Advanced Pharmaceutical Analysis: Solvatochromism in UV-Vis Spectroscopy Lab

To ensure accurate absorption of the compound of interest without interference from the solvent.

The absorption bands are broad due to the higher magnitude and wide range of wavelengths absorbed.

The solvent should not absorb radiation in the region under investigation and should have minimal interaction with the solute molecule.

95% ethanol.

They are usually determined in the vapor phase or in very dilute solution.

To ensure that the compound dissolves at low concentrations, allowing for accurate analysis.

High polarity solvents shift the color of a chromophore towards blue, while low polarity solvents shift the color towards red. This phenomenon is known as solvatochromism.

A chromophore is a covalently bonded, unsaturated group that is capable of absorbing radiation, while an auxochrome is a fully saturated group with lone pair of electrons that can shift the absorption peak of a chromophore towards longer wavelengths and increase the intensity of absorption.

Solvatochromism is the ability of a chemical substance to change color due to a change in solvent polarity. The sign of solvatochromism depends on the difference in the dipole moment of the ground and excited state of the chromophore.

The presence of specific and non-specific interactions between the solvent and solute molecules can affect the molecular geometry, electronic structure, and dipole moment of the solute, which in turn affects the solute's electronic absorption spectrum. This is the phenomenon known as solvatochromism.

A chromophore may have $\pi$ electrons or $n$ electrons that can undergo $\pi \to \pi^$ and $n \to \pi^$ electronic transitions.

When an auxochrome is attached to a chromophore, it shifts the absorption peak of the chromophore towards longer wavelengths and also increases the intensity of absorption.

Increased polarity leads to a bathochromic (red) shift for -* transitions. This is due to increased interaction between the solvent and the absorption species, which lowers the energy level of the excited and unexcited states. The effect is more pronounced on the excited state.

For n-* transitions, increasing polarity leads to a hypsochromic (blue) shift. This is due to increased solvation of the non-bonded electron pairs, which lowers the energy of the n-orbital. When using polar solvents like water or alcohol, hydrogen bonding between the solvent protons and the non-bonded electron pair further lowers the n-orbital energy, resulting in the blue shift.

In neutral solution, aniline exhibits a conjugation between the lone pair of electrons on the nitrogen and the benzene ring, leading to an absorption at 280 nm. However, in acidic solution, the lone pair is removed due to the formation of the anilinium cation. This removal of conjugation results in a blue shift, where the absorption occurs at a shorter wavelength of around 200 nm.

A bathochromic shift refers to a red shift in the UV-Vis absorption spectrum, where the absorption maximum moves to a longer wavelength. This is typically associated with increased conjugation or polarity. In contrast, a hyperchromic shift refers to an increase in the intensity or absorptivity of the absorption band, without a change in the wavelength position.

Increased conjugation leads to a bathochromic (red) shift in the UV-Vis absorption spectrum. This is because conjugation delocalizes the $\pi$-electrons, lowering the energy of the $\pi^*$ orbital and decreasing its antibonding character. This results in a smaller energy gap between the ground and excited states, shifting the absorption maximum to longer wavelengths.

A hypochromic shift refers to a decrease in the intensity or absorptivity of an absorption band, without a change in the wavelength position. In contrast, a hypsochromic shift, also known as a blue shift, refers to a shift of the absorption maximum to a shorter wavelength. Hypsochromic shifts are typically associated with n-$\pi^*$ transitions, where increased polarity lowers the energy of the n-orbital, resulting in the blue shift.