Synthesis, Spectroscopic Properties, and Electropolymerization of Azulene Dyads

Four azulene dyads have been synthesized and studied by spectroscopic and electrochemical methods. A triarylamine, a boron-dipyrromethene (BDP or BODIPY), a porphyrin, and an isoalloxazine moiety have been linked to an extended π electron system at the 2-position of azulene, leading to the dyads <b>1</b>–<b>4</b>, respectively. For the synthesis of <b>1</b>–<b>4</b>, first 2-(4-ethynyl-phenyl)azulene (EPA) was prepared, which was further reacted with the halogenated chromophores by Pd-catalyzed cross-coupling reactions. The dyads <b>1</b>–<b>4</b> exhibit strong absorption bands in the visible range, which are dominated by the absorption spectra of the individual subchromophores. Fluorometric studies of <b>2</b>–<b>4</b> revealed that after excitation of the subchromophoric unit attached to the parent azulene moiety, quenching mainly through energy transfer to azulene is effective, whereas possible charge-transfer interactions play only a minor role. Potentiodynamic oxidation of the dyads <b>1</b>–<b>4</b> leads to the formation of polymer films, which are deposited at the electrode. The polymer film derived from <b>1</b> was further characterized by spectroelectrochemistry. During positive doping of poly-<b>1</b>, a strong absorption band appears at 13,200 cm<sup>–1</sup>, which is typical for triarylamine radical cations. This band is overlapping with a broad absorption band in the low-energy region that might be caused by charge-transfer interactions within the polymer.