Auxochrome: Auxochromes are the chemical groups that result in a bathochromic shift when attached to a chromophore. The strongest auxochromes like –OH, –NH2, –OR, etc. possess nonbonding electrons. They exhibit bathochromism by extending conjugation through resonance.
The auxochrome modified chromophore is a new chromophore in real sense. The term auxochrome is therefore rarely used these days, and the entire group (basic chromophore + auxochrome) can be considered as a chromophore different from the basic chromophore. Alkyl groups also result in the bathochromic shifts in the absorption spectra of alkenes. Alkyl groups do not have non-bonded electrons, and the effect is brought about by another type of interaction called hyperconjugation.
Solvents: The solvents used in any spectroscopic method should ideally be transparent (non-absorbing) to the electromagnetic radiation being used. Table 5.1 shows the wavelength cutoffs (the lowest working wavelength) of some of the solvents used in UV/visible spectroscopy.
Water, the solvent of biological systems, thankfully is transparent to the UV/visible region of interest i.e. the regions above λ > 190 nm. Solvents also play important role on the absorption spectra of molecules. Spectrum of a compound recorded in one solvent can look significantly different in intensity, wavelength of absorption, or both from that recorded in another. This is not something unexpected because energies of different electronic states will depend on their interaction with solvents. Polarity of solvents is an important factor in causing shifts in the absorption spectra. Conjugated dienes and aromatic hydrocarbons are little affected by the changes in solvent polarity. α,β-unsaturated carbonyl compounds are fairly sensitive to the solvent polarity. The two electronic transitions π → π* and n → π* respond differently to the changes in polarity. Polar solvents stabilize all the three molecular orbitals (n, π, and π*), albeit to different extents (Figure 5.4). The non-bonding orbitals are stabilized most, followed by π*. This results in a bathochromic shift in the π → π* absorption band while a hypsochromic shift in n → π* absorption band. Shift to different extents of the two bands will result in the different shape of the overall absorption spectrum.
Figure 5.4 Differential stabilization of molecular orbitals in polar solvents |