Traditionally vibrational infrared and Raman spectra are used by synthetic chemists for the simple chemical characterization of materials. Most chemists just give a quick look at the spectrum to judge the success of their synthesis or the chemical purity of the sample. A smaller scientific community has focussed on the more detailed vibrational assignments based on symmetry arguments and/or on isotopic rules for the determination of the molecular shapes and sometimes for the actual measurement of the molecular geometrical parameters. An even smaller community has accounted for the vibrational spectra in terms of molecular dynamics supported by theoretical arguments and by numerical computations.A more modern approach to the understanding of the molecular vibrational spectra is the use of numerical simulations based on 'molecular mechanics' or semiempirical or ab initio quantum mechanical calculations. These calculations have reached a level of automation which we feel is seriously damaging to science when used in an uncritical way.We hope to show that the vibrational spectra of conjugated oligomers and polymers are a very rich source of data which are lost by quick and superficial analysis based on molecular correlations; are a unique source of specific information on the structure and properties of these materials if an attempt is made to go beyond simple diagnosis based on chemical correlations; and are a prototype for the simultaneous use of vibrational frequencies and intensities which carry the understanding of molecular properties to extreme limits.In the past vibrational spectra of conjugated materials have been dealt with both by 'problem oriented' material scientists and by 'technique oriented' molecular spectroscopists. The two approaches are different and the information derived differs in depth and detail. Our aim is to provide material scientists with the technical tools developed by specialized spectroscopists in order to derive unique and specific information on the molecular properties of such technologically relevant materials.First we briefly sketch the physics and the derived vibrational frequency and intensity spectroscopy of chain molecules considered first as ideal infinite and periodic systems and then reduced to realistic finite and possibly disordered molecular systems. The purpose is to provide the reader with the techniques used in spectroscopy for the study of chain molecules and the derived conclusions of general validity. Then we offer an overview of the electronic and optical properties of conjugated oligomers and polymers and present a detailed list of the particular
