title: Influence of the Conjugation Length on the Optical Spectra of Single Ladder-Type (p-Phenylene) Dimers and Polymers.
abstract: We employ low-temperature single-molecule photoluminescence spectroscopy on a π-conjugated ladder-type (p-phenylene) dimer and the corresponding polymer methyl-substituted ladder-type poly(p-phenylene), MeLPPP, to study the impact of the conjugation length (π-electron delocalization) on their optical properties on a molecular scale. Our data show that the linear electron-phonon coupling to intramolecular vibrational modes is very sensitive to the conjugation length, a well-known behavior of organic (macro-) molecules. In particular, the photoluminescence spectra of single dimers feature a rather strong low-energy (150 cm(-1)) skeletal mode of the backbone, which does not appear in the spectra of individual chromophores on single MeLPPP chains. We attribute this finding to a strongly reduced electron-phonon coupling strength and/or vibrational energy of this mode for MeLPPP with its more delocalized π-electron system as compared to the dimer. In contrast, the line widths of the purely electronic zero-phonon lines (ZPL) in single-molecule spectra do not show differences between the dimer and MeLPPP; for both systems the ZPLs are apparently broadened by fast unresolved spectral diffusion. Finally, we demonstrate that the low-temperature ensemble photoluminescence spectrum of the dimer cannot be reproduced by the distribution of spectral positions of the ZPLs. The dimer's bulk spectrum is rather apparently broadened by electron-phonon coupling to the low-energy skeletal mode, whereas for MeLPPP the inhomogeneous bulk line shape resembles the distribution of spectral positions of the ZPLs of single chromophores.
authors.affiliation: Experimentalphysik IV and Bayreuth Institute for Macromolecular Research (BIMF), Universität Bayreuth , 95440 Bayreuth, Germany.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Baderschneider
authors.affiliation: Fachbereich C - Mathematik und Naturwissenschaften and Institut für Polymertechnologie, Universität Wuppertal , Gauss-Strasse 20, 42097 Wuppertal, Germany.
authors.forename: Uli
authors.initials: U
authors.lastname: Scherf
authors.affiliation: Experimentalphysik IV and Bayreuth Institute for Macromolecular Research (BIMF), Universität Bayreuth , 95440 Bayreuth, Germany.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Köhler
authors.affiliation: Experimentalphysik IV and Bayreuth Institute for Macromolecular Research (BIMF), Universität Bayreuth , 95440 Bayreuth, Germany.
authors.forename: Richard
authors.initials: R
authors.lastname: Hildner