Electronic Structure and Properties of Poly- and Oligoazulenes

Abstract

One of the most important research interests in the field of organic photovoltaic devices (OPVs) is the development of new materials which can serve as light-absorbing electron donors and hole-conducting (p-type) semiconductors. In this context, 1,3-polyazulenes were synthesized chemically and electrochemically. Their spectroscopic and electrochemical properties are compared with those of the 1,3-oligoazulenes Az1−Az6. The UV−vis spectra of the neutral azulenes Az1−Az6 show a linear correlation between the lowest absorption maximum and the inverse chain length 1/n leading to a band gap of Eg = 1.90 eV for infinite chain length. Derived from this correlation the effective conjugation length of chemically synthesized polyazulene is only about 10. By an alternative approach, a band gap of Eg = 1.46 eV was determined. Depending on the applied potential the oligomers Az2−Az6 undergo up to two reversible oxidation processes or further polymerization which results in the formation of polymer films at the electrode. The potentiodynamic oxidation of chemically synthesized polyazulene leads to electrocrystallization at the electrode, whereas films of polyazulenes are obtained directly upon oxidation of Az1−Az6. Chemically and electrochemically generated polyazulenes adsorbed on Pt show similar electrochemical behavior upon positive doping. The spectroelectrochemical investigations in combination with density functional theory (DFT) calculations lead to the conclusion that polyazulene can be oxidized up to a doping level of one charge per three or four azulene units. At this stage polarons or polaron pairs are formed (depending on the doping level) but not bipolarons. At higher doping levels the polymers start to decompose.