6.9 Chemistry of Colour

1. 6.9 Chemistry of Colour

2. 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.

3. 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.

4. 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.

5. Groups, energy and colour • -OH, -NH2 , or NR2 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.

6. 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.

7. 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.