Structure−Property Relationships for Electron−Vibrational Coupling in Conjugated Organic Oligomeric Systems

Abstract

A series of π-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.

On the Origin of Green Emission Bands in Fluorene‐Based Conjugated Polymers

“…The Stokes' shift observed in the green products is higher than that observed in the blue products, indicating a poorer conjugation of the compound formed upon electrochemical reduction. [23] Very similar results were obtained for the electrochemically reduced PFC6 plate (Fig. S3 is equivalent to Fig.…”

Observation of the β‐Phase in Two Short‐Chain Oligofluorenes

“…It can be seen that the Stokes shift (D) also decreases with increasing chain length; an effect attributed to restricted structural relaxation of longer molecular chains. [25] In general, we find that the absorption and PL spectra recorded in thin-film are similar to that of the solutions apart from the fact that the measurements made in thin-film are all (except absorption of PF8) shifted to slightly lower energies (around 20 meV (absorption) and 50 meV (PL)). This effect may have a range of origins, including an increase in conjugation length and also the effect of the difference in dielectric constant of thin-film and solution.…”

Single Molecule Electronic Devices

“…In general, the HOMO − LUMO gap of π -conjugated molecules decreases with an increase in conjugation length. [ 126 ] It is thus reasonable to expect longer conjugated molecules to exhibit a smaller value of V trans than shorter conjugated molecules within a given molecular series. [ 121 ] Recently, TVS has also facilitated the calibration of orbital energy positions in molecular transistors.…”