Colour

Principles of Colour

Light and Colour

  • Colour is the result of light being absorbed, transmitted or reflected by a substance.
  • Substances appear coloured because they absorb light in the visible wavelength range (400-700nm) and reflect or transmit other wavelengths.
  • Chromophores are parts of molecules that absorb light in the UV-visible range, giving the molecule its colour.

Transition Metal Ions and Colour

  • Transition metal ions often produce coloured compounds. This is because they have partly filled d orbitals which can absorb light in the UV-visible region, promoting an electron to a higher energy level.
  • The exact colour is dependent on the difference in energy between the d orbitals, which can be affected by factors such as the type of transition metal, its oxidation state, and the type of ligands coordinated to it.

Organic Compounds and Colour

  • Many organic compounds are colourful due to the presence of an extended system of conjugated double bonds, also known as a chromophore.
  • Increasing the length of conjugation shifts the absorption of light to longer wavelengths, resulting in a change in the perceived colour.
  • These principles are important in the design of organic dyes and pigments.

Chromatography and Spectroscopy

Chromatography

  • Chromatography is a technique used to separate the different components in a mixture based on their solubility in a particular solvent.
  • This can be used to separate out the different coloured compounds in a mixture, allowing each individual colour to be analyzed.

Spectroscopy

  • Spectroscopy is a powerful tool for identifying the makeup of coloured substances.
  • UV-visible spectroscopy provides an absorption spectrum which shows which wavelengths of light are absorbed by a substance. This can be used to identify the chromophores present.
  • Correlating the spectra with the known structures of compounds can help predict the colour of new materials.

Applications in Design

Dyes and Pigments

  • Dyes are soluble substances used to give colour to other materials, while pigments are insoluble and provide colour by reflecting selective wavelengths of light.
  • The colour of dyes and pigments can be tweaked by modifying their molecular structure, allowing a wide range of colours to be produced.

Colour Change Indicators

  • Some compounds change colour in response to changes in their environment, for example, pH indicators.
  • By understanding the chemical basis for these changes, such compounds can be designed for use in a variety of applications, like litmus paper in chemistry or pH strips in aquariums.

Medicine and Imaging

  • Certain compounds can be designed to change colour upon reacting with particular substances, for instance, glucose strips for diabetics.
  • Coloured compounds can also be used in medical imaging, where they provide contrasts and highlight certain areas.