6.9 Chemistry of Colour

Recapping from earlier Coloured substances absorb radiation in the visible region of the EM spectrum. Absorb energy - outermost electrons promoted to excited state. Same electrons involved in bonding or lone pairs. Innermost electrons more tightly held by attraction to the nucleus - more energy needed for excitation.

Pulling theory together Some transitions needing less energy are brought about by visible light Excitation energy is greater when UV light is absorbed. Compounds absorbing UV radiation appear colourless.

Coloured organic compounds Often contain unsaturated groups, -C=O, -C=C, -N=N- Usually part of extended delocalised electron system called the chromophore. Electrons in double bonds more spread out - require less energy to excite than those in single bonds particularly in conjugated system. Absorption of radiation in visible region.

Groups, energy and colour -OH, -NH 2, or NR 2 attached to chromophores to enhance or modify the colours. Lone pair electrons become involved in the delocalised system. Small changes change the energy of light absorbed and therefore the colour. Dyes often different colours in acids and alkalis - useful indicators. Methyl orange bond to H + at pH 3.5 and below - red, above pH3.5 there is no H + bonded and the dye is yellow.

Coloured inorganic compounds Often contain transition metals. Ligands cause 5 d orbitals to split into two levels Energy needed to excite to a higher level depends on the oxidation state of the metal and the type of ligand. Redox reactions often accompanied by colour changes.

Changing a ligand in a complex can change the colour - different ligands have different splitting powers. Electrons can move from the ground state in one atom to the excited state in another atom - electron transfer. Often bright colours, Chrome yellow and Prussian blue.