We present the synthesis of regioregular polythiophenes with alkylthiophene side chains P3TC16 prepared by Ni-catalyzed polymerization from the branched, thiophene-based monomer 5-bromo-5″-hexadecyl-[2,2′;3′,2″]terthiophene. The optical properties in solution and thin films of the polymer were investigated in situ as a function of temperature and compared to the low regioregularity analogue FeP3TC6 synthesized by Fe(III) mediated oxidative polymerization of 5″-hexyl-[2,2′;3′,2″]terthiophene. It was found that due to the regioregular structure, P3TC16 tends to strong aggregation in solution, which is ascribed to π−π interactions. The bandgap in thin films of 1.88 eV is slightly smaller than the bandgap of the reference polymer poly(3-hexylthiophene) (P3HT, 1.91 eV). Interestingly, it was found that the HOMO and LUMO levels of P3TC16 are shifted to significantly lower values as compared to P3HT. First results regarding the application of P3TC16 in FETs are shown and mobilities of up to 3.1 × 10 −2 cm 2 /(V s) were achieved. Open circuit voltages of up to 710 mV in combination with PC[60]BM in organic solar cells were found, which is about 30% higher than for P3HT, which can be attributed to the low HOMO energy level.

A zoo of chemical structures is accessible when the branched unit 2,2':3',2″-terthiophene (3T) is included both in structurally well-defined small molecules and polymer-like architectures. The first part of this review article highlights literature on all-thiophene based branched oligomers including dendrimers as well as combinations of 3T-units with functional moieties for light-harvesting systems. Motivated by the perfectly branched macromolecular dendrimers both electropolymerization as well as chemical approaches are presented as methods for the preparation of branched polythiophenes with different branching densities. Structure-function relationships between the molecular architecture and optical and electronic properties are discussed throughout the article.

The potential of 2,2';3,2″-terthiophene (3T) as branching units in 3D copolymers is presented with EDOT as an example comonomer. Branched EDOT/3T polythiophenes were prepared by electropolymerization, and their electrochemical and optical properties are discussed. Two different approaches were employed: (i) the direct electropolymerization of a novel branched thiophene monomer (3TE3) consisting of a 3T core that contains three outer EDOT end groups and (ii) the electrochemical copolymerization of a EDOT/3T mixture in different ratios from [1:1] to [1:10]. Cyclic voltammetric and vis spectrometric experiments show that the EDOT content within the polymer has a strong influence on the electronic properties of the material: with increasing EDOT content, the HOMO-LUMO gap is decreased. To prove copolymer formation of EDOT and 3T, chemically synthesized reference copolymers of EDOT and 3T were prepared by oxidative coupling using FeCl3, and their optical and electronic properties were compared to those of the electrodeposited films. In addition, the copolymer formation is indicated by the comparison of the electrochemical and spectroscopic results with those of the homopolymers P3T and PEDOT.

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