jp5b10879_si_001.pdf (1.12 MB)
Influence of the Conjugation Length on the Optical Spectra of Single Ladder-Type (p‑Phenylene) Dimers and Polymers
journal contribution
posted on 2016-01-21, 00:00 authored by Sebastian Baderschneider, Uli Scherf, Jürgen Köhler, Richard HildnerWe 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.