Fluorescence anisotropy and rotational diffusion of polyene-like molecules in solution

Oligo(phenylenevinylene)s and oligothiophenes of different chain-lengths are investigated in solution by UV–VIS absorption and fluorescence spectroscopy. Up to six absorption bands are identified in the spectral region below 45,000 cm−1. Applying the criteria of spectral position, chain-length dependence of spectral position, oscillator strength and polarization, the bands are assigned to electronic transitions calculated by semiempirical LCAO–MO–SCF methods. Two types of molecular orbitals (MOs) are distinguished. In polyene-like MOs the nodal surfaces run exclusively perpendicularly to the long molecular axis. In aromatic MOs, there are only nonbonding interactions between the rings. It is shown, that electronic transitions with chain-length dependent spectral positions involve exclusively MOs with prevailing polyene-like character. Absorption bands with approximately chain-length independent spectral positions are due to transitions between polyene-like MOs and MOs of mainly aromatic character. The chain-length dependences of the rotational diffusion constants of nBPVs and ECnTs in solution are determined by time-resolved fluorescence depolarisation measurements and rationalized by a simple model.