Photophysics of two prototypical molecular-wire building blocks: solvent-induced conformational dynamics?

The photophysics of two molecular wire building blocks of different lengths based on p-phenyleneethylene, namely, 1,4-bis[p-(N,N-dimethylamino)phenyl]-1,2-ethyne and 1,4-bis[p-(N,N-dihexylamino)phenylethynyl]benzene, are studied experimentally in a wide range of organic solvents. The band shape and position of the electronic absorption and fluorescence emission of both compounds are discussed in terms of the empirical Catalán linear solvent energy relationship and the analytical solvation model of Liptay. It is found that solute polarizability plays an important part in the description of the pronounced solvatochromism for these highly symmetric molecules. In addition, the dependence of the emission quantum yield and the excited-state lifetime on the solvent are measured. The experimental findings can only be partially rationalized by the common theoretical models. They indicate that not only torsion about the triple bonds but also solvent-solute reorganization must be taken into account.