{"id":56,"date":"2023-02-23T01:16:53","date_gmt":"2023-02-23T06:16:53","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/instanchem1\/?post_type=chapter&p=56"},"modified":"2025-01-03T13:49:27","modified_gmt":"2025-01-03T18:49:27","slug":"uv-visible-absorption","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/instanchem1\/chapter\/uv-visible-absorption\/","title":{"raw":"UV-Visible Absorption","rendered":"UV-Visible Absorption"},"content":{"raw":"

Absorption of Light by Molecules<\/strong><\/h2>\r\n

A molecule at room temperature is found in its ground electronic state (S0<\/sub>)\u00a0and, most frequently, its ground vibrational state (v<\/em>0<\/sub>). This combination is written as S0<\/sub>v<\/em>0<\/sub>. The most energetic (i.e.<\/em> valence) electrons are found in the highest (energy) occupied molecular orbital (HOMO).<\/p>\r\n

When a molecule absorbs a UV or visible photon, a transition occurs to an excited electronic state. Commonly, this new state corresponds to an electron in what is normally the lowest (energy) unoccupied molecular orbital (LUMO), with either a ground or excited vibrational state (vm<\/sub><\/em>). Such a state is denoted as S1<\/sub>vm<\/sub><\/em>. The energy of the absorbed photon matches the energy difference between the S0<\/sub>v<\/em>0<\/sub> and S1<\/sub>vm<\/sub><\/em> states\u2014a condition known as resonance<\/em>. A transition to the second lowest unoccupied molecular orbital (SLUMO; S0<\/sub>v<\/em>0<\/sub> to S2<\/sub>vm<\/sub><\/em>) may occur upon absorption of a higher-energy visible or UV photon.<\/p>\r\n

Molecules or parts of molecules that strongly absorb UV and visible light are called chromophores<\/em>. Organic chromophores tend to have conjugated double bonds, and often heteroatoms, in their structures. For simple molecules, the HOMO-LUMO transition frequently corresponds to an electron moving from a \u03c0 bonding or non-bonding orbital to a \u03c0 anti-bonding orbital.<\/p>\r\n

Charge transfer absorption is common with inorganic chromophores and also occurs with some organic chromophores. An electron moves from one part of a molecule to another part upon transition to the excited state. Transitions involving d <\/em>or f<\/em> electrons may also occur with inorganic chromophores (cf.<\/em> the s<\/em> and p<\/em> electrons that form molecular orbitals in organic chromophores).<\/p>\r\n \r\n\r\n\"Simple\r\n

Beer-Lambert Law<\/strong><\/h2>\r\n

Transmittance<\/em> is the fraction of incident light absorbed by a sample of molecules. That is, the ratio of light intensity transmitted through the sample, P<\/em>(\u03bb), and the original light intensity, P<\/em>0<\/sub>(\u03bb). The absorbance<\/em>, A<\/em>, is the negative logarithm of this fraction (Eqn. 6.1), where \u03b5(\u03bb) is the molar absorption coefficient of the molecule at a defined wavelength (\u03bb), b<\/em> is the path length of light through the sample, and c<\/em> is the concentration of the molecule. Note that absorbance and the molar absorption coefficient are wavelength-specific values.<\/p>\r\n\u00a0 \u00a0 \u00a0 \u00a0 (Eqn. 6.1) \u00a0 \u00a0 \u00a0 \u00a0[latex] A(\u03bb) = \u00a0-log[P(\u03bb)\/P_{0}(\u03bb)] = \u03b5(\u03bb)bc [\/latex]\r\n

The molar absorption coefficient is a measure of the probability that a photon of wavelength \u03bb is absorbed by a molecule. Typical peak values of \u03b5(\u03bb) are 104<\/sup>\u2013105<\/sup> M\u20131<\/sup> cm\u20131<\/sup> for strongly absorbing organic molecules. Absorption spectra measure and plot absorbance as a function of wavelength.<\/p>\r\n

At sufficiently dilute concentrations, the linear relationship between absorbance and concentration in Eqn. 6.1 will be observed experimentally. UV-visible spectrophotometry measures the absorbance of samples. It is a simple and popular method of determining analyte concentrations when the molar absorption coefficient is known or can be determined by a calibration curve derived from standard solutions. When there are multiple absorbing molecules in the same solution, the observed absorbance is the sum of the absorbances from each type of absorbing molecule (i.e.<\/em> a sum of terms analogous to Eqn. 6.1).<\/p>\r\n \r\n\r\n

\r\n\r\n

Connections<\/h3>\r\n