Acceptor End‐functionalization of Naphthalenediimide Bithiophene Oligomers

Oligomeric materials combine advantageous properties of both their small molecule and polymeric counterparts. Utilizing oligomers as non‐fullerene acceptors (NFAs) has been shown to be extremely useful for the development of organic solar cells with high efficiency, reproducible performance and long‐term stability. Here we report on two series of synthetically simple acceptor‐terminated oligomers A−T2‐(NDI−T2)n‐A with naphthalene diimide (NDI) and bithiophene (T2) cores up to the trimer (n =1,2,3). Termination of the oligomers is done using the strong acceptors (A) dicyanomethylene‐indanone (IC) and rhodanine (RD). Upon acceptor termination in the presence of piperidine (pip) as base, oligomers with pip‐substituted tricyclic end groups are obtained in high yield. We investigate the effect of oligomer length and acceptor end group on opto‐electronic properties and crystallinity. Both IC‐ and RD‐termination increase electron affinity compared to the parent, non‐functionalized cores. UV‐vis absorption in solution slightly redshifts as the chain length increases without showing a distinct aggregation. Asymmetric termination with hexylphenyl‐substituted indacenodithiophene (IDT) and IC is also possible. All symmetric oligomers show a strong tendency for crystallization, with the oligomer having the tricyclic end group exhibiting the highest melting enthalpy and temperature. The asymmetric IDT−T2‐NDI−T2‐IC oligomer is amorphous.