Determination of the Molar Absorptivity of a Light Absorbing Molecule
In addition to the optical path length and the concentrationof the absorbing species, the intensity of UV-Visible absorption band depends on the oscillator strength of the absorbing species. Oscillator strength of an absorbing species measures the allowedness of an electronic transition. It represents the strength of absorption relative to a completely allowed transition. Oscillator strength ~1 implies a completely allowed transition. The oscillator strength depends on the transition probability and the nature of the absorbing species. The oscillator strength is related to the absorptivity (formerly called the extinction coefficient) which can be easily determined experimentally. Thus absorptivity is a useful quantity giving us idea about the transition probabilities in the molecules and the effective light capture area (the cross-section for light absorption) of the species. Absorptivity is a characteristic of the absorbing substance under given conditions (wavelength, solvent and temperature). Experimentally molar absorptivity (ε) can be determined from absorbance measurements. The values of ε (M-1·cm-1) usually range from about 1 to 106. Very high values of ε indicate a favored (or, spin-allowed) transition: high intensity absorptions havevalues in the range 104-106 whereas absorptions with ε values in the range of 103-104 are termed as low intensity absorptions. For example, ε values can be over 10000 for the UVvisible pi– pi* transition in conjugated polyenes. Absorptions with ε values of 0 to 103 are less favored or forbidden transitions. For example, ε values less than 100 for the n –pi* absorption of ketones indicate that the transition is ‘forbidden’. If molar abosrptivity value is known, then concentration of an unknown solution of the absorbing species can be determined from its absorbance measurements (by applying Beer-Lambert Law). The molar absorptivity (ε) plays an important role in the quantitative determinations of an absorbing species. ε of the species to be determined must be reasonably large. For example, copper or cobalt in water as the aquo complex can be determined accurately down to about 1% since for such complexes ε ~10 m2 mol-1. On the other hand, even a solution of about 0.5 ppm anthracene (C14H10) with a ε ~18 000 m2 mol-1 gives an absorbance of ~0.1 in a cell of 1 cm path length. In this context it is worth noting that an isosbestic point is a wavelength at which the molar absorptivity (ε) is the same for two absorbing species that are interconvertible. Isosbestic points are used as reference points in many studies, since the absorbance at those wavelengths remains constant throughout the whole conversion process and its position is concentration independent.