Inhomogeneous bands emission

The width of a band in the absorption or emission spectrum of a fluorophore located in a particular microenvironment is a result of two effects homogeneous and inhomogeneous broadening. Homogeneous broadening is due to the existence of a continuous set of vibrational sublevels in each electronic state. Absorption and emission spectra of moderately large and rigid fluorophores in solution could therefore be almost structureless at room temperature. However, in some cases, many of the vibrational modes are not active, neither in absorption nor in emission, so that a dear vibrational structure is observed (e.g. naphthalene, pyrene). 

The width of the 0-0 line in single-tryptophan proteins at 77 K has been interpreted to reflect inhomogeneous broadening arising because the protein exists as a distribution of conformations. 30 34 The width of the 0-0 band of liver alcohol dehydrogenase is 500 cm-1 at 22°C.(10 31 35) The widths of the 0-0 transition for other proteins are somewhat greater. In many cases for the spectra taken at room temperature, low-resolution optics were used (as in Figure 3.2), and hence the published spectra may overestimate the width of the emission band. 

Hole burning The photohleaching of a feature, normally a narrow range, within an inhomogeneous broader absorption or emission band. The holes are produced by the disappearance of resonantly excited molecules as a result of photophysical or photochemical processes. The resulting spectroscopic technique is site-selection spectroscopy. 

As is easy to see from Eq. [80] and Figure 7, the Q model predicts the breaking of the symmetry between the absorption and emission widths (Eq. [11]) generated by a statistical distribution of solvent configurations around a donor-acceptor complex (inhomogeneous broadening). This fact may have a significant application to the band shape analysis of optical transitions since unequal absorption and emission width are often observed experimently. " ... 

The solubilized molecules are in a dynamic equilibrium between complexed and uncomplexed forms. Therefore, a fraction of the molecules will react in the uncom-plexed form in solution during exposure to radiation. Some molecules are likely to interact with the hydroxyl residues or substituents on the outer surface of the cyclodextrin unit and thereby experience a partly hindered degree of freedom. The remaining fraction will experience a less polar environment, protection from oxygen, and a decrease of the intramolecular rotational freedom inside the cavity during excitation. The inhomogenity of cyclodextrin systems can be demonstrated by a typically broad lifetime distribution and a red shift of the emission maximum when excited in the red edge of the absorption band (Bortolus and Monti, 1996). 

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