Have you ever wondered why some compounds are colourless whereas some are coloured. The answer lies in the presence or absence of chromophores. A chromophore has a functional group present in a molecule that is capable of electronic transitions in the UV – VIS spectral range resulting in colour of a compound.
Types of Chromophores
Commonly three types of compounds show colour characteristics, namely, organic, inorganic and complex forming compounds resulting from charge transfer between metals and ligands.
An organic molecule absorbs light in the UV – VIS region depending on its molecular structure. Electronic transitions take place between the ground state and the excited electronic states of molecules having some degree of unsaturation or a heteroatom.
Transitions in the UV region are generally not accompanied by colour changes whereas those in lower energy visible region are capable of producing colour changes.
Some examples of chromophores are
Inorganic compounds containing atoms with electrons in d-orbitals give weak absorptions in the visible region. Metals in transition series are often coloured on account of such transitions, e.g., blue colour of aqueous copper sulphate solution
Charge Transfer Complexes
In some cases a compound is colourless naturally but in presence of a complex forming agent becomes coloured. In such cases one speices is an electronic donor group and the other is an electron acceptor.On interaction the charge transfer complex formed is intensely coloured, e.g., a blood red complex is formed when a \(Fe^3^+\) combines with \( SCN^-^2\) .Tthe complex formed absorbs light resulting in transfer of an electron from \( SCN^-^1\) to \( Fe^3^+\) .
You would have often observed that the colour of a compound deeperns or fades when either the environmental conditions are changed or on reaction with other species. In such situations chromophoric shifts take place.
Bathochromic Shift (red shift) results in shift to longer wavelengths i.e. colourless to the colour or deepening of colour. Examples – increase in conjugation or increase in number of aromatic rings can result in colouration or deepening of colour. The increase in intensity is referred to as hyper chromic effect
Hypochromic shift (blue shift) is a shift of absorption to shorter wavelengths resulting in a coloured solution becoming colourless or a deep colour to become lighter. The fading of colours is also referred to hypsochromic effect.