Dendrimers are generally synthesized in two ways: one is the divergent approach where the molecule is built up from the inside (core) to the outside (periphery). The other is the convergent approach where the molecule is synthesized from the periphery to the core. Our recently developed synthesis allows preparation of all-thiophene dendrimers in either a convergent or a divergent approach. All-thiophene dendrimers based on branched terthiophene units were synthesized up to a 4^th generation (G4) which consists of 90 thiophene units (Figure 1).^[1] In the meanwhile, we also synthesized various all-thiophene dendrimers with different repeating units.

Figure 1. Chemical structure of G4 all-thiophene dendrimer 90T.

By using this novel synthetic approach, optical or redox-active groups can be introduced to the dendrimer skeleton at the core-, arm- and/or periphery. Due to their branched 3D structures, these dendrimers showed unique physicochemical properties. As an example, Figure 2 depicts a dendritic oligothiophene-perylene bisimide hybrid molecule (DOT-PBI). Interesting competitive intramolecular fluorescence resonance energy transfer (FRET) and photoinduced electron transfer (PET) from the DOT to the PBI moiety occur within the molecule.^[2]

Figure 2. Intramolecular energy transfer within DOT-PBI molecule

Conjugated all-thiophene dendrimers synthesized in our group represent semiconducting materials with band gaps between 2.2-2.5 eV. In combination with fullerene derivatives as electron acceptor, these DOTs showed excellent power conversion efficiencies (up to 2.8%) in solution-processed bulk heterojunction solar cells.^[3]