Color of substances

From WikiLectures

Many substances contain a valence electron that can be excited to a higher energy level by electromagnetic radiation. Such a substance then absorbs radiation of a certain wavelength with an energy photon corresponding to the energy difference of the two electron levels. If the absorbed radiation lies in the visible part of the spectrum, the substance will appear coloured to the human eye (it will have a colour complementary to the colour of the absorbed light).

Absorbed wavelength (nm)      Matching colour Complementary colour     
380–435 purple yellow-green
435–480 blue yellow
480–490 green-blue orange
490–500 blue-green red
500–560 green purple
560–580 yellow-green purple
580–595 yellow blue
595–650 orange green-blue
650–760 red blue-green

Unfortunately, it is not easy to predict the colour of a substance on the basis of its chemical structure, nor is it possible to infer the composition of a substance unambiguously from the absorption spectrum. However, from our point of view, three groups of substances that are often coloured are relevant:

  1. Substances containing a system of conjugated double bonds whose molecule is not symmetrical. If we imagine a symmetric conjugated double bond system, it can exist in two resonance states that are energetically equivalent: The presence of an asymmetric substituent will cause the energies of the two states to be different. The energy difference often corresponds to the energy of a photon in the visible part of the spectrum. Typical representatives may be dyes with a polymethine chain (–CH=CH–CH=CH–) nor azo (-N=N-). Substances with aromatic or heterocyclic structures bonded to a common central atom (e.g. triphenylmethane dyes) behave similarly.
  2. Also, the d and f valence electrons often determine the color of the compound. They tend to be present in coordination covalent bonds complex compounds. For example, anhydrous copper sulfate CuSO4 is colorless, while its pentahydrate CuSO4•5H2O and its aqueous solution are blue: in both cases, because copper enters a complex with water [Cu(H2O)4]2+. In a similar way, colored and complex compounds of other transition metals (Fe, Cu, Cr, Mn, Ni, Co) tend to be complex-bound metal also in the colored proteins of hemoglobin and cytochromes.
  3. Ions containing a transition metal with a high oxidation number as the central atom are also coloured, e.g. MnO4-, Cr2O72-.

Analytical methods used in medicinal chemistry and biochemistry make use of all three groups of colored compounds. Systems of conjugated double bonds are often formed in reactions in which the analyte condenses with a suitable chromogen (e.g. creatinine with picric acid in the Jaffé reaction, diazo-coupling reactions in the detection of bilirubinu), or are formed by oxidation of a chromogen that contains one double bond less (oxidation of benzidine derivatives in peroxidase reactions). The formation of colored complexes is used, for example, in the determination of proteins by the so-called biuret reaction (Cu2+ complexes with O and N peptide bonds) or in the detection of a number of substances, e.g. with FeCl3 . Color changes during the reduction of Cr6+ to Cr3+ are used, for example, in the detection of ethanol in exhaled air.

Absorption of monochromatic light can also be conditioned by events other than electron excitation. These are primarily changes in the various oscillation energies of atoms in molecules and the rotational energies of entire molecules. These principles are used more in fluorimetry. From the point of view of medical biochemistry, they are much less important than the above principles.