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.

Direct observation of backbone planarization via side-chain alignment in single bulky-substituted polythiophenes

“…For other conjugated polymers, such as P3HT (22) and ladder-type poly(p-phenylenes) (37), we showed that the ZPLs are broadened by unresolved spectral diffusion, i.e., by random jumps of the transition energy on time scales faster than the temporal resolution of our measurement (here 1 s to 3 s). This dynamic disorder is induced by structural fluctuations of groups of atoms or molecules in the local environment of the emitting site (31,(37)(38)(39)(40). Accordingly, we ascribe the broad ZPLs of PDOPT to unresolved spectral diffusion processes as well.…”

Electronic Communication in Closely Connected BODIPY-Based Bichromophores

“…4,5 Apart from experimental and theoretical studies aimed at better appreciating the nuances of the special pair, considerable attention has been given to the detailed spectroscopic examination of artificial molecular dimers. [6][7][8][9][10] Notable among such studies have been dimers formed from porphyrins, 11 nucleobases, 12 aryl mono-and diimides, 13 and aryl hydrocarbons. 14 Many more studies have addressed the electronic properties of intermolecular 15 and intramolecular excimers.…”

Exciton Migration and Surface Trapping for a Photonic Crystal Displaying Charge‐Recombination Fluorescence

“…A temperature dependence emission study carried out with a single crystal of MBIC indicates that the re‐organization energy associated with charge‐recombination is approximately 1,530 cm −1 and the Huang–Rhys factor is 0.50. The latter is fully consistent with the exciton being localized on a single molecule, rather than being spread over several aligned molecules within the lattice. The re‐organization energy and Huang–Rhys factor are small compared to typical values determined in solution but, as mentioned above, solvent stabilization is not possible in the crystal.…”