Shape-Persistent Oligothienylene-Ethynylene-Based Dendrimers: Synthesis, Spectroscopy and Electrochemical Characterization

A series of novel structurally well-defined oligothienylene-ethynylene-based dendritic macromolecules up to the 3rd generation (G3) were successfully synthesized by a combination of Pd-catalyzed Sonogashira-type cross-coupling and oxidative homocoupling steps. Oxidative homocoupling of dendrons successfully afforded dendrimers up to the 2nd generation (G2). In contrast, the G3 dendrimer was effectively prepared by a four-fold Sonogashira-type cross-coupling reaction. All compounds showed broad and structureless absorption and emission spectra arising from the presence of different pi-conjugated chromophores. With increasing generation, a bathochromic shift of the pi-pi* absorption band and an increase of the absorption coefficient were observed. The insertion of ethynylene groups into the conjugated dendrimer backbone resulted in a hypsochromic shift compared to all-thiophene dendrimers reported earlier by our group. All dendritic compounds are fluorescent and showed moderate quantum efficiencies due to an effective intramolecular charge-transfer (ICT) process. Cyclic voltammetry measurements also revealed the presence of multiple pi-conjugative pathways that show very broad oxidation waves for higher generations. HOMO-LUMO energy levels of these dendrons and dendrimers were estimated from optical and redox measurements and the calculated band gaps were within the range of 3.3 to 2.4 eV, typical for oligo- and polythiophenes. Electrochemical polymerizations of several desilylated compounds were performed and characterization of the films is reported. Preliminary bulk heterojunction solar cells that utilise these ethynylated dendrimers as the donor and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM[60]) as the acceptor showed moderate efficiencies ranging from 0.18-0.64%.