Question

Why do conjugated compounds absorb light in the Ultraviolet region? Why do more conjugated compounds absorb at higher wavelengths/lower energies? What is required for organic compounds to absorb in the Visible light region?

Answer 1

When ultraviolet or visible light is absorbed by an atom or molecule, an electron is excited-in other words, it is raised to an orbital of higher energy than the one it usually occupies. In an atom, for example, an electron might move from an s to a p orbital upon absorption of light; in a molecule an electron moves from one molecular orbital to another, more energetic one.

As a general rule, molecules containing conjugated systems of pi electrons absorb light closer to the visible region than saturated molecules or those with isolated double or triple bonds. The longer the conjugated system, the longer the wavelength of the light absorbed .

Due to greater conjugation the orbitals or electronic energy levels come closer due to which the energy difference decreases thus wavelength increases and absorption shifts to visible region of the spectrum.