Fluorescence spectroscopic properties and single aggregate structures of π-conjugated wire-type dendrimers

The spectroscopic properties of seven varieties of wire-type dendrimers, which consist of different generation numbers of dendritic wedges and molecular weights, were examined in tetrahydrofuran solution, in the solid state, and in good/poor solvent mixture for the purpose of revealing the relation between molecular conformation and spectral behavior. The second and third generation wire-type dendrimers showed a sharp aggregate band, which is ascribed to the interchain pi stacked aggregate, in the solid state and in the good/poor solvent mixture, while the fourth generation wire-type dendrimer did not show the aggregate band. The absence of the aggregate band indicates that the bulky fourth generation dendritic wedges prevent the conjugated backbone from forming the interchain pi stacked aggregate even in the solid state and in the good/poor solvent mixture. In the experiment on poor solvent-induced aggregation, we observed the progressive spectral change in the process of forming the aggregate, and simultaneously the aggregate formation was confirmed by observation of the fluorescence microscope image. The progressive spectral change suggested the molecular conformational change during the aggregate formation. Furthermore, the difference of molecular conformation depending on each aggregate and on some positions of the single aggregate was revealed using fluorescence microspectroscopy with the spatial resolution of about 1 mum.