy with Polarized Light 

Abstract In the first part of this chapter we will illustrate circular dichroism and we will discuss the optical activity of chemical compounds with respect to light absorption which is at the basis of this technique. Moreover, we will introduce the phenomena that lie behind the technique of optical rotatory dispersion. We thought appropriate to include a brief description of linear dichroism spectroscopy, although this technique has nothing to do with optical activity. In the final part of the chapter we will introduce the basic principles of the luminescence teehniques based on polarized (either circularly or linearly) excitation. The experimental approach to the determination of steady-state and time resolved fluorescence anisotropy will be illustrated. For all the teehniques examined in this chapter the required instrumentation will be schematieally deseribed. A few examples of application of these techniques to molecular and supramolecular systems will also be presented. 

The orientation of transition moments in the molecular framework can be predicted by quantum mechanical calculations (Sections 2.1.4 and 4.4). Experimentally, the direction of transition moments can be assessed by studying the absorption of linearly polarized light by samples in which the molecules are preferentially oriented. Complete orientation is available in single crystals. However, optical measurements on single crystals are demanding and rarely practicable, not least because extremely thin crystals are required for absorption studies. 

Partial orientation is much easier to achieve and generally sufficient to determine the direction of the transition moments, particularly with respect to any axes of symmetry that the molecules may have. Partial orientation can be achieved by photoselection using a linearly polarized excitation source, by application of an electric field or by dissolving the solute in a transparent anisotropic medium (liquid crystals, stretched polymers).182 Photoselection is the basis of emission anisotropy measurements discussed below. It is also used for molecules that can be either generated or destroyed photochemically in a rigid medium such as poly(methyl methacrylate) or glassy solvents at low temperature. Preferential alignments of dipolar molecules that are achievable by electrostatic fields are unfortunately fairly small. 

Most light sources, lasers in particular, are linearly polarized to some degree. As a result, the emission from the sample may also be polarized. The degree of anisotropy R is defined by Equation 3.3  

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Scattering and other forms of spectroscopy Rely on the fact that electromagnetic radiation has other interactions with matter beyond that of simple absorption and emission. These interactions generate other measurable quantities such as scattering of polarized light (e.g. circular dichroism), and changes of spectral features of chemical bonds (e.g. Raman spectroscopy). 

All the above EPR spectroscopy was carried out in the steady state. With the use of fast-response spectrometers, however, it was discovered a decade ago that when measured early (at room temperature a few /u,s) after a light flash, the EPR spectra of primary reactants in PS I [105,106] and in bacterial RC [107] showed EPR lines characteristic of systems out of Boltzmann equilibrium (Table 2). Part or all of these so-called spin-polarized lines may then either be in emission or show absorption that is enhanced or decreased compared to the equilibrium absorption (Fig. 7). Electron spin polarization occurs through magnetic interactions between two simultaneously induced donor-acceptor radicals (reviewed in Ref. R14). Thus, a study of spin-polarized EPR lines yields information on these magnetic interactions and therefore on the configuration (distance, relative orientation, etc.) of the radicals (see e.g. Ref. R14). 

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