Principle of Photochemical Hole Burning

5 Photochemical Hole Burning A. Principle of Photochemical Hole Burning 

The principle of photochemical hole burning (PHB) is described in Table 2.11(1). Photoabsorption occurs according to the energy difference between the g and e states of a certain molecule (the guest). The linewidth of the absorption of an individual molecule is called the homogeneous width, Arui, and is given by Eq. (2.2), where T, is the lifetime of the excited state and T2 is the dephasing time (transverse relaxation time). 

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Fig. 19 Principle of photochemical hole-burning. Irradiation at the inhomogeneously broadened band of a tautomeric molecule in a matrix creates a hole due to photo-induced tautomerization.

Many tetrazines undergo an irreversible photochemical fragmentation with reasonable quantum yield even at very low temperatures. This fact has been used in several applications of the new spectroscopic technique of photochemical hole burning (HB). In the following sections brief descriptions of the principles of this method with references to applications to tetrazines, especially 3,6-dimethyl-1,2,4,5-tetrazine are given. 

An interesting ansatz for this problem is based on photochemical or photophysical hole-burning. The principle is explained in Fig. 12.16. The emission as well as the absorption spectra of molecules in a solid matrix, or also in an organic glass, consists in general of relatively broad bands. They are inhomogeneously broadened, since the molecules, which have intrinsically sharp absorption and emission lines, experience a multiplicity of different local environments in the matrix, with different values of the so-called solvent shift. The observed spectrum is then a superposition of many individual sharp lines, and one observes only their broad envelope. 

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