Why Absorption and Intensity Shifts Occur in UV-Visible Spectroscopy

 

Introduction

The number of double bond and aromatic conjugations inside a molecule is determined using ultraviolet and visible spectroscopy, sometimes referred to as electronic spectroscopy.

 In this process, electrons are moved from the HOMO to the LUMO orbital (HOMO stands for Highest Occupied Molecular Orbital and LUMO for Lowest Unoccupied Molecular Orbital).  As conjugation increases, the HOMO-LUMO gap narrows.

A popular analytical method known as UV visible spectroscopy uses the absorption of light in the ultraviolet to visible region to provide details about the electrical structure of molecules. Inorganic chemistry, biology, and analytical chemistry all often employ UV-visible spectroscopy.

A sample is exposed to a variety of light wavelengths in the ultraviolet-visible range during UV-visible spectroscopy. This may affect how some sample components absorb or do not absorb photons of particular energies. The molecular structure and chemical make-up of the sample can be ascertained by the absorption or non-absorption of specific light wavelengths.

Absorption and intensity shifts caused by changes in the electronic structure of a sample or changes in the experimental circumstances are important issues to address in UV visible spectroscopy. This article will go into depth about these two events, including their causes and how they impact the spectrum analysis process.

Absorption Shifts

Absorption shifts are variations in the wavelength of a species' maximum absorbance or the placement of absorption peaks within the UV-visible spectrum induced by changes in the sample's electronic structure. Changes in the electrical structure of the sample might cause absorption shifts. 

Absorption shifts refer to changes in the wavelength of the maximum absorbance of a species or the location of absorption peaks within the UV-visible spectrum, which are caused by changes in the electronic structure of the sample. Absorption shifts can occur due to changes in the electronic structure of the sample. A shift to a longer wavelength indicates an increase in energy required for excitation, while a shift to a shorter wavelength indicates a decrease in energy required. The causes of these shifts are varied and include changes in the oxidation state of a species, the presence of conjugated systems within the sample, and changes in coordination number.

A) Oxidation State

One cause of absorption shifts in UV-visible spectroscopy is the change in oxidation state of the sample. Oxidation is a process that involves the addition of electrons to a molecule by others, leading to the conversion of this molecule or an atom to its higher oxidation state. The reverse reaction is called reduction, which involves the loss of electrons from a molecule.

In cases where a molecule undergoes oxidation, the transition energy of the molecule may change, causing the absorption maxima to shift to a longer wavelength. An example of this is the shifting of the Fe^2+ ion from 507 nm to 535 nm upon oxidation to Fe^3+. The electronic structure of Fe^3+ involves a single unpaired electron based on the complex ion's d orbitals.

The shifting of the absorption maxima of Fe^2+ to Fe^3+ is due to the formation of a new molecular orbital (MO) when electrons are added to the complex ion. In Fe^2+ ions, the energy of the MO is lower, corresponding to a maximum absorbance at 507 nm. Upon oxidation, the energy of the MO increases, which shifts the maximum absorbance to a longer wavelength (535 nm).

b) Conjugation

We know that, E = hc As the conjugation increase transition energy (E) between the orbitals is decrease and therefore wavelength (max) increase. If double bonds (chromophore) present in the molecule are in conjugation, then absorption shift towards longer wavelength. In compound 'A', double bonds are in conjugation therefore 'A' possessing higher wavelength (max) as compared to compound 'B' (non-conjugated derivative).

Effect of Conjugation

The set of atoms in a molecular structure that alternately possess single and double bonds is referred to as a conjugated system. Conjugation alters the molecule's electrical structure significantly, changing the absorption of UV and visible light.

There is considerable -conjugation in inorganic dyes such food colouring, azo dyes, and acid-base indicators. As a result, the system interacts with light, producing the distinctive colours that can be seen. As an example, the presence of a conjugated double bond in the system causes azo dyes to absorb at longer wavelengths than their comparable aniline dyes.

c) Coordination Number

A third cause of absorption shifts in UV-visible spectroscopy is a change in coordination number. Coordination compounds such as metal complexes can undergo changes in their coordination number that lead to changes in their UV-visible absorbance.

For example, the coordination number of Co in a complex containing a diamine ligand is four. Its electron configuration is d6, and this requires ligands with low-energy orbitals, such as halides. However, when the number of ligands increases, the energy required to excite an electron from the t2g set to the eg set decreases. The energy required for the d-d transition is lowered upon the formation of the hexacoordinate complex, leading to a maximum absorbance at a longer wavelength (from 515 to 550 nm).

 

 

Intensity Shifts

Intensity shifts refer to changes in the intensity of the absorbance peaks. Intensity shifts are possible and can indicate changes in the sample or experimental conditions. An increase in intensity may be due to changes in the concentration of the analyte or the introduction of a co-solvent. In contrast, a decrease in intensity may be indicative of sample degradation, decreased analyte concentration, or the presence of a scavenging agent.

a) Concentration

A change in concentration can lead to an intensity shift in the UV-visible spectrum. When a sample's concentration is too low, the signals may be weak or barely detectable. On the other hand, high concentrations of a sample can lead to overloading of the detector or produce signal saturation. Both effects reduce the sensitivity of the analysis and can lead to erroneous results.

The sensitivity of the analysis can be controlled by changing the sample concentration, but it is challenging to find the optimum concentration. The optimum concentration is usually dependent on the instrument's sensitivity, the wavelength range, and the sample's identity.

b) Environment

The sample environment can also play a role in intensity shifts in UV-visible spectroscopy. The environment may include the type of solvent used, temperature, or pressure. These factors can lead to changes in the sample's structure or chemical configuration, leading to a change in the sample's spectral behavior.

For example, an increase in temperature may cause a shift in the maximum absorbance wavelength due to a change in the electronic structure of the sample. The pH of the solvent used can also affect the maximum absorbance, depending on the pKa of the sample's constituents.

c) Aggregation

Aggregation can also cause intensity shifts in UV-visible spectroscopy. Aggregation occurs when molecules in solution come together to form larger particles. The formation of aggregates can cause a shift in the absorbance spectrum since the interaction between the particles can affect the electronic structure of the sample.

For example, aggregation of proteins occurs when proteins come together to form multimers. This can lead to changes in the maximum absorbance of the protein due to the changes in the electronic structure of the protein as the proteins aggregate.

Conclusion

In conclusion, UV-visible spectroscopy is a powerful analytical technique used in the identification and quantification of analytes in a sample. Spectral changes in absorption and intensity of UV-visible light can be used to identify and quantify the analytes concentration present in the sample. Shifts in the absorbance maxima and intensity of UV-visible spectra can be caused by changes in the electronic structure of the sample, solution environment, and sample concentration. Analysis of these shifts can be crucial in providing insights into the molecular structure and composition of the sample under investigation.