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 p henylenevinylene oligomer with five phenylene units (OPV(5)) have been recorded employing the site selection technique. The spectra of the f ormer 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 estim ated. It decreases with increasing molecular length. Zero-phonon featu res are absent in the OPV(5) spectrum. Comparison with spectra of shor ter oligomers indicates an increase of excited state coupling to low e nergy phonons with increasing molecular length. The average Huang-Rhys factor for vibronic coupling is approximate to 0.8 for OPV(5).