Analytical Chemiluminescence/Chemiluminescence spectroscopy

A2. Chemiluminescence spectroscopy

The wavelengths of chemiluminescence emission that are analytically useful depend on the characteristics of the detector. Visible emission (though it is seldom visible to the naked eye) has a wavelength range of about 400-750 nm, corresponding to enthalpy changes of exothermic reactions of between 180 and 300 kJ mol⁻¹, provided that there is a pathway to an excited state that relaxes with the loss of a photon (see Figure 1.1). Emission intensity is proportional to the concentration of the emitting species, which is either an intermediate or a product in an electronically excited state. This concentration depends on the rate of the reaction producing it. Analytical detection of chemiluminescence usually involves no wavelength selection, i.e., it is emission photometry rather than emission spectrophotometry. Selectivity is achieved by on-line treatments rather than by processing of the signal, which has little fine-structure.

Because of this, the importance of chemiluminescence spectroscopy lies more in elucidating the mechanisms of chemiluminescence reactions rather than in analytical applications. In particular, spectroscopic investigations have been found useful for the identification of the emitter species in particular chemiluminescence reactions. Thus, experimental evidence has shown that manganese(II) ion is a common emitter in chemiluminescence arising due to the reduction either of permanganate or of other higher oxidation states of manganese. Using a variety of reductants, chemiluminescence spectra, corrected for wavelength-related differences in detector sensitivity, showed maximum emission at 689 nm (in hexametaphosphate) and 734 nm, (in phosphate/orthophosphoric acid) which corresponds to the emission maxima of manganese(II) phosphorescence, and is clearly distinguishable from the intense emission at 634 nm and 703 nm from singlet oxygen, which had been earlier proposed as the emitter. Diagnosis of the emitter usually cannot be based on spectroscopic evidence alone, but must make use of chemical evidence also. Identifying the emitting species in luminol chemiluminescence in aqueous solutions is an example of such an investigation. The product of the oxidation of luminol is 3-aminophthalate. The maximum emission wavelength in these conditions is 424 nm, which corresponds to the maximum wavelength of fluorescence emission from the 3-aminophthalate dianion and this species was originally accepted as the emitter. However, chemical evidence suggests that emission is from the monoanion, which has a fluorescence maximum of 451 nm. Closer examination of the chemiluminescence reaction suggests that the emitter is one particular conformer of the 3-aminophthalate monoanion that has a maximum emission wavelength resembling that of the dianion.