A comparative site-selective fluorescence study of ladder-type para-phenylene oligomers and oligo-phenylenevinylenes

Fluorescence spectra of the monomer, dimer and trimer of a ladder-type para-phenylene with perfectly planarized rr-systems as well as of a phenylenevinylene oligomer with five phenylene units (OPV(5)) have been recorded employing the site selection technique. The spectra of the former compounds, embedded in a 6 K MTHF glass, show zero-phonon lines associated with the various vibronic features built on an origin that is resonant with the laser. For the trimer the Huang-Rhys factors for coupling of the electronic transition to molecular vibrations is estimated. It decreases with increasing molecular length. Zero-phonon features are absent in the OPV(5) spectrum. Comparison with spectra of shorter oligomers indicates an increase of excited state coupling to low energy phonons with increasing molecular length. The average Huang-Rhys factor for vibronic coupling is approximate to 0.8 for OPV(5).