Solid-state absorbance spectra

Figure 5.1 Resonant absorption of y-radiation by a nucleus can only take place in the solid state because of recoil effects. The excited nucleus of a free atom emits a y-photon with an energy EirER, whereas the nucleus in the ground slate of a free atom can only absorb a photon if it has an energy equal to Eo+ER. As the linewidth of nuclear transitions is extremely narrow, the emission spectrum does not overlap with the absorption spectrum. In a solid, a considerable fraction of events occurs recoil free (ER=0), and here the emission spectrum overlaps completely with the absorption spectrum (provided source and absorber have the same chemical environment).

Fig. 15 Experimental spectrum from solid state (dashed line) and calculated spectrum of a heptamer of molecule 1 (solid line). Upper panel. Absorbance spectrum, lower panel. CD spectrum [23]...

Subphthalocyanines contain a delocalized 14 -electron conjugated pathway, and are brightly colored compounds, both in the solid state and in organic solution. As such, they exhibit fairly strong absorption bands in their visible electronic spectrum. For instance, subphthalocyanine 2.284 exhibits a Soret-like absorbance band at ca. 305 nm and a more intense Q-like transition at 565 nm (e = 50 100 M cm and 89 100 M cm , respectively, in CHCI3). These bands are blue-shifted relative to those of the metallophthalocyanines (by c. 20-30 nm and 120-130 nm in the Soret and Q-band regions, respectively), and exhibit absorption coefficients that typically are smaller than those of the metallophthalocyanines. ... 

The acquisition of solid-state FTIR spectra suitable for use in the characterization of polymorphic impurities is performed using either the Nujol mull technique, diffuse reflectance (DRIFT), or attenuated total reflectance (ATR). One should avoid the use of pelleting techniques to eliminate any spurious effects associated with compaction of the KBr pellet. The simplest approach is to prepare a mull of the sample in mineral oil, sandwich this between salt plates, and measure the spectrum using ordinary transmission techniques. The main drawback of the mull technique is that regions in the IR spectrum overlapping with carbon-hydrogen vibrational modes will be obliterated because of absorbance from the oil. 

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