Fluorescence depolarization in poly(p-phenylphenylenevinylene) and related oligomers

Polarized optical spectroscopy applied to investigate two poly(phenylene-vinylene) polymers with different side chain structures

Two related poly(phenylene-vinylene) (PPV) light-emitting polymers have been investigated by means of polarized optical spectroscopy. The purpose of the investigation was to investigate the nature of the interactions in thin films and to examine what impact the difference in side chain structure and molecular weight in poly(2'-methoxy-5-2-ethyl-hexoxy)-1,4-phenylene vinylene (MEH-PPV) and poly(2-(3',7'-dimethyloctyloxy)-5-methoxy-1,4-phenylene-vinylene) (OC1C10-PPV) has on the electronic and optical properties of the two polymers. Aligning the polymers by dispersing them in anisotropic solvents and stretched films shows that the side chains have an impact on the relative orientations of the transition dipole moments. In anisotropic solvents the linear dichroism is larger for MEH-PPV than for the related polymer OC1C10-PPV, while in stretched films the opposite situation prevails. A lower polarization of the luminescence from OC1C10-PPV, relative to MEH-PPV, was also obtained independent of alignment medium used. The data therefore suggest that while mechanical stretching may align the OC1C10-PPV to a greater degree, the emitting species is distinct from the absorbing species. The circular dichroism (CD) spectra of both polymers undergo dramatic changes when the liquid phase and the solid state (film) are compared. The solution CD spectra shows no evidence of interchain interactions; instead the spectra of both systems indicate a helical conformation of the polymers. The CD spectra of films are dramatically different with the strong Cotton effect being observed. This points to the formation of an aggregate in the film, with an associated ground state interaction, an interchain species such as a physical dimer, or a more complex higher aggregate.

An investigation into the excitation migration in polyfluorene solutions via temperature dependent fluorescence anisotropy

Fluorescence anisotropy of dilute polyfluorene polymer solutions has been used to show that two processes, exciton migration and conformational relaxation (twisting of part of the chain), occur within polyfluorene polymers of 20 or more repeat units. The former process is dominant and temperature independent for high excitation energy but, as the chain length is decreased, exciton migration is eliminated and the conformational relaxation becomes the only mechanism by which excited state energy relaxation to the emission site can occur. When the polymers are excited in the absorption band tails, again no migration is observed but conformational relaxation is still present.

Steps to demarcate the effects of chromophore aggregation and planarization in poly(phenyleneethynylene)s. 1. Rotationally interrupted conjugation in the excited states of 1,4-bis(phenylethynyl)benzene

A series of photophysical measurements and semiempirical calculations were carried out with 1,4-bis(phenylethynyl)benzene in search of evidence on the effects of phenyl group rotation and chromophore aggregation of oligo- and poly(phenyleneethynylene)s. It is suggested that planarization gives rise to relatively modest shifts of ca. 20-30 nm, which preserve the vibronic structure of the monomer and retain a high emission quantum yield. In contrast, it is proposed that aggregation gives rise to larger shifts and loss of vibronic structure.