Structure-property relationships for electron-vibrational coupling in conjugated organic oligomeric systems

A series of pi-conjugated oligomers containing one to six monomer units were studied by absorption and photoluminescence spectroscopy. As is common for these systems, a linear relationship between the positioning of the lowest-energy absorption and the highest-energy photoluminescence maxima plotted versus inverse conjugation length is observed, in good agreement with a simple nearly free electron model, one of the earliest descriptions of the properties of one-dimensional organic molecules. It was observed that the Stokes shift and therefore Huang-Rhys factor also exhibit a well-defined relationship with increasing conjugation length, implying a correlation between the electron-vibrational coupling and chain length. This correlation is further examined using Raman spectroscopy, whereby the integrated relative Raman scattering is seen to behave superlinearly with chain length. The Stokes shift and the Raman activity are also well-correlated in these systems. There is a clear indication that the vibrational activity and thus nonradiative decay processes are controllable through molecular structure.