Particle in a Box : Absorption Spectrum of Conjugated Dyes

Absorption bands in the visible region of the spectrum (350 700 nm) correspond to transitions from the ground state of a molecule to an excited electronic state which is 160 to 280 kJ above the ground state. In many substances, the lowest excited electronic state is more than 280 kJ above the ground state and no visible spectrum is observed. Those compounds which are colored (i.e., absorb in the visible) generally have some weakly bound or delocalized electrons such as the odd electron in a free radical or the electrons in a conjugated organic molecule. In this experiment we are concerned with the determination of the visible absorption spectrum of several symmetrical polymethine dyes and with the interpretation of these spectra using the "free-electron" model.